Monazite geochronology

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Illustration showing age map and zonation pattern of a monazite grain. Brighter colour represents older age. Edited after Williams, 1999.

 

Monazite geochronology is a dating technique to study geological history using the mineral monazite. It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as igneous, sedimentary and hydrothermal rocks. The dating uses the radioactive processes in monazite as a clock. 

The uniqueness of monazite geochronology comes from the high thermal resistance of monazite, which allows age information to be retained during the geological history. As monazite grows, it forms successive generations of different compositions and ages, commonly without erasing the previous ones, forming zonation patterns in monazite. Because of the age zonation, dating should be done on individual zones, rather than the whole crystal. Also, textures of monazite crystals may represent certain type of events. Therefore, direct sampling techniques with high spatial resolution are required, in order to study these tiny zones individually, without damaging the textures and zonations.

The advantage of monazite geochronology is the ability to relate monazite compositions with geological processes. Finding the ages of compositional zones can mean finding the ages of geological processes.

Decay of U and Th to Pb

Monazite is a rare-earth-element phosphate mineral, with the chemical formula e.g. (Ce, La, Nd, Th, Y)PO₄. It appears in a small amount as an accessory mineral in many igneous, metamorphic and sedimentary rocks. Monazite minerals contain significant amounts of radioactive elements Th and U, which trigger radioactive processes. These two elements are what make this mineral suitable for radiometric dating.

In the radioactive processes, the three unstable parent isotopes decay into their respective stable daughter isotopes of Pb. Each following a decay chain consisting of alpha and beta decays, parent isotopes ²³⁸U, ²³⁵U and ²³²Th, decay into a series of intermediate daughter isotopes, and finally lead to stable isotopes, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb, respectively. Each decay chain has a unique half-life, which means the daughter isotopes are generated at different rates.

The decay processes can be simplified as the following equations, which omit all the intermediate daughter isotopes.

${\displaystyle ^{238}U\rightarrow ^{206}Pb+8\alpha +6\beta ^{-}+Energy\quad \lambda _{238}=1.55125{e}^{-10}yr^{-1}\quad t_{1/2}=9842yr}$

${\displaystyle ^{235}U\rightarrow ^{207}Pb+7\alpha +4\beta ^{-}+Energy\quad \lambda _{235}=9.8485{e}^{-10}yr^{-1}\quad t_{1/2}=1550yr}$

${\displaystyle ^{232}Th\rightarrow ^{208}Pb+6\alpha +4\beta ^{-}+Energy\quad \lambda _{232}=4.9475{e}^{-10}yr^{-1}\quad t_{1/2}=3086yr}$

where α represents alpha particle, β⁻ represents beta particle, λ represents decay constant and t_½ represents half-live.

Monazite geochronology studies the ratio of parent isotopes to daughter isotopes (isotopic ratio), and calculates how much time has passed since daughter isotopes start accumulating.

Radiometric age and geological age

Radiometric age represents the time when the decay process starts. Geological age represents the time when a geological event occurs. Manipulating the isotopic ratios can only give us radiometric age. To obtain the geological age, we need to know the relationship between the two. In other words, how do geological events affect the radioactive system in monazite? Actually, the radioactive system is like a digital 'clock', while the geological processes can be like replacing a battery. When a new battery is inserted, this 'clock' starts counting from 00:00. This process is what we call the age resetting mechanism. In monazite, the age resetting is caused by the loss of Pb. Pb is produced continuously by the decays of U and Th since the radioactive system (clock) starts running. The more Pb (or less U and Th) the system contains means the longer period has been passed. If all Pb are suddenly removed from monazite by a geological event (replacing battery), the age become zero (00:00) again. Before thinking what exact geological events trigger Pb loss (see section: Interpretation and application), it is important to know the two mechanisms causing Pb loss in monazite.

Mechanisms of Pb loss

Solid-state diffusion

Closure temperature for U-Pb dating

Mineral	Tc for U-Pb dating (°C)
Titanite	600—650
Rutile	400—450
Apatite	450—500
Zircon	>1000
Monazite	>1000

Solid-state diffusion is the net movement of atoms in solid phase, from region of higher concentration to that of lower concentration. It is easy to imagine diffusion in liquid phase as ink spreading in water. Solid-state diffusion of Pb is the net exchange of Pb in the solid mineral with the external environment, which is usually a fluid. In most of the cases, Pb is transported from the mineral to the fluid, resulting in Pb loss and thus age resetting.

The rate of diffusion increases with temperature as atoms are moving faster. However, as the mineral cools and the crystal structure becomes more complete, the diffusions of parent and daughter isotopes slows down and finally become insignificant at a certain temperature. This closure temperature (T_c) depends on the crystal size, shape, cooling rate and diffusion coefficient, which in turn varies for each mineral and radioactive systems. That is, above T_c, Pb is continuously lost and the radioactive clock is keeping zero. Once the temperature falls below T_c, the system is closed and the clock starts counting.

Monazite is characterised for its high Pb retention ability even at high temperatures for a prolonged period. The closure temperature of monazite in U-Th-Pb system is higher than 800 °C, much higher than the other common minerals.

Fluid-assisted dissolution-precipitation

 

Successive growth of monazite grain by fluid-assisted dissolution-precipitation. (1) Monazite (orange) dissolves along reaction front the contact with the fluid (yellow) (2) Monazite reprecipitates as an altered monazite with a new chemical composition (pink) (3) Reaction continues with fluid being transported to the reaction front by infiltration paths. (A) Reaction ceased due to recrystallisation of precipitating phase (dark orange). (B) Reaction ceased due to change in reaction system (blue).

 

Unlike solid-state diffusion, fluid-assisted dissolution-precipitation occurs below T_c. Interaction between mineral phase and coexisting fluid phase during geological events directly contributes to this process. It is a chemical reaction driven by the system stabilisation from minimising Gibbs free energy. A reactive fluid is present as a catalyst and a source of reactants for the reaction.

If a geological process gives a suitable fluid and temperature, monazite dissolves along the contact with the fluid (reaction front), and reprecipitates as an altered monazite with a new chemical composition. The rates of the dissolution and reprecipitation are the same, so that the original mineral phase is always in contact with the precipitating phase, separated by only a thin layer of fluid as a reaction medium. Once the reaction is activated, it is self-continuing. The reaction front migrates towards the centre of the parent monazite, leaving behind the newly formed monazite, forming a core-rim structure.

The composition of the precipitating phase depends on the fluid composition and temperature. During most of the reactions, Pb is efficiently removed and the precipitating phase is Pb-free. Therefore, the age of the newly formed rim is reset, representing the time of this alternation.

There are basically two factors causing the reaction to cease. (A) Reaction ceases due to the recrystallisation of precipitating phase, removing all the fluid infiltration paths. This results in fluid inclusion in monazite. (B) Reaction ceases due to change in system such as composition of fluid and monazite, making this reaction no longer reactive.

Implications for monazite geochronology

Range of geological processes at different temperature recorded by monazite, zircon and apatite in U-Pb dating

 

Since the diffusion of reactants between dissolving phase and precipitating phase is slow, the fluid is essential to provide an easy transport for the reactants. Yet as reaction proceeds, dissolving phase and the fluid are separated by the solid precipitating phase, blocking the transport of reactants. Therefore, there must be some inter-connected porosity in the precipitating phase, which allows the fluid to infiltrate and fuel the reaction front.

Most other geochronometers usually have a much lower closure temperature. Once they are subject to a temperature higher than T_c, all age information will be reset, losing information of the past geological events. In contrast, since monazite has a high T_c, even it experiences younger high-grade metamorphism with high temperatures, it is likely that the previous geological history is preserved. Furthermore, dissolution-precipitation is usually triggered by geological events such as metamorphism, deformation and hydrothermal alternation below T_c. Each of these events writes a new age information by precipitating a new domain without erasing the older information. Therefore, it is likely that monazite preserves a complete history of generations.

Monazite and zircon are two minerals that commonly employed in geochronology to study geological history. They both exhibits high closure temperatures which make them suitable to reveal igneous and metamorphic events. However, they behave differently throughout the geological history. Generally, monazite performs better in recording metamorphism (recrystallisation ages) with different zonation patterns in ages and composition. Zircon is not as reactive as monazite during metamorphism reaction and better in recording igneous events (cooling ages). Moreover, monazite is suitable in dating relatively low-temperature metamorphism for example amphibolite-facies than zircon.

Monazite zonation

Zonation is a characteristic of monazite. A single monazite grain can contain domains of distinctively different compositions and ages. These domains are widely accepted to represent episodes in geological history during monazite growth or recrystallisation. The key to monazite geochronology is to find out what geological events or environments a domain is representing, by comparing its chemical composition with mineral stability and reactions. The age of the event is thus represented by the domain age. 

The ideal formula of monazite is [LREE(PO₄)], the variation in composition is mainly due to the chemical substitutions of light rare earth elements (REE) in monazite by other elements. One of the common substitutions are the exchange between LREE with Th and Ca, and P with Si to form huttonite [Th(SiO₄)] and brabantite [CaTh(PO₄)₂]. Since all three minerals share the same chemical structure, they are the three endmembers in their solid solution, meaning that they appear in a same solid phase where substitutions happen. It is important to note that the composition zonation pattern may not be the same when we are considering different elements. And age zonation may have no relationship with composition zonation at all. (see images from the section: analysis procedures) Thus, it needs to be very careful in linking among zonations. In natural monazite, the zonation pattern maybe complex and hard to interpret. Below we describe some simple chemical zonation patterns and the associated interpretations. Zonation patterns associated with igneous activity are usually easy to interpret. However, those associated with metamorphism are more complicated.

Concentric zoning

Concentric zoning: monazite grows with new successive layers with different compositions

 

Sector zoning: different elements crystallised preferentially at different faces of the crystal

 

Core-rim zoning: altered rim formed surrounding the original core under dissolution-precipitation reaction

One of the monazite formations is crystallization of an igneous melt. The concentric zoning pattern reflects the changing composition of the melt which affect the crystallising composition of monazite.

Sector zoning

Sector zoning is also associated with the crystallization of monazite in a melt. However, some elements may have a tendency to crystallise onto a specific crystal face. It results in uneven growth and composition around monazite.

Core-rim zoning

Core-rim zoning is usually associated with the fluid-assisted dissolution-precipitation in metamorphic reactions, forming rims with new composition. The fluid composition and metamorphic grade (H/T) are important factors on the rim composition.

Other zoning patterns

Mottled and patchy zoning patterns are the more complicated zonations. The interpretations are usually not simple.

Dating approaches

Isotopic dating and chemical dating are the two typical dating approaches used in monazite geochronology. Both methods make use of the radioactive nature of Th and U in monazite.

Isotopic dating

Isotopic dating requires measuring the isotopic concentration of radioactive U and Th, and radiogenic Pb in monazite. By treating each decay chain in the U-Th-Pb system independently, three classic isochron equations can be obtained: 

${\displaystyle \left({\frac {^{206}Pb}{^{204}Pb}}\right)=\left({\frac {^{206}Pb}{^{204}Pb}}\right)_{0}+\left({\frac {^{238}U}{^{204}Pb}}\right){\bigl (}e^{\lambda _{238}t}-1{\bigr )}}$

${\displaystyle \left({\frac {^{207}Pb}{^{204}Pb}}\right)=\left({\frac {^{207}Pb}{^{204}Pb}}\right)_{0}+\left({\frac {^{235}U}{^{204}Pb}}\right){\bigl (}e^{\lambda _{235}t}-1{\bigr )}}$

${\displaystyle \left({\frac {^{208}Pb}{^{204}Pb}}\right)=\left({\frac {^{208}Pb}{^{204}Pb}}\right)_{0}+\left({\frac {^{232}Th}{^{204}Pb}}\right){\bigl (}e^{\lambda _{232}t}-1{\bigr )}}$

where ${\displaystyle {\bigl (}\quad {\bigr )}_{0}}$ represents the initial isotopic ratio when the system resets, t represents the time after the system reset, and λ²³⁸, λ²³⁵ and λ²³² are the decay constants of ²³⁸U, ²³⁵U and ²³²Th respectively.

Combinations of the use of the above equations, such as U-Th-Pb dating, U-Pb dating and Pb-Pb dating, require different levels of analysis techniques and offer variable levels of precision and accuracy. The general uncertainty in the ages measured is 2σ (e.g.).

Chemical dating/ Total Pb dating

Chemical dating requires measuring the elemental abundances of U, Th and Pb but not isotopes. U-Th-total Pb dating, also known as electron microprobe U–Th–Pb dating, measures the elemental abundances of the three elements by an electron microprobe, and calculate the age (t) by the below equation.

$${\displaystyle Pb={\frac {Th}{232}}[\exp(\lambda ^{232}t)-1]208+{\frac {U}{238.04}}0.9928\times [\exp(\lambda ^{238}t)-1]206+{\frac {U}{238.04}}0.0072\times [\exp(\lambda ^{235}t)-1]207}$$

 

where Pb, Th and U are concentrations in parts-per-million, and λ²³², λ²³⁵ and λ²³⁸ are the decay constants of ²³²Th, ²³⁵U and ²³⁸U respectively.

 

For chemical dating results to be valid, the following assumptions are required:

1. Non-radiogenic Pb is negligible compared to radiogenic Pb.
2. No modification of U/Th/Pb has occurred except radioactivity.

The first assumption tends to be true since monazite is very unlikely to incorporate Pb during its growth. The non-radiogenic Pb content in many laboratory tests were found to be very low, nearly always lower than 1 ppm. The most common error arose from this assumption is the contamination with lead during sample preparation. The second assumption is usually justified by the concordant behavior of the mineral observed in tests. That means the system is either reset totally or unaffected totally by geological processes, there is not partial resetting of the system. Minor errors are arose due to negligible disturbance during mass transfer.

The theory is that monazite has high contents of Th (generally 3-15% and up to 25% of its weight) and U (generally hundreds of ppm and up to 5% in concentration). Thus, Pb accumulates at a high rate by radioactive processes. In less than hundreds of years, it reaches a level high enough to be measured accurately by an electron microprobe.

Analysis techniques

Age and composition zonations as well as the texture of monazite provide evidence on the successive growth of the crystal during geological events. The scope of such information that can be obtained largely depends on the analysis techniques employed in geochronology.

Comparison between convectional and in situ analysis

Convectional analysis

 

Conventionally, monazite is separated from samples by dissolution and chemical methods. Single or fractions of crystals are selected for dating, usually by thermal ionization mass spectrometry (TIMS). That means one age is generated for a single monazite crystal or for a group of crystals. The age information obtained is obviously inconsistent and inaccurate, because even a single monazite crystal contains zones of different ages. Also, the mechanical separation for monazite often destroy the associated textual and spatial information of monazite, which is crucial in interpreting relationship between domains and geological environment.

In situ analysis

 

	Convectional analysis	In situ analysis
Sampling	Physical/chemical separation	Direct sampling
Dating target	Single/ fractions of grains	Age domains
Dated age	Inconsistent	Consistent
Texture preserved?	No	Yes

For the above reasons, the demand for in situ analysis is increasing. In situ means analyzing monazite at its original place without monazite separation (refer to in situ) such that the texture and zonation pattern are kept intact in order to reveal a more comprehensive geological history of the host rock. Direct sampling techniques, high spatial resolution and precision are the requirements for an in situ analysis. With technological advancement, more and more measurement tools such as laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and laser microprobe mass spectrometer (LMMS) become capable for such analysis.

Analysis procedures

Below shows a general procedure for monazite dating. The characteristics and procedure are different for each measurement tool, especially sample preparation and dating method. Details of some common measurement tools are described in the section: Measurement tools.

1. Sample preparation
2. Monazite identification and mapping
3. Monazite compositional mapping
4. Monazite age mapping
5. Quantitative dating

Sample preparation: Thin sections of limestone rocks

 

Monazite identification: Illustration showing backscattered electron image of a rock sample with monazite at the centre with white colour. Edited after Williams, 1999.

 

Compositional mapping: Illustration showing X-ray Th composition map of a monazite grain. Brighter colour represents higher concentration. Edited after Williams, 1999.

 

Quantitative dating: Histogram of age measured, showing two age zonations in monazite. Edited after Williams, 1999.

