ARGON INCREMENTAL HEATING AS AN IDENTIFIER OF AGE-RELATED ODDITIES BETWEEN PLAGIOCLASE AND GROUNDMASS IN EASTERN OREGON

Sunstones are copper-bearing plagioclase feldspar crystals found in basaltic lava flows in eastern Oregon. Sunstone-bearing lavas are found in two discrete locations ~150 km apart, both located within the spatial extent of the Columbia River Basalt Group (CRBG). In addition to being spatially distinct, preliminary geochronological work suggests these unique basaltic lavas are also temporally distinct.

⁴⁰Ar/ ³⁹Ar dating on sunstone-bearing lavas indicate age discrepancies between phases and between mine locations. For example, samples from the northern location (Ponderosa Mine) indicated a groundmass age of 16.2 Ma, coeval with CRBG volcanism. However, the associated plagioclase age yielded a younger date of ~9 Ma. Samples from the southern location (Plush claims) yield groundmass ages of ~8.9-9.2 Ma, but a variety of discordant plagioclase ages. We collected sunstone-bearing lava and tuffaceous material from the northern open pit mine (Ponderosa Mine) and dated groundmass and plagioclase from four samples. Our resulting age data (1) confirms the age of the sunstone-bearing basalts, and (2) suggests plagioclase ages from this mine consistently yield discordant age data. These data suggest that these sunstone-bearing lava erupted in two distinct events, separated by ~7 million years. Quantifying the age relationship between phases and mine locations provides insight into how these lava flows are related and may help us understand processes of argon loss.