In situ versus bulk analyses of oxygen isotope ratios: improved thermometry for corundum and calcite at Naxos, Greece

Rachelle B. Turnier, Yaron Katzir, Kouki Kitajima, I. J. Orland, M. J. Spicuzza, John W. Valley

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Corundum (Crn), including sapphire, occurs with calcite (Cc), Fe-Ti oxides, rutile, + or - 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 (Delta (super 18) O(Cc-Crn)) of -1.97 to 8.36 ppm were measured from 17 rocks that are inconsistent with previous temperature estimates (420 to 620 degrees 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 mu m spots) that shows variability of delta (super 18) O within volumes smaller than analyzed by bulk methods: Crn varies by up to 2.3 ppm/100 mu m and Cc varies by up to 4.6 ppm/400 mu m. 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 Delta Cc-Crn that systematically decrease from 7.6 to 5.4 ppm up-grade. However, the modified increment method (Zheng et al. 1991) indicates values of Delta Cc-Crn that are nearly equal 3.5 ppm larger (7.3 ppm at 650 degrees 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.
Original languageEnglish
Title of host publicationGSA Annual Meeting in Denver, Colorado, USA
PagesAbstract no. 35-8
Volume48
Edition7
StatePublished - 2016

Publication series

NameAbstracts with Programs - Geological Society of America
ISSN (Print)0016-7592

Keywords

  • Isotope geochemistry

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