 

Illustration of age map of a monazite grain. Brighter colour corresponds to older age. Edited after Williams, 1999.

Sample preparation

In both conventional and in-situ dating, a thin section of the rock in interest is prepared. First, a thin layer of rock is cut by a diamond saw and ground to become optically flat. Then, it is mounted on a slide made of glass or resin, and ground smooth using abrasive grit. The final sample is usually only 30 μm thick.

Monazite identification and mapping

Monazite grains are identified by backscattered electron imaging survey or/and electron microprobe analysis (EMPA) by mapping concentration of distinctive Ce in monazite. The two images are usually superimposed to reflect sample texture and monazite locations at the same time.

Monazite compositional mapping

Monazite grains which show useful relations with microtextures or host minerals are selected for compositional mapping. Major elemental and sometimes trace elemental maps are created at high magnification by electron microprobe X-ray mapping to show composition zonation patterns. Maps of elemental Y, Th, Pb, U have been proven useful in identifying composition domains in monazite.

Monazite age mapping

Estimated ages are calculated across the compositional map by analysing the concentration of Th, Ph and U by total-Pb dating method. The result is then used to generate an age map which approximately identifies all the age domains.

Quantitative dating

A number of spots within an age domains are selected and further dated accurately with the measurement tools by isotopic dating method. The results are then analysed statistically to give an accurate age of each age domain.

Measurement techniques

Employment of different analysis techniques (conventional or in situ analysis) provides selection of different measurement techniques. Choice between these techniques in turn affects the resolution, precision, detection limits and costs of monazite geochronology. The recent analytical progress in U-Th-Pb system in natural monazite has been mainly achieved by (1) Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS), (2) Secondary Ion Mass Spectrometry (SIMS), (3) Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and (4) Electronic Microprobe Analyses (EMPA).

Conventional analysis

Isotope dilution thermal ionization mass spectrometry

In the 1950s, Alfred Nier developed the technique of ID-TIMS, which later become the first tool used in monozite geochronology. Since this method involves the chemical separation of monazite (isotope dilution), it is regarded as a conventional analysis technique. Generally, it takes several hours for a U-Pb measurement. The precision of date is nearly 0.1%, provided that the ages are concordant (not dating with mixing of zonations). It is regarded as the most precise method in monozite geochronology.

Monazite mineral grains selected are carefully hand-picked for dating. They are spiked with a tracer solution and dissolved in HF or HCl. Using ion exchange chemistry, U, Th and Pb are separated from other elements. The purposes of the separation are (1) potential isobaric interference should be removed before analysis because of the high-sensitivity and low-mass resolution nature of TIMS; (2) ionization of the interested elements maybe impeded by other elements, which results in reduced signal size and precision.

The separated U, Th and Pb samples are put carefully onto a metal filament, which is usually made from Re. The elements are heated and ionize to the respective ions, which accelerate under strong magnetic field and are measured by a detector. 

The tracer solution is a solution with a known amount of U and Pb tracer isotopes. Due to elemental fractionation, both elements cannot be measured simultaneously by TIMS. The tracer solution is therefore used to measure ratios of sample isotope to tracer isotopes. The ratios are converted to moles of sample isotopes for dating.

In situ analysis

The following measurement techniques applies in in situ analysis which involves direct sampling of monazite grains using an incident ion beam or a laser.

Secondary ion mass spectrometry

Old magnetic sector SIMS by the manufacturer CAMECA

 

SIMS is a mass spectrometry method to measure small-scale elemental and isotopic variations of samples. Its ability to measure in spots with small-diameter (10-40 μm) makes it a useful tool to date small (less than 100 μm) minerals and domains in a single crystal for monozite geochronology. SIMS can achieve a precision of ~3% on the dates. Sensitive high-resolution ion microprobe (SHRIMP) is widely regarded as a powerful tool among SIMS.

SIMS analyzes the mineral surface (a few μm) composition by sputtering the surface with a focused primary ion beam under vacuum. The secondary ions liberated from the mineral are accelerated, analysed and measured in the mass spectrometer. Sample are analysed in rotation with standard with known elemental or isotopic ratios in order to measure the ratios in the sample for dating.

Laser ablation inductively coupled plasma mass spectrometry

The application of LA-ICPMS in U-Pb geochronology started in the 1990s. Since it enables relatively short and cheap yet high-spatial-resolution analysis, it has become the most utilized method of monazite geochronology. The precision of LA-ICPMS is limited by standard variability, which is about 2% on date.

Mineral sample surface is sputtered by a laser inside a sample cell. The ablated particles are collected and incorporated into a carrier gas. The resulting aerosols is analyzed by a mass spectrometer for dating. A solid-state or gas-source laser with short wavelength is commonly used as the laser ablation system in geochronology.

Electronic microprobe analyses

EMPA is employed in monazite geochronology especially in in situ chemical dating (total-Pb dating). The high content of U, Th and Pb in monazite match with the requirement arose from the relatively higher lower detection limit. Therefore, EMPA is thus a high-resolution (approximately 1 μm), rapid and inexpensive method in chemical dating to resolve growth histories of monazite. It can achieve a precision of 5—10 myr in Pb-rich monazite, and 10—20 myr in Pb-poor monazite.

Interpretation and application

Monazite geochronology can reveal complex geological history recorded in the monazite mineral grains. The characteristic composition and age zonations are the basic for carrying out such analysis, with each domain representing a past geological event with a certain age. The most important issue in monazite geochronology is to relate textures and compositions in each domain to the associated geological events.

Even for a single monazite grain may reveal complex history, in which events maybe inter-related or even happen at the same time, making it hard to clearly separate each event for discussion. The below section aims to provide briefly how composition and age data are interpreted to link different types of events.

Crystallisation of melt

Understanding the igneous petrology of monazite is important to date crystallisation age of igneous rocks. Monazite is commonly present as accessory mineral in low-CaO peraluminous granitoids, from diorites, micaceous granites to pegmatites. The reason of the low CaO content is probably that melts with high CaO content promotes the formation of apatite and allanite but not monazite. It is commonly formed from the magmatism involving carbonatic melts but not mafic plutons or lavas. Those rocks usually host economic REE ore deposits, making monazite geochronology important in mining exploration.

The simplest monazite zonation showing successive crystallisation of melts is concentric zonation, with new monazite crystallised as rims by rims surrounding the core. The rims often shows compositional variation due to the preferential incorporation of certain elements in the crystal lattice. For example, considering a closed system, Th is preferentially incorporated into the monazite mineral structure, leaving Th-depleted melt. Therefore, older monazite is rich in Th while younger monazite contains less Th. This results in a rimward decrease of Th in a concentric zoning pattern. Investigating composition and age variation of these rims help to constrain the timing and rate of crystallisation as well as the composition of the melt, especially for rocks where zircon is not present for zircon dating.

Monazite – cheralite – huttonite system

 

Monazite geochronology can also reveal igneous differentiation events such as magma mixing, where the magma chamber is evolved into different composition. Isomorphous substitution is one of the examples. It is a form of substitution where one element is replaced by another without changing the crystal structure. In the case of monazite, the rare earth elements are replaced by Ca and Th.

${\displaystyle 2REE^{3+}\leftrightarrow Ca^{2+}+Th^{4+}}$

Different level of substitution form a range of compositions, with endmembers monazite [2REE(PO₄)], brabantite [Ca,Th(PO₄)₂] and huttonite [2ThSiO₄]. The level of substitution usually depends composition of melt and thus the geological environment.

Hydrothermal alternation

Illustration showing clusters formed by multiple crystals. Edited after Schandl (2004)

 

Hydrothermal process is usually coupled with igneous process. Monazite geochronology helps studying the development from igneous process to Hydrothermal process, and revealing later hydrothermal alternation, which is vital in the study of ore formation. 

Although it is hard to distinguish between magmatic monazite and hydrothermal monazite, analysing the texture and the occurrence pattern of monazite may help distinguishing them. Hydrothermal monazites tend to appear in clusters formed by multiple crystals, while igneous monazites tend to appear homogenous throughout the rock. Also, hydrothermal monazites usually contain low ThO₂ content. These distinctive features can be easily identified with textual and compositional analysis in monazite geochronology.

