An Excel-based tool for evaluating and visualizing geothermobarometry data

摘要

Application of geothermobarometry based on equilibrium exchange of chemical components between two mineral phases in natural samples frequently leads to the dilemma of either: (1) relying on relatively few measurements where there is a high likelihood of equilibrium, or (2) using many analysis pairs, where a significant proportion may not be useful and must be filtered out. The second approach leads to the challenges of (1) evaluation of equilibrium for large numbers of analysis pairs, (2) finding patterns in the dataset where multiple populations exist, and (3) visualizing relationships between calculated temperatures and compositional and textural parameters. Given the limitations of currently-used thermobarometry spreadsheets, we redesign them in a way that eliminates tedium by automating data importing, quality control and calculations, while making all results visible in a single view. Rather than using a traditional spreadsheet layout, we array the calculations in a grid. Each color-coded grid node contains the calculated temperature result corresponding to the intersection of two analyses given in the corresponding column and row. We provide Microsoft Excel templates for some commonly-used thermometers, that can be modified for use with any geothermometer or geobarom-eter involving two phases. Conditional formatting and ability to sort according to any chosen parameter simplifies pattern recognition, while tests for equilibrium can be incorporated into grid calculations. A case study of rhyodacite domes at Parinacota volcano, Chile, indicates a single population of Fe-Ti oxide temperatures, despite Mg-Mn compositional variability. Crystal zoning and differing thermal histories are, however, evident as a bimodal population of plagioclase-amphibole temperatures. Our approach aids in identification of suspect analyses and xenocrysts and visualization of links between temperature and phase composition. This facilitates interpretation of whether heat transfer was accompanied by bulk mass transfer, and to what degree diffusion has homogenized calculated temperature results in hybrid magmas.