T107 Gemological Research in the 21st Century—Gem Minerals and Localities

Monday, 11 October 2021
1:30 PM - 5:30 PM
A107–A109

VARIABLE OXYGEN ISOTOPE RATIOS FROM MONTANA CORUNDUM: IMPLICATIONS FOR PROTOLITH AND GENESIS OF IN SITU SIMS MEASUREMENTS

Paper number: T107-6
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

Abstract

Prolific deposits of gem corundum occur in southwestern Montana, most as secondary deposits. Only a couple primary occurrences are known: Yogo Gulch (lamprophyre), and non-gem deposits at Elk Creek (gneiss) and the French Bar Sill (basaltic trachyandesite). Despite decades of research, connections among the Montana gem corundum deposits are uncertain, both in terms of source rocks and genesis.

Our research investigates corundum genesis with in situ oxygen isotope ratios collected by secondary ion mass spectrometry (SIMS) in 196 corundum crystals from Montana deposits (Table 1), including Elk Creek (EC), Silver Bow (SB), Rock Creek (RC), Dry Cottonwood Creek (DCC), Lowland Creek (LC), Big Spring Creek (BSC), the French Bar Sill (FBS), Missouri River (MR), specifically at El Dorado Bar (EDB), and Yogo Gulch (YG). Most analyses are of sapphires but a few pink to red rubies were studied. Rubies from DCC and RC fall within the range of sapphires at these deposits. But rubies from EDB range to lower δ18O values.

Corundum from Montana deposits have a much larger δ18O range than previously known. EC gneiss and YG corundum have homogeneous oxygen isotope ratios. Other deposits exhibit similar averages, from 5 to 6‰, but have large δ18O ranges. This suggests corundum may form in different protoliths before entrainment and eruption in the source rock. Many values are outside the range of mantle-equilibrated corundum (4.4 to 5.7‰). Higher δ18O values may result from crustal assimilation into mantle-like magmas and low δ18O values may indicate crystallization from gabbro, eclogite, or protoliths altered by low-δ18O fluids. While many deposits overlap, some capture corundum populations that are distinct. The large δ18O ranges among Montana corundum deposits suggest a wide range of protoliths can form corundum under the right conditions, perhaps through peritectic melting or the action of volatiles on restite or low-degree partial melts of aluminum-rich protoliths.

Table 1. Summary of minimum, maximum, and average δ18O for Montana corundum, along with number of crystals analyzed (# Crn).

Sapphire

Ruby

EC

SB

RC

DCC

LC

BSC

FBS

MR

EDB

YG

RC

DCC

EDB

Min (‰)

4.0

4.3

0.6

1.9

1.1

4.7

4.0

3.7

1.7

4.3

0.3

4.5

-3.3

Max (‰)

4.4

7.2

6.6

12.7

7.3

7.4

6.7

8.1

7.8

5.9

5.0

5.6

6.6

Avg. (‰)

4.2

5.7

2.7

5.9

4.9

5.6

4.9

5.8

5.1

5.2

2.3

4.9

1.9

# Crn

2

9

44

55

13

7

7

15

23

11

4

3

3

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-367740

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