Metamorphism

Monazite geochronology is generally regarded as a powerful tool to reveal metamorphic history. Metamorphism is the mineral change of textural change in preexisting rocks in response to a change in environment with different temperatures and pressures. It occurs at a temperature above diagenesis (~200 °C) and below melting (>800 °C). The mineral assemblage formed under metamorphism depends on the composition of the parent rock (protolith) and more importantly, the stability of different minerals in varying temperature and pressure (P-T). A set of mineral assemblage that formed under similar temperature and pressure is called metamorphic facies. Actually, most mineral changes during rock burial, uplift, hydrothermal processes and deformation are associated with metamorphic reactions.

Monazite is commonly found in many metamorphic rocks, especially in those formed from pelites and sandstones. The zonation in monazite reflects the successive monazite forming events. They may be formed from reactions along a single pressure-temperature (P-T) loop in a phase diagram, or reactions without changing P-T. For a metamorphic event, monazite is formed by the reactions with more than one P-T loop.

The objective of monazite geochronology is to relate these monazite forming events/reactions with P-T conditions. We can then put time constrains on the P-T loops, forming a comprehensive pressure-temperature-time loops revealing the metamorphic history of the rocks.

Monazite inclusions in metamorphic porphyroblasts and matrix

(1-3) A simplified diagram showing generations of monazite inclusions in different porphyroblasts and matrix.

 

P-T path associated with generation of monazite inclusion bearing porphyroblast and matrix

Different porphyroblasts like garnet and quartz are often formed during metamorphism in different ranges of P-T. Monazite grains are often found as inclusion in porphyroblasts. Since the host mineral monazite is quite thermally resistant, these inclusions are protected from age resetting, even at a prolonged exposure at temperature higher than 800 °C, this enables us to restrict an upper limit of the age of the porphyroblasts, and thus the associated metamorphic events. 

For example, a metamorphic rock in the Neil Bay area of northern Saskatchewan underwent high grade (high P/T) metamorphism followed by exhumation (uplift). Porphyroblast garnet is formed during high grade metamorphism while porphyroblast cordierite is formed during exhumation afterwards. Both porphyroblasts contain monazite inclusions which dated 1910 Ma and 1840 Ma respectively. And matrix monazite is dated 1800 Ma. Thus, it is interpreted that high grade metamorphism occurred after 1910 Ma and before 1840 Ma, while exhumation after 1840 Ma, and the final annealing (cooling and coarsening of minerals) at 1800 Ma.

Within the same setting as above, monazite inclusions in garnet maybe either younger, older than or have similar ages with the matrix monazite. Both of them may even have a wide range of ages with no systematic distribution. These scenarios are interpreted to represent different metamorphic paths and conditions, giving varying or complex sequences of metamorphic reactions.

Elemental fractionation between monazite and silicates

Elemental fractionation refers to the difference between the amount of element incorporated into the solid mineral phase and the amount of element stayed in the liquid fluid phase. Minerals have the characteristic of preferential intake of certain elements during its growth. For example, as monazite grows in size, it preferentially incorporate Th in the crystal structure. It results in less available Th in the environment for future growth. Thus, younger monazite tends to have lower Th contents. It provides one of the reasons for the compositional variation of monazite. 

When considering the whole system of metamorphic rocks, there are also other minerals which shows elemental fractionation. The interplay between fractionations in monazite and these minerals has a great impact on the compositional zonation of monazite. The interplay is often caused by the formation and breakdown of the minerals, which is in turn a result of different stages in P-T paths. Dating fractionating zonation thus help putting time constraint on metamorphism. 

P-T path corresponding to formation of low-Y core and high-Y rim of monazite

 

The mostly studied system is the yttrium (Y) fractionation between monazite and silicates garnet and xenotime. All three minerals preferentially fractionate Y, yet they form and break down at different stage of metamorphism. Xenotime has the highest fractionating power, then garnet and then monazite. In a simplified case of a clockwise P-T path involving garnet and monazite, garnet grows along prograde path with Y continuously incorporated, thus the Y content in monazite formed at this stage (prograde) should decrease progressively with higher grade. However, as temperature increases to a certain point, partial melting (anatectic) of monazite occurs and it dissolves along the rim, releasing Y into the melts. As the system later cools and melt crystallises, regrowing monazite will have higher Y content. Partial melting usually happen during peak metamorphism (highest temperature in P-T path), but age and chemical information during this stage are not recorded since the monazite is melting. However, the ages of last prograde growth rim (lowest Y) and the first post-anatectic growth rim (highest Y) usually bracket the time of partial melting.

Another scenario involves the formation or breakdown of garnet, influencing the Y and HREE (heavy rare earth elements) content in the environment, thus the content of growing monazite. Basically, monazite growth before garnet formation has a higher Y and HREE content than those during or after garnet formation. As garnet start breaking down in the later stage of metamorphism, the monazite forms rims rich in Y and HREE. 

The extent of fractionation of Y between garnet and monazite is also found to be related to temperature. It is thus used as a thermometer, providing the temperature constrain on the P-T path.

Deformation

Timing deformation events is one of the important components in tectonic study. Large scaled cross-cutting relationships between rocks, dikes and plutons easily provide certain but relatively broad time constrain on deformation. In contrast, monazite can itself be participated in deformation fabric, reaction and fracture, thus studying microfabrics and microtextures of monazite offers a more straightforward method of dating a deformation event.

Deformation metamorphic reaction

Deformation events may trigger metamorphic reactions which produce monazite. For example, a metamorphic reaction associated with the movement in the Legs Lake shear zone partly replaced garnet with cordierite. This reaction also generated new monazite with high content of Y, and dated around 1850 Ma. The age is treated as the timing of shearing.

One point to notice is those monazite forming reactions may happen a bit later than the shearing after the rocks have been in re-equilibrium in response to a new pressure environment. That means monazite age may not be equivalent to shearing age, yet it provides a more precise age than the other methods.

Monazite deformation fabric

Monazite grain is aligned with foliation S1. New monazite overgrowth grows along S1 direction. Edited after Mccoy, 2005.

 

Monazite mineral itself can form fabric caused by deformation. Monazite may be present as elongate grains aligned in foliation. It can be interpreted that the monazite is formed before the shearing and align during shearing, or formed at the same time of shearing. It thus provides an upper limit of the shearing age. For example, if the monazite is dated 800 Ma, the age of shearing cannot be older than 800 Ma. 

However, it can also be interpreted that the monazite grew along the foliation of other minerals long after the shearing. This problem can be solved by analysing the compositional domains of monazite. Monazite along exiting foliation would have a tendency to grow at the two ends along the foliation. If we can find monazite overgrowth with different composition and age along at the two opposite ends of the grain, it is likely that date of monazite overgrowth is younger than shearing.

Monazite fracture

Schematic diagram showing monazite fracture and refilling monazite. The monazite crystal with lighter colour is fractured by shearing. Later new monazite with new composition with darker colour forms along the fracture. Modified from Shaw (2001).

 

Fracture and offset in a single monazite crystal have been observed mimicking a bookshelf fault in a large-scale fracturing event. The fractured grain is dated 1375 Ma, indicating that the large-scale displacement happened after this date. Moreover, new monazite may later grow and fill up the space created by the fracture, enclosing the time constrain completely. For example, if the new monazite is dated 1200 Ma, the displacement probably occurred within 1375—1200 Ma.

Sedimentary events

Detrital monazite

Detrital monazite is the monazite particles that produced from the weathering and erosion of pre-existing rocks. The weathered monazite grains are produced in the source and then transferred into sedimentary basins by erosion. The detrital monazite contains zonation patterns which preserve the geological history of the source region. Investigating detrital monazite in the basin not only help constructing the metamorphic, tectonic and hydrothermal history of the source region, but also finding deposition age, structural evolution and sediment source of the basin. For example, the domain with youngest age may represent exhumation of source rock, which is followed by immediate erosion and deposition.

Diagenetic monazite

Diagenetic monazite is the monazite that formed during or after the lithification of sedimentary rocks. Monazite has been observed to grow on the other minerals or in the pore spaces during diagenesis of sediments. Studying diagenetic monazite provides a good method to study age, geochemical and thermal evolution of sedimentary basins, in particular those in Precambrian that with little fossil controls.

