COMPARING PT PATHS OF METAMORPHIC ROCKS DETERMINED BY QUANTITATIVE AND SEMI-QUANTITATIVE APPROACHES: A CASE STUDY FROM THE MONVISO OPHIOLITE, ITALY

Historically, the field of metamorphic petrology has advanced largely due to the implementation of techniques for quantifying the chemical compositions of rocks in situ. Today, concentrations of major and trace elements are routinely measured on the micrometer scale with instruments like the Electron Probe Microanalyzer (EPMA) and others. Quantitative chemical maps of minerals produced by EPMA (using raw data corrections and standards) are normally favored over semi-quantitative chemical maps produced by Scanning Electron Microscopes (SEM; using raw data corrections without standards). Although significantly more cost effective, the latter technique is generally regarded as unsuitable for calculating pressures and temperatures (PTs) with calibrated thermobarometers. However, we hypothesize that a monte-carlo resampling technique applied to semi-quantitative chemical maps can reproduce PT paths determined by alternative methods. To test this hypothesis, samples of various HP metamorphic rocks were collected from the well-studied Monviso Ophiolite, Italy. Multiple samples, including eclogite facies metasedimentary (phengite + quartz + garnet ± epidote ± chlorite) and metagabbroic rocks (omphacite + garnet + rutile ± phengite ± glaucophane ± quartz) were sectioned and analyzed by optical microscopy and SEM. High-resolution chemical maps (up to 1024x1024µm) were collected using a Zeiss Supra 35 VP FEG SEM with a Bruker Quantax 100 EDS system at Miami University. Hundreds of thermobarometric calculations applied to the peak (eclogite facies) and retrograde (epidote-blueschist facies) assemblages with XMapTools 4 yield a distribution of possible PT conditions that can be directly compared to previously-determined PT paths derived from EMPA analyses. The PT comparisons will indicate whether or not our semi-quantitative workflow with XMapTools 4 is a viable alternative for a currently redundant and expensive workflow for metamorphic petrology.