HR: 1340h
AN: V53B-1322    [Abstracts]
TI: Individual magma batches related to the origin of Yellowstone 516-70 ka post-caldera rhyolitic lava flows : results from glass geochemistry.
AU: Stix, J
EM: stix@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AU: * Girard, G
EM: ggirard@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AB: Since its last collapse at 640 ka, Yellowstone caldera has been filled by at least 25 voluminous rhyolitic lava flows totalling more than 900 km³. The oldest identified flows have been erupted at 516 ka, 486-479 ka and 198 ka around the two resurgent domes. Most of the known effusions occurred during relatively short episodes from 165 to 147 ka, 117 to 102 ka, and 72 to 70 ka (Christiansen, 2001), erupting along 2 NNW-trending lineaments which are parallel to regional faults and do not intersect the resurgent domes. The intermittent nature of the volcanism and its spatial distribution remain largely unresolved questions. We used a combination of XRF whole rock major and trace element analyses, electron microprobe and laser ablation-ICP-MS major and trace element glass micro-analyses to examine the temporal and spatial links among the eruptions. Petrologically, we observe a broad-scale differentiation of the lavas over time. Lavas dated at 516 ka have SiO₂ contents of 71-72 wt% for bulk rock and 76-76.5 wt% in glass, while 165 ka and younger lavas exhibit SiO₂ contents of 76.5-77.5 wt% for both the bulk rock and glass. Mineralogically, the lavas evolve from plagioclase-dominated to sanidine and quartz with no plagioclase. These observations suggest an origin from a common source undergoing slow differentiation. Temporal changes of certain trace elements such as Sr and Ba also suggest a similar evolution. For instance, Sr evolves from 115 ppm in bulk rock in the most primitive lavas of the 516 ka group, to 60 ppm at 198 ka, declining to 2.5 ppm at 70 ka. A simultaneous increasingly negative Eu anomaly is observed in the glass. Such behavior is expected from feldspar fractionation in a long-lived reservoir. Elements such as Y, Th and REE also behave compatibly in the Yellowstone lavas, their concentrations being systematically higher in bulk rock compared to glass. Should the system be a long-lived magma chamber undergoing fractionation, their concentrations over time would therefore decrease. However, Y generally increases from 60 ppm in bulk rock at 516 ka to 65-70 ppm at 165-147 ka and 85 ppm at 70 ka. Of the analyzed elements, only Rb exhibits concentrations higher in glass than bulk rock and therefore is incompatible. Although it increases slightly over time in bulk rocks from 160 ppm in the most primitive of the 516 ka lavas to 170 ppm at 198 ka, it is generally buffered at 180-200 ppm from 165 to 70 ka. In addition, spatial compositional differences are observed between the flows, despite comparable mineralogy and crystal contents. For instance, during the 165-147 ka period, Ba in the western lavas ranges from 50 to 70 ppm, while it varies from 95 to 120 ppm in the eastern lavas. During the 117-102 ka period, two western lavas erupted at 6 km distance from each other at 112 and 108 ka have respective contents of 40 vs 80 ppm Ba in glass, 40 vs 160 ppm Ba in bulk rock and 65 vs 80 ppm La in glass. These spatial compositional differences, together with the decoupled temporal changes in various trace elements, suggest that these lava flows did not originate from a single homogeneous long-lived liquid reservoir. It is possible that eruptions are the result of localized magma injections at various depths into the mush, which mixed with the mush or merely provided heat to remelt the mush and initiate magma ascent. Each batch of magma may then have undergone variable degrees of differentiation before eruption.
DE: 1065 Major and trace element geochemistry
DE: 3618 Magma chamber processes (1036)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8425 Effusive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting