DIFFUSION CHRONOMETRY IN SANIDINE: HELPING UNRAVEL THERMAL HISTORIES OF LARGE SILICIC MAGMA RESERVOIRS

Correctly interpreting processes that govern how silicic magma is generated, stored, and differentiated within the upper crust is fundamental to understanding explosive eruptions. Constraining the timescales on which these processes occur can provide insight with regards to eruption timing, interpretation of volcano monitoring data, and also constrain the long-term conditions of magma storage. Despite the importance of understanding these conditions, the thermal evolution of large silicic magma reservoirs and the timescales on which they exist in an eruptible state are not fully understood.

From this ambiguity, two end-member hypotheses have been proposed: (1) Magma reservoirs spend the vast majority of their time in a state of 'cold storage' at near solidus temperatures, where they are not eruptible, and experience punctuated thermal events that generate eruptible magma. (2) Magma reservoirs spend the vast majority of their time in a state of 'warm storage' at temperatures much closer to the liquidus, where they contain a significant and eruptible melt fraction throughout much of their history.

Here we utilize electron probe micro-analyzer (EPMA) derived X-Ray maps of sanidine crystals and diffusion chronometry to better constrain the thermal evolution of the 900 km³ Kneeling Nun Tuff (New Mexico, USA) and discern how some of the largest volcanic eruptions on the planet are formed. Recent work has suggested that the Kneeling Nun Tuff, prior to eruption, underwent roughly 600,000 years of magma assembly between the granitic solidus (680-700°C) and the temperature of titanite crystallization (720-730°C). Diffusion timescale estimates generated from sanidine grains, after comparison with previous work, indicate that the Kneeling Nun Tuff spent the vast majority of its time in a state of cold storage, unable to erupt until thermal rejuvenation of the reservoir immediately prior to eruption.