Geocheminar fall-2012

A Refined Model of Zircon Saturation in Crustal Magmas Patrick Boehnke (UCLA)

Improvements in experimental, analytical and computation methodologies together with published studies yielding seemingly contradictory results prompted us to return to the determination of zircon stability in the range of felsic to intermediate melts expected in continental environments. We re-analyzed both the run products from the 30 year old zircon crystallization study of Harrison and Watson (1983) and a new style of zircon dissolution experiments (up to 25 kbars) using a large radius ion microprobe to constrain a refined zircon solubility model. The new data yield broadly similar patterns as before when arrayed for temperature and confirm that the parameter M [=(K+Na+2Ca)/(Si Al) as molar abundances] is an appropriate compositional proxy for the mechanism by which zircon is dissolved. We used a Bayesian approach to optimize calculation of the coefficients in the zircon solution model, which is given by: ln DZr = (10108±32)/T(K) – (1.16±0.15) . (M-1) – (1.48±0.09), where DZr is the distribution coefficient of Zr between zircon and melt and the errors are at one sigma. Sensitivity tests indicate that temperature and composition are the two dominant controls on zircon solubility in crustal melts with, surprisingly, no observable pressure effect. Our new data together with literature results suggest a weaker dependence of zircon solubility on water content than previously thought. Comparison of the down-temperature extrapolation with natural examples confirms the validity of the model at ca. 700°C.

Temporal Record of Magma Accumulation and Storage in the Pastos Grandes Caldera Complex Jason Kaiser (Oregon)

The Pastos Grandes Caldera Complex in SW Bolivia is a nested caldera system that has produced 3 regionally extensive ignimbrites as well as small lava flows and domes. Volcanic activity in the complex spanned from 5.45 Ma to 85 ka. The most recent eruptions produced a string of domes in the caldera which contain granodiorite xenoliths. These xenoliths represent the remnant pluton that sourced the youngest ignimbrite of the complex at 2.89 Ma. Zircons were separated from fresh pumice and lavas in order to obtain U-Pb crystallization ages. The combined zircon crystallization age spectra of each volcanic unit reveal a temporal record of magma accumulation and storage. These data support the idea that each ignimbrite eruption was sourced by discrete pulses of magma into the shallow crust. The youngest ignimbrite eruption was also associated with subsequent effusive eruptions. Together, the ignimbrite, post-caldera lavas, and the plutonic xenolith represent a complete magma cycle related to a super-volcanic eruption. Combining the age spectra of these magma stages has allowed us to compare magma residence times before and after a climactic eruption. The U-Pb crystallization data will be compared with in-situ trace element geochemical data to provide a chronologic record of changing melt composition.