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.