Determining the Metamorphic Temperatures of Rocks in the Raspas Complex using Zirconium -in- Rutile Thermometry to Compare to Computer Models

Overview

The Raspas Complex in Ecuador, estimated to be approximately 120+ Ma old, is a metamorphic fossil subduction zone with diverse rock types including eclogites. This study determines the precise temperature conditions under which this rock formed enabling comparative analysis with computer model predictions to assist with seismic-event information. Three rock samples were used, which contain rutile, a mineral with trace amounts of zirconium oxide which helps determine temperature conditions. Specifically, rut ile in garnet inclusions provides a more accurate measure of temperature than rutile crystals found in the matrix of the rock. The location of approximately 40 rutiles in each sample was determined using a petrographic microscope and mapped using Adobe Illustrator. Utilizing an Electron Probe Microanalyzer (EPMA), oxide concentrations at each rutile location were determined. Once confirmed as rutile, Zirconium (Zr) parts per million (ppm) were calculated from ZrO2 concentrations. Using zirconium in Rutile thermometry techniques (Kohn, 2020), temperatures for the three samples were derived by averaging Zr concentration within each sample using the mean-max Zr content method following the approach of Penniston-Dorland et al. (2018). The temperature range spann ed from 500°Cto 650°C, with one sample exceeding 600°C, possibly influenced by sample location or sub-microscopic inclusions of zircon. The obtained temperature generally aligns with previous predictions of the Pressure-Temperature conditions of the Raspas Complex; however, computer models still predict cooler temperatures on average. This discrepancy may result from the omission of shear heating in computer models or the neglect of the effects of exhumation on temperatures recorded by rocks.