Mathematics – Logic
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.162..123b&link_type=abstract
Earth and Planetary Science Letters, Volume 162, Issue 1-4, p. 123-136.
Mathematics
Logic
6
Scientific paper
Low-18O silicic magmas are reported from only a small number of localities (e.g., Yellowstone and Iceland), yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization (AFC) during magma genesis for nearly all silicic magmas. The rarity of low-18O magmas in intracontinental caldera settings is remarkable given the evidence of intense low-18O meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems. In the Platoro caldera complex, regional ignimbrites (150-1000 km3) have plagioclase δ18O values of /6.8+/-0.1‰, whereas the Middle Tuff, a small-volume (est. 50-100 km3) post-caldera collapse pyroclastic sequence, has plagioclase δ18O values between 5.5 and 6.8‰. On average, the plagioclase phenocrysts from the Middle Tuff are depleted by only 0.3‰ relative to those in the regional tuffs. At Yellowstone, small-volume post-caldera collapse intracaldera rhyolites are up to 5.5‰ depleted relative to the regional ignimbrites. Two important differences between the Middle Tuff and the Yellowstone low-18O rhyolites elucidate the problem. Middle Tuff magmas reached water saturation and erupted explosively, whereas most of the low-18O Yellowstone rhyolites erupted effusively as domes or flows, and are nearly devoid of hydrous phenocrysts. Comparing the two eruptive types indicates that assimilation of low-18O material, combined with fractional crystallization, drives silicic melts to water oversaturation. Water saturated magmas either erupt explosively or quench as subsurface porphyries before the magmatic 18O can be dramatically lowered. Partial melting of low-18O subvolcanic rocks by near-anhydrous magmas at Yellowstone produced small-volume, low-18O magmas directly, thereby circumventing the water saturation barrier encountered through normal AFC processes.
Balsley Steven D.
Gregory Robert T.
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