VOLCANOGENIC DETRITAL ZIRCONS FROM THE KIMAMA DRILL CORE (PROJECT HOTSPOT): EVIDENCE FOR SOURCE EVOLUTION IN THE YELLOWSTONE-SNAKE RIVER PLAIN MAGMATIC SYSTEM, CENTRAL SNAKE RIVER PLAIN, ID

Project Hotspot produced continuous core from three deep drill holes in the Snake River Plain of southern Idaho. The Kimama drill site is located on the axial volcanic high of the central SRP, NE of Twin Falls, Idaho. The 1912 m (6,275 ft) Kimama core provides a depositional record of basaltic lava flows, aeolian loess, and fluvial sand on the central Snake River Plain from the late Miocene through Pleistocene epochs. Ar/Ar and paleomagnetic dating establish a relatively linear basalt accumulation rate of ~300 m/my (~1,000 ft/my), and a projected bottom hole age of 6.2 Ma.

Detrital zircons were recovered from two upward fining sandstone interbeds at 1842-1844 m (6044-6050 ft) and 1707-1748 m (5602-5737 ft) depth and analyzed for U-Pb and εHf at the University of Arizona LaserChron laboratory. In addition to older recycled grains, a large fraction of the recovered grains represent primary zircons formed during eruptions of rhyolite ash. The fresh, rod- and blade-shapes, with minimal rounding, suggests a primary fallout origin of these volcanic zircon grains. These volcanogenic detrital zircons were ejected during caldera-forming eruptions in the central Snake River Plain, transported by aeolian or fluvial processes, and deposited within the fluvial deposits shortly after their eruption. This implies that the depositional age of the sediment intervals corresponds to the age of youngest zircons in each unit (5.8 Ma and 6.2 Ma).

Hf isotopic compositions vary from eHf of -7.3 to -3.9, and the variation is highly correlated with age: volcanogenic zircons ages of 7.1, 6.7, 6.2 Ma, and 5.8 Ma have corresponding eHf of -7.3, -6.4, -4.9. -3.9. This progressive decrease in εHf in the younger zircons indicates an increase in mantle derived Hf through time, with a corresponding decrease in crustal Hf.