Ayla S. Pamukcu

Timescales of Quartz Crystallization and the Longevity of the Bishop Giant Magma Body

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.

Quantitative 3D petrography using X-ray tomography 3: Documenting accessory phases with differential absorption tomography

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.

Melt inclusion shapes: Timekeepers of short-lived giant magma bodies

Geology , v. 43, no. 11, p. [947–950][1], doi:10.1130/G37021.1 In the third paragraph of the Methods section, the following equation was incorrect: t = D Ti L 2/4. The correct version of the equation is: t = L 2/(4 D Ti). [1]: /lookup/volpage/43/947

Quantitative 3D petrography using X-ray tomography 4: Assessing glass inclusion textures with propagation phase-contrast tomography

Textures (sizes, shapes, distributions) of glass inclusions in crystals can provide important insight into magmatic processes. Three-dimensional X-ray tomography is an excellent nondestructive method for assessing igneous rock textures in general; however, standard absorption-contrast tomography is not useful for imaging glass inclusions because it typically does not provide enough contrast between the inclusion and crystal host to allow quantitative separation of these phases during image processing. Propagation phase-contrast tomography enhances contacts between different phases, allowing the use of edge-detection algorithms to separate glass inclusions from their host crystal. The sample-to-detector distance is increased to achieve edge enhancement at the cost of some loss in image resolution. We qualitatively assessed images acquired at a series of distances (30, 100, 150, 200 mm) to determine the best distance for imaging given this trade-off. Typical image processing approaches used for absorption-contrast tomography are not useful for edge-detection problems, so we developed an IDL-based graphical user interface (GUI) to process propagation phase-contrast tomograms. A combination of grayscale filters, distance thresholding, erosion processes , and individual picking of outlier voxels is used to fit a convex hull to an individual inclusion, and the output provides information on its size and shape. The processed inclusions can be placed in the context of the original host, such that size, shape, and location of multiple inclusions can be assessed individually or in combination. We apply this method to five glass inclusions in quartz crystals from early erupted Bishop Tuff. Results indicate that inclusions range in shape from nearly spherical to nearly ellipsoidal and from round to faceted. The relationship between degree of faceting, size, and location within the host is consistent with melt inclusion faceting under magmatic conditions over centennial to millennial time scales. We also show application of this method to other kinds of studies, such as glass inclusion size distributions.

Climbing the crustal ladder: Magma storage-depth evolution during a volcanic flare-up

Very large eruptions in the TVZ (New Zealand) reveal rapid magma assembly and eruption and progressive magma shallowing with time. Very large eruptions (>50 km3) and supereruptions (>450 km3) reveal Earth’s capacity to produce and store enormous quantities (>1000 km3) of crystal-poor, eruptible magma in the shallow crust. We explore the interplay between crustal evolution and volcanism during a volcanic flare-up in the Taupo Volcanic Zone (TVZ, New Zealand) using a combination of quartz-feldspar-melt equilibration pressures and time scales of quartz crystallization. Over the course of the flare-up, crystallization depths became progressively shallower, showing the gradual conditioning of the crust. Yet, quartz crystallization times were invariably very short (<100 years), demonstrating that very large reservoirs of eruptible magma were transient crustal features. We conclude that the dynamic nature of the TVZ crust favored magma eruption over storage. Episodic tapping of eruptible magmas likely prevented a supereruption. Instead, multiple very large bodies of eruptible magma were assembled and erupted in decadal time scales.