LOW-O-18 SILICIC MAGMAS - WHY ARE THEY SO RARE

Low-O-18 silicic magmas are reported from only a small number of local ities (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 o f low-O-18 magmas in intracontinental caldera settings is remarkable g iven the evidence of intense low-O-18 meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems. In the Platoro caldera complex, regional ignimbrites (150-1000 km(3)) have plagiocla se delta(18)O values of 6.8 +/- 0.1 parts per thousand, whereas the Mi ddle Tuff, a small-volume (est. 50-100 km(3)) post-caldera collapse py roclastic sequence, has plagioclase delta(18)O values between 5.5 and 6.8 parts per thousand. On average, the plagioclase phenocrysts from t he Middle Tuff are depleted by only 0.3 parts per thousand relative to those in the regional tuffs. At Yellowstone, small-volume post-calder a collapse intracaldera rhyolites are up to 5.5 parts per thousand dep leted relative to the regional ignimbrites. Two important differences between the Middle Tuff and the Yellowstone low-O-18 rhyolites elucida te the problem. Middle Tuff magmas reached water saturation and erupte d explosively, whereas most of the low-O-18 Yellowstone rhyolites erup ted effusively as domes or flows, and are nearly devoid of hydrous phe nocrysts. Comparing the two eruptive types indicates that assimilation of low-O-18 material, combined with fractional crystallization, drive s silicic melts to water oversaturation. Water saturated magmas either erupt explosively or quench as subsurface porphyries before the magma tic O-18 can be dramatically lowered. Partial melting of low-O-18 subv olcanic rocks by near-anhydrous magmas at Yellowstone produced small-v olume, low-O-18 magmas directly, thereby circumventing the water satur ation barrier encountered through normal AFC processes. (C) 1998 Elsev ier Science B.V. All rights reserved.