T35 Go Small or Go Home: Microbeam Techniques Applied to Igneous, Metamorphic, and Sedimentary Petrology of Earth and Planetary Materials

Sunday, 25 September 2016
1:30 PM - 5:30 PM
Room 404

IN SITU VERSUS BULK ANALYSES OF OXYGEN ISOTOPE RATIOS: IMPROVED THERMOMETRY FOR CORUNDUM AND CALCITE AT NAXOS, GREECE

Paper number: T35-8
Program: Technical Sessions
Time: 3:25 PM – 3:40 PM

Authors

Abstract

Corundum (Crn), including sapphire, occurs with calcite (Cc), Fe-Ti oxides, rutile, ± diaspore, paragonite-muscovite, and margarite with minor amounts of kyanite, tourmaline, and anorthite in emery pods surrounded by marble on Naxos, Greece that are metamorphosed karst-bauxite. Naxos marbles experienced a complex polymetamorphic history including heterogeneous fluid flow, tectonic unroofing and retrograde metamorphism. Nevertheless, many of these rocks appear well suited for Refractory Accessory Mineral (RAM) thermometry, which uses oxygen isotope fractionation between a refractory accessory mineral (Crn) and a modally dominant phase with faster diffusion of oxygen (Cc) to determine peak metamorphic temperatures. Application of RAM thermometry to corundum-calcite mineral pairs from Naxos will aid current understandings of tectonic evolution and formation of emery, some of which contain relatively coarse, deep-blue sapphire.

Based on bulk oxygen isotope analyses by laser fluorination (Crn) and phosphoric acid extraction (Cc) fractionations (Δ18O(Cc-Crn)) of -1.97 to 8.36‰ were measured from 17 rocks that are inconsistent with previous temperature estimates (420 to 620 °C) based on mineral isograds for M2 metamorphism of corundum-bearing rocks across Naxos. These erratic results are explained by in situ secondary ion mass spectrometry (SIMS, 10 μm spots) that shows variability of δ18O within volumes smaller than analyzed by bulk methods: Crn varies by up to 2.3‰/100 μm and Cc varies by up to 4.6‰/400 μm. Thus bulk oxygen isotope values represent mixtures and not equilibrated sample pairs.

By SIMS, Crn and Cc analyses can be located in adjacent grains where textures suggest equilibrium was attained. Preliminary SIMS analyses of adjacent mineral pairs in three rocks with texturally equilibrated assemblages yield values of ΔCc-Crn that systematically decrease from 7.6 to 5.4‰ up-grade. However, the modified increment method (Zheng et al. 1991) indicates values of ΔCc-Crn that are ~3.5‰ larger (7.3‰ at 650 °C) and unreasonable temperatures for the Naxos data. These preliminary results are very promising, but further analysis is needed to empirically evaluate the Cc-Crn fractionation.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-279011

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.