926 research outputs found
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Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti
We present 36 new ⁴⁰Ar-³⁹Ar incremental heating age determinations from the Caribbean Large
Igneous Province (CLIP) providing evidence for extended periods of volcanic activity and suggest a new
tectonomagmatic model for the province’s timing and construction. These new ⁴⁰Ar-³⁹Ar ages for the
Curaçao Lava Formation (CLF) and Haiti’s Dumisseau Formation show evidence for active CLIP
volcanism from 94 to 63 Ma. No clear changes in geochemical character are evident over this period. The
CLF has trace element signatures (e.g., Zr/Nb=10–20) and flat rare earth element (REE) trends
consistent with plume volcanism. The Dumisseau Formation also has plume-like geochemistry and
steeper REE trends similar to ocean island basalts. Volcanism in the Dumisseau Formation appears to
have largely ceased by 83 Ma while at Curaçao it continued until 63 Ma. A rapidly surfacing and melting
plume head alone does not fit this age distribution. Instead, we propose that the residual Galapagos plume
head, following initial ocean plateau construction, was advected eastward by asthenospheric flow induced
by subducting oceanic lithosphere. Slab rollback at the Lesser Antilles and Central America subduction
zones created an extensional regime within the Caribbean plate. Mixing of plume with upwelling
asthenospheric mantle provided a source for intermittent melting and eruption through the original
plateau over a ~30 Ma period.Keywords: Caribbean large igneous province, Geochronology, Mantle plume, Plume-subduction interactionKeywords: Caribbean large igneous province, Geochronology, Mantle plume, Plume-subduction interactio
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Origin of minor and trace element compositional diversity in anorthitic feldspar phenocrysts and melt inclusions from the Juan de Fuca Ridge
Melt inclusions trapped in phenocryst phases are important primarily due to their potential of preserving a significant proportion of the diversity of magma composition prior to modification of the parent magma array during transport through the crust. The goal of this investigation was to evaluate the impact of formational and post-entrapment processes on the composition of melt inclusions hosted in high anorthite plagioclase in MORB. Our observations from three plagioclase ultra-phyric lavas from the Endeavor Segment of the Juan de Fuca Ridge document a narrow range of major elements and a dramatically greater range of minor and trace elements within most host plagioclase crystals. Observed host/inclusion partition coefficients for Ti are consistent with experimental determinations. In addition, observed values of D[subscript]Ti are independent of inclusion size and inclusion TiO₂ content of the melt inclusion. These observations preclude significant effects from the re-homogenization process, entrapment of incompatible element boundary layers or dissolution/precipitation. The observed wide range of TiO₂ contents in the host feldspar, and between bands of melt inclusions within individual crystals rule out modification of TiO₂ contents by diffusion, either pre-eruption or due to re-homogenization. However, we do observe comparatively small ranges for values of K₂O and Sr compared to P₂O₅ and TiO₂ in both inclusions and crystals that can be attributed to diffusive processes that occurred prior to eruption.KEYWORDS: crustal processes, magma transport, plagioclas
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The petrogenesis of plagioclase-phyric basalts at mid-ocean ridges
Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with
ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin
assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of
plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined,
this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and
densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of
PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a
magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have
never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial
magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt
(MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction
between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by
loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater
than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for
these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB
eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit
system that bypassed an axial chamber.Keywords: Mid-ocean ridges, Plagioclase, Magma transport, PU
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Species and Life History Affect the Utility of Otolith Chemical Composition for Determining Natal Stream of Origin for Pacific Salmon
To test the utility of otolith chemical composition as a tool for determining the natal stream of origin for salmon, we examined water chemistry and otoliths of juvenile and adult Chum Salmon Oncorhynchus keta and Coho Salmon O. kisutch from three watersheds (five rivers) in the Norton Sound region of Alaska. The two species are characterized by different life histories: Coho Salmon rear in freshwater for up to 3years, whereas Chum Salmon emigrate from freshwater shortly after emergence. We used laser ablation (LA) inductively coupled plasma (ICP) mass spectrometry (MS) to quantify element: Ca ratios for Mg, Mn, Zn, Sr, and Ba, and we used multicollector LA-ICP-MS to determine Sr-⁸⁷:Sr-⁸⁶ ratios in otolith regions corresponding to the period of freshwater residence. Significant differences existed in both water and otolith elemental composition, suggesting that otolith composition could be used to discriminate the natal origin of Coho Salmon and Chum Salmon but only when Sr-⁸⁷:Sr-⁸⁶ ratios were included in the discriminant function analyses. The best discriminant model included Sr-⁸⁷:Sr-⁸⁶ ratios, and without Sr-⁸⁷:Sr-⁸⁶ ratios it was difficult to discriminate among watersheds and rivers. Classification accuracy was 80% for Coho Salmon and 68% for Chum Salmon, indicating that this method does not provide sufficient sensitivity to estimate straying rates of Pacific salmon at the scale we studied.Keywords: Plasma mass spectrometry, Fish otoliths, Stock identification, Chemistry, Oncorhynchus tshawytscha, Population structure, Microchemistry, Elemental analysis, Describe movements, Trace element
The touchscreen operant platform for assessing executive function in rats and mice.
