Oxygen isotope thermometry using quartz inclusions in garnet

Autor: M. J. Spicuzza, Daisuke Nakashima, Ryan Joel Quinn, John W. Valley, Kouki Kitajima
Rok vydání: 2016
Předmět:
Zdroj: Journal of Metamorphic Geology. 35:231-252
ISSN: 1525-1314
0263-4929
DOI: 10.1111/jmg.12230
Popis: Oxygen isotope ratios of quartz inclusions within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analyzed and used to determine metamorphic temperatures. Primary quartz inclusions for eight of 12 samples have δ18O values significantly lower than matrix quartz. The primary quartz inclusions retain δ18O values representative of thermal conditions during garnet crystallization whereas the δ18O values of matrix quartz were raised by diffusive exchange with other matrix minerals (e.g., mica and feldspar) during cooling. The δ18O differences between quartz inclusions and matrix quartz show that garnet (a mineral with slow diffusion of oxygen) can armour quartz inclusions from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in matrix quartz and quartz inclusions can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying quartz inclusions that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in quartz inclusions, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of quartz inclusions preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm 60–75) from each sample was measured by laser–fluorination mass–spectrometry (LF–MS) for δ18O and compared with ion microprobe measurements of δ18O in quartz inclusions for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640 to 740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625 to 800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the Anorthosite–Mangerite–Charnockite–Granite suite in the central Adirondacks and yield temperatures of 640 to 665 °C, approximately 100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post-peak) crystallization of garnet on the retrograde path. This article is protected by copyright. All rights reserved.
Databáze: OpenAIRE
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