ZIRCON INCLUSIONS IN IGNEOUS-RELATED GEM-CORUNDUM: A SURVEY OF OXYGEN ISOTOPE FRACTIONATION AND GEOCHRONOLOGY

Igneous-related corundum (Crn) can form in a wide variety of geological settings, but the details of its genesis are uncertain. Genesis determination is complicated because many gem-Crn deposits are detrital. Inclusions and correlative geochemistry are critical sources of information.

To evaluate the timing, duration, and protolith, we imaged growth textures in Crn with photoluminescence (PL) mapping of Cr³⁺ fluorescence; imaged zircon (Zrc) inclusions by cathodoluminescence (CL); measured apparent pressures by Raman (Turnier et al., GSA 2018); and analyzed δ¹⁸O and U-Pb ages by SIMS to place the geochemical signatures in a temporal context. Luminescence imaging (PL and CL) is important for distinguishing any growth domains or zoning in the Crn and Zrc to target by SIMS.

A total of 47 Zrc inclusions in 21 Crn were studied from Australia, Brazil, Cambodia, Ethiopia, Madagascar, Malawi, Montana, Myanmar, and Nigeria. These samples are from secondary deposits (mostly alkali basalt-related), except two Myanmar Crn (syenite). Results vary by locality, but Zrc-Crn oxygen isotope fractionation suggests not all Zrc inclusions are co-genetic with host Crn. CL images are useful for evaluating any resorption or alteration of Zrc and to identify inherited cores.

The Crn δ¹⁸O values range from mantle-like (4.6 to 6.1‰; Australia, Cambodia, Ethiopia, Madagascar, Montana, Nigeria, Thailand) to evolved or sediment-derived values (Myanmar, 16‰). Southeast Asia Crn with generally mantle-like δ¹⁸O values can range to mildly elevated values (~8‰); Zrc from mantle-like Crn have 4 Ma rims (Pailin, Kanchanaburi) and higher δ¹⁸O samples have Zrc with 2 Ma rims (Pailin, Chanthaburi) that match ages of likely host basalts. Geochronology of Zrc inclusions shows crustal interaction or assimilation during some Crn formation—some inherited cores are older and have a non-mantle signature (δ¹⁸O >6.4‰; up to 17‰ at Myanmar). Other Crn contains Zrc formed during separate magmatic events (Ethiopia, 3 and 18 Ma). Madagascar (534 and 483 Ma Zrc) and Malawi (633 and 591 Ma Zrc) samples may be related to Gondwana assembly.

Details vary among deposits, but the approach of combining correlative, in situ geochemical analyses with luminescence imaging is proven useful for evaluating differences in Crn genesis among geologically similar deposits.