This protocol details a subset of assays developed within the touchscreen platform to measure various aspects of executive function in rodents. Three main procedures are included: extinction, measuring the rate and extent of curtailing a response that was previously, but is no longer, associated with reward; reversal learning, measuring the rate and extent of switching a response toward a visual stimulus that was previously not, but has become, associated with reward (and away from a visual stimulus that was previously, but is no longer, rewarded); and the 5-choice serial reaction time (5-CSRT) task, gauging the ability to selectively detect and appropriately respond to briefly presented, spatially unpredictable visual stimuli. These protocols were designed to assess both complementary and overlapping constructs including selective and divided visual attention, inhibitory control, flexibility, impulsivity and compulsivity. The procedures comprise part of a wider touchscreen test battery assessing cognition in rodents with high potential for translation to human studies
Discovery of Stable and Selective Antibody Mimetics from Combinatorial Libraries of Polyvalent, Loop-Functionalized Peptoid Nanosheets.
The ability of antibodies to bind a wide variety of analytes with high specificity and high affinity makes them ideal candidates for therapeutic and diagnostic applications. However, the poor stability and high production cost of antibodies have prompted exploration of a variety of synthetic materials capable of specific molecular recognition. Unfortunately, it remains a fundamental challenge to create a chemically diverse population of protein-like, folded synthetic nanostructures with defined molecular conformations in water. Here we report the synthesis and screening of combinatorial libraries of sequence-defined peptoid polymers engineered to fold into ordered, supramolecular nanosheets displaying a high spatial density of diverse, conformationally constrained peptoid loops on their surface. These polyvalent, loop-functionalized nanosheets were screened using a homogeneous Förster resonance energy transfer (FRET) assay for binding to a variety of protein targets. Peptoid sequences were identified that bound to the heptameric protein, anthrax protective antigen, with high avidity and selectivity. These nanosheets were shown to be resistant to proteolytic degradation, and the binding was shown to be dependent on the loop display density. This work demonstrates that key aspects of antibody structure and function-the creation of multivalent, combinatorial chemical diversity within a well-defined folded structure-can be realized with completely synthetic materials. This approach enables the rapid discovery of biomimetic affinity reagents that combine the durability of synthetic materials with the specificity of biomolecular materials
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Magma transport and olivine crystallization depths in Kīlauea's east rift zone inferred from experimentally rehomogenized melt inclusions
Concentrations of H₂O and CO₂ in olivine-hosted melt inclusions can be used to estimate crystallization depths for the olivine host. However, the original dissolved CO₂ concentration of melt inclusions at the time of trapping can be difficult to measure directly because in many cases substantial CO₂ is transferred to shrinkage bubbles that form during post-entrapment cooling and crystallization. To investigate this problem, we heated olivine from the 1959 Kīlauea Iki and 1960 Kapoho (Hawai‘i) eruptions in a 1-atm furnace to temperatures above the melt inclusion trapping temperature to redissolve the CO₂ in shrinkage bubbles. The measured CO₂ concentrations of the experimentally rehomogenized inclusions (⩽590 ppm for Kīlauea Iki [n = 10]; ⩽880 ppm for Kapoho, with one inclusion at 1863 ppm [n = 38]) overlap with values for naturally quenched inclusions from the same samples, but experimentally rehomogenized inclusions have higher within-sample median CO₂ values than naturally quenched inclusions, indicating at least partial dissolution of CO₂ from the vapor bubble during heating. Comparison of our data with predictions from modeling of vapor bubble formation and published Raman data on the density of CO₂ in the vapor bubbles suggests that 55–85% of the dissolved CO₂ in the melt inclusions at the time of trapping was lost to post-entrapment shrinkage bubbles. Our results combined with the Raman data demonstrate that olivine from the early part of the Kīlauea Iki eruption crystallized at <6 km depth, with the majority of olivine in the 1–3 km depth range. These depths are consistent with the interpretation that the Kīlauea Iki magma was supplied from Kīlauea’s summit magma reservoir (∼2–5 km depth). In contrast, olivine from Kapoho, which was the rift zone extension of the Kīlauea Iki eruption, crystallized over a much wider range of depths (∼1–16 km). The wider depth range requires magma transport during the Kapoho eruption from deep beneath the summit region and/or from deep beneath Kīlauea’s east rift zone. The deeply derived olivine crystals and their host magma mixed with stored, more evolved magma in the rift zone, and the mixture was later erupted at Kapoho.This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.sciencedirect.com/science/journal/00167037Keywords: Melt inclusions, Kīlauea, Geochemistry, Volcanology, Hawaiian geolog
The Atacama Cosmology Telescope: the stellar content of galaxy clusters selected using the Sunyaev-Zel'dovich effect
We present a first measurement of the stellar mass component of galaxy
clusters selected via the Sunyaev-Zel'dovich (SZ) effect, using 3.6 um and 4.5
um photometry from the Spitzer Space Telescope. Our sample consists of 14
clusters detected by the Atacama Cosmology Telescope (ACT), which span the
redshift range 0.27 < z < 1.07 (median z = 0.50), and have dynamical mass
measurements, accurate to about 30 per cent, with median M500 = 6.9 x 10^{14}
MSun. We measure the 3.6 um and 4.5 um galaxy luminosity functions, finding the
characteristic magnitude (m*) and faint-end slope (alpha) to be similar to
those for IR-selected cluster samples. We perform the first measurements of the
scaling of SZ-observables (Y500 and y0) with both brightest cluster galaxy
(BCG) stellar mass and total cluster stellar mass (M500star). We find a
significant correlation between BCG stellar mass and Y500 (E(z)^{-2/3} DA^2
Y500 ~ M*^{1.2 +/- 0.6}), although we are not able to obtain a strong
constraint on the slope of the relation due to the small sample size.
Additionally, we obtain E(z)^{-2/3} DA^2 Y500 ~ M500star^{1.0 +/- 0.6} for the
scaling with total stellar mass. The mass fraction in stars spans the range
0.006-0.034, with the second ranked cluster in terms of dynamical mass (ACT-CL
J0237-4939) having an unusually low total stellar mass and the lowest stellar
mass fraction. For the five clusters with gas mass measurements available in
the literature, we see no evidence for a shortfall of baryons relative to the
cosmic mean value.Comment: Accepted for publication in MNRAS; 12 pages, 10 figure
Cosmological Parameters from Pre-Planck CMB Measurements
Recent data from the WMAP, ACT and SPT experiments provide precise
measurements of the cosmic microwave background temperature power spectrum over
a wide range of angular scales. The combination of these observations is well
fit by the standard, spatially flat LCDM cosmological model, constraining six
free parameters to within a few percent. The scalar spectral index, n_s =
0.9690 +/- 0.0089, is less than unity at the 3.6 sigma level, consistent with
simple models of inflation. The damping tail of the power spectrum at high
resolution, combined with the amplitude of gravitational lensing measured by
ACT and SPT, constrains the effective number of relativistic species to be
N_eff = 3.28 +/- 0.40, in agreement with the standard model's three species of
light neutrinos.Comment: 5 pages, 4 figure
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