Abstract

Zoned crystals are ubiquitous in volcanic rocks. Their zoning patterns are hold clues to the magmatic processes that shaped their host rocks. An enduring question is whether this information pertains to kinetic process, changes in intensive variables, open system behaviour, or combinations thereof. The challenge is to unpick the mute testimony of zoned crystals as an archive of pre-eruptive magmatic processes. This challenge has been lessened by the advent of microbeam techniques capable of high spatial and analytical resolution, thermodynamic models for plagioclase-melt equilibrium and element partitioning, and numerical models of intracrystalline diffusion. We have developed new models for “uncrystallising” zoned plagioclase feldspars from the 1980-86 eruptions of Mount St. Helens volcano in the USA . We have been able to show that individuals crystals record ascent from 12 to 4 km depth before eruption. The crystal cores are old, and resided in the deeper (12 km) reservoir for more than 10,000 years. In contrast, the rims whose growth pertains to ascent to shallower depeth, developed in timescales of 3 years or less prior to eventual eruption. A picture emerges of a long-lived, vertically extensive magmatic system that became abruptly destabilised, probably gravitationally, shortly before eruption.

All welcome to attend, free for members of the Sedgwick Club, £2 for non-members. Refreshments provided!