THE THERMAL HISTORY OF SHALLOW MAGMA STORAGE

Many magmas erupted in continental convergent margins experience extended (>> ~10 ka) periods of storage within the shallow continental crust (< 10 km). Thermal conditions during storage largely dictate the physical state of stored magma, and thus exert an important control over the behavior of volcanic systems – particularly with respect to processes that mobilize magma and initiate eruptions. The thermal state of the magma also controls crystallization, assimilation and vapor saturation, and strongly influences magmatic evolution.

Despite this, there are few direct constraints on the thermal state through time – a.k.a. the thermal history – experienced by stored magmas during extended crustal residence. Phase equilibria and mineral thermometry record the temperature of final mineral equilibration and typically indicate supersolidus conditions. In contrast widespread evidence for eruption of crystal-rich mushes suggests that many erupted magmas are stored for extended periods at near or even sub solidus temperatures. To date the relative duration of storage at “mushy” vs. supersolidus conditions has been hard to quantify.

Differences between U-Th mineral ages and crystal residence estimates based on diffusion or crystal growth provide one of the few means to quantify magma thermal histories. For example at Mount Hood, Oregon, U-Th systematics show erupted magmas are stored for > 21 ka, but diffusion modeling of Sr in plagioclase suggests <12%, and probably < 1%, of this storage was at temperatures high enough for magma to be readily mobilized (~750°C). Globally this general pattern is widespread, although existing data sets are incomplete and largely restricted to smaller volume intermediate eruptions.

Further quantification of the thermal history of magma storage in a range of volcanic systems is likely to provide insight into the nature of crustal magma storage and volcanism. Important questions remain concerning the thermal history of magmas stored in systems that erupt large volumes of material, and whether a pre-eruption “maturation” phase at higher temperatures is required to accumulate large magma volumes. In addition thermal histories of magma storage provide important constraints for numerical modeling of the processes of magma accumulation, storage, and eruption.