Experimental Investigation of the Blueschist to Greenschist Facies Transition

蓝片岩向绿片岩相转变的实验研究

• 批准号: 0609184
• 负责人: David Jenkins
• 金额: $ 18.11万
• 依托单位: SUNY at Binghamton
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0609184&HistoricalAwards=false
• 关键词: Experimental; Investigation; Blueschist; Greenschist; Facies

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.

角闪石蓝闪石（Na_2 Mg_3Al_2Si_8 O_（22）（OH）_2）是岩石受蓝片岩相变质作用的重要标志矿物。 基于本实验室在理想蓝闪石合成方面取得的最新进展，本提案将侧重于定义蓝闪石低压稳定性的反应的实验研究，并模拟从蓝片岩到绿片岩变质岩的过渡。 确定这些反应在压力-温度空间中发生的位置限制了产生蓝片岩相岩石的地质过程，反过来，提高了我们对近地表岩石在俯冲到地幔深处或深部岩石折返过程中所遵循的压力-温度路径的理解，在这个提议中，将对几种反应进行实验研究，这些反应都是节水的，也就是说，它们既不消耗也不释放水，以避免片状硅酸盐（蛭石）的介入。 研究具有最小量的水（1- 5wt%）的保水反应的可行性已经用以下反应证明：蓝闪石+2石英=滑石+2钠长石（1），其是蓝片岩到绿片岩转变的天然类似物。 本文对Na_2 O-CaO-MgO-Al_2 O_3-SiO_2-H_2O含钙体系中蓝闪石+2透辉石+2石英=透闪石+2钠长石的反应进行了初步的实验研究。 该提案的主要部分致力于研究Fe-Mg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质压力计。 在Na_2 O-MgO-Al_2 O_3-SiO_2-H_2 O系统中发现的其他保水反应可能提供了这种转变的更好模型，包括：7蓝闪石+28石英+6橄榄石+8蓝晶石= 17滑石+14钠长石; 7蓝闪石+14钠长石+3滑石= 28钠长石+8蓝晶石+6橄榄石。