Experimental Investigation of Glaucophane Solid Solution with Several Calcic Amphiboles

本文拟通过实验研究钠质角闪石与几种常见钙质角闪石之间矿物固溶体组成的范围和性质。这些类型的钠钙角闪石是世界各地观察到的典型高压变质岩。虽然已有经验观察将这类角闪石的成分变化与其构造环境联系起来，但这些成分变化尚未得到定量校准。这些信息将有助于地质学家从钠钙角闪岩的组成变化中确定造山带或俯冲带的压力-温度历史。本研究将重点研究透闪石-蓝透石、镁角闪石-蓝透石和马尾石-蓝透石在Na2O-MgO-CaO-Al2O3-SiO2-H2O化学体系中的组成。这些成分连接构成了钠钙角闪石中最常见的成分变化。从现场证据和本实验室获得的初步实验数据可以很好地确定透闪石-蓝透石连接处存在混相间隙。当前提议的一个主要目标是尽可能准确地定义这种混相间隙的位置，因为它对衍生的端元组分的热力学混合特性施加了严格的约束。混相间隙在温度-组成空间中的位置可以通过不同的组分反转来确定，如端元透闪石和蓝透石沿混相间隙向组分的溶解，以及可能通过初始均质角闪洞的溶出。在其它连接处也要寻找混相间隙。对于所研究的每个连接，将通过oh -拉伸区域的红外光谱组合和使用适当大晶体的单晶结构精化的键长与离子半径关系来确定长、短程阳离子顺序的程度。从每个连接处选择的晶体将通过高分辨率透射电子显微镜进行检查，以记录链多重缺陷的存在，这些缺陷可能会导致某种程度的明显而非真正的固溶体。本研究的信息可用于校准钠钙角闪石组成随压力、温度和共存矿物组合的变化。反过来，这对于那些利用钠钙角闪洞组成的变化来确定增生带或碰撞带的构造历史的人来说将是特别感兴趣的。该提案包括为研究生和本科生提供资金。与当地一所高中合作，将指导选定的地球科学荣誉学生作为本研究项目的一部分。

This proposal is to investigate experimentally the extent and nature of mineral solid solution compositions between the sodic amphibole glaucophane and several common calcic amphiboles. These types of sodic-calcic-amphiboles are typical of high-pressure metamorphic rocks observed around the world. Although there have been empirical observations relating compositional changes in such amphiboles to their tectonic settings, these compositional changes have yet to be calibrated quantitatively. The information obtained in this study will help geologists to determine the pressure-temperature history of orogenic belts or subduction zones from the compositional variations of sodic-calcic-amphiboles.This study will focus on the compositional joins tremolite-glaucophane, magnesiohornblende-glaucophane, and pargasite-glaucophane in the chemical system Na2O-MgO-CaO-Al2O3-SiO2-H2O. These compositional joins comprise the most common compositional variations in sodic-calcic-amphiboles. The presence of a miscibility gap for the tremolite-glaucophane join is well established from field evidence and supported by preliminary experimental data obtained in this lab. A major goal of the current proposal will be to define the location of this miscibility gap as accurately as possible because of the tight constraints that it places on the derived thermodynamic mixing properties of the end-member components. Location of the miscibility gap in temperature-composition space will be determined by compositional reversals using various methods, such as the dissolution of end-member tremolite and glaucophane to compositions along the miscibility gap and possibly by exsolution of initially homogeneous amphibole. Miscibility gaps will also be sought along the other joins. For each of the joins being studied, care will be taken to determine the extent of long- and short-range cation order by a combination of infrared spectroscopy in the OH-stretching region and the use of bond-length versus ionic-radius relationships from single-crystal structural refinements of suitably large crystals. Selected crystals from each join will be examined by high-resolution transmission electron microscopy to document the presence of chain-multiplicity faults that might induce a certain degree of apparent rather than true solid solution. The information from this study can be used to calibrate variations in the compositions of sodic-calcic-amphiboles with pressure, temperature, and coexisting mineral assemblage. This, in turn, will be of particular interest to those using variations in sodic-calcic-amphibole compositions to determine the tectonic history of accretionary or collision zones. This proposal includes funding for graduate and undergraduate students. In cooperation with a local high school, mentoring of selected Earth science honors students will be included as part of this research project.