Industrial Use

U-Th-Pb data and monazite ages can be used as a valuable tool for prospecting. It was shown for 3 localities in Pisecke Hory Region, the Czech Republic.

Indoor air quality

From Wikipedia, the free encyclopedia

A common air filter, being cleaned with a vacuum cleaner

 

Indoor air quality (IAQ) is the air quality within and around buildings and structures. IAQ is known to affect the health, comfort and well-being of building occupants. Poor indoor air quality has been linked to Sick Building Syndrome, reduced productivity and impaired learning in schools. 

IAQ can be affected by gases (including carbon monoxide, radon, volatile organic compounds), particulates, microbial contaminants (mold, bacteria), or any mass or energy stressor that can induce adverse health conditions. Source control, filtration and the use of ventilation to dilute contaminants are the primary methods for improving indoor air quality in most buildings. Residential units can further improve indoor air quality by routine cleaning of carpets and area rugs.

Determination of IAQ involves the collection of air samples, monitoring human exposure to pollutants, collection of samples on building surfaces, and computer modelling of air flow inside buildings. 

IAQ is part of indoor environmental quality (IEQ), which includes IAQ as well as other physical and psychological aspects of life indoors (e.g., lighting, visual quality, acoustics, and thermal comfort).

Indoor air pollution in developing nations is a major health hazard. A major source of indoor air pollution in developing countries is the burning of biomass (e.g. wood, charcoal, dung, or crop residue) for heating and cooking. The resulting exposure to high levels of particulate matter resulted in between 1.5 million and 2 million deaths in 2000.

Common pollutants

Second-hand smoke

Second-hand smoke is tobacco smoke which affects people other than the 'active' smoker. Second-hand tobacco smoke includes both a gaseous and a particulate phase, with particular hazards arising from levels of carbon monoxide (as indicated below) and very small particulates (fine particular matter at especially PM2.5 size, and PM10) which get into the bronchioles and alveoles in the lung. The only certain method to improve indoor air quality as regards second-hand smoke is to eliminate smoking indoors.

Radon

Radon is an invisible, radioactive atomic gas that results from the radioactive decay of radium, which may be found in rock formations beneath buildings or in certain building materials themselves. Radon is probably the most pervasive serious hazard for indoor air in the United States and Europe, and is probably responsible for tens of thousands of deaths from lung cancer each year. There are relatively simple test kits for do-it-yourself radon gas testing, but if a home is for sale the testing must be done by a licensed person in some U.S. states. Radon gas enters buildings as a soil gas and is a heavy gas and thus will tend to accumulate at the lowest level. Radon may also be introduced into a building through drinking water particularly from bathroom showers. Building materials can be a rare source of radon, but little testing is carried out for stone, rock or tile products brought into building sites; radon accumulation is greatest for well insulated homes. The half life for radon is 3.8 days, indicating that once the source is removed, the hazard will be greatly reduced within a few weeks. Radon mitigation methods include sealing concrete slab floors, basement foundations, water drainage systems, or by increasing ventilation. They are usually cost effective and can greatly reduce or even eliminate the contamination and the associated health risks. 

Radon is measured in picocuries per liter of air (pCi/L), a measurement of radioactivity. In the United States, the average indoor radon level is about 1.3 pCi/L. The average outdoor level is about 0.4 pCi/L. The U.S. Surgeon General and EPA recommend fixing homes with radon levels at or above 4 pCi/L. EPA also recommends that people think about fixing their homes for radon levels between 2 pCi/L and 4 pCi/L.

Molds and other allergens

These biological chemicals can arise from a host of means, but there are two common classes: (a) moisture induced growth of mold colonies and (b) natural substances released into the air such as animal dander and plant pollen. Mold is always associated with moisture, and its growth can be inhibited by keeping humidity levels below 50%. Moisture buildup inside buildings may arise from water penetrating compromised areas of the building envelope or skin, from plumbing leaks, from condensation due to improper ventilation, or from ground moisture penetrating a building part. Even something as simple as drying clothes indoors on radiators can increase the risk of exposure to (amongst other things) Aspergillus - a highly dangerous mould that can be fatal for asthma sufferers and the elderly. In areas where cellulosic materials (paper and wood, including drywall) become moist and fail to dry within 48 hours, mold mildew can propagate and release allergenic spores into the air. 

In many cases, if materials have failed to dry out several days after the suspected water event, mold growth is suspected within wall cavities even if it is not immediately visible. Through a mold investigation, which may include destructive inspection, one should be able to determine the presence or absence of mold. In a situation where there is visible mold and the indoor air quality may have been compromised, mold remediation may be needed. Mold testing and inspections should be carried out by an independent investigator to avoid any conflict of interest and to insure accurate results; free mold testing offered by remediation companies is not recommended.

There are some varieties of mold that contain toxic compounds (mycotoxins). However, exposure to hazardous levels of mycotoxin via inhalation is not possible in most cases, as toxins are produced by the fungal body and are not at significant levels in the released spores. The primary hazard of mold growth, as it relates to indoor air quality, comes from the allergenic properties of the spore cell wall. More serious than most allergenic properties is the ability of mold to trigger episodes in persons that already have asthma, a serious respiratory disease.

Carbon monoxide

One of the most acutely toxic indoor air contaminants is carbon monoxide (CO), a colourless and odourless gas that is a byproduct of incomplete combustion. Common sources of carbon monoxide are tobacco smoke, space heaters using fossil fuels, defective central heating furnaces and automobile exhaust. By depriving the brain of oxygen, high levels of carbon monoxide can lead to nausea, unconsciousness and death. According to the American Conference of Governmental Industrial Hygienists (ACGIH), the time-weighted average (TWA) limit for carbon monoxide (630-08-0) is 25 ppm.

Volatile organic compounds

Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors. VOCs are emitted by a wide array of products numbering in the thousands. Examples include: paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions.

Chlorinated drinking water releases chloroform when hot water is used in the home. Benzene is emitted from fuel stored in attached garages. Overheated cooking oils emit acrolein and formaldehyde. A meta-analysis of 77 surveys of VOCs in homes in the US found the top ten riskiest indoor air VOCs were acrolein, formaldehyde, benzene, hexachlorobutadiene, acetaldehyde, 1,3-butadiene, benzyl chloride, 1,4-dichlorobenzene, carbon tetrachloride, acrylonitrile, and vinyl chloride. These compounds exceeded health standards in most homes.

Organic chemicals are widely used as ingredients in household products. Paints, varnishes, and wax all contain organic solvents, as do many cleaning, disinfecting, cosmetic, degreasing, and hobby products. Fuels are made up of organic chemicals. All of these products can release organic compounds during usage, and, to some degree, when they are stored. Testing emissions from building materials used indoors has become increasingly common for floor coverings, paints, and many other important indoor building materials and finishes.

Several initiatives envisage to reduce indoor air contamination by limiting VOC emissions from products. There are regulations in France and in Germany, and numerous voluntary ecolabels and rating systems containing low VOC emissions criteria such as EMICODE, M1, Blue Angel and Indoor Air Comfort in Europe, as well as California Standard CDPH Section 01350 and several others in the USA. These initiatives changed the marketplace where an increasing number of low-emitting products has become available during the last decades.

At least 18 Microbial VOCs (MVOCs) have been characterised including 1-octen-3-ol, 3-methylfuran, 2-pentanol, 2-hexanone, 2-heptanone, 3-octanone, 3-octanol, 2-octen-1-ol, 1-octene, 2-pentanone, 2-nonanone, borneol, geosmin, 1-butanol, 3-methyl-1-butanol, 3-methyl-2-butanol, and thujopsene. The first of these compounds is called mushroom alcohol. The last four are products of Stachybotrys chartarum, which has been linked with sick building syndrome.

Legionella

Legionellosis or Legionnaire's Disease is caused by a waterborne bacterium Legionella that grows best in slow-moving or still, warm water. The primary route of exposure is through the creation of an aerosol effect, most commonly from evaporative cooling towers or showerheads. A common source of Legionella in commercial buildings is from poorly placed or maintained evaporative cooling towers, which often release water in an aerosol which may enter nearby ventilation intakes. Outbreaks in medical facilities and nursing homes, where patients are immuno-suppressed and immuno-weak, are the most commonly reported cases of Legionellosis. More than one case has involved outdoor fountains in public attractions. The presence of Legionella in commercial building water supplies is highly under-reported, as healthy people require heavy exposure to acquire infection. 

