caa a22 4500 445300361 CHVBK 20180317142555.0 cr unu---uuuuu 170323e20101001xx s 000 0 eng 10.1007/s11430-010-4038-4 doi (NATIONALLICENCE)springer-10.1007/s11430-010-4038-4 Genesis of 230Th 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 [Elektronische Daten] [GuoLiang Zhang, ZhiGang Zeng] 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, P o, Na8, and Fmelt (Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%, P o=pressure of initial melting and F melt=degree of melt), while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting. Furthermore, compared with the MORBs, higher (230Th/238U) in OIBs actually corresponds to a lower melting degree. This 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 control. The 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 D U>D Th for Al-clinopyroxene at pressures of >1.0 GPa. The 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 source. Accordingly, excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K2O/TiO2 with (230Th/238U) and initial melting pressure (P o) of MORBs, so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone. OIBs and MORBs in both (230Th/238U) vs. K2O/TiO2 and (230Th/238U) vs. P o plots fall in two distinct areas, indicating that the mineral phases which dominate their excess 230Th are different. Ce/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 garnet. The mechanisms generating excess 230Th in MORBs and OIBs are significantly different, with formation of excess 230Th in the garnet zone only being suitable for OIBs. Science China Press and Springer-Verlag Berlin Heidelberg, 2010 excess 230Th nationallicence clinopyroxene nationallicence garnet nationallicence spinel-lherzolite nationallicence mid-ocean ridge nationallicence ocean island nationallicence Zhang GuoLiang Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 266071, Qingdao, China aut Zeng ZhiGang Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 266071, Qingdao, China aut Science China Earth Sciences SP Science China Press 53/10(2010-10-01), 1486-1494 1674-7313 53:10<1486 2010 53 11430 https://doi.org/10.1007/s11430-010-4038-4 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s11430-010-4038-4 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Zhang GuoLiang Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 266071, Qingdao, China aut NATIONALLICENCE 700 1- Zeng ZhiGang Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 266071, Qingdao, China aut NATIONALLICENCE 773 0- Science China Earth Sciences SP Science China Press 53/10(2010-10-01), 1486-1494 1674-7313 53:10<1486 2010 53 11430 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer