Guilherme A. R. Gualda

A method for estimating the activity of titania in magmatic liquids from the compositions of coexisting rhombohedral and cubic iron–titanium oxides

A method is described for estimating the activity of titania (TiO2) in a magmatic liquid from the compositions of coexisting cubic oxide (spinel) and rhombohedral oxide (ilmenite). These estimates are derived from the thermodynamic models of Ghiorso and Evans (Am J Sci 308:957–1039, 2008; see also Sack and Ghiorso in Contrib Mineral Petrol 106:474–505, 1991a; Am Mineral 76:827-847, 1991b) and may be computed self consistently along with temperature and oxygen fugacity for an assumed pressure. The method is applied to a collection of 729 naturally occurring oxide pairs from rhyolites and dacites. For this suite of oxides, values of titania activity relative to rutile saturation range from 0.3 to 0.9. Genetically related groups of oxide pairs display activity–temperature trends with negative slopes at higher activities (0.6–0.9) or positive slopes at lower activities (0.3–0.7). Thermodynamic analysis supports the assumption of two-oxide, liquid equilibrium for the former group, but suggests that such an interpretation for oxide sequences with positive activity–temperature trends may be problematic. Application of the estimation method to oxide pairs from the Shiveluch Volcano and the Bishop Tuff reveals that the former are consistent with having equilibrated with known matrix glass compositions, whereas the latter pairs are inconsistent with equilibration with pre-eruptive liquids trapped in quartz inclusions.

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.

MELTS_Excel: A Microsoft Excel‐based MELTS interface for research and teaching of magma properties and evolution

The thermodynamic modeling software MELTS is a powerful tool for investigating crystallization and melting in natural magmatic systems. Rhyolite-MELTS is a recalibration of MELTS that better captures the evolution of silicic magmas in the upper crust. The current interface of rhyolite-MELTS, while flexible, can be somewhat cumbersome for the novice. We present a new interface that uses web services consumed by a VBA backend in Microsoft Excel©. The interface is contained within a macro-enabled workbook, where the user can insert the model input information and initiate computations that are executed on a central server at OFM Research. Results of simple calculations are shown immediately within the interface itself. It is also possible to combine a sequence of calculations into an evolutionary path; the user can input starting and ending temperatures and pressures, temperature and pressure steps, and the prevailing oxidation conditions. The program shows partial updates at every step of the computations; at the conclusion of the calculations, a series of data sheets and diagrams are created in a separate workbook, which can be saved independently of the interface. Additionally, the user can specify a grid of temperatures and pressures and calculate a phase diagram showing the conditions at which different phases are present. The interface can be used to apply the rhyolite-MELTS geobarometer. We demonstrate applications of the interface using an example early-erupted Bishop Tuff composition. The interface is simple to use and flexible, but it requires an internet connection. The interface is distributed for free from http://melts.ofm-research.org.

Low-Pressure Origin of High-Silica Rhyolites and Granites

High-silica rhyolites and granites (>75 wt% SiO2, anhydrous basis) are common features of the crust as part of both the volcanic and the plutonic records. While low crystallization pressure (<250 MPa) is typically inferred, it has been suggested that they form via polybaric evolution, with initial crystallization at relatively high pressures (>500 MPa). We use glass compositions derived from the EarthChem portal, selected natural examples from the literature, and rhyolite-MELTS calculations to show that the phase relations in the quartz-albite-orthoclase ternary dictate the silica content of silicic melts. In particular, we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz-sanidine cotectic toward the Qz apex with decreasing pressure. It follows from our analysis that (1) the crust is expected to be stratified in terms of the silica content of residing melts; (2) high-silica glasses form at low pressure, requiring shallow-level crystallization, and preclude a polybaric evolution for many systems (e.g., Bishop Tuff); (3) the existence of high-silica pumice requires fractionation (or melting) at low pressure, showing that high-silica rhyolites are intrinsic to the shallow crust; and (4) low-pressure cumulates (or melting residues) must exist in the shallow crust, weighing in favor of the cumulatic nature of many granitoids found in plutons.

Fragmentation, nucleation and migration of crystals and bubbles in the Bishop Tuff rhyolitic magma

The Bishop Tuff (USA) is a large-volume, high-silica pyroclastic rhyolite. Five pumice clasts from three early stratigraphic units were studied. Size distributions were obtained using three approaches: (1) crushing, sieving and winnowing (reliable for crystals >100 μm); (2) microscopy of ∼1 mm 3 fragments (preferable for crystals 3 pumice pieces. Phenocryst fragments coated with glass are common, and the size distributions for all crystals are concave-upward, indicating that crystal fragmentation is an important magmatic process. Three groups are recognised, characterised by: (1) high-density (0·759–0·902 g cm −3 ), high-crystal content (14·4–15·3 wt.%) and abundant large crystals (>800 μm); concave-downward size distributions for whole crystals indicate late-stage growth with limited nucleation, compatible with the slow cooling of a large, gas-saturated, stably stratified magma body; (2) low-density (0·499 g cm −3 ), low-crystal content (6·63 wt.%) and few large crystals; the approximately linear size distribution reveals that nucleation was locally important, perhaps close to the walls; and (3) intermediate characteristics in all respects. The volumetric fraction of bubbles inversely correlates with the number of large crystals. This is incompatible with isobaric closed-system crystallisation, but can be explained by sinking of large crystals and rise of bubbles in the magma

