GEOCHEMISTRY OF KEANAKĀKO‘I TEPHRA FROM KĪLAUEA VOLCANO, HAWAI‘I

The last 2500 years of eruptive activity at Kīlauea Volcano have been dominated (60%) by centuries-long periods of intermittent explosive eruptions (Swanson et al., 2014). The Keanakāko‘i Tephra (KT) represents the most recent explosive period (~1500-~1820 CE), which began during or just after the formation of Kīlauea Caldera. Most of the KT eruptions were phreatomagmatic, with only one known lava flow erupted at Kīlauea’s summit. The compositions of the KT glasses and olivines are generally more MgO-rich than those from historical eruptions (e.g., 9-11 vs. 7-8 wt.% for glasses and forsterite contents of 87-89 vs. 80-85 for olivine). The more primitive compositions for many KT glasses and olivine are indicative of a shorter crustal storage times than is typical for historical eruptions. Compositional zoning in olivine indicates that most KT magmas were stored in the crust for weeks to months prior to eruption. Shorter magma residence times may be a result of a smaller summit reservoir after the formation of the new caldera. Products from three KT eruptions that span its entire ~300 year period have been analyzed for Pb and Sr isotopes. The variation of these isotopes, and in MgO-normalized major and trace element abundances, for KT glasses and flow is small compared to those for lavas erupted during Kīlauea's historical period (1832 to present; Pietruszka and Garcia, 1999) and even the ongoing Puʻu ʻŌʻō eruption (Greene et al., 2013). The compositions of KT eruptive products overlap with those from the Puʻu ʻŌʻō eruption, which is indicative of the cyclic nature of the volcano's compositional variation (Pietruszka and Garcia, 1999; Greene et al., 2013). The cause of the limited compositional variation in isotopes and MgO-normalized major and trace element abundances for KT glasses is problematic. Generally, it is thought that lower magma eruption rates lead to more compositional variation. However, the average eruption rate during the KT period was much smaller (~50 times) than during the historical period (Swanson et al., 2014). Perhaps during periods of explosive activity, Kilauea may be sampling a smaller volume of the mantle that is more fusible.

Greene et al., 2013, Geochem. Geophys. Geosyst., 14, doi: 0.1002/2013GC004719; Pietruszka and Garcia, 1999, J. Petrol., 40, 1321-1342; Swanson et al., 2014, Geology, 42, 631–634.