CAN A SUPERVOLCANO ERUPTION ERADICATE EXISTING ZIRCON?

The zircon age spans observed in silicic plutonic and volcanic rocks are similar up to volumes of about 500 km³, indicating they are assembled at similar rates (on the order of 10^-3 km³/a). However, at volumes >500 km³, the relationship breaks down. Whereas plutons maintain the same slow assembly rates, large-volume volcanic rocks (supervolcanoes) show rates that are at least an order of magnitude faster. This discrepancy may reflect a different assembly mechanism for the magma reservoirs related to supereruptions. An alternative is that supervolcano magma bodies are assembled slowly in the upper crust and remobilized and erupted after a major reheating event, resulting in dissolution of the zircon crystallized during slow assembly.

A better understanding of zircon dissolution during reheating is necessary to evaluate these hypotheses. The preservation of inherited zircon in ignimbrites (e.g., preservation of Precambrian zircon with Tertiary overgrowths in Tertiary ignimbrites of the Southern Rocky Mountain volcanic field) suggest that zircon dissolution may be inefficient. Published models for zircon dissolution/crystallization during mush rejuvenation calculated under similar conditions show wide-ranging results. For example, a zircon with a radius of 100 µm is calculated to fully dissolve under magmatic conditions (750ºC, Zr-undersaturated) in a peraluminous magma anywhere between 8 ka and 180 ka depending on the model used. The disparity in these models is significant because the timescales hypothesized for magma body rejuvenation are on the order of 0.1 to 10 ka. If zircon dissolves rapidly under these conditions, it is possible that information recorded over several Ma in a growing magma body may be erased prior to eruption. However, if zircon dissolution is slow, it suggests that large ignimbrites are assembled much more quickly than plutons, requiring a different physical process for their generation. Our preliminary modeling indicates that very slow dissolution rates, consistent with the preservation of inherited cores, are correct. These preliminary results suggest that it is unlikely for incrementally assembled, intrusive suite-sized magma bodies to be rejuvenated in supervolcanic eruptions.