PHENOCRYST ZONING: FRACTURE VERSUS RESORPTION

Phenocrysts in the Bishop Tuff rhyolite are zoned in several ways: cathodoluminescence of quartz phenocrysts reveals variously bright concentric zones that parallel crystal faces; some zones are truncated along straight (flat) surfaces; other zones are truncated along curved surfaces; some crystals have pockets and hourglass inclusions extending inward from a crystal face. It is commonly assumed or argued that truncated zones and melt pockets are the result of resorption. In most cases, however, resorption is neither necessary nor plausible to explain the truncation textures. Rather, fragmentation is indicated. This is important because resorption is an indication of undersaturation (superheating or fluxing). Fragmentation, on the other hand, requires no undersaturation, and it can be the result of rupture of melt and/or gas inclusions with magma ascent and decompression. Whereas decompression of magma poor in H2O can cause superheating and resorption, decompression of magma that is saturated with an H2O-rich gas causes exsolution of H2O and this tends to drive supercooling and crystallization. Without a textural indicator of resorption, undersaturation is problematic, because, in general, crystals record growth; otherwise they disappear.