首页 | 本学科首页   官方微博 | 高级检索  
     


Combined iron and magnesium isotope geochemistry of pyroxenite xenoliths from Hannuoba,North China Craton: implications for mantle metasomatism
Authors:Xin?Miao?Zhao  author-information"  >  author-information__contact u-icon-before"  >  mailto:xinmiao@mail.iggcas.ac.cn"   title="  xinmiao@mail.iggcas.ac.cn"   itemprop="  email"   data-track="  click"   data-track-action="  Email author"   data-track-label="  "  >Email author,Hui?Hui?Cao,Xue?Mi,Noreen?J.?Evans,Yu?Han?Qi,Fang?Huang,Hong?Fu?Zhang
Affiliation:1.State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,China;2.University of Chinese Academy of Sciences,Beijing,China;3.John de Laeter Center, TIGeR, Applied Geology,Curtin University,Perth,Australia;4.CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences,University of Science and Technology of China,Hefei,China;5.State Key Laboratory of Continental Dynamics, Department of Geology,Northwest University,Xi’an,China
Abstract:We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ57Fe = ?0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ26Mg = ? 0.28 to ?0.25 with an average of ?0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ57Fe ranging from 0.12 to 0.30. The δ57Fe values of minerals in these garnet-bearing pyroxenites also vary widely (?0.25 to 0.08 in olivines, ?0.04 to 0.25 in orthopyroxenes, ?0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ26Mg (?0.43) relative to the mantle. The δ26Mg of minerals in the garnet-bearing pyroxenite range from ?0.35 for olivine and orthopyroxene, to ?0.34 for clinopyroxene, 0.04 for spinel and ?0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ57Fe (as high as 1.00) but the lightest δ26Mg (as low as ?1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ57Fe and δ26Mg in the Hannuoba garnet-bearing pyroxenite and in the phlogopite clinopyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle.
Keywords:
本文献已被 SpringerLink 等数据库收录!
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号