Genesis of ^(230)Th excess in basalts from mid-ocean ridges and ocean islands:Constraints from the global U-series isotope database and major and rare earth element geochemistry
Genesis of ^(230)Th excess in basalts from mid-ocean ridges and ocean islands:Constraints from the global U-series isotope database and major and rare earth element geochemistry作者机构：Key Laboratory of Marine Geology and Environment Institute of Oceanology Chinese Academy of Sciences Qingdao China Graduate University of Chinese Academy of Sciences Beijing China
出 版 物：《Science China Earth Sciences》 (中国科学（地球科学英文版）)
年 卷 期：2010年第53卷第10期
页 面：1486-1494页
核心收录：
学科分类：070902[理学-地球化学] 0709[理学-地质学] 07[理学] 0708[理学-地球物理学] 0704[理学-天文学]
基 金：supported by National Natural Science Foundation of China (Grant No.40830849) Special Foundation for the Eleventh Five Plan of COMRA (Grant No.DYXM-115-02-1-03) National Natural Science Foundation of China (Grant No.40906029)
主 题：excess 230Th clinopyroxene garnet spinel-lherzolite mid-ocean ridge ocean island
摘 要：Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th excess.(230Th/238U) in global MORBs shows a positive correlation with Fe8,Po,Na8,and Fmelt(Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%,Po=pressure of initial melting and Fmelt=degree of melt),while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle ***,compared with the MORBs,higher(230Th/238U) in OIBs actually corresponds to a lower melting *** suggests that the 230Th excess in MORBs is controlled by mantle melting conditions,while the 230Th excess in OIBs is more likely related to the deep garnet *** vast majority of calculated initial melting pressures of MORBs with excess 230Th are between 1.0 and 2.5 GPa,which is consistent with the conclusion from experiments in recent years that DUDTh for Al-clinopyroxene at pressures of 1.0 *** initial melting pressure of OIBs is 2.2-3.5 GPa(around the spinel-garnet transition zone),with their low excess 226Ra compared to MORBs also suggesting a deeper mantle ***,excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability *** addition,there is no obvious correlation of K2O/TiO2 with(230Th/238U) and initial melting pressure(Po) of MORBs,so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition *** and MORBs in both(230Th/238U) vs.K2O/TiO2 and(230Th/238U) *** plots fall in two distinct areas,indicating that the mineral phases which dominate their excess 230Th are ***/Yb-Ce curves of fast and slow ridge MORBs are similar,while,in comparison,the Ce/Yb-Ce curve for OIBs shows more influence from *** mechanism
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