Lithium elemental and isotopic variations in rock-melt interaction |
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| Institution: | 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Centre National de la Recherche Scientifique, CRPG, BP20, 54501 Vandoeuvre-Les-Nancy Cedex, France;3. Department of Earth Sciences, University of Hong Kong, Hong Kong, China;4. Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon, Accra, Ghana;1. Department of Mineralogy, Petrology and Economic Geology, School of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece;2. Laboratory of Nuclear Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece;1. Laboratoire d’Hydrologie et de Géochimie de Strasbourg, UMR 7517, 1 rue Blessig, 67084 Strasbourg Cedex, France;2. 4, rue de Marlenheim, 67000 Strasbourg, France;3. SQU, College of Science, Oman;4. Università degli Studi di Messina, Dipartimento di Fisica e di Scienze della Terra Viale F, Stagno d’Alcontres 31, 98100 Messina, Italy;1. Geochemistry Department of Chengdu University of Technology, Sichuan Province 610059, China;2. Key Laboratory of Nuclear Techniques in Geosciences, Sichuan Province 610059, China;1. Department of Geology, University of Patras, Panepistimiopolis of Rion, Patras 265 04, Greece;2. School of Earth Sciences and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, PR China |
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| Abstract: | Despite the occurrence of highly variable lithium (Li) elemental distribution and isotopic fractionation in mantle mineral, the mechanism of Li heterogeneity and fractionation remains a controversial issue. We measured Li contents and isotopic compositions of olivine and clinopyroxene xenocrysts and phenocrysts from kamafugite host lavas, as well as minerals in melt pockets occurring as metasomatic products in peridotite xenoliths from the Western Qinling, central China. The olivine xenocrysts in the kamafugites show compositional zonation. The cores have high Mg# (100 × Mg/(Mg+Fe); 91.0–92.2) and Li abundances (5.63–21.7 ppm), low CaO contents (≤0.12 wt%) and low δ7Li values (?39.6 to ?6.76‰), which overlap with the compositional ranges of the olivines in the melt pockets as well as those in peridotite xenoliths. The rims of the olivine xenocrysts display relatively low Mg# (85.9–88.2), high CaO contents (0.19–0.38 wt%) and high δ7Li values (18.3–26.9‰), which are comparable to the olivine phenocrysts (Mg#: 86.4–87.1; CaO: 0.20–0.28 wt%; Li: 12.4–36.8 ppm; δ7Li: 18.1–26.0‰) and the silicate-melt metasomatized olivines. The clinopyroxene phenocrysts and clinopyroxenes in the melt pockets have no distinct characteristics with respect to the Li abundances and δ7Li values, but show higher and lower CaO contents, respectively, than the clinopyroxenes from silicate and carbonatite metasomatized samples. These features indicate that Li concentration and isotopic signatures of the cores of the xenocrysts recorded carbonatite melt-peridotite reaction (carbonatite metasomatism) at mantle depth, and the variations in the rims probably resulted from xenocryst–host magma interaction during ascent. Our results reveal that the interaction with carbonatite and silicate melts gave rise to an increase in Li abundance in minerals of peridotite xenoliths at mantle depth or during transportation. In terms of δ7Li, the carbonatite and silicate melts produced remarkably contrasting δ7Li variations in olivine. Based on the systematic variations of Li abundances and Li isotopes in olivines, we suggest that the δ7Li value of olivine is a more important indicator than that of clinopyroxene in discriminating carbonatite and silicate melt interaction agents with peridotites. |
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| Keywords: | Li isotopes Xenocryst Phenocrysts Mantle metasomatism Mineral–host magma interaction |
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