Pre-eruptive History of Taapaca Volcano (N. Chile) Constrained by Non-isothermal Diffusion Chronometry on Sanidine Megacrysts

Abstract:

Hybrid dacite magmas from Taapaca volcano (Chile) contain sanidine megacrysts (≤15 cm in size) from the silicic endmember. These sanidines occur throughout the ~1.5 My-long eruption history of Taapaca. They are rich in mineral inclusions and strongly zoned in Ba with growth bands separated by multiple resorption interfaces. The zonation takes a “saw-tooth pattern” where each resorption is followed by a sudden increase (by ≤2.3 wt%) in Ba followed by a slow decrease.

We argue that the Ba-jumps reflect distinct heating and dissolution events, suggesting significant differences in temperature. Amphibole-plagioclase thermo-barometry gave temperatures (~720-820 °C) and pressures (1-3 kbar) for individual growth zones. This control on zone-specific temperature allowed for a “non-isothermal” diffusive analysis where calculated diffusion times from Ba-profiles would give crystal residence and pre-eruptive reactivation times. Individual diffusion times of the boundaries range from 0.4 to 490 ky which add up to total residence times of 9 to 499 ky for different crystals from different eruptions.

A combination of temperatures, pressure, diffusion times and R-MELTS modeling suggests periods of tens to hundreds of ky of storage in a rhyodacite crystal mush at near eutectic composition and temperature (≥720 °C) at shallow depth (4-10 km). Repeated recharges of basaltic-andesitic magma, documented by mafic enclaves and bi-modal amphibole & plagioclase compositions in all the erupted dacites, kept the entire reservoir thermally "alive". Such recharge events became more frequent (~0.33-0.67 ky^-1) towards the end of the sanidines’ growth history, and after many “unsuccessful” recharges, a “critical” (rate of) recharge was “successful” in entraining a particular set of sanidine megacrysts into the hybrid dacite that carried it to the surface. This process was repeated at similar timescales throughout the 1.5 My-long history of Taapaca, indicating a slow and long-lived magma system in thermal steady state of long periods of time. The observed features of these sanidines are identical to those seen in granite intrusions and typical age ranges of zircon crystallization. Taapaca sanidines, thus, provide “smoking gun” evidence that links intrusive to extrusive activity of silicic magmas in arc settings.