EMPIRICAL CALIBRATION OF OXYGEN ISOTOPE FRACTIONATION IN CORUNDUM

Oxygen isotope ratios in corundum (Crn) are important for determining genesis, temperatures, and provenance of Crn, but no experimental data exist for the A-factor defining isotope fractionation with other phases. We empirically calibrated the calcite-corundum A-factor (A_Cc-Crn) from metamorphosed karst-bauxite deposits (emery) at Naxos, Greece, using SIMS measurements of δ¹⁸O in adjacent calcite (Cc) and Crn, and independent thermometry from mineral isograds (420 to 620°C). In situ analysis avoids zoning, veins, and mineral inclusions that impeded previous studies. This empirical calibration yields A_Cc-Crn = 2.87 ±0.10 (Eqn. 1). At 1000 K, this equates to Δ_Cc-Crn = 2.87‰. In contrast, the increment method yields Δ_Cc-Crn = 5.9‰ (Zheng 1991, 1993), and would produce unreasonably high T (greater than 1050°C for the staurolite-in isograd on Naxos). Derived using A_Cc-Crn = 2.87 and published data, these A-factors are tabulated below (see Valley 2003, RIMG 53 for details).

Oxygen isotope thermometry can be used to help understand the origin of gem Crn. Determinations of equilibrium fractionation between Crn and inclusions, other minerals, or melt rely upon accurate A-factors. Previous studies have discussed uncertainties of the A-factor (Yui et al., 2003; Palke et al., 2017). The new A_Cc-Crn value of 2.87 yields Δ_Zircon-Crn ~0.3‰ at 1000-1300°C. Zircons equilibrated at magmatic temperatures with typical mantle have δ¹⁸O = 5.3±0.6‰ (Valley et al. 2005) and thus “mantle-like” Crn δ¹⁸O = 5.0±0.6‰. Compilations of gem Crn (Wong & Verdel 2017) show that many igneous-related Crn, including basalt-hosted megacrysts, have higher δ¹⁸O values, suggesting evolved or contaminated source rocks.

Pv	-3.55
Mt	-3.04	0.51	Matrix of A-factors for a Selection of Mineral Pairs
Ru	-1.44	2.11	1.60	Eqn. (1) ΔY-X ≈ 1000 ln αY-X = AY-X × 106 T-2
Fo	-0.42	3.13	2.62	1.02
Ttn	-0.41	3.14	2.63	1.03	0.01	Where Y = Y-axis and X = X-axis mineral
Gr	0.22	3.77	3.26	1.66	0.64	0.63	(T in K)
Di	0.50	4.05	3.54	1.94	0.92	0.91	0.28
Alm	0.54	4.09	3.58	1.98	0.96	0.95	0.32	0.04
Zc	0.61	4.16	3.65	2.05	1.03	1.02	0.39	0.11	0.07
Ap	0.74	4.29	3.78	2.18	1.16	1.15	0.52	0.24	0.20	0.13
An	1.26	4.81	4.30	2.70	1.68	1.67	1.04	0.76	0.72	0.65	0.52
Ab	2.31	5.86	5.35	3.75	2.73	2.72	2.09	1.81	1.77	1.70	1.57	1.05
Cc	2.87	6.42	5.91	4.31	3.29	3.28	2.65	2.37	2.33	2.26	2.13	1.61	0.56
	Crn	Pv	Mt	Ru	Fo	Ttn	Gr	Di	Alm	Zc	Ap	An	Ab