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.Williams, M. L., Jercinovic, M. J., & Hetherington, C. J. (2007). Microprobe monazite geochronology: understanding geologic processes by integrating composition and chronology. Annual Review of Earth and Planetary Sciences, 35(1), 137.Williams, M. L., & Jercinovic, M. J. (2002). Microprobe monazite geochronology: putting absolute time into microstructural analysis. Journal of Structural Geology, 24(6), 1013–1028. 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.Crowley, J. L., & Ghent, E. D. (1999). An electron microprobe study of the U–Th–Pb systematics of metamorphosed monazite: the role of Pb diffusion versus overgrowth and recrystallization. Chemical Geology, 157(3), 285–302. 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.Parrish, R. R. (1990). U-Pb dating of monazite and its application to geological problems. Canadian Journal of Earth Sciences, 27(11), 1431–1450.
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.
where α represents alpha particle, β− represents beta particle, λ represents decay constant and t1/2 represents half-life.
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.Seydoux-Guillaume, A. M., Paquette, J. L., Wiedenbeck, M., Montel, J. M., & Heinrich, W. (2002). Experimental resetting of the U–Th–Pb systems in monazite. Chemical geology, 191(1), 165–181. 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)Flowers, R. M., Bowring, S. A., Tulloch, A. J., & Klepeis, K. A. (2005). Tempo of burial and exhumation within the deep roots of a magmatic arc, Fiordland, New Zealand. Geology, 33(1), 17–20. 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 a region of higher concentration to one of lower concentration. It is easy to imagine diffusion in a 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 slow down and finally become insignificant at a certain temperature.Dodson, M. H. (1973). Closure temperature in cooling geochronological and petrological systems. Contributions to Mineralogy and Petrology, 40(3), 259–274. This closure temperature (Tc) depends on the crystal size, shape, cooling rate and diffusion coefficient, which in turn varies for each mineral and radioactive systems. That is, above Tc, Pb is continuously lost and the radioactive clock is keeping zero. Once the temperature falls below Tc, the system is closed and the clock starts counting.
Monazite is characterized by 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.Smith, H. A., & Giletti, B. J. (1997). Lead diffusion in monazite. Geochimica et Cosmochimica Acta, 61(5), 1047–1055.Cherniak, D. J., Watson, E. B., Grove, M., & Harrison, T. M. (2004). Pb diffusion in monazite: a combined RBS/SIMS study. Geochimica et Cosmochimica Acta, 68(4), 829–840.Braun, I., Montel, J. M., & Nicollet, C. (1998). Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India. Chemical Geology, 146(1), 65–85.
Fluid-assisted dissolution-precipitation
Unlike solid-state diffusion, fluid-assisted dissolution-precipitation occurs below Tc. Interaction between the mineral phase and a coexisting fluid phase during geological events directly contributes to this process. It is a chemical reaction driven by the system stabilization from minimizing Gibbs free energy.Putnis, A. (2002). Mineral replacement reactions: from macroscopic observations to microscopic mechanisms. Mineralogical Magazine, 66(5), 689–708. A reactive fluid is present as a catalyst and a source of reactants for the reaction.
If a geological process creates 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.Williams, M. L., Jercinovic, M. J., Harlov, D. E., Budzyń, B., & Hetherington, C. J. (2011). Resetting monazite ages during fluid-related alteration. Chemical Geology, 283(3), 218–225.Harlov, D. E., Wirth, R., & Hetherington, C. J. (2011). Fluid-mediated partial alteration in monazite: the role of coupled dissolution–reprecipitation in element redistribution and mass transfer. Contributions to Mineralogy and Petrology, 162(2), 329–348. 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 which can cause the reaction to cease. (A) Reaction ceases due to the recrystallisation of precipitating phase, removing all the fluid infiltration paths. This results in fluid inclusions in monazite. (B) Reaction ceases due to a change in the system such as the composition of fluid and monazite, making this reaction no longer reactive.
Implications for monazite geochronology Since the diffusion of reactants between the dissolving phase and the precipitating phase is slow, the fluid is essential for providing easy transport for the reactants. Yet as the reaction proceeds, the dissolving phase and the fluid are separated by the solid precipitating phase, blocking the transport of reactants. Therefore, there must be some interconnected 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 subjected to a temperature higher than Tc, all age information will be reset, losing information from past geological events. In contrast, since monazite has a high Tc, even though it may experience younger high-grade metamorphism with high temperatures, it is likely that the previous geological history will be preserved. Furthermore, dissolution-precipitation is usually triggered by geological events such as metamorphism, deformation and hydrothermal alternation below Tc. Each of these events writes 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.