TY - JOUR
T1 - Calibration of oxygen isotope fractionation and calcite-corundum thermometry in emery at Naxos, Greece
AU - Turnier, Rachelle B.
AU - Katzir, Yaron
AU - Kitajima, Kouki
AU - Orland, Ian J.
AU - Spicuzza, Michael J.
AU - Valley, John W.
N1 - Funding Information:
We thank Brian Hess for sample preparation, the WiscSIMS group for SIMS support, John Fournelle and Aurélien Moy for EPMA help, Bil Schneider and Phil Gopon for SEM training and aid, Maciej Śliwiński for corrections of SIMS bias for carbonates, and Noriko Kita for SIMS advice. We are grateful to Alan Matthews for helpful discussions and a thoughtful review. We also thank an anonymous reviewer for constructive comments and Donna Whitney for great editorial contributions and suggestions for our manuscript. This work was supported by the National Science Foundation (EAR‐1525336). WiscSIMS is supported by NSF (EAR‐1355590, 1658823) and UW‐Madison. RT received a UW‐Madison Advanced Opportunity Fellowship. Fieldwork was aided by the UW Geoscience Department. We declare no conflict of interest.
Funding Information:
We thank Brian Hess for sample preparation, the WiscSIMS group for SIMS support, John Fournelle and Aur?lien Moy for EPMA help, Bil Schneider and Phil Gopon for SEM training and aid, Maciej ?liwi?ski for corrections of SIMS bias for carbonates, and Noriko Kita for SIMS advice. We are grateful to Alan Matthews for helpful discussions and a thoughtful review. We also thank an anonymous reviewer for constructive comments and Donna Whitney for great editorial contributions and suggestions for our manuscript. This work was supported by?the National Science Foundation (EAR-1525336). WiscSIMS is supported by NSF (EAR-1355590, 1658823) and UW-Madison. RT received a UW-Madison Advanced Opportunity Fellowship. Fieldwork was aided by the UW Geoscience Department. We declare no conflict of interest.
Publisher Copyright:
© 2019 John Wiley & Sons Ltd
PY - 2020/1/1
Y1 - 2020/1/1
N2 - Corundum (Crn), including sapphire, occurs in emery pods surrounded by marble on the island of Naxos, Greece. The emery formed from bauxite deposited in karst that was metamorphosed to 400–700°C at 20–15 Ma. Many of these rocks initially appeared well suited for refractory accessory mineral (RAM) thermometry, which uses oxygen isotope fractionation between a RAM – corundum – and a modally dominant phase with faster diffusion of oxygen – calcite (Cc) – to determine peak metamorphic temperatures. However, previous attempts at oxygen isotope thermometry were confounded by highly variable fractionations (Δ18O) measured at mm-scale and the uncertain calibration of Δ18O(Cc-Crn) versus temperature. Secondary ion mass spectrometry (SIMS) permits in situ analysis of δ18O in corundum and calcite at the 10-μm scale in adjacent grains where textures suggest peak metamorphic equilibrium was attained. SIMS analyses of adjacent mineral pairs in eight rocks yield values of Δ(Cc-Crn) that systematically decrease from 7.2 to 2.9‰ at higher metamorphic grade. Pairing these data with independent temperature estimates from mineral isograds yields an empirical calibration of 1,000 lnα(Cc-Crn) = 2.72 ± 0.3 × 106/T2 (T in K). The new fractionations (2.7‰ at 1,000 K) are significantly smaller than those calculated from the modified increment method (6.5‰ at 1,000 K; Zheng, Geochimica et Cosmochimica Acta, 1991, 55:2299–2307; Zheng, Mineral Mag, 1994, 58A:1000–1001), which yield unreasonably high temperatures of 630 to 1,140°C when applied to the new Naxos data. The new calibration of Δ(Cc-Crn) can be combined with published fractionations to calculate A-factors for corundum versus a range of 14 other minerals. These new fractionation factors can be used for thermometry or to constrain the genesis of corundum. A compilation of gem corundum δ18O values shows that many igneous sapphires, including important deposits of basalt-associated sapphire, are mildly elevated in δ18O relative to the calculated range in equilibrium with mantle values (4.4–5.7‰) and formed from evolved magmas.
AB - Corundum (Crn), including sapphire, occurs in emery pods surrounded by marble on the island of Naxos, Greece. The emery formed from bauxite deposited in karst that was metamorphosed to 400–700°C at 20–15 Ma. Many of these rocks initially appeared well suited for refractory accessory mineral (RAM) thermometry, which uses oxygen isotope fractionation between a RAM – corundum – and a modally dominant phase with faster diffusion of oxygen – calcite (Cc) – to determine peak metamorphic temperatures. However, previous attempts at oxygen isotope thermometry were confounded by highly variable fractionations (Δ18O) measured at mm-scale and the uncertain calibration of Δ18O(Cc-Crn) versus temperature. Secondary ion mass spectrometry (SIMS) permits in situ analysis of δ18O in corundum and calcite at the 10-μm scale in adjacent grains where textures suggest peak metamorphic equilibrium was attained. SIMS analyses of adjacent mineral pairs in eight rocks yield values of Δ(Cc-Crn) that systematically decrease from 7.2 to 2.9‰ at higher metamorphic grade. Pairing these data with independent temperature estimates from mineral isograds yields an empirical calibration of 1,000 lnα(Cc-Crn) = 2.72 ± 0.3 × 106/T2 (T in K). The new fractionations (2.7‰ at 1,000 K) are significantly smaller than those calculated from the modified increment method (6.5‰ at 1,000 K; Zheng, Geochimica et Cosmochimica Acta, 1991, 55:2299–2307; Zheng, Mineral Mag, 1994, 58A:1000–1001), which yield unreasonably high temperatures of 630 to 1,140°C when applied to the new Naxos data. The new calibration of Δ(Cc-Crn) can be combined with published fractionations to calculate A-factors for corundum versus a range of 14 other minerals. These new fractionation factors can be used for thermometry or to constrain the genesis of corundum. A compilation of gem corundum δ18O values shows that many igneous sapphires, including important deposits of basalt-associated sapphire, are mildly elevated in δ18O relative to the calculated range in equilibrium with mantle values (4.4–5.7‰) and formed from evolved magmas.
KW - SIMS
KW - calcite
KW - corundum
KW - oxygen isotope fractionation
KW - thermometry
UR - http://www.scopus.com/inward/record.url?scp=85075061864&partnerID=8YFLogxK
U2 - 10.1111/jmg.12512
DO - 10.1111/jmg.12512
M3 - Article
AN - SCOPUS:85075061864
SN - 0263-4929
VL - 38
SP - 53
EP - 70
JO - Journal of Metamorphic Geology
JF - Journal of Metamorphic Geology
IS - 1
ER -