Phase stability fields in calc-alkaline magmatic systems and textural evolution of crystalline aggregates

点击翻译按钮获取中文摘要

Magma plumbing systems of calc-alkaline volcanoes are complex. The magma storage conditions (P, T, fO2, fH2O), the geochemical processes occurring at depth as well as the lifetime of magma reservoirs are difficult to constrain. One approach to determine time scales of magmatic processes in arc systems is to analyze compositionally zoned minerals and to apply diffusion chronometry, but the correct interpretation of the compositional profiles requires additional information to be known: (1) the magma storage conditions (e.g. temperature, pressure) and (2) the lifetime of the individual crystals used for diffusion chronometry. This information is particularly difficult to constrain for arc magmas because they contain high and variable water concentrations, which affect crystallization temperatures significantly, and because crystals may be the subject of dissolution/growth processes, especially due to numerous replenishment and mixing processes. The goals of this proposal are (1) to provide better general constraints on the role of P, T, fO2, fH2O on the stability of main silicate phases in calc-alkaline systems and (2) to understand the rate of textural evolution of polycrystalline partially molten mineral aggregates (simulating processes in a crystal mush) and to evaluate the influence of growth and dissolution on diffusion profiles (and on estimated time scales). High pressure experimental studies will be conducted to constrain the crystallization temperatures of the main phases olivine, clino- and orthopyroxene, and plagioclase as a function of pressure (100 - 500 MPa), fO2 (QFM+1.5 -QFM+3.5) and fH2O (2 to 7 wt% H2O in melt). The starting compositions will be representative of the large variety of primitive calc-alkaline melts and of two volcanoes from Kamchatka (Bezymianny and Klyuchevskoy), which have different plumbing systems. The experiments with compositions from Bezymianny (andesitic-dacitic) and Klyuchevskoy (basaltic) will also be used to constrain the magma storage conditions, which are needed for accurate diffusion chronometry. These two volcanoes are ideal candidates to check if differences between time scales recorded in mineral phases can be detected between andesitic-dacitic systems with probably large volume magma reservoirs and basaltic systems with low-volume magma channel (see Project #8 of the research unit). The basaltic compositions that will be used to understand the textural evolution of polycrystalline aggregates will be designed so that they are in equilibrium with Fo-rich olivine crystals only and the results will be directly applied to simulation of such processes (see Project #5 of the research unit). The experimental results will also allow us to test and recalibrate mineral-melt thermobarometers available so far and are crucial to improve thermodynamic models that are designed to predict phase relations and liquid lines of descent.