HR: 10:50h
AN: V11F-03    [PDF]
TI: A History of one Olivine Crystal: Microsampling Melt Inclusions by Wire Saw
AU: * Roggensack, K
EM: kxr@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Hervig, R L
EM: Richard.Hervig@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Hervig, R L
EM: Richard.Hervig@asu.edu
AF: Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704 United States
AB: Melt inclusions record magma composition, magma volatiles and magma depth (from volatile saturation pressures), but multiple melt inclusions are difficult to study within a single crystal. Often the sample geometry and preparation requirements (e.g. FTIR wafer) limit sampling density to one melt inclusion per crystal. Thus suites of crystals are studied but the relationship between multiple crystals and their melt inclusions must be inferred. Crystal mass has been used as a temporal constraint for melt inclusions (EPSL 187 p.221), but questions remain about crystal growth mechanisms, growth rates and possible hybridism. This study uses microsampling to investigate the melt inclusion record preserved within a single phenocryst. A large euhedral olivine crystal (2.2 x 4.5 mm) from the 1999 eruption of Cerro Negro volcano, Nicaragua has been sectioned with a wire saw to isolate multiple melt inclusions. The size of the host crystal and the wide spatial distribution of the melt inclusions indicate that inclusion formation occurred repeatedly throughout the crystal's growth history rather than during a single event. The crystal was first photographed to record melt inclusion position and then sectioned to recover 22 individual melt inclusions and several gas inclusions. The host phenocryst is unzoned (Fo$_{82}$) except for slight zoning at the rim (Fo$_{77}$). Locally the rim is resorbed or displays unsealed hourglass inclusions. Most melt inclusions have a single vapor bubble and the volume, as percentage of total volume, is lowest near the core of the crystal (1.8%\ ) and shows an apparent increase outward (3-4%\ ). All seven identified gas inclusions, containing little or no silicate glass, occur near the outer crystal edge consistent with the observed bubble volume trend in silicate inclusions. Melt inclusion compositions are restricted (all but four 0.18 to 0.26 wt.%\ K$_{2}$O, others 0.41 to 0.43 wt.%\ K$_{2}$O) relative to those found in more common, small to moderate-size phenocrysts from the same eruptive unit (0.18 to 0.72 wt.%\ K$_{2}$O). The melt inclusions with elevated K$_{2}$O ($>$ 0.40 wt.%\ ) are located near the core of the crystal and are also distinguished by high S and low Cl relative to melt inclusions in small to moderate-size phenocrysts. This compositional pattern apparently records an early perturbation (decreased K$_{2}$O, S and moderate Cl increase) followed by relatively stable conditions. The variation can be explained by fractional crystallization and repeated magma recharge. The high K$_{2}$O and sulfur within interior melt inclusions is interpreted as due to early closed-system enrichment, although low Cl requires previous gas loss. Later recharge events produced relatively constant K$_{2}$O, sulfur and chlorine. FTIR measurements on microsampled melt inclusions are currently under investigation and will be compared to associated small and moderate-size phenocrysts hosting compositionally similar melt inclusions. It is expected that the results for the large phenocryst will exceed volatile saturation pressures (H$_{2}$O and CO$_{2}$) of associated small and moderate-size phenocrysts ($\sim$1.5 to 3 kb) indicating deep magma storage and movement prior to the 1999 earthquake and eruption sequence.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting