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.
