超高压变质作用不同阶段的流体变化:碧溪岭榴辉岩的氢、硼同位素分析

Fluid evolution of different stages during UHP metamorphism: evidence from hydrogen and boron isotope compositions of Bixiling eclogite from Dabieshan

大别山碧溪岭榴辉岩中有三种含水矿物:多硅白云母、角闪石和黑云母,它们分别是超高压(UHP)阶段(即柯石英榴辉岩相阶段)或者石英榴辉岩相阶段、退变质后成合晶阶段和角闪岩相退变质阶段的产物,本文利用离子探针技术对它们进行了氢同位素和硼同位素的分析。三种矿物内部的同位素组成都是均一的,多硅白云母的δD为-105‰±9‰,δ^(11)B为-25.9‰±2.0‰;角闪石的δD为-100‰±9‰,δ^(11)B为-24.4‰±0.9‰;黑云母的δD为-65‰±4‰,δ^(11)B为-19.3‰±1.3‰。多硅白云母和角闪石的氢-硼同位素组成在误差范围内是相同的,而和黑云母则有明显的差别,这表明,从UHP阶段或者石英榴辉岩相阶段到随后的后成合晶阶段,变质流体是内部缓冲的,而在角闪岩相变质阶段,则有了外来流体的加入,这个流体是相对富集D和^(11)B的。碧溪岭榴辉岩矿物相对于其地壳原岩表现出低δ^(11)B的特征,说明俯冲过程中板块经历了强烈的脱硼。

There are three types of hydrous minerals in Bixiling eclogites of Dabieshan: phengite, amphibole and biotite. They are formed at UHP metamorphism, retrograde symplectite metamorphism and retrograde amphibolite-facies metamorphism respectively. Hydrogen and boron isotope compositions of them have been obtained by ion microprobe. They show homogeneous hydrogen and boron isotopic ratios within the same grain: deltaD and delta(11)B values of phengite are - 105parts per thousand +/- 9parts per thousand and - 25. 9parts per thousand +/- 2.0parts per thousand respectively, those of amphibole are - 100parts per thousand +/- 9parts per thousand and - 24.4parts per thousand +/- 0.9parts per thousand, and those of biotite are - 65parts per thousand +/- 4parts per thousand and - 19.3parts per thousand +/- 1.3parts per thousand deltaD and delta(11)B values of phengite are the same as those of amphibole within analytical error, but higher than those of biotite. These results demonstrate that metamorphic fluid was internally buffered from UHP metamorphism to retrograde symplectite metamorphism, but at amphibolitefacies metamorphic stage exotic fluid with D- and B-11-enrichment entered into the system. delta(11)B values of Bixiling eclogite are lower than their crustal protolith, suggesting that subducted plate lose the original boron at high grade.