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

Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers

Chemical interdiffusion of Fe-Mg along the c-axis [001] in natural diopside crystals (XDi = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10-11 to 10-17 bar. Diffusion couples were prepared by ablating an olivine (XFo = 0.3) target to deposit a thin film (20-100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe-Mg on composition of clinopyroxene (Fe/Mg ratio between Di93-Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe-Mg, can be described by a single Arrhenius relation with (Formula presented). DFe-Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa-Mg) while it is slower than DFe-Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks)

Urban geochemical mapping studies : how and why we do them

Geochemical mapping is a technique rooted in mineral exploration but has now found worldwide application in studies of the urban environment. Such studies, involving multidisciplinary teams including geochemists, have to present their results in a way that nongeochemists can comprehend. A legislatively driven demand for urban geochemical data in connection with the need to identify contaminated land and subsequent health risk assessments has given rise to a greater worldwide interest in the urban geochemical environment. Herein, the aims and objectives of some urban studies are reviewed and commonly used terms such as baseline and background are defined. Geochemists need to better consider what is meant by the term urban. Whilst the unique make up of every city precludes a single recommended approach to a geochemical mapping strategy, more should be done to standardise the sampling and analytical methods. How (from a strategic and presentational point of view) and why we do geochemical mapping studies is discussed. Keywords Background - Baseline - Geochemical mapping - Heavy metals - Pollution - Soil - Urba

Direct measurements of latent heat during crystallization and melting of a ugandite and an olivine basalt

Step-scanning calorimetric measurements using a Setaram HT1500 calorimeter were performed between 800 and 1400°C on two natural samples: a ugandite from the East African rift and an olivine basalt from the western Mexican arc. Our measurements provide the first in-situ quantitative assessment of enthalpy during melting of initially crystalline natural samples. The distribution of latent heat across the liquidus-solidus intervals of the two samples is distinctly different, reflecting significant variation in the sequence and abundance of mineral phases during melting (clinopyroxene and leucite in the ugandite; olivine, clinopyroxene, and plagioclase in the basalt). Our data further indicate that the common assumption of a uniform distribution of latent heat across the liquidus-solidus interval of a magma is a reasonable approximation for the olivine basalt, but is grossly in error for the ugandite. This is due to cotectic precipitation of leucite and clinopyroxene, leading to a large, disproportionate release of latent heat early in the crystallization sequence. The implication for the thermal history of a crystallizing ugandite magma is that the rate of heat loss during conductive cooling will unitially be more rapid than the average rate. The net result will be to produce lower magmatic temperatures after a given cooling interval relative to models assuming a uniform release of latent heat. An additional series of scanning calorimetric experiments were performed at variable rates (1,2 and 3°/min) to evaluate the role of kinetics on the distribution of enthalpy during both melting and crystallization of the ugandite and olivine basalt. The results indicate that clinopyroxene is the most important mineral phase in controlling the shapes of the enthalpy profiles during cooling; this is due to its large enthalpy of fusion and its tendency for sluggish nucleation, followed by rapid crystallization at temperatures that vary with cooling rate. The resolution of the calorimeter (in terms of heat detected per unit time) is also important in determining the shapes of the observed enthalpy profiles during these rapid scans. Estimates based on the observed calorimetric signal associated with melting of olivine, and the lack of a calorimetric signal during melting of leucite, combined with known enthalpies of fusion for the two phases, indicate detection limits of approximately 0.6–1.2 kJ per 5 min increments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47310/1/410_2004_Article_BF01052867.pd

The MintAct project and molecular interaction databases

Molecular interaction databases collect, organize, and enable the analysis of the increasing amounts of molecular interaction data being produced and published as we move towards a more complete understanding of the interactomes of key model organisms. The organization of these data in a structured format supports analyses such as the modeling of pairwise relationships between interactors into interaction networks and is a powerful tool for understanding the complex molecular machinery of the cell. This chapter gives an overview of the principal molecular interaction databases, in particular the IMEx databases, and their curation policies, use of standardized data formats and quality control rules. Special attention is given to the MIntAct project, in which IntAct and MINT joined forces to create a single resource to improve curation and software development efforts. This is exemplified as a model for the future of molecular interaction data collation and dissemination