Rapid remobilization of magmatic crystals kept in cold storage

Abstract

The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage-that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.

Field-trip guide to Mount Hood, Oregon, highlighting eruptive history and hazards

“…238 U-230 Th data from Eppich and others (2012) also provide age information, and because the ~76,000 year half-life of 230 Th is significantly longer, can be used to resolve ages as old as 350,000 years. Groundmass-plagioclase isochrons for Old Maid and Timberline samples suggest that the population 2 plagioclase in these lavas have a minimum age of 17,000 years and a mean age of 126,000 years (Eppich and others, 2012;Cooper and Kent, 2014). Once corrections are made for the presence of eruptionage rims, the minimum age of population 2 plagioclase crystals is estimated to be 21,000 years.…”

“…12), as we noted before, these typically reflect the temperature of mineral formation or equilibration and not the long-term thermal history experienced by magmas. In an alternate approach, Cooper and Kent (2014) combined crystal ages from the uranium-thorium-radium studies with estimates of crystal residence based on the diffusion of strontium in plagioclase and CSDs to investigate this issue. As we noted before, 238 U-230 Th isochrons constrain the average age of the cores of population 2 plagioclase (which formed from the silicic parental magma) to more than ~21,000 years.…”

“…Magma storage at relatively cool temperatures will result in shorter magma residence time estimates from strontium diffusion and from CSDs. For population 2 plagioclase from Old Maid and Timberline age samples, Cooper and Kent (2014) determined maximum magma residence times of <<2,000 years at 750 °C-a temperature that marks the transition from a rheologically mobile magma to crystal-dominated rheologic solid (Marsh, 1981). As a result, Cooper and Kent (2014) concluded that the silicic parental magma stored beneath Mount Hood has only been in a mobile state, and thus in a condition where it could be erupted, for a small fraction of its total residence (at least <12 percent and probably <1 percent).…”

“…This scenario of cold storage for the silicic parental magma makes intuitive sense given the relatively small volumes erupted at Mount Hood (typically <<1 km 3 ; for example Scott and others, 1997) and the inference that magmatic fluxes are relatively low. However ongoing studies of other volcanoes suggest that cold storage of magmas may be relatively common in many arc volcanoes (Cooper and Kent, 2014), even where eruption volumes are much larger than those typical for Mount Hood.…”

“…Before 2000 there were relatively few studies in the peer-reviewed literature concerning the petrological aspects of Mount Hood (for example, Wise, 1969;Eichelberger, 1978;. Since 2005 we have been involved in a series of papers and graduate theses Kent and others, 2010;Eppich, 2010;Eppich and others, 2012;Koleszar and others, 2012;Kent, 2013;Cooper and Kent, 2014) exploring the petrology, petrography, geochemistry, geochronology, and other features of Mount Hood lavas. Other studies (for example, Woods, 2004) have also provided additional important data.…”