Supereruptions violently transfer huge amounts (100 s–1000 s km3) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted ∼760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500–3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies.

No abstract

Very large eruptions in the TVZ (New Zealand) reveal rapid magma assembly and eruption and progressive magma shallowing with time.

No abstract

No abstract

X-ray tomography is a nondestructive technique that can be used to study rocks and other materials in three dimensions over a wide range of sizes. Samples that range from decimeters to micrometers in size can be analyzed, and micrometer-to centimetersized crystals, vesicles, and other particles can be identifi ed and quantifi ed. In many applications, quantifi cation of a large spectrum of sizes is important, but this cannot be easily accomplished using a single tomogram due to a common trade-off between sample size and image resolution. This problem can be circumvented by combining tomograms acquired for a single sample at a variety of resolutions. We have successfully applied this method to obtain crystal size distributions (CSDs) for magnetite, pyroxene + biotite, and quartz + feldspar in Bishop Tuff pumice. Five cylinders of systematically varying size (1-10 mm diameter and height) were analyzed from each of fi ve pumice clasts. Cylinder size is inversely proportional to image resolution, such that resolution ranges from 2.5 to 17 μm/voxel with increasing sample size. This allows quantifi cation of crystals 10-1000 μm in size. We obtained CSDs for each phase in each sample by combining information from all resolutions, each size bin containing data from the resolution that best characterizes crystals of that size. CSDs for magnetite and pyroxene + biotite in late-erupted Bishop pumice obtained using this method are fractal, but do not seem to result from crystal fragmentation. CSDs for quartz + feldspar reveal a population of abundant crystals <35 μm in size, and a population of crystals >50 μm in size, which will be the focus of a separate publication.

No abstract

Accessory minerals preserve important records of the evolution of magmatic systems, but study of their textures and contact relations is hindered by the lack of suitable methods for characterization. We show here that differential absorption X-ray tomography can be used to yield three-dimensional maps of selected elements, particularly Zr and rare earth elements (REE), making it possible to qualitatively and quantitatively document the textures of zircon and REE minerals in situ and in three dimensions. We apply this method to pumice from the Peach Spring Tuff (Nevada, Arizona, California) and Mount St. Helens (Washington State) and present a few illustrative examples of the kinds of data that can be extracted using elemental and conventional tomographic data. Particularly when combined with compositional and age data, the ability to visualize and document accessory minerals in three dimensions and in textural context opens exciting new possibilities for the study of accessory minerals and the rocks that contain them.