Low-18O silicic magmas:: why are they so rare?

Low-O-18 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-O-18 magmas in intracontinental caldera settings is remarkable given 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 plagioclase delta(18)O values of 6.8 +/- 0.1 parts per thousand, whereas the Middle Tuff, a small-volume (est. 50-100 km(3)) post-caldera collapse pyroclastic sequence, has plagioclase delta(18)O values between 5.5 and 6.8 parts per thousand. On average, the plagioclase phenocrysts from the Middle Tuff are depleted by only 0.3 parts per thousand relative to those in the regional tuffs. At Yellowstone, small-volume post-caldera collapse intracaldera rhyolites are up to 5.5 parts per thousand depleted relative to the regional ignimbrites. Two important differences between the Middle Tuff and the Yellowstone low-O-18 rhyolites elucidate the problem. Middle Tuff magmas reached water saturation and erupted explosively, whereas most of the low-O-18 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-O-18 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 O-18 can be dramatically lowered. Partial melting of low-O-18 subvolcanic rocks by near-anhydrous magmas at Yellowstone produced small-volume, low-O-18 magmas directly, thereby circumventing the water saturation barrier encountered through normal AFC processes. (C) 1998 Elsevier Science B.V. All rights reserved.