A Numerical Treatment of Melt/Solid Segregation: Size of the Eucrite Parent Body and Stability of the Terrestrial Low-Velocity Zone

Crystal sinking to form cumulates and melt percolation toward segregation in magma pools can be treated with modifications of Stokes' and Darcy's laws, respectively. The velocity of crystals and melt depends, among other things, on the force of gravity (g) driving the separations and the cooling time of the environment. The increase of g promotes more efficient differentiation, whereas the increase of cooling rate limits the extent to which crystals and liquid can separate. The rate at which separation occurs is strongly dependent on the proportion of liquid that is present. As a result, cumulate formation is a process with a negative feedback; the more densely aggregated the crystals become, the slower the process can proceed. In contrast, melt accumulation is a process with a positive feedback; partial accumulation of melt leads to more rapid accumulation of subsequent melt. This positive feedback can cause melt accumulation to run rapidly to completion once a critical stability limit is passed. The observation of cumulates and segregated melts among the eucrite meteorites is used as a basis for calculating the g (and planet size) required to perform these differentiations. The eucrite parent body was probably at least 10-100 km in radius. The earth's low velocity zone (LVZ) is shown to be unstable with respect to draining itself of excess melt if the melt forms an interconnecting network. A geologically persistent LVZ with a homogeneous distribution of melt can be maintained with melt fractions only on the order of 0.1% or less

Silicon isotopes indicate enhanced carbon export efficiency in the North Atlantic during deglaciation

Today’s Sargasso Sea is nutrient starved, except for episodic upwelling events caused by wind-driven winter mixing and eddies. Enhanced diatom opal burial in Sargasso Sea sediments indicates that silicic acid, a limiting nutrient today, may have been more available in subsurface waters during Heinrich Stadials, millennial-scale climate perturbations of the last glacial and deglaciation. Here we use the geochemistry of opal-forming organisms from different water depths to demonstrate changes in silicic acid supply and utilization during the most recent Heinrich Stadial. We suggest that during the early phase (17.5–18 ka), wind-driven upwelling replenished silicic acid to the subsurface, resulting in low Si utilization. By 17 ka, stratification reduced the surface silicic acid supply leading to increased Si utilization efficiency. This abrupt shift in Si cycling would have contributed to high regional carbon export efficiency during the recent Heinrich Stadial, despite being a period of increasing atmospheric CO2

Crystal Growth Kinetics of Boron Oxide Under Pressure

We have measured the crystal growth rate u of B2O3-I in the amorphous phase, as it varied over five orders of magnitude with changes in temperature and pressure. We eliminated the crystal nucleation barrier by seeding the surface of boron oxide glass with crystals. u became measurable only when the pressure exceeded a threshold level near 10 kbar. Using the published thermodynamic information on the B2O3 system and a crude free-energy model for the crystal and glass phases, we account qualitatively for our results with the theory of crystal growth limited by the rate of two-dimensional nucleation of monolayers. The constants for the prefactor, activation energy, activation volume, and ledge tension are determined by fitting. By adjusting the thermodynamic parameters to a set of values that are well within the ranges delineated by their experimental uncertainties, we account quantitatively for the measured growth rates from 300 to 500 °C and from 0 to 30 kbar with the following relation: u(T,P)=(785 m/s)[||DeltaGm||/(RT)]1/6 ×exp[pi×3 Å(420 erg/cm2)2(28 cm3/mole)/(3 kTDeltaGm)]exp[−10 366 cal/mole/(RT)] ×exp[−P×16 cm3/mole/(RT)]×{1−exp[DeltaGm/(RT)]}2/3, with the driving free energy given by DeltaGm(T,P)=(13 cm3/mole) [PM(T)−P] and the melting curve given by PM(T)=(T−450 °C)/(42.6 K/kbar). The "B2O3 crystallization anomaly", that crystals have never been observed to grow at atmospheric pressure, is explained, since according to our model, the frequency of two-dimensional nucleation is negligible at all temperatures at pressures less than 10 kbarEngineering and Applied Science

