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 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 DU>DTh 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(Po) 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.Po 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
ZHANG GuoLiang1,2 & ZENG ZhiGang1 1 Key Laboratory of Marine Geology and Environment,Institute of Oceanology,Chinese Academy of Sciences,Qingdao 266071,China
This paper reports silicon and oxygen isotopes of 20 kinds of igneous rocks and their major elements from the eastern Manus Basin. Combining silicon and oxygen isotopic data from other studies, we suppose that both δ30Si and δ18O values increase with the increasing of SiO2 content. It means that the fractionation of silicon and oxygen isotopes are affected by the silica content. The positive correlation between CaO/Al2O3 ratios and MgO and that between Si/Al and SiO2 content indicate that clinopyroxene is the predominant mineral phase in our samples. We suppose that the fractionation of silicon and oxygen isotopes are influenced by mineral fractional crystallization. Probably, it is due to their different silicon and oxygen bridges. In this study, the δ30Simean value=-0.17‰±0.17‰ and δ18Omean value= +6.07‰±0.57‰ are higher than normal δ30Si and δ18O values of mantle, and we propose that these igneous rocks in the eastern Manus Basin are affected by hydrothermal alteration.
The rare earth element(REE)geochemistry of hydrothermal vent systems has been investigated intensively,but few studies have been carried out on marine shallow-water hydrothermal systems like that at Kueishantao.Here we present novel data sets of REE in hydrothermal fluids from Kueishantao,off northeastern Taiwan.The total REE(REE)concentrations of yellowish fluids are similar to those of whitish fluids,813–1212 ng/L,and are significantly higher than that of ambient seawater.The yellowish fluids have chondrite-normalized REE(REEN)distribution patterns with slight convex-downward curvatures at Eu;and the REE patterns of the whitish fluids are smooth at Eu,which is related to the lower temperature and more oxidizing conditions.The Kueishantao hydrothermal fluids are slightly enriched in light REE(La-Nd)relative to the heavy REE(Gd-Lu).The behaviors of REE in both yellowish and whitish fluids are affected by the short time of water-rock interaction.The REE distributions in the yellowish fluids are also affected by very low pH(2.81 and 2.29),boiling of the fluid and precipitation of native sulfur.In the whitish fluids,adsorption by small particles and formation of REE-chloride complexes has played a role in the distribution of REE.
WANG XiaoYuanZENG ZhiGangCHEN ShuaiYIN XueBoCHEN Chen-Tung Arthur
