Evaluation of self-consistency among geothermometers: a step toward more accurate temperature-time paths of crystallizing magmas
Final Report Abstract
Geothermobarometry in igneous systems, i.e. the precise determination pressure (P) and temperature (T) conditions of the storage and crystallization in magma reservoirs prior to eruption, is important because it yields critical information on the past behavior and future risk of eruption at volcanoes. Volcanoes that erupt highly evolved silica-rich magmas tend to produce the most hazardous eruptions, yet particularly for these compositions, there are difficulties in implementing the various geothermobarometers that currently exist. The goal of this project was to find approaches to improve the application of these tools, and in so doing, improve the accuracy with which these critical pieces of information about volcanoes are calculated. This had the result of yielding information on the pressure-temperature-time (P-T-t) history of the case study system, Parinacota Volcano in the central Andes, a result that can likely be generalized to the behavior of how stagnant silicic systems in the crust respond to influx of new magma. Some thermobarometers are based on the exchange of elements (e.g. Fe and Ti) between two minerals (e.g. magnetite and ilmenite). If crystals experience a long history of crystallization, matching the appropriate parts of the history to calculate a temperature can be difficult. If mineral grains in mutual contact are rare, a common tactic is to analyze many grains in equilibrium with melt to arrive at a solution. This approach can involve many possible pairings and risks that some pairs do not correspond to minerals that were in equilibrium. We developed a tool that evaluates equilibrium and calculates a result for every possible pairing in a grid that is easy to evaluate visually. Using such an approach, the presence of populations of mineral grains with distinct histories can be readily identified in a mixed magma. At Parinacota Volcano, a population of higher-T amphibole and plagioclase indicate that the stagnant silicic magma had been intruded by more mafic magma. It not only input heat into the system, but the magmas must have physically mixed such that both populations of minerals mingled in the hybrid melt. In these same magmas the temperatures calculated for pairs of Fe-Ti oxides are uniform, indicating that for oxide minerals rapid diffusion has likely wiped away any pre-mixing history. The tools developed here have been made available to the broader petrologic community to use and modify for a wide variety of applications. Another significant difficulty in using thermobarometers is that the result depends not only on the composition of minerals (that are measured) but on parameters that are difficult to constrain (e.g. melt composition at the time of crystallization, water content of the melt, and pressure). For some minerals, this is further complicated by the existence of several calibrations for the same thermobarometer, which indicate different dependences on these unconstrained variables. We deal with these problems together, and in doing so, resolve both of these issues to some degree. In a system where there are more equations relating unknown variables than unknown variables, these can be simultaneously solved for. We apply this approach to rhyodacitic magmas at Parinacota Volcnao, which have many minerals on which thermobarometry can be done. We find that most thermobarometers converge on a region where P=2.75 kbar, T=750°C, and activity of titania in the melt (aTiO2) = 0.85. The latter is important to constrain since it has been a source of controversy in previous application of thermobarometers in silicic rocks. This method also can be used to identify which equations do not return results near the consensus values of these variables. Among the calibrations that perform poorly are those where the element of interest diffuses rapidly, or those where the calculated result is strongly dependent on inputs of melt composition and water content, both of which are expected to change significantly during the time that a magma crystallizes. The approach developed here is useful because it provides validation for certain calibrations of a class of thermobarometers that depend on analysis of only one mineral. That class of thermobarometers, which applies to partitioning of Ti and Zr in the minerals zircon, titanite, amphibole, and quartz, holds immense promise for reading highly detailed thermal histories from the chemical zonation that developed in these minerals as they grew. Zircon has the advantage that it can be used for age determination by in-situ analysis of U-Th isotopic disequilibrium. This allows pairing of temperature and time for a given growth zone. Parinacota zircons are much older than the volcano itself, and record a wide variety of temperatures during their long storage time prior to eruption. Over the past several years, there has been paradigm shift in thinking about how magmatic systems behave. It has been increasingly recognized that magmas likely spend significant amounts of their pre-eruptive histories as crystal-rich mushes that experience periodic excursions to high temperatures as they are recharged by hotter more mafic magmas, experiencing a period of crystallization as they cool. The Parinacota zircon data are entirely consistent with this emerging model. Zircon, however has the disadvantage that crystals are small, and time resolution is limited. We evaluated amphibole and titanite as complements to the zircon record, and find that if aTiO2 can be constrained, these minerals can, owing to their much larger crystal sizes and ease of analysis, provide some of the same information as zircon, with higher spatial resolution, albeit with only relative timing constraints. The overall result at Parinacota shows that the output of some volcanoes is highly dependent on recharge. At low recharge rates, silicic magmas stagnate as crystal mushes at depth. If a critical recharge rate (and melt fraction) is attained, they become eruptible, forming domes. Once recharge purges old crystals from the system, these mafic magmas can pass unimpeded to the surface.
Publications
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(2013) An Excel-based tool for evaluating and visualizing geothermobarometry data. Computers and Geosciences 56: 178-185
Hora JM, Kronz A, Möller-McNett S, Wörner G
