Monazite geochronology

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.

238 U → 206 P b + 8 α + 6 β − + E n e r g y λ 238 = 1.55125 e − 10 y r − 1 t 1 / 2 = 4.4683 x 10 9 y r {\displaystyle ^{238}U\rightarrow ^{206}Pb+8\alpha +6\beta ^{-}+Energy\quad \lambda _{238}=1.55125{e}^{-10}yr^{-1}\quad t_{1/2}=4.4683x10^{9}yr}

235 U → 207 P b + 7 α + 4 β − + E n e r g y λ 235 = 9.8485 e − 10 y r − 1 t 1 / 2 = 7.0310 x 10 8 y r {\displaystyle ^{235}U\rightarrow ^{207}Pb+7\alpha +4\beta ^{-}+Energy\quad \lambda _{235}=9.8485{e}^{-10}yr^{-1}\quad t_{1/2}=7.0310x10^{8}yr}

232 T h → 208 P b + 6 α + 4 β − + E n e r g y λ 232 = 4.9475 e − 11 y r − 1 t 1 / 2 = 1.4010 x 10 10 y r {\displaystyle ^{232}Th\rightarrow ^{208}Pb+6\alpha +4\beta ^{-}+Energy\quad \lambda _{232}=4.9475{e}^{-11}yr^{-1}\quad t_{1/2}=1.4010x10^{10}yr}

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.

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