Experimental Study of Plagioclase Textures and Amphibole Reaction Rims: Implications for Rates of Magmatic Processes

University Of Alaska Fairbanks Campus, Fairbanks AK

Investigators

Abstract

Larsen and Gardner EAR-0106658 Mineral phases in crustal magma bodies record information about magma mixing, heating, and degassing in the form of reaction textures that occur as the mineral responds to the change in state of the magma. Two minerals that commonly show these features are plagioclase and amphibole, exhibiting sieve, dusty, and patchy zoning (dissolution and growth) and reaction rims (degassing and heating) respectively. These textures are often used to estimate the rates of magma mixing and magmatic ascent processes. We propose to experimentally constrain the rates at which these textures form at conditions applicable to shallow-level crustal reservoirs, using natural (rhyo)dacite and andesite starting material, and clean, unzoned, euhedral seed crystals. Our goal for this proposed study is to provide the vital calibrations needed to make observations of these reaction textures quantitatively useful for volcanologists when examining the products of crustal magma reservoirs. Previous work (refs) investigating the rates and causes of these textures have provided a good basis for our proposed study. However, limitations to this work exist, necessitating further study to make constraints useful to a wide range of mineral-melt compositions and P-T conditions applicable to crustal magma reservoirs. We will accomplish our goals in two tasks that are designed to experimentally constrain the rates of dissolution and regrowth textures in plagioclase resulting from heating and compositional disequilibrium, and amphibole reaction rims produced by melt degassing or heating. The first task will focus on plagioclase dissolution and rim growth in Aniakchak rhyodacite and andesite, using plagioclase seed phenocrysts of An20, An37, and An80 composition, at pressures between 0.1 and 300 MPa and temperatures between 825 and 1100 degrees C. The goal of this task is to constrain rates of plagioclase dissolution as a function of heating above the plagioclase solidus, and to examine dissolution and re-growth of plagioclase due to chemical disequilibrium in melts in which plagioclase of another composition is stable. The second task will investigate pargasite and hornblende amphibole breakdown in Redoubt dacite and andesite at P-T conditions appropriate for amphibole stability in these compositions. The goal of this task is to extend known calibrations of this reaction to different melt compositions and magmatic conditions. In addition, the thermal breakdown of amphibole will be investigated in order to help petrologists distinguish between decompression and heating rims. The results of these studies will be constraints on the rates of formation of important plagioclase and amphibole reaction textures in shallow-level crustal reservoirs.