马里亚纳俯冲带弧内坡由于俯冲的太平洋板片的侵蚀而出露了相当于弧下地幔成分的橄榄岩, 这些橄榄岩是研究岛弧岩浆作用及弧下地幔流体交代作用难得的样品。报道了马里亚纳南部关岛以南的弧内坡橄榄岩的矿物组成、主量元素以及微量元素地球化学特征。这些岩石具有非常难熔的矿物组成: 橄榄石具有很高的Mg, 斜方辉石中的Al2O3含量极低, 尖晶石具有很高的Cr。滑石、蛇纹石和角闪石(透闪石为主, 有少量镁角闪石和浅闪石)则指示了流体在中低温度下对橄榄岩的交代作用, 并表明流体可能含有较高的SiO2、Al2O3和Na2O。全岩的主量元素中, CaO和Al2O3的质量百分比含量很低, 而MgO的含量较高(干体系下多在43%左右), 显示岩石经历了高度熔融。微量元素中, U和Sr的含量较高, 相对富集, 可能是来源较浅的板片流体化学特点的反映。
The peridotites beneath arc outcrop at the inner slop of the Mariana Trench because of subduction erosion, which provides us a window to see the geological process in mantle wedge. In this paper, mineral composition, major elements, and trace element data of the peridotites dragged from southern Mariana Trench inner slope are reported to discuss petrogeness and fluid/rock interaction. High Mg of olivine, extremely low Al2O3 of orthopyroxene, and high Cr of spinel indicate the peridotites are refractory. Hydrous minerals in the peridotites such as Talc, amphibole (mainly tremolite with a little magnesiohornblende and edenite) and serpentine indicate fluid that might be rich in SiO2, Na2O and Al2O3 interacting with the refractory peridotites in mantle wedge. Whole rocks contain low concentration of CaO and Al2O3 and high MgO (~43% calculate to anhydrous system) coincidence with refractory nature revealed by mineral composition. The relative enrichment of U and Sr might reflect the chemical characteristics of the fluid released from the subducting slab at shallow depth.
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