GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 35-8
Presentation Time: 3:25 PM


TURNIER, Rachelle B.1, KATZIR, Yaron2, KITAJIMA, Kouki1, ORLAND, I.J.1, SPICUZZA, M.J.1 and VALLEY, John W.1, (1)Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton Street, Madison, WI 53706, (2)Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Be'er Sheva, 8410501, Israel,

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.