T99 A Matter of Perspective: Integrating Macro- and Micro-Scale Evidence in Investigations of Magmatic Origins and Histories

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 341

SIMPLY COMPLICATED: PERSPECTIVES ON SUPERERUPTIVE SYSTEMS FROM MULTIPLE AND VARIED DATASETS

Paper number: T99-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

Systems that produce giant supereruptions evacuate devastatingly large quantities of crystal-poor, melt-rich, high-silica rhyolite (HSR) magma from the crust in explosive and swift eruptions. The lack of any supereruptions in recorded history requires that deposits from past ones be studied in order to understand how, when, where, and why these magmas are generated, stored, evolve, and erupt.

We are exploring such questions in multiple large-volume silicic systems from a variety of tectonic settings: 767 ka Bishop Tuff, CA, USA; 18.8 Ma Peach Spring Tuff (PST), SW, USA; 25.4 ka Oruanui, 240 ka and Ohakuri-Mamaku (Oh-Ma), TVZ, New Zealand. We are using a plethora of methods, including: field observations; crystal and melt inclusion textures; whole-rock, glass, and mineral geochemistry; and phase-equilibria modeling. Such a broad dataset provides us with a nuanced perspective on individual systems and reveals both striking similarities and notable distinctions in the pre- and syn-eruptive timescales, crustal storage conditions, and geometry of these magmas more generally:

  • Crystal size distributions (CSD), melt inclusion faceting, and Ti diffusion chronometry in quartz crystals indicate that the final pre-eruptive accumulation of melt-rich HSR magmas occurs over impressively short centennial-millennial timescales.

  • CSDs and diffusion chronometry of quartz rims suggest that eruptive decompression is a short-lived event (days-a year). In some cases (Oruanui), we cannot resolve such groundmass or rim crystallization; this may indicate that ascent and eruption can occur even more quickly.

  • Crystal textures and compositions suggest that late-stage thermal events may be important in the final destabilization of giant systems (PST); however, these indicators are not ubiquitous, intimating that such events are either not always recorded or are not required for the triggering of an eruption.

  • Geobarometry indicates that HSR magmas are restricted to shallow upper-crustal depths. Yet, in combination with field relations, whole-rock, glass, and mineral geochemistry and textures, phase-equilibria geobarometry also suggests multiple potential models for the geometry of large-volume HSR systems in the crust, from a single zoned magma body (PST) to multiple simultaneously erupted batches (Bishop, Oh-Ma).