TIMESCALES OF MAGMATIC FRACTIONATION IN THE SNAKE RIVER PLAIN VOLCANIC PROVINCE RECORDED IN DRILL CORE

The Snake River Plain of Idaho traces the impingement of the Yellowstone-Snake River Plain hotspot into continental crust and is the youngest and best-preserved record of hotspot-crustal interactions on Earth. While thick welded tuffs on the margins of the plain provide evidence of explosive rhyolite volcanism, the upper 2 km of the Snake River Plain records over 6 million years of nearly continuous basalt volcanism in the aftermath of caldera-forming eruptions.

Deep drilling in the Snake River Plain of southern Idaho documents the eruption of monogenetic basalts that post-date passage over the Yellowstone-Snake River Plain hotspot. Detailed logging of core from these deep drill holes, along with geophysical logging, chemical stratigraphy, and paleomagnetic inclinations document individual eruptive units, which can be grouped together based on their chemical compositions and paleomagnetic inclinations into compound lava flows and flow groups. Hiatuses between flow groups are frequently marked by loess interbeds between >0.1 and 20 meters thick. Ar-Ar, detrital zircon, and paleomagnetic timescale ages show the deepest hole (Kimama, 1912 m) accumulated over ~6 Ma, while Ar-Ar and paleomagnetic ages indicate the shallower hole (USGS 135, 363 m) accumulated over ~1.1 Ma. Within these lava flows and flow groups, cycles of fractional crystallization and recharge are recognized as up-section shifts in Mg, Fe, Ti, K, P, and trace elements. Individual fractionation cycles that reflect at least 38-50% fractional crystallization typically occur with little to no change in magnetic inclination (0-2°), whereas adjacent cycles may differ by several degrees from one another.

Rates of paleosecular variation in Holocene lavas and sediments dated using 14C document significant shifts in magnetic inclination over short time scales, ranging from about 0.5°/century to 2°/decade, with an average of about 0.5°/decade or 5°/century. This implies that fractionation cycles with <1° to 2° variation in magnetic inclination formed on timescales of a few decades to two centuries.