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

Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: a comparison of various methods

An important drawback of the electron microprobe is its inability to quantify Fe3+/Fe2+ ratios in routine work. Although these ratios can be calculated, there is no unique criterion that can be applied to all amphiboles. Using a large data set of calcic, sodic-calcic, and sodic amphibole analysis from A-type granites and syenites from southern Brazil, weassess the choices made by the method of Schumacher (1997, Canadian Mineralogist, 35: 238-246), which uses the average between selected maximum and minimum estimates. Maximum estimates selected most frequently are: 13 cations excluding Ca, Na, and K (13eCNK - 66%); sum of Si and Al equal to 8 (8SiAl - 17%); 15 cations excluding K (15eK - 8%). These selections are appropriate based on crystallochemical considerations. Minimum estimates are mostly all iron as Fe2+ (all Fe2 - 71%), and are clearly inadequate. Hence, maximum estimates should better approximate the actual values. To test this, complete analyses were selected from the literature, and calculated and measured values were compared. 13eCNK and maximum estimates are precise and accurate (concordance correlation coefficient- rc » 0.85). As expected, averages yield poor estimates (rc = 0.56). We recommend, thus, that maximum estimates be used for calcic, sodic-calcic, and sodic amphiboles.

Dissolution of ophiuroid ossicles on the shallow Antarctic Shelf: Implications for the fossil record and ocean acidification

ABSTRACT The brittlestar, Ophionotus victoriae, is abundant in Explorers Cove, offshore Taylor Valley. However its ossicles, composed of high-Mg calcite, have not been reported from Cenozoic cores taken from McMurdo Sound. To identify taphonomic processes we analyzed (1) ossicle dissolution and silhouette area loss during a 2-year in situ experiment in which whole dead brittlestars were suspended above or placed on the sediment-water interface at water depths of 7–25 m; (2) ossicle dissolution in a 27-day, in situ experiment using ossicles freed of soft tissue; (3) porosities of experimental and pristine ossicles; and (4) abundance of ossicles in short cores taken at shallow depths in Explorers Cove. SEM analysis demonstrates significantly higher levels of dissolution in ossicles submerged for two years than in pristine ossicles. Submerged ossicles also had significant breakage reflected in silhouette area loss. During the 27-day experiment, submerged ossicles lost between 0.07 wt% and 1.31 wt%. At the observed rate of dissolution it would take between 6 and 105 years for vertebral ossicles to dissolve completely. Ossicles submerged for two years had a slightly higher mean porosity than pristine ossicles; porosity is controlled by variability in the porous stereom structure as well as dissolution. Results demonstrate that ossicle dissolution starts soon after death and that the stratigraphic record does not accurately reflect the presence and abundance of ophiuroids, thus complicating their use in paleoenvironmental, paleoclimatic, and paleoecologic reconstructions. These results also provide baseline information about CaCO3 skeletal dissolution needed to monitor the ocean acidification that is predicted to affect high-latitude benthic ecosystems within decades.

The Year Leading to a Supereruption

Supereruptions catastrophically eject 100s-1000s of km3 of magma to the surface in a matter of days to a few months. In this study, we use zoning in quartz crystals from the Bishop Tuff (California) to assess the timescales over which a giant magma body transitions from relatively quiescent, pre-eruptive crystallization to rapid decompression and eruption. Quartz crystals in the Bishop Tuff have distinctive rims (<200 μm thick), which are Ti-rich and bright in cathodoluminescence (CL) images, and which can be used to calculate Ti diffusional relaxation times. We use synchrotron-based x-ray microfluorescence to obtain quantitative Ti maps and profiles along rim-interior contacts in quartz at resolutions of 1–5 μm in each linear dimension. We perform CL imaging on a scanning electron microscope (SEM) using a low-energy (5 kV) incident beam to characterize these contacts in high resolution (<1 μm in linear dimensions). Quartz growth times were determined using a 1D model for Ti diffusion, assuming initial step functions. Minimum quartz growth rates were calculated using these calculated growth times and measured rim thicknesses. Maximum rim growth times span from ~1 min to 35 years, with a median of ~4 days. More than 70% of rim growth times are less than 1 year, showing that quartz rims have mostly grown in the days to months prior to eruption. Minimum growth rates show distinct modes between 10−8 and 10−10 m/s (depending on sample), revealing very fast crystal growth rates (100s of nm to 10s of μm per day). Our data show that quartz rims grew well within a year of eruption, with most of the growth happening in the weeks or days preceding eruption. Growth took place under conditions of high supersaturation, suggesting that rim growth marks the onset of decompression and the transition from pre-eruptive to syn-eruptive conditions.