Browsing by Author "Ying, J.-F."
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Item Breakdown of orthopyroxene contributing to melt pockets in mantle peridotite xenoliths from the Western Qinling, central China: Constraints from in situ LA-ICP-MS mineral analyses(Mineralogy and Petrology, 2012-03) Su, B.-X.; Zhang, H.-F.; Yang, Y.-H.; Sakyi, P.A.; Ying, J.-F.; Tang, Y.-J.Major and trace element compositions of constituent minerals, partly decomposed rims of orthopyroxenes (DRO), 'closed' melt pockets (CMP) and open melt pockets (OMP) in some Western Qinling peridotite xenoliths were obtained by LA-ICP-MS. Systematic core-to-rim compositional variations of garnet, clinopyroxene and orthopyroxene demonstrate that these minerals underwent variable degrees of subsolidus breakdown or partial melting. Both DROs and CMPs consist of similar mineral assemblages and are characterized by high TiO 2, CaO + Na 2O and low MgO contents; they are enriched in LREE and LILE compositions, have positive anomalies in Pb, Sr and particularly Ti, negative Th and U, and variable Zr and Hf anomalies. These chemical features are distinct and reflect reactions involving the orthopyroxenes. Compared to the CMPs, the OMPs, which are composed of a complex assemblage of minerals, display lower FeO and MgO contents, larger ranges in SiO 2 and Na 2O, higher TiO 2, Al 2O 3, CaO and trace element concentrations, slightly negative Zr and Hf anomalies, and apparently negative Ti anomalies. Modeling calculations of partial fusion of orthopyroxenes and clinopyroxenes suggest that the CMPs most likely originated from the breakdown of orthopyroxenes with variably minor contribution of external melts from the melting of clinopyroxenes, whereas the OMPs were probably formed from the modification of the CMPs through the interaction with large amount of external melts. © 2011 Springer-Verlag.Item Extremely high Li and low δ 7Li signatures in the lithospheric mantle(Chemical Geology, 2012-01) Su, B.-X.; Zhang, H.-F.; Deloule, E.; Sakyi, P.A.; Xiao, Y.; Tang, Y.-J.; Hu, Y.; Ying, J.-F.; Liu, P.-P.Geochemical behavior of lithium (Li) in mantle processes is generally explained by: (1) Li diffuses into minerals from melts, faster in clinopyroxene than in olivine, creating isotopically lighter-Li clinopyroxene and heavier-Li olivine; (2) Partitioning of Li changes with temperature, and as cooling proceeds Li diffuses into clinopyroxene from olivine, also resulting in low-δ 7Li clinopyroxene and high-δ 7Li olivine, similar to the result of process (1). Our results obtained from highly carbonatite-metasomatized peridotite xenoliths from the Western Qinling reveal that the minerals of these xenoliths contain extremely high Li in a range of 2-16ppm for olivine, 2-43ppm for orthopyroxene, and up to 75ppm for clinopyroxene. Secondly, olivines have much lower δ 7Li (down to -42‰) relative to co-existent pyroxenes. Above observations provide important insights into the behavior of Li during mantle processes. One explanation is that the fractionation of Li strongly depends on the metasomatic medium. Alkaline melts, particularly Na-rich carbonatitic melts, which have high Li contents (~200ppm) but mantle level δ 7Li (3-5‰), tend to effectively transfer 6Li into olivine and 7Li into pyroxene, while silicate melts behave on the contrary. Alternative might be the existence of a reservoir with a high Li content but low δ 7Li, like eclogite, in the mantle since the 7Li released into the overlying mantle wedge during dehydration. This study reveals the distinctive behavior of Li between the carbonatite and silicate metasomatism in the mantle. © 2011.Item Large lithium isotopic variations in minerals from peridotite xenoliths from the eastern north China craton(Journal of Geology, 2015-01) Xiao, Y.; Zhang, H.-F.; Deloule, E.; Su, B.-X.; Tang, Y.-J.; Sakyi, P.A.; Hu, Y.; Ying, J.-F.To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and d7Li of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) for six peridotite xenoliths from the eastern North China Craton. These xenoliths contain two lherzolites, two Cpx-rich lherzolites, and two wehrlites and are variably metasomatized. Lithium isotope zonation is observed in most peridotite minerals. The majority of Cpx grains display isotopically light cores with lower Li concentrations than the heavy rims. However, the Opx grains show a different style of zonation fromCpx, where higher Li concentrations in the cores are associated with much lighter δ7Li. Olivines in most peridotites have a restricted range of Li concentration and δ7Li within individual grains, whereas the olivines in a lherzolite show isotopically light cores (210.3) with high Li concentrations (2.3 ppm) and heavy rims (5.5) with low Li concentrations (1.7 ppm). These Li isotopic variations in mineral phases may reflect the combined effects of diffusiondriven kinetic fractionation of Li isotopes during melt/fluid-peridotite interactions and slow cooling. Intersample heterogeneity of Li isotopes is also apparent. Olivine with forsterite (Fo) content of 91.3 in one lherzolite sample has “normal” mantle-like Li concentrations (1.1∼2.4 ppm) and light δ7Li (210.3∼5.5), while Ol with Fo content of 89.7 in another lherzolite has slightly high Li concentrations (2.0∼3.0 ppm) but similar δ7Li (1.6∼6.4) relative to normalmantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier d7Li (5.0∼22.0) than those in lherzolites and normalmantle. The δ7LiOl value correlates positively with Li concentration and negativelywith Fo fromlherzolites toCpx-rich lherzolites, indicating a reaction between lherzolites andmeltswith isotopically heavy Li- and Fe-rich signatures. By contrast, olivines in wehrlites have extremely lower Fo contents (82.2∼83.2) and higher Li concentrations (2.4∼4.2 ppm) than those in normal mantle, while their δ7Li values are within the range of normal mantle, reflecting metasomatism of the peridotites by asthenospheric melt. Overall, the large intragrain and intersample variations in Li concentrations and isotopic compositions reflect kinetic isotope fractionation during multiple metasomatisms. © 2015 by The University of Chicago. All rights reserved.