Legionella testing typically involves collecting water samples and surface swabs from evaporative cooling basins, shower heads, faucets/taps, and other locations where warm water collects. The samples are then cultured and colony forming units (cfu) of Legionella are quantified as cfu/Liter.

Legionella is a parasite of protozoans such as amoeba, and thus requires conditions suitable for both organisms. The bacterium forms a biofilm which is resistant to chemical and antimicrobial treatments, including chlorine. Remediation for Legionella outbreaks in commercial buildings vary, but often include very hot water flushes (160 °F; 70 °C), sterilisation of standing water in evaporative cooling basins, replacement of shower heads, and in some cases flushes of heavy metal salts. Preventative measures include adjusting normal hot water levels to allow for 120 °F (50 °C) at the tap, evaluating facility design layout, removing faucet aerators, and periodic testing in suspect areas.

Other bacteria

There are many bacteria of health significance found in indoor air and on indoor surfaces. The role of microbes in the indoor environment is increasingly studied using modern gene-based analysis of environmental samples. Currently efforts are under way to link microbial ecologists and indoor air scientists to forge new methods for analysis and to better interpret the results.

Bacteria (26 2 27) Airborne microbes

 

"There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with large numbers of bacteria on the skin and as gut flora." A large fraction of the bacteria found in indoor air and dust are shed from humans. Among the most important bacteria known to occur in indoor air are Mycobacterium tuberculosis, Staphylococcus aureus, Streptococcus pneumoniae.

Asbestos fibers

Many common building materials used before 1975 contain asbestos, such as some floor tiles, ceiling tiles, shingles, fireproofing, heating systems, pipe wrap, taping muds, mastics, and other insulation materials. Normally, significant releases of asbestos fiber do not occur unless the building materials are disturbed, such as by cutting, sanding, drilling, or building remodelling. Removal of asbestos-containing materials is not always optimal because the fibers can be spread into the air during the removal process. A management program for intact asbestos-containing materials is often recommended instead. 

When asbestos-containing material is damaged or disintegrates, microscopic fibers are dispersed into the air. Inhalation of asbestos fibers over long exposure times is associated with increased incidence of lung cancer, in particular the specific form mesothelioma. The risk of lung cancer from inhaling asbestos fibers is significantly greater to smokers, however there is no confirmed connection to damage caused by asbestosis . The symptoms of the disease do not usually appear until about 20 to 30 years after the first exposure to asbestos.

Asbestos is found in older homes and buildings, but occurs most commonly in schools, hospitals and industrial settings. Although all asbestos is hazardous, products that are friable, eg. sprayed coatings and insulation, pose a significantly higher hazard as they are more likely to release fibers to the air. The US Federal Government and some states have set standards for acceptable levels of asbestos fibers in indoor air. There are particularly stringent regulations applicable to schools.

Carbon dioxide

Carbon dioxide (CO₂) is a relatively easy to measure surrogate for indoor pollutants emitted by humans, and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, to get headaches, or to function at lower activity levels. Outdoor CO₂ levels are usually 350-450 ppm whereas the maximum indoor CO₂ level considered acceptable is 1000 ppm. Humans are the main indoor source of carbon dioxide in most buildings. Indoor CO₂ levels are an indicator of the adequacy of outdoor air ventilation relative to indoor occupant density and metabolic activity. 

To eliminate most complaints, the total indoor CO₂ level should be reduced to a difference of less than 600 ppm above outdoor levels. The National Institute for Occupational Safety and Health (NIOSH) considers that indoor air concentrations of carbon dioxide that exceed 1,000 ppm are a marker suggesting inadequate ventilation. The UK standards for schools say that carbon dioxide in all teaching and learning spaces, when measured at seated head height and averaged over the whole day should not exceed 1,500 ppm. The whole day refers to normal school hours (i.e. 9:00am to 3:30pm) and includes unoccupied periods such as lunch breaks. In Hong Kong, the EPD established indoor air quality objectives for office buildings and public places in which a carbon dioxide level below 1,000 ppm is considered to be good. European standards limit carbon dioxide to 3,500 ppm. OSHA limits carbon dioxide concentration in the workplace to 5,000 ppm for prolonged periods, and 35,000 ppm for 15 minutes. These higher limits are concerned with avoiding loss of consciousness (fainting), and do not address impaired cognitive performance and energy, which begin to occur at lower concentrations of carbon dioxide. Given the well established roles of oxygen sensing pathways in cancer and the acidosis independent role of carbon dioxide in modulating immune and inflammation linking pathways, it has been suggested that the effects of long-term indoor inspired elevated carbon dioxide levels on the modulation of carcinogenesis be investigated.

Carbon dioxide concentrations increase as a result of human occupancy, but lag in time behind cumulative occupancy and intake of fresh air. The lower the air exchange rate, the slower the buildup of carbon dioxide to quasi "steady state" concentrations on which the NIOSH and UK guidance are based. Therefore, measurements of carbon dioxide for purposes of assessing the adequacy of ventilation need to be made after an extended period of steady occupancy and ventilation - in schools at least 2 hours, and in offices at least 3 hours - for concentrations to be a reasonable indicator of ventilation adequacy. Portable instruments used to measure carbon dioxide should be calibrated frequently, and outdoor measurements used for calculations should be made close in time to indoor measurements. Corrections for temperature effects on measurements made outdoors may also be necessary. 

CO₂ levels in an enclosed office room can increase to over 1,000 ppm within 45 minutes.

 

Carbon dioxide concentrations in closed or confined rooms can increase to 1,000 ppm within 45 minutes of enclosure. For example, in a 3.5-by-4-metre (11 ft × 13 ft) sized office, atmospheric carbon dioxide increased from 500 ppm to over 1,000 ppm within 45 minutes of ventilation cessation and closure of windows and doors

Ozone

Ozone is produced by ultraviolet light from the Sun hitting the Earth's atmosphere (especially in the ozone layer), lightning, certain high-voltage electric devices (such as air ionizers), and as a by-product of other types of pollution. 

Ozone exists in greater concentrations at altitudes commonly flown by passenger jets. Reactions between ozone and onboard substances, including skin oils and cosmetics, can produce toxic chemicals as by-products. Ozone itself is also irritating to lung tissue and harmful to human health. Larger jets have ozone filters to reduce the cabin concentration to safer and more comfortable levels.

Outdoor air used for ventilation may have sufficient ozone to react with common indoor pollutants as well as skin oils and other common indoor air chemicals or surfaces. Particular concern is warranted when using "green" cleaning products based on citrus or terpene extracts, because these chemicals react very quickly with ozone to form toxic and irritating chemicals as well as fine and ultrafine particles. Ventilation with outdoor air containing elevated ozone concentrations may complicate remediation attempts.

Ozone is on the list of six criteria air pollutant list. The Clean Air Act of 1990 required the United States Environmental Protection Agency to set National Ambient Air Quality Standards (NAAQS) for six common indoor air pollutants harmful to human health.  There are also multiple other organizations that have put forth air standards such as Occupational Safety and Health Administration (OSHA), National Institute for Occupational Safety and Health (NIOSH), and the World Health Organization (WHO). The OSHA standard for Ozone concentration within a space is 0.1 ppm. While the NAAQS and the EPA standard for ozone concentration is limited to 0.07 ppm. The type of ozone being regulated is ground-level ozone that is within the breathing range of most building occupants

Particulates

Atmospheric particulate matter, also known as particulates, can be found indoors and can affect the health of occupants. Authorities have established standards for the maximum concentration of particulates to ensure indoor air quality.

Prompt cognitive deficits

In 2015, experimental studies reported the detection of significant episodic (situational) cognitive impairment from impurities in the air breathed by test subjects who were not informed about changes in the air quality. Researchers at the Harvard University and SUNY Upstate Medical University and Syracuse University measured the cognitive performance of 24 participants in three different controlled laboratory atmospheres that simulated those found in "conventional" and "green" buildings, as well as green buildings with enhanced ventilation. Performance was evaluated objectively using the widely used Strategic Management Simulation software simulation tool, which is a well-validated assessment test for executive decision-making in an unconstrained situation allowing initiative and improvisation. Significant deficits were observed in the performance scores achieved in increasing concentrations of either volatile organic compounds (VOCs) or carbon dioxide, while keeping other factors constant. The highest impurity levels reached are not uncommon in some classroom or office environments.

