Ag在橄榄石、斜方辉石、单斜辉石和玄武质熔体间的分配

Partitioning of silver between olivine,orthopyroxene,clinopyroxene,and basaltic melts

地幔部分熔融过程中亲S元素Ag的分配行为能够限定地幔中Ag含量,同时可以追踪地幔中S含量和去向。在地幔部分熔融过程中, Ag的分配行为与其在地幔硅酸盐矿物–玄武质熔体间的分配系数密切相关。因此准确地测定Ag在除硫化物之外地幔硅酸盐矿物和共存熔体之间的分配系数是预测Ag分配行为的必要条件之一。目前缺乏Ag在地幔矿物(橄榄石、辉石等)和玄武质熔体之间分配系数的报道,阻碍了对Ag在地幔部分熔融过程中分配行为的研究。本研究通过高温高压实验获得了在洋中脊玄武岩(MORB)产生的温度和压力条件下,Ag在橄榄石(ol)、斜方辉石(opx)、单斜辉石(cpx)与玄武质熔体(melt)之间的分配系数,分别为DAgol/melt=0.0005±0.0002、DAgopx/melt=0.007±0.003和DAgcpx/melt=0.046±0.009。根据实验获得的分配系数进行模拟的计算结果表明,太古代科马提岩和洋中脊苦橄岩源区的地幔Ag含量为(8±2)×10^(-9)。如果这个值能代表MORB源区地幔的Ag含量,那么当MORB地幔中的S含量为170×10^(-6)~240×10^(-6)时,其低于15%部分熔融产生熔体中Ag含量与原始MORB中Ag含量吻合。因此MORB源区地幔中的S含量为(205±35)×10^(-6)。另外,原始MORB熔体产生时,其源区普遍有硫化物残留。

The partitioning behavior of the chalcophile element silver(Ag)during mantle partial melting has the potential to constrain the abundance of Ag and facilitate tracking of the abundance and budget of sulfur in the mantle.Ag partitioning behavior during mantle partial melting is closely related to partition coefficients between silicate minerals and basaltic melts.Therefore,accurate Ag partition coefficients for silicate minerals in addition to sulfide are necessary to predict Ag partitioning behavior during mantle partial melting.However,accurate Ag partition coefficients between silicate minerals and basaltic melts are rare,hindering the understanding of Ag partitioning behavior during mantle partial melting.Using the high temperature and pressure experimental method,this study obtained the Ag partition coefficients between olivine(ol),orthopyroxene(opx),clinopyroxene(cpx)and basaltic melts under the P-T conditions for MORB generation,which are D^(ol/melt)_(Ag)=0.0005±0.0002,Dopx/melt Ag=0.007±0.003,and D^(cpx/melt)_(Ag)=0.046±0.009,respectively.Applying these partition coefficients to the upper mantle partial melting model,the calculated Ag contents in the sources of three Archean komatiite and one mid-ocean ridge picrite are(8±2)×10−9.If the predicted Ag content in the sources of komatiite and picrite represents Ag abundance of MORB mantle,the calculated Ag contents of partial melts are consistent with those of primitive MORBs with melting degree<15%when MORB mantle contains 170×10^(−6) to 240×10^(−6) sulfur.Therefore,the abundance of sulfur in MORB mantle is(205±35)×10^(−6).On the other hand,the calculated results indicate that sulfide is present commonly in the source mantle during primitive MORB generation.