Behavior of trace elements concentrated at grain-boundaries during partial melting of Peridotites

采用电子探针分析、水浸和原位部分熔融等方法对地幔橄榄岩中不相容元素的分布进行了研究，以解释玄武岩岩浆中钾等不相容元素的含量。矿物化学性质很难解释母岩橄榄岩的化学性质。不相容元素沿晶界富集是通过EPMA跨晶界阶跃分析证实的。通过对酸洗前后组分表面的EPMA分析也证明了边界富集。表面的AIKbeta发射光谱表明，矿物表面的原子与周围原子的结合不如矿物表面的原子紧密。因此它们很容易被滤出。为了估计不相容元素在样品表面的含量，用去离子水在20、75和160的温度下反复洗涤样品和浸出液，然后用离子色谱分析仪分析更多的溶液。浸出元素的累积量在每个温度下趋于几乎不变，但随着温度的升高而增加。在160℃温度下，1g样品的钾浸出量为63.3 ~ 255mug，约占全岩丰度的40 ~ 80%。因此，我们发现橄榄岩中的大部分不可解释元素都储存在晶界上。钾离子和其他不相容元素的晶界富集在橄榄岩的部分熔融中应是特别重要的。为了验证这一点，将橄榄岩碎片在气体混合炉中以1100-1370的温度加热。在各温度下，沿晶界均形成宽3 ~ 30 μ m的玻璃脉。在1300℃时，玻璃占4- 8%，呈玄武岩成分。结果表明，熔体中不相容元素的丰度主要由晶界决定，因为晶界首先熔化。目前的研究表明，玄武岩岩浆可以产生于不含钙云母和/或辉云母的地幔橄榄岩。少

The distribution of the incompatible elements in mantle peridotites was examined by means of electron microprobe analysis, water leaching and in situ partial melting to explain the contents of potash and other incompatible elements in basaltic magmas. The mineral chemistry can hardly account for the chemistry of the parental peridotites. The enrichment of incompatible elements along grainboundaries is confirmed by the EPMA step analysis across boundaries of the constituent minerals. The boundary enrichment is demonstrated also by the EPMA analysis of the surfaces of the constituents before and after acid washing. AIKbeta emission spectrum of the surfaces suggests that atoms on mineral surfaces are less tightly bonded with surrounding atoms than those in minerals. ahd hence they are leached out easily. To estimate the amount of incompatible elements stored on the surfaces, elements were leached out through repeated washing of samples with deionized water at 20, 75 and 160ﾟC and the leac … More hing solutions were analyzed with an ion chromatographic analyzer. The accumulated amount of leached elements tends to become almost constant at each temperature, but increases with increase in temperature. The potash leached out from 1g sample at 160ﾟC amounts to 63.3-255mug, about 40-80 % of the whole-rock abundance. Thus, we find that most of the unaccountable elements in peridotites is stored on the grain-boundaries. The grain-boundary enrichment of potash and other incompatible elements should be particularly important in partial melting of peridotites. To examine this, chips of peridotites were heated at 1100-1370ﾟC in a gas-mixing furnace. Glass veins, 3-30 mum in width, are formed along grain-boundaries of the constituents at every temperatures. Glass makes up 4-8 vol.% at 1300ﾟC, and shows a basaltic composition. This result leads us to a conclusion that the abundance of incompatible elements in the melt at the small degree of melting is governed mainly by the grain-boundaries, because the grain-boundaries can melt first. The present study demonstrates that basaltic magmas can be generated from kaersutite- and/or phlogopite-free mantle peridotites. Less