Handling Editor: Donna L. Whitney Abstract 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.

中文翻译：

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希腊 Naxos 金刚砂中氧同位素分馏和方解石-刚玉温度计的校准

处理编辑：Donna L. Whitney 摘要 刚玉 (Crn)，包括蓝宝石，出现在希腊纳克索斯岛上被大理石包围的金刚砂荚中。金刚砂由沉积在岩溶中的铝土矿形成，在 20-15 Ma 时变质到 400-700°C。这些岩石中的许多最初似乎非常适合难熔副矿物 (RAM) 温度测量，它使用 RAM（刚玉）和具有更快氧扩散的模态主导相（方解石（Cc））之间的氧同位素分馏来确定峰值变质温度。然而，之前在氧同位素温度测量方面的尝试被毫米级测量的高度可变的分馏 (Δ18O) 和 Δ18O(Cc-Crn) 与温度的不确定校准相混淆。二次离子质谱法 (SIMS) 允许对相邻晶粒中 10 微米尺度的刚玉和方解石中的 δ18O 进行原位分析，其中纹理表明达到了峰值变质平衡。八种岩石中相邻矿物对的 SIMS 分析屈服值 Δ(Cc-Crn) 在较高变质等级时系统地从 7.2‰降低到 2.9‰。将这些数据与来自矿物等梯度的独立温度估计值配对，得出 1,000 lnα(Cc-Crn) = 2.72 ± 0.3 × 106/T2（T in K）的经验校准。新的馏分（1,000 K 时为 2.7‰）显着小于通过修正增量法计算的那些（1,000 K 时为 6.5‰；Zheng, Geochimica et Cosmochimica Acta, 1991, 55:2299–2307; Zheng, Mineral Mag, 1994, 58A:1000–1001)，产生 630 到 1 的不合理高温，应用于新的 Naxos 数据时为 140°C。新的 Δ(Cc-Crn) 校准可以与已发布的分馏物相结合，以计算刚玉与一系列其他 14 种矿物的 A 因子。这些新的分馏因子可用于测温或限制刚玉的成因。宝石刚玉 δ18O 值的汇编表明，许多火成岩蓝宝石，包括玄武岩伴生蓝宝石的重要矿床，相对于与地幔值平衡的计算范围 (4.4–5.7‰) 的 δ18O 轻度升高，并由演化的岩浆形成。