Experimental Investigation of the Blueschist to Greenschist Facies Transition

角闪石蓝闪石(Na2Mg3Al2Si8O22(OH)2)是遭受蓝片岩相变质作用的岩石的关键指示矿物。基于本实验室在合成理想蓝闪石方面取得的最新进展，这项建议将重点放在定义蓝闪石低压稳定性的反应的实验研究上，以及模拟蓝片岩向绿片岩变质岩转变的反应。确定这些反应在压力-温度空间中发生的位置对产生蓝片岩相岩石的地质过程施加了限制，这反过来又提高了我们对近地表岩石在俯冲到地球地幔深处或返回地球表面时深部岩石折返时遵循的压力-温度路径的理解。在这个建议中，将对几种都是节水的反应进行实验研究，即它们既不消耗也不释放水，以避免片状硅酸盐(蛭石)的干扰。蓝闪石+2石英=滑石+2钠长石(1)是蓝片岩向绿片岩转变的粗略类似物，证明了用最少的水(1-5%)研究保水反应的可行性。本文对Na2O-CaO-MgO-Al_2O_3-SiO_2-H_2O体系中蓝闪石+2透辉石+2石英=透闪石+2钠长石的反应进行了初步实验。这一建议的主要部分致力于研究铁-镁交换对反应位置的作用(1)。热力学计算表明，亚铁将降低蓝闪石和滑石的稳定压力，并可能定义一个非常有用的地压计。在Na2O-MgO-Al_2O_3-SiO_2-H_2O体系中已确定的其他保水反应包括：7蓝闪石+28石英+6绿泥石+8蓝晶石=17滑石+14副辉石；7蓝闪石+14副辉石+3滑石=28钠长石+8蓝晶石+6绿泥石。

The amphibole glaucophane (Na2Mg3Al2Si8O22(OH)2) is the key index mineral of rocks that have been subjected to blueschist-facies metamorphism. Building on the recent advances made in this lab on the synthesis of ideal glaucophane, this proposal will focus on the experimental investigation of reactions that define the lower-pressure stability of glaucophane and that model the transition from blueschist to greenschist metamorphic rocks. Determining where these reactions occur in pressure-temperature space places constraints on the geological processes that produce blueschist-facies rocks which, in turn, improves our understanding of the pressure-temperature path followed by near-suface rocks during their subduction deep into Earth's mantle or exhumation of deep-seated rocks on their return to the Earth's surface.In this proposal several reactions will be experimentally investigated which are all water-conserving, that is, they neither consume nor liberate water, in order to avoid the intervention of a sheet silicate (vermiculite). The feasibility of studying a water-conserving reaction with minimal amounts of water (1-5 wt%) has been demonstrated with the reaction: glaucophane + 2 quartz = talc + 2 albite (1), which is a crude analogue of the blueschist-to-greenschist transition. Preliminary experiments have been done on the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite in the Ca-bearing system Na2O-CaO-MgO-Al2O3-SiO2-H2O, which will be continued in this proposal. A major part of this proposal is devoted to investigating the role of Fe-Mg exchange on the location of reaction (1). Thermodynamic calculations indicate that ferrous iron will lower the pressure of stability of glaucophane and talc and may define a very useful geobarometer. Other water-conserving reactions that have been identified in the system Na2O-MgO-Al2O3-SiO2-H2O which may provide a better model of this transition include: 7 glaucophane + 28 quartz + 6 chlorite + 8 kyanite = 17 talc + 14 paragonite; and 7 glaucophane + 14 paragonite + 3 talc = 28 albite + 8 kyanite + 6 chlorite.