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

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