HR: 1340h
AN: V43A-1108    [Abstracts]
TI: Interacting Convective Processes in Kilauea Iki Lava Lake, Hawaii
AU: * Helz, R T
EM: rhelz@usgs.gov
AF: Rosalind T. Helz, U.S. Geological Survey M.S. 926A, Reston, VA 20192, United States
AB: Kilauea Iki lava lake formed in 1959 as a closed magma chamber of 40 million m³ of picritic magma. Repeated drilling and sampling of the lake allows recognition of processes of magmatic differentiation, and places time restrictions on the periods when they operated. Two processes, double-diffusive convection and finger diapirism, occurred because melt density decreases as olivine crystallization and re-equilibration proceeds, until after plagioclase begins to crystallize. Finger diapirism, described in previous work, occurred from 1961 to 1971 and affected most the lava lake between depths of 13 to 94 m. The period of inferred double- diffusive convection occurred between mid-1962 and 1964 and affected only the most olivine-poor part of the lava lake. Recent re-evaluation of petrographic and chemical data refine our understanding of this second process. The overall variation of bulk MgO content with depth in Kilauea Iki is an S-curve, consistent with gravitative redistribution of the abundant olivine phenocrysts present in the erupted lava. The olivine-poor zone (MgO <11 weight percent) is a sill-like volume found between depths of 21 to 43 m in the lake. This zone is bisected by a median layer containing more and slightly coarser olivine phenocrysts, which has an MgO content 2 weight percent higher than the minimum in the layers above and below. This configuration, not achievable by gravitative settling, suggests that the olivine-poor zone at some point contained a two-layer convective system. The upper and median layers of the olivine-poor zone contain a sparse population of augite microphenocrysts (0.2-0.4 mm in length), often in monomineralic clusters (1-3 mm in length), while the lower layer contains only olivine. Plagioclase and other phases occur only in the groundmass in all samples. If the layers developed before groundmass crystallization began, then the assemblage in the upper layer was olivine + augite, and was olivine-only in the lower. Because melt density decreases as temperature decreases in this part of the crystallization range and because the lava lake was strongly cooled from above, the conditions for double- diffusive convection, with splitting of the melt column into layers, were met. Core samples and temperature data obtained by drilling the lake in mid-1962 and late 1967 constrain the period of double-diffusive convection to the first half of that period. The process ceased without shifting the position of the median olivine-enriched layer downward, suggesting that it was very brief. Finger diapirism, already active in Kilauea Iki, was volumetrically more important, and passed through both layers. This overlapping process may have ended the broader convective process by reducing the thermal gradient that drove it. Although double- diffusive convection was a minor process in Kilauea Iki, it did occur in this closed magma system.
DE: 8145 Physics of magma and magma bodies
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting