Diffusion-controlled spherulite growth in obsidian inferred from H<Subscript>2</Subscript>O concentration profiles

Spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian. The growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass, and rejection of non-crystal forming components from the crystallizing region. Water concentration profiles measured by synchrotron-source Fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth, and that it diffuses outward ahead of the advancing crystalline front. We compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth. Using a diffusion-controlled growth law, we find that spherulites can grow on the order of days to months at temperatures above the glass transition. The diffusion-controlled growth law also accounts for spherulite size distribution, spherulite growth below the glass transition, and why spherulitic glasses are not completely devitrified. An erratum to this article can be found at     

Amsterdam Island,an isolated volcano in the southern Indian ocean

Twelve lavas have been sampled on Amsterdam Island, which is an extinct shield volcano located on a transform fault crossing the south-eastern branch of the Indian Ocean rise. Chemical analyses show that the lavas have mainly tholeiitic affinities, ranging from olivine tholeiites to plagioclase basalts with accumulation of bytownite. The high alumina lavas are similar to bytownite cumulates dredged from the Indian Ocean rise, and the low alumina lavas resemble tholeiites of Hawaii. The volcano appears to be in an early stage of evolution from oceanic ridge basalt to an alkaline volcano.     

Propagation and source of Pc5 frequency range pulsation at cusp latitude

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Straight from the source's mouth: Controls on field-constrained sediment export across the entire active Corinth Rift,central Greece

The volume and grain-size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grain-size exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grain-sizes of channel bars and full-weighted grain-size distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grain-size increases westward along the southern coast of the Gulf of Corinth, with the coarse-fraction grain-sizes (84^th percentile of weighted grain-size distribution) ranging from approximately 19 to 91 mm. We find that the median and coarse-fraction of the sieved grain-size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grain-size export is primarily controlled by the input grain-size within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarse-fraction of the grain-size distribution are predominantly transported in bedload; however, typical sand-grade particles are transported as suspended load at bankfull conditions, suggesting disparate source-to-sink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grain-size-specific bedload discharged into the Gulf of Corinth using the grain-size measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475–16 cm). Our results suggest that the grain-size of sediment export at the rift scale is particularly sensitive to catchment lithology and fluvial mophodynamics, which complicates our ability to make direct inferences of tectonic and palaeoenvironmental forcing from local stratigraphic characteristics.     

Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Samples produced in piston cylinder experiments were used to document the thermal isotopic fractionation of all the major elements of basalt except for aluminum and the fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic fractionations are summarized in terms of a parameter Ω_i defined as the fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report Ω_Ca = 1.6, Ω_Fe = 1.1, Ω_Si = 0.6, Ω_O = 1.5. In an earlier paper we reported Ω_Mg = 3.6. These fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt-rhyolite diffusion couple showed significant fractionation that are parameterized in terms of a parameter β_Fe = 0.03 when the ratio of the diffusion coefficients D₅₄ and D₅₆ of ⁵⁴Fe and ⁵⁶Fe is expressed in terms of the atomic mass as D₅₄/D₅₆ = (56/54)^β_Fe. This value of β_Fe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., β_Li = 0.215, β_Ca = 0.05, β_Mg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic fractionations driven by either or both chemical and thermal gradients.     

The science process for selecting the landing site for the 2011 Mars Science Laboratory

The process of identifying the landing site for NASA’s 2011 Mars Science Laboratory (MSL) began in 2005 by defining science objectives, related to evaluating the potential habitability of a location on Mars, and engineering parameters, such as elevation, latitude, winds, and rock abundance, to determine acceptable surface and atmospheric characteristics. Nearly 60 candidate sites were considered at a series of open workshops in the years leading up to the launch. During that period, iteration between evolving engineering constraints and the relative science potential of candidate sites led to consensus on four final sites. The final site will be selected in the Spring of 2011 by NASA’s Associate Administrator for the Science Mission Directorate. This paper serves as a record of landing site selection activities related primarily to science, an inventory of the number and variety of sites proposed, and a summary of the science potential of the highest ranking sites.     

The fluid regime of high-temperature metamorphism during granitoid magma genesis

The strong negative correlation between the temperatures and melt H₂O contents of granitoid magmas implies that the crustal melting reactions that produced the magmas were strongly buffered, with T and aH₂O co-varying within a narrow band. This observation can only be explained if the partial melting reactions that created the magmas were either fluid-absent from the outset or evolved toward this condition as melting progressed. Since these melting reactions occur during upper amphibolite- to granulite-facies metamorphism, it is reasonable to conclude that metamorphic events responsible for the generation of granitoid magmas generally occur in the absence of excess pervasive fluid.