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

本文通过电子探针分析、水浸和原地部分熔融等方法研究了地幔橄榄岩中不相容元素的分布，解释了玄武岩浆中钾等不相容元素的含量。矿物化学很难解释其母岩的化学性质。电子探针跨矿物边界阶跃分析证实了不相容元素沿着晶界富集。酸洗前后组分表面的电子探针分析也证明了边界富集。表面的AIKbeta发射光谱表明，矿物表面上的原子与周围原子的结合不如矿物中的原子紧密。因此它们很容易被滤出。为了估计储存在表面上的不相容元素的量，通过在20、75和160 ℃下用去离子水反复洗涤样品来沥滤元素，并将沥滤物与样品中的不相容元素进行比较。 关于我们 用离子色谱分析仪分析洗涤溶液。淋溶元素的累积量在每个温度下趋于几乎恒定，但随着温度的升高而增加。在160 ℃下，每克样品浸出的钾碱量为63.3- 255 μ g，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。钾和其他不相容元素的晶界富集在橄榄岩的部分熔融中应特别重要。为了检验这一点，在气体混合炉中将橄榄岩碎片加热到1100-1370摄氏度。在各个温度下，沿组分的晶界沿着形成宽度为3-30 μ m的玻璃脉。玻璃占4-8体积%在1300摄氏度，并显示出玄武岩的组成。这一结果使我们得出一个结论，即在熔融程度较低时，熔体中不相容元素的丰度主要由晶界决定，因为晶界可以首先熔融。本研究表明，玄武岩浆可以产生于钾霞石和/或金云母的地幔橄榄岩。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