Melt Segregation from Partially Molten Source Regions: The Importance of Melt Density and Source Region Size

The compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals. Consequently, the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth. The increasingly olivine-normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth. Once vertical permeability is established by melt generated during partial melting, buoyancy-driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale. Limits to this process are provided by cooling of the source region (freezing melt in) and rigidity of the crystalline matrix (mechanically trapping melt). Source region size influences these limits strongly: consequently, small, partially molten diapirs (∼km in diameter) may be able to trap large melt fractions (≳30%), but larger source regions would be unable to do so. The reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation. Diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent; this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma. Melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets—∼500 km on the moon, ∼250 km on Mars, ∼100 km on earth—correspond to similar pressures (25–35 kbar); at greater pressures, melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy. This may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time

Correlation of PROMIS scales and clinical measures among chronic obstructive pulmonary disease patients with and without exacerbations

The Patient-Reported Outcomes Measurement Information System (PROMIS®) initiative was developed to advance the methodology of PROs applicable to chronic diseases. Chronic obstructive pulmonary disease (COPD) is a progressive chronic disease associated with poor health. This study was designed to examine the correlation of PROMIS health-related quality of life (HRQOL) scales and clinical measures among COPD patients

Spectrum of Infection and Risk Factors for Human Monkeypox, United States, 2003

Infection is associated with proximity to virus-infected animals and their excretions and secretions

Observations of Milky Way Dwarf Spheroidal galaxies with the Fermi-LAT detector and constraints on Dark Matter models

We report on the observations of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf spheroidal galaxies, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10^-9 photons cm^-2 s^-1. Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in 8 of the 14 candidate dwarfs to place limits on the pair annihilation cross-section of WIMPs in several widely studied extensions of the standard model. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e.g. in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e^+e^- data, including low-mass wino-like neutralinos and models with TeV masses pair-annihilating into muon-antimuon pairs. (Abridged)Comment: 25 pages, 4 figures, accepted to ApJ, Corresponding authors: J. Cohen-Tanugi, C. Farnier, T.E. Jeltema, E. Nuss, and S. Profum

Environmental and societal factors affect food choice and physical activity: Rationale, influences, and leverage points

Dietary and physical activity behaviors that affect health are influenced by a wide variety of forces; changes in these behaviors require interventions and commitment to action at multiple levels.l.2 Education-based obesity-prevention strategies (e.g., mass-media promotion of healthy foods and promotion of healthy physical activity habits through schools) are viewed as the most useful and the most feasible to im~lementIm.~p licit in these strategies is the focus on the individual? Education-based strategies have met with limited long-term success in changing behavior: however, perhaps owing to a general lack of supporting environmental modifications. There is increasing recognition of the importance of the environment in shaping behavior, yet strategies that focus on changing environmental factors are much less familiar, and may therefore require partnerships with relevant sectors outside traditional health domains. As described in greater detail by Economos et al.: partnerships among researchers, educators, government, and industry have demonstrated success in smoking reduction at the population level. Interventions such as taxation and advertisement regulations have been instrumental in promoting smoking cessation in the United States and are used by agriculture and agribusiness interests to promote specific food consumption patterns. Similar models of collaborations or interventions may be successful in changing food intake and physical activity, and may potentially result in such desirable outcomes as prevention and reduction of obesity.6 It is important to appreciate the interaction among multiple environmental factors and that complex behavior changes are dependent on different influences at different levels. In Working Group 11, we took on the task of identifying broader contextual, environmental, societal, and policy variables that may improve our understanding of people's eating and physical activity behaviors and may lead to new directions for influencing shifts in behavior. Ecologic models of behavior, and most health promotion models, specify that health behaviors be influenced by biologic, demographic, psychological, sociaYcultura1, environmental, and policy variables. However, the research base that identifies specific important environmental and policy variables is very limited.'.""' Nevertheless, there are several reasons that support the need to identify environmental and policy influences on physical activity and eating behaviors

A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices $(0.7 < \Gamma <1.4)$ and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J. Kn\"odlseder, N. Webb, B. Pancraz