T189 Frontiers in Non-Traditional Stable Isotopes: In Honor of Fang-Zhen Teng, Recipient of the 2014 MSA Award
Tuesday, 21 October 2014
8:00 AM - 12:00 PM
213
EMPIRICAL CALIBRATION OF THE OMPHACITE–GARNET MAGNESIUM ISOTOPE GEOTHERMOMETER BASED ON ECLOGITES FROM THE DABIE OROGEN, CHINA
Paper number: T189-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM
Authors
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Wang-Ye Li Presenting AuthorSchool of Earth and Space Sciences, University of Science and Technology of China, Hefei, N/A
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Fangzhen TengUniversity of Washington, Seattle, WA
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Yilin XiaoSchool of Earth and Space Sciences, University of Science and Technology of China, Hefei, N/A
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Hai-Ou GuSchool of Earth and Space Sciences, University of Science and Technology of China, Hefei, N/A
Abstract
The large equilibrium Mg isotope fractionation between omphacite and garnet in eclogites may serve as a potential geothermometer but comprehensive calibration of this thermometer by natural samples is still lacking. Here, we report Mg isotopic compositions of whole rocks, Mg and O isotopic compositions of omphacites and garnets for twenty eclogites that formed at variable temperatures in the Dabie orogen, China. Fifteen out of twenty samples have whole-rock δ26Mg values from -0.36 to -0.18‰, which is similar to those of their protoliths and, thus, consistent with limited Mg isotopic variation of rocks during eclogite-facies metamorphism. By contrast, δ26Mg values vary largely in omphacite from -0.37 to +0.54‰ and in garnet from -1.94 to -0.66‰, with inter-mineral fractionation (Δ26Mgomphacite–garnet = δ26Mgomphacite – δ26Mggarnet) ranging from 1.05 to 2.15‰. According to the “δomphacite vs. δgarnet diagram” and Δ18Oomphactie–garnet (= δ18Oomphacite – δ18Ogarnet) values, the large fractionation reflect both equilibrium and disequilibrium inter-mineral Mg isotope fractionation in the investigated samples. The equilibrium fractionation equation between omphacite and garnet is calibrated by using the equilibrium Δ26Mgomphacite–garnet values and corresponding temperatures calculated by the clinopyroxene–garnet Fe–Mg thermometer for natural eclogites, which yields the following function: Δ26Mgomphacite–garnet = (0.99 ± 0.09) × 106/T2, where T is temperature in Kelvin. The novel omphacite–garnet Mg isotope thermometer has a better precision of temperature estimation than the clinopyroxene–garnet Fe–Mg thermometer (< 60 ºC vs. >100 ºC). Hence, it provides a new thermometer for precise temperature constraints of eclogite-facies metamorphism.