Intellectual Merit. An essential component of studies of magmatic systems is the ability to place the thermal, physical, and compositional evolution of magmas within a temporal context. In recent years there has been increased interest in constraining the rates of igneous processes and also an emerging awareness that crystals in magmatic systems often record complex histories of magmatic processes that are obscured in the liquid fractions of magmas. Three effective techniques used to constrain the timescales of crystallization and crystal residence in intermediate and silicic magma systems include textural studies, studies of diffusional relaxation of chemical gradients in crystals, and uranium-series crystal dating. However, efforts to apply these techniques have also revealed large variations (up to four orders of magnitude) in the apparent timescales of crystal storage and magma residence. Although this likely reflects natural variations in crystal residence times between different types of magmatic systems, there also appear to be systematic differences in the results recorded by each technique, which implies that each technique may respond to and record somewhat different signals. Accordingly, this project has two goals: (1) to quantify the characteristic timescales of crystal residence prior to eruption and to investigate whether these timescales vary over time at a single volcano, and (2) to apply and compare three different techniques for estimating magmatic timescales on the same suite of rock samples. To this end, this project addresses timescales of crystal residence and magma mixing at Mount Hood, Oregon. Existing data indicate that mixing between felsic and mafic magmas there has been remarkably consistent throughout the ~500 ka life of the modern edifice, Crystal populations derived from the respective endmembers are readily recognized, and importantly, initial results suggest that the pre-eruptive residence times of crystals from each of these populations have remained broadly constant throughout the volcano?s history. This hypothesis will be tested by measuring pre-mixing crystal residence times, and estimating the time elapsed between mixing and eruption. It is proposed to quantify the timescales of crystal and magma storage and magma mixing at Mount Hood by applying textural quantification (CSD), diffusional relaxation and 238U-230Th-226Ra dating. There have been few studies that directly compare any two of these techniques on the same sample suites, and none that have applied all three. Direct comparison will provide an increased level of confidence in estimates of magmatic timescales and provide a methodology that can be used for other studies. Moreover, the three techniques interact to provide more information than would be provided by using any one technique in isolation.
Broader Impacts. This study will provide valuable insight into the behavior of intermediate composition silicic volcanoes and likely will have valuable implications for understanding the hazard potential of such volcanoes to nearby transportation corridors (Columbia River) and metropolitan regions (Portland). The project will promote collaboration between scientists from Oregon State and UC-Davis, as well as from Durham Univ. (Dougal Jerram - an expert in quantification of magmatic textures) and the USGS. It will advance the careers of a female PI and a promising female graduate student. Jerram will teach an informal workshop while in Corvallis, and his visit will be scheduled so as to allow for participation by the graduate students supported by this proposal in addition to others in the Northern California and Pacific Northwest region. The proposal will also support continued outreach efforts to demonstrate basic volcanologic principles to undergraduate and K-12 students in the Corvallis area.
The big-picture goal of this project was to contribute to a better understanding of how volcanic plumbing systems operate. In particular, we aimed to understand how long magmas are stored beneath volcanoes, and what are the interactions between magmas in the subsurface that eventually lead to eruption. We combined chemical measurements of erupted lavas with three different methods of measuring ages of crystals in volcanic rocks. The three methods are based on 1) crystal sizes combined with growth rates, 2) movement of elements (by diffusion) within the crystals in order to equilibrate with new surroundings, and 3) radioactive decay of 238U and its daughter products. Each method has been used independently in the past, but rarely in combination and (as far as we are aware) never all three in the same sample. The advantage of using the combined technique is that each age estimate gives somewhat different information. For example, the size of crystals depends on crystal growth rates (determined experimentally) and on the time that crystals spend within a temperature interval that is neither too hot nor too cold for crystals to grow, and therefore provides information about the amount of time that the magma spent at crystallization temperatures. The movement of elements by diffusion is strongly dependent on temperature and only happens rapidly at high temperatures, so that age estimate captures the amount of time a crystal spent at high temperature. Finally, the radiometric dating is independent of temperature or crystal growth rates and depends only on time since crystallization, but on the other hand it gives an average age if crystallization happened in multiple stages. The main outcomes of this study were: 1) the lavas erupted at Mt Hood have a very restricted chemical composition compared to lavas erupted from many other volcanoes. We determined from chemical analysis of the crystals and rocks that these restricted compositions came about by mixing two different types of magmas in the subsurface. Interestingly, the mixture always ends up with approximately the same proportions of the two, and the pure subsurface magmas are rarely (if ever) erupted at Mt Hood. This can be explained if the magma sitting below the surface requires input of a new, hotter, magma from below in order to activate the system and cause eruption. 2) Volcanic gas contents are tied to eruptive style, where more gas-rich magmas tend to erupt explosively. Yet Mt Hood dominantly has less explosive eruptions than other Cascades volcanoes. We analyzed chemical compositions of tiny drops of magma caught within crystals, and found that the volcanic gas content of the magmas below Mt Hood are not unusual. Therefore, the less-explosive eruptive style at Mt Hood has to do with some other factor; calculations of temperatures of formation for crystals in Mt Hood lavas suggests that the new, hotter magmas entering the system heat up the resident magmas enough that volcanic gases can escape the magma on its way to the surface, rather than remaining trapped and causing an explosive eruption at the surface. 3) Ages of crystals in the two most recent Mt Hood lavas show younger ages for the crystals that came from the hot new input magma (less than a few thousand years), and older ages for the crystals that came from the cooler magma stored in the subsurface (older than 5 thousand years). The details of the age data indicate that the average of five thousand years for the older crystals is actually an average of some crystallization happening right before eruption mixed with some crystals that formed at least 20 thousand years before eruption. This indicates that magmas are stored beneath the surface for long periods of time and parts of the magma body is brought to the surface during each eruption. 4) The different methods of estimating the timing of crystallization and the amount of time at high temperatures indicate that, of the over 20 thousand years of history recorded by the crystals, the magma system was hot enough to erupt for less than 1-5% of that time. This has clear implications for volcanic hazards – for example, we would not expect in general to see an "eruptible body" of magma in geophysical measurements of Mt Hood, and in cases where we do an eruption will likely happen within years to decades. These conclusions likely apply to other volcanoes, but more work is needed in order to determine how much of the time eruptible bodies of magma exist in other systems.