Effect of indoor plants

Spider plants (Chlorophytum comosum) absorb some airborne contaminants

 

Houseplants together with the medium in which they are grown can reduce components of indoor air pollution, particularly volatile organic compounds (VOC) such as benzene, toluene, and xylene. Plants remove CO₂ and release oxygen and water, although the quantitative impact for house plants is small. Most of the effect is attributed to the growing medium alone, but even this effect has finite limits associated with the type and quantity of medium and the flow of air through the medium. The effect of house plants on VOC concentrations was investigated in one study, done in a static chamber, by NASA for possible use in space colonies. The results showed that the removal of the challenge chemicals was roughly equivalent to that provided by the ventilation that occurred in a very energy efficient dwelling with a very low ventilation rate, an air exchange rate of about 1/10 per hour. Therefore, air leakage in most homes, and in non-residential buildings too, will generally remove the chemicals faster than the researchers reported for the plants tested by NASA. The most effective household plants reportedly included aloe vera, English ivy, and Boston fern for removing chemicals and biological compounds. 

Plants also appear to reduce airborne microbes and molds, and to increase humidity. However, the increased humidity can itself lead to increased levels of mold and even VOCs.

When carbon dioxide concentrations are elevated indoors relative to outdoor concentrations, it is only an indicator that ventilation is inadequate to remove metabolic products associated with human occupancy. Plants require carbon dioxide to grow and release oxygen when they consume carbon dioxide. A study published in the journal Environmental Science & Technology considered uptake rates of ketones and aldehydes by the peace lily (Spathiphyllum clevelandii) and golden pothos (Epipremnum aureum) Akira Tani and C. Nicholas Hewitt found "Longer-term fumigation results revealed that the total uptake amounts were 30−100 times as much as the amounts dissolved in the leaf, suggesting that volatile organic carbons are metabolized in the leaf and/or translocated through the petiole." It is worth noting the researchers sealed the plants in Teflon bags. "No VOC loss was detected from the bag when the plants were absent. However, when the plants were in the bag, the levels of aldehydes and ketones both decreased slowly but continuously, indicating removal by the plants." Studies done in sealed bags do not faithfully reproduce the conditions in the indoor environments of interest. Dynamic conditions with outdoor air ventilation and the processes related to the surfaces of the building itself and its contents as well as the occupants need to be studied.

While results do indicate house plants may be effective at removing some VOCs from air supplies, a review of studies between 1989 and 2006 on the performance of houseplants as air cleaners, presented at the Healthy Buildings 2009 conference in Syracuse, New York, concluded "...indoor plants have little, if any, benefit for removing indoor air of VOC in residential and commercial buildings." This conclusion was based on a trial involving an unknown quantity of indoor plants kept in an uncontrolled ventilated air environment of an arbitrary office building in Arlington, Virginia.

Since extremely high humidity is associated with increased mold growth, allergic responses, and respiratory responses, the presence of additional moisture from houseplants may not be desirable in all indoor settings if watering is done inappropriately.

HVAC design

Environmentally sustainable design concepts also include aspects related to the commercial and residential heating, ventilation and air-conditioning (HVAC) industry. Among several considerations, one of the topics attended to is the issue of indoor air quality throughout the design and construction stages of a building's life. 

One technique to reduce energy consumption while maintaining adequate air quality, is demand-controlled ventilation. Instead of setting throughput at a fixed air replacement rate, carbon dioxide sensors are used to control the rate dynamically, based on the emissions of actual building occupants.

For the past several years, there have been many debates among indoor air quality specialists about the proper definition of indoor air quality and specifically what constitutes "acceptable" indoor air quality.

One way of quantitatively ensuring the health of indoor air is by the frequency of effective turnover of interior air by replacement with outside air. In the UK, for example, classrooms are required to have 2.5 outdoor air changes per hour. In halls, gym, dining, and physiotherapy spaces, the ventilation should be sufficient to limit carbon dioxide to 1,500 ppm. In the USA, and according to ASHRAE Standards, ventilation in classrooms is based on the amount of outdoor air per occupant plus the amount of outdoor air per unit of floor area, not air changes per hour. Since carbon dioxide indoors comes from occupants and outdoor air, the adequacy of ventilation per occupant is indicated by the concentration indoors minus the concentration outdoors. The value of 615 ppm above the outdoor concentration indicates approximately 15 cubic feet per minute of outdoor air per adult occupant doing sedentary office work where outdoor air contains 385 ppm, the current global average atmospheric CO₂ concentration. In classrooms, the requirements in the ASHRAE standard 62.1, Ventilation for Acceptable Indoor Air Quality, would typically result in about 3 air changes per hour, depending on the occupant density. Of course the occupants are not the only source of pollutants, so outdoor air ventilation may need to be higher when unusual or strong sources of pollution exist indoors. When outdoor air is polluted, then bringing in more outdoor air can actually worsen the overall quality of the indoor air and exacerbate some occupant symptoms related to outdoor air pollution. Generally, outdoor country air is better than indoor city air. Exhaust gas leakages can occur from furnace metal exhaust pipes that lead to the chimney when there are leaks in the pipe and the pipe gas flow area diameter has been reduced.

The use of air filters can trap some of the air pollutants. The Department of Energy's Energy Efficiency and Renewable Energy section suggests that "[Air] Filtration should have a Minimum Efficiency Reporting Value (MERV) of 13 as determined by ASHRAE 52.2-1999." Air filters are used to reduce the amount of dust that reaches the wet coils. Dust can serve as food to grow molds on the wet coils and ducts and can reduce the efficiency of the coils.

Moisture management and humidity control requires operating HVAC systems as designed. Moisture management and humidity control may conflict with efforts to try to optimize the operation to conserve energy. For example, moisture management and humidity control requires systems to be set to supply make-up air at lower temperatures (design levels), instead of the higher temperatures sometimes used to conserve energy in cooling-dominated climate conditions. However, for most of the US and many parts of Europe and Japan, during the majority of hours of the year, outdoor air temperatures are cool enough that the air does not need further cooling to provide thermal comfort indoors. However, high humidity outdoors creates the need for careful attention to humidity levels indoors. High humidities give rise to mold growth and moisture indoors is associated with a higher prevalence of occupant respiratory problems.

The "dew point temperature" is an absolute measure of the moisture in air. Some facilities are being designed with the design dew points in the lower 50s °F, and some in the upper and lower 40s °F. Some facilities are being designed using desiccant wheels with gas-fired heaters to dry out the wheel enough to get the required dew points. On those systems, after the moisture is removed from the make-up air, a cooling coil is used to lower the temperature to the desired level. 

Commercial buildings, and sometimes residential, are often kept under slightly positive air pressure relative to the outdoors to reduce infiltration. Limiting infiltration helps with moisture management and humidity control.

Dilution of indoor pollutants with outdoor air is effective to the extent that outdoor air is free of harmful pollutants. Ozone in outdoor air occurs indoors at reduced concentrations because ozone is highly reactive with many chemicals found indoors. The products of the reactions between ozone and many common indoor pollutants include organic compounds that may be more odorous, irritating, or toxic than those from which they are formed. These products of ozone chemistry include formaldehyde, higher molecular weight aldehydes, acidic aerosols, and fine and ultrafine particles, among others. The higher the outdoor ventilation rate, the higher the indoor ozone concentration and the more likely the reactions will occur, but even at low levels, the reactions will take place. This suggests that ozone should be removed from ventilation air, especially in areas where outdoor ozone levels are frequently high. Recent research has shown that mortality and morbidity increase in the general population during periods of higher outdoor ozone and that the threshold for this effect is around 20 parts per billion (ppb).

