The origin and role of fluid associated with lower crustal metamorphism

下地壳流体可能对高压高温下各种变质岩和火成岩矿物组合的稳定性起着重要作用。虽然人们普遍认为CO_2是中下地壳中丰富的流体成分，而H_2O在浅层中占主导地位，但这种流体的来源及其在地壳岩石P-T演化中的作用尚不清楚。因此，本研究将重点放在麻粒岩-角闪岩相岩石中的流体包裹体上，确定了流体标本，并试图揭示流体在高变质作用中的作用。所有分析的岩石都含有CO_2，这些CO_2被困在石榴石和石英等各种矿物中，这证实了CO_2是下地壳的主要流体。包裹体碳同位素研究表明，部分CO_2来源于地幔。还发现了含水流体，但它们大多被困在逆行阶段。印度南部正正性矿物组合的相平衡模拟表明，含H_2O的还原流体（可能是H_2O + CH_4）在700℃左右的逆行过程中渗透形成含正辉石麻粒岩相组合，这表明流体的氧化状态和H_2O活性可能控制了下地壳高变质期矿物共生。

Fluid in lower crust probably plays an important role on the stability of mineral assemblages in various metamorphic and igneous rocks at high pressure and temperature. Although it is generally known that CO_2 is an abundant fluid component in middle to lower crust, while H_2O is dominant in shallower level, the origin of the fluid and its role during P-T evolution of crustal rocks are not known. In this study we therefore focused on fluid inclusions in granulite to amphibolite facies rocks, identified fluid specimens, and attempted to unravel the role of fluid during high-grade metamorphism. All the analyzed rocks contain CO_2 trapped in various minerals such as garnet and quartz, which confirms thatCO_2 is the dominant fluid in the lower crust. Carbon isotope study on some inclusions suggest that some CO_2 were derived from mantle. H2O bearing fluids were also identified, but they are mostly trapped in a retrograde stage. Phase equilibrium modeling of mineral assemblages on orthogniess from southern India suggest that infiltration of H_2O-bearing reduced fluid (probably H_2O + CH_4) during a retrograde (around 700℃) formed orthopyroxene-bearing granulite-facies assemblages, which indicates oxidation state, as well as H_2O activity, of fluid probably controls mineral paragenesis during high-grade metamorphism in lower crust.