T20 Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Skagit 3

EMPIRICAL CALIBRATION OF OXYGEN ISOTOPE FRACTIONATION IN CORUNDUM

Paper number: T20-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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 (ACc-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 ACc-Crn = 2.87 ±0.10 (Eqn. 1). At 1000 K, this equates to ΔCc-Crn = 2.87‰. In contrast, the increment method yields ΔCc-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 ACc-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 ACc-Crn value of 2.87 yields ΔZircon-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

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-303621

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.