Building ecology

It is common to assume that buildings are simply inanimate physical entities, relatively stable over time. This implies that there is little interaction between the triad of the building, what is in it (occupants and contents), and what is around it (the larger environment). We commonly see the overwhelming majority of the mass of material in a building as relatively unchanged physical material over time. In fact, the true nature of buildings can be viewed as the result of a complex set of dynamic interactions among their physical, chemical, and biological dimensions. Buildings can be described and understood as complex systems. Research applying the approaches ecologists use to the understanding of ecosystems can help increase our understanding. “Building ecology “ is proposed here as the application of those approaches to the built environment considering the dynamic system of buildings, their occupants, and the larger environment.

Buildings constantly evolve as a result of the changes in the environment around them as well as the occupants, materials, and activities within them. The various surfaces and the air inside a building are constantly interacting, and this interaction results in changes in each. For example, we may see a window as changing slightly over time as it becomes dirty, then is cleaned, accumulates dirt again, is cleaned again, and so on through its life. In fact, the “dirt” we see may be evolving as a result of the interactions among the moisture, chemicals, and biological materials found there.

Buildings are designed or intended to respond actively to some of these changes in and around them with heating, cooling, ventilating, air cleaning or illuminating systems. We clean, sanitize, and maintain surfaces to enhance their appearance, performance, or longevity. In other cases, such changes subtly or even dramatically alter buildings in ways that may be important to their own integrity or their impact on building occupants through the evolution of the physical, chemical, and biological processes that define them at any time. We may find it useful to combine the tools of the physical sciences with those of the biological sciences and, especially, some of the approaches used by scientists studying ecosystems, in order to gain an enhanced understanding of the environments in which we spend the majority of our time, our buildings.

Building ecology was first described by Hal Levin in an article in the April 1981 issue of Progressive Architecture magazine.

Institutional programs

The topic of IAQ has become popular due to the greater awareness of health problems caused by mold and triggers to asthma and allergies. In the US, awareness has also been increased by the involvement of the United States Environmental Protection Agency, who have developed an "IAQ Tools for Schools" program to help improve the indoor environmental conditions in educational institutions (see external link below). The National Institute for Occupational Safety and Health conducts Health Hazard Evaluations (HHEs) in workplaces at the request of employees, authorised representative of employees, or employers, to determine whether any substance normally found in the place of employment has potentially toxic effects, including indoor air quality.

A variety of scientists work in the field of indoor air quality including chemists, physicists, mechanical engineers, biologists, bacteriologists and computer scientists. Some of these professionals are certified by organisations such as the American Industrial Hygiene Association, the American Indoor Air Quality Council and the Indoor Environmental Air Quality Council.

On the international level, the International Society of Indoor Air Quality and Climate (ISIAQ), formed in 1991, organises two major conferences, the Indoor Air and the Healthy Buildings series. ISIAQ's journal Indoor Air is published 6 times a year and contains peer-reviewed scientific papers with an emphasis on interdisciplinary studies including exposure measurements, modeling, and health outcomes.

Actinides in the environment

From Wikipedia, the free encyclopedia

Actinides in the environment refer to the sources, environmental behaviour and effects of actinides in Earth's environment. Environmental radioactivity is not limited solely to actinides; non-actinides such as radon and radium are of note.

Inhalation versus ingestion

Generally, ingested insoluble actinide compounds, such as high-fired uranium dioxide and mixed oxide (MOX) fuel, will pass through the digestive system with little effect since they cannot dissolve and be absorbed by the body. Inhaled actinide compounds, however, will be more damaging as they remain in the lungs and irradiate the lung tissue.

Ingested low-fired oxides and soluble salts such as nitrate can be absorbed into the blood stream. If they are inhaled then it is possible for the solid to dissolve and leave the lungs. Hence, the dose to the lungs will be lower for the soluble form.

Thorium in the environment

Monazite, a rare-earth-and-thorium-phosphate mineral, is the primary source of the world's thorium

 

In India, a large amount of thorium ore can be found in the form of monazite in placer deposits of the Western and Eastern coastal dune sands, particularly in the Tamil Nadu coastal areas. The residents of this area are exposed to a naturally occurring radiation dose ten times higher than the worldwide average.

Occurrence

Thorium is found at low levels in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide. There are substantial deposits in several countries. ²³²Th decays very slowly (its half-life is about three times the age of the earth). Other isotopes of thorium occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than ²³²Th, though on a mass basis they are negligible.

Effects in humans

Thorium has been linked to liver cancer. In the past thoria (thorium dioxide) was used as a contrast agent for medical X-ray radiography but its use has been discontinued. It was sold under the name Thorotrast.

Uranium in the environment

Uranium is a natural metal which is widely found. It is present in almost all soils and it is more plentiful than antimony, beryllium, cadmium, gold, mercury, silver, or tungsten, and is about as abundant as arsenic or molybdenum. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources). 

Seawater contains about 3.3 parts per billion of uranium by weight as uranium (VI) forms soluble carbonate complexes. The extraction of uranium from seawater has been considered as a means of obtaining the element. Because of the very low specific activity of uranium the chemical effects of it upon living things can often outweigh the effects of its radioactivity. Additional uranium has been added to the environment in some locations as a result of the nuclear fuel cycle and the use of depleted uranium in munitions.

Neptunium in the environment

Like plutonium, neptunium has a high affinity for soil. However, it is relatively mobile over the long term, and diffusion of neptunium-237 in groundwater is a major issue in designing a deep geological repository for permanent storage of spent nuclear fuel. ²³⁷Np has a half-life of 2.144 million years and is therefore a long-term problem; but its half-life is still much shorter than those of uranium-238, uranium-235, or uranium-236, and ²³⁷Np therefore has higher specific activity than those nuclides.

Plutonium in the environment

Sources

Plutonium in the environment has several sources. These include:

• Atomic batteries
  • In space
  • In pacemakers
• Bomb detonations
• Bomb safety trials
• Nuclear accidents (such as Chernobyl)
• Nuclear crime
• Nuclear fuel cycle

Environmental chemistry

Plutonium, like other actinides, readily forms a plutonium dioxide (plutonyl) core (PuO₂). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH⁻, NO₂⁻, NO₃⁻, and SO₄²⁻) to form charged complexes which can be readily mobile with low affinities to soil.

• PuO₂CO₃²⁻
• PuO₂(CO₃)₂⁴⁻
• PuO₂(CO₃)₃⁶⁻

PuO₂ formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO₂ which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium. 

Plutonium is known to bind to soil particles very strongly, see above for an X-ray spectroscopic study of plutonium in soil and concrete. While caesium has very different chemistry to the actinides, it is well known that both caesium and many of the actinides bind strongly to the minerals in soil. Hence it has been possible to use ¹³⁴Cs labeled soil to study the migration of Pu and Cs is soils. It has been shown that colloidal transport processes control the migration of Cs (and will control the migration of Pu) in the soil at the Waste Isolation Pilot Plant.

Americium in the environment

Americium often enters landfills from discarded smoke detectors. The rules associated with the disposal of smoke detectors are very relaxed in most municipalities. For instance, in the UK it is permissible to dispose of an americium containing smoke detector by placing it in the dustbin with normal household rubbish, but each dustbin worth of rubbish is limited to only containing one smoke detector. The manufacture of products containing americium (such as smoke detectors) as well as nuclear reactors and explosions may also release the americium into the environment.

Picture illustrating David "Radioactive Boyscout" Hahn.

 

In 1999, a truck transporting 900 smoke detectors in France had been reported to have caught fire; it is claimed that this led to a release of americium into the environment. In the U.S., the "Radioactive Boy Scout" David Hahn was able to buy thousands of smoke detectors at remainder prices and concentrate the americium from them. 

There have been cases of humans being exposed to americium. The worst case was that of Harold McCluskey, who was exposed to an extremely high dose of americium-241 after an accident involving a glove box. He was subsequently treated with chelation therapy. It is likely that the medical care which he was given saved his life; because of the difference in the chemistry of americium (the +3 oxidation state is very stable) to plutonium (where the +4 state can form in the human body) the americium has very different biochemistry to plutonium.

The most common isotope americium-241 decays (half-life of 431 years) to neptunium-237 which has a much longer half-life, so in the long term, the issues discussed above for neptunium apply.

Americium released into the environment tends to remain in soil and water at relatively shallow depths and may be taken up by animals and plants during growth; shellfish such as shrimp take up americium-241 in their shells, and parts of grain plants can become contaminated with exposure.