Evaluation of self-consistency among geothermometers: a step toward more accurate temperature-time paths of crystallizing magmas

• Applicant: John Hora, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2012 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 221141776

One of the main goals of igneous petrology is to describe the series of conditions experienced by a magma leading up to eruption: a pressure-temperature-time path. The two most common routes to this goal are: (1) to interpret information from several phases that have a variety of closure temperatures, and therefore record conditions at different points in time, or (2) to interpret zoning in crystals. Several geothermometers based on equilibrium exchange of chemical components among phases exist for use in silicic magmas. However, performance of their calibrations are, in most cases, verified by comparison to only one or two other thermometers in natural samples, and no reliable measure of self-consistency exists for the entire suite of thermometers in aggregate. Thermometers that rely on analysis of a single phase (or growth zone in a crystal) offer a way forward, in that a sequence of temperatures can be read from core to rim of a crystal, but recently the calibrations of these, too, have been questioned and the various extant formulations of calibrations need to be rigorously tested against as many benchmarks as possible.This project shall compare a large number of thermometers in a single natural sample, with the goal of evaluating self-consistency among available thermometers. Having more equations than intensive variables is important if the values of those variables are not known a priori (as is often the case with natural samples). Identification of outliers from the ensemble model can be used as a means of identifying xenocrysts. If on the other hand, equilibrium can be independently verified (e.g. though trace element partitioning), discordance may point to ways in which the underlying thermometer calibrations can be improved to give more consistent results.Self-consistency among geothermometers will allow more robust interpretation of a sequence of closure temperatures as a rough T-t path. A higher-resolution T-t path requires finer spatial resolution, such that both temperatures and ages of individual growth zones can be measured. We aim to evaluate the feasibility of this idea via limited-scope ion probe work that, if successful, is intended to form the basis for subsequent work.

• DFG Programme: Research Grants
• International Connection: USA
• Participating Person: Professor Axel Karl Schmitt, Ph.D.