HR: 12:10h
AN: V52C-08    [Abstracts]
TI: Magma chamber rejuvenation associated with Yellowstone 516-70 ka post-caldera rhyolitic lava flows : results from crystal geochemistry
AU: * Girard, G
EM: ggirard@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AU: Stix, J
EM: stix@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AB: After Yellowstone caldera collapsed at 640 ka, it has been the focus of many voluminous rhyolite effusions. Over 900 km³ of magma have been erupted during relatively short periods, at 516 ka, 486-479 ka, 198 ka, and principally from 165 to 70 ka when the largest lava flows were emplaced. The last period itself consists of three episodes, from 165 to 147 ka, 117 to 102 ka, and 72 to 70 ka (Christiansen, 2001). The first-erupted lavas contain mainly plagioclase and sanidine. Quartz becomes increasingly abundant in younger flows, and clearly dominates over plagioclase in the 165 ka and younger rocks. Plagioclase is not present in the 117-102 ka and 72-70 ka lavas. Recent micro-analytical techniques have led to a better understanding of pre-eruptive conditions in magmatic systems by studying trace element concentrations and zoning in volcanic crystals. We analyzed plagioclase by electron microprobe and Ti in quartz by laser ablation-ICP-MS in order to assess changes in crystallization conditions from one period to the next, and also to reconstruct the pre-eruptive history of individual lava flows, by searching for potential dissolution, overgrowth or zoning features. Plagioclase in the lavas shifts progressively from a median value of An₂₈ at 516 ka to An₂₁ at ~ 160 ka. The plagioclase also contains decreasing amounts of K over time for a given level of Ca; for example, K₂O at An₂₂ declines from ~ 1.9 wt % in the oldest lavas to ~ 1.4 wt % in the 160 ka episode lavas, suggesting a reduced ability of plagioclase to incorporate K. This may translate into crystallization in a progressively cooler or shallower environment. Average Ti contents in quartz rims decrease from ~ 150- 200 ppm in the older lavas to ~ 100-125 ppm at ~ 160 ka and ~ 75-90 ppm at 70 ka. Zoned quartzes are found in most flows. They show higher Ti contents in the cores, with up to 240 ppm in the older lavas, 150 ppm at ~ 160 ka and 120 ppm at 70 ka. The temporal Ti decline in quartz rims and the normal zoning patterns for Ti in individual crystals are also both suggestive of formation in a cooling environment. Texturally, the plagioclase is commonly sieved, especially in the most primitive flows of the 516-479 ka period. The rims are typically thin, discontinuous and/or absent and do not appear to reflect overgrowths; analyses of sieved cores and fresh rims did not reveal any compositional zoning. For quartz, long narrow glass re-entrants commonly penetrate through crystals and are connected to the matrix glass. Major and trace element glass analyses did not reveal any noticeable compositional change between the re-entrants and the matrix. Sieved textures are commonly formed by dissolution resulting from heating, and we envisage a similar mechanism for the re-entrants in quartz. We hypothesize that discrete episodes of magma recharge occurred after caldera collapse, with further recharge episodes related to the 165 to 70 ka period when most of the lava was erupted. Neither quartz nor plagioclase bears an imprint of remobilization in a more mafic melt, so two possibilities present themselves. (1) The replenishing material never came into contact with the crystals which were erupted, and the crystals may have been merely heated causing embayments and partial dissolution. (2) The replenishing magma was high-silica rhyolite. Where present, rims on plagioclase are not well developed, suggestive of only a short interval of time from replenishment to eruption.
DE: 3618 Magma chamber processes (1036)
DE: 3625 Petrography, microstructures, and textures
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8425 Effusive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting