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Brian R. Jicha Clark M. Johnson Wes Hildreth Brian L. Beard Garret L. Hart Steven B. Shirey Brad S. Singer 《Earth and Planetary Science Letters》2009,277(1-2):38-49
A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) > 1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization. 相似文献
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M. Magliocchetti S. J. Maddox J. V. Wall C. R. Benn G. Cotter 《Monthly notices of the Royal Astronomical Society》2000,318(4):1047-1067
We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼18.6, corresponding to z ∼0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15≲ z ≲0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05≲ z ≲0.2; starbursting galaxies are a small fraction (∼6 per cent), and are very nearby ( z ≲0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z ≳0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts ( z ∼0.2).
Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N ( z ) at S1.4 GHz ≥1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ≲5 per cent of the radio population at S ∼1 mJy. 相似文献
Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N ( z ) at S
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We introduce a mixed discontinuous/continuous finite element pair for ocean modelling, with continuous quadratic layer thickness and discontinuous velocity. We investigate the finite element pair applied to the linear shallow-water equations on an f-plane. The element pair has the property that all geostrophically balanced states which strongly satisfy the boundary conditions have discrete divergence equal to exactly zero and hence are exactly steady states of the discretised equations. This means that the finite element pair has excellent geostrophic balance properties. We also show that the element pair applied to the non-rotating linear shallow-water equations does not have any spurious small eigenvalues. We illustrate these properties using numerical tests and provide convergence calculations which show that the numerical solutions have errors which decay quadratically with element edge length for both velocity and layer thickness. 相似文献
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R. C. Bolton C. J. Chandler G. Cotter T. J. Pearson G. G. Pooley A. C. S. Readhead J. M. Riley E. M. Waldram 《Monthly notices of the Royal Astronomical Society》2006,367(1):323-330
In this paper, we present subarcsecond resolution observations of 36 compact sources from the 15h region of the 15-GHz 9th Cambridge survey. These sources all have previously measured simultaneous continuum radio spectra spanning 1.4–43 GHz and we classify each source by fitting a quadratic function to its spectrum. Using the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array, both at 5 GHz, we resolve all six steep-spectrum objects and four of the 13 flat-spectrum objects. However, none of the 16 objects with convex spectra peaking above 2.5 GHz is resolved even at <3-mas resolution. These results, in combination with the findings of a 15-GHz variability study, suggest that emission from the high-frequency peaking objects is affected by relativistic beaming, and that these objects are not necessarily as young as the synchrotron self-absorption interpretation of their peak frequencies would imply. 相似文献
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Brian R. Jicha Garret L. Hart Clark M. Johnson Wes Hildreth Brian L. Beard Steven B. Shirey John W. Valley 《Contributions to Mineralogy and Petrology》2009,157(2):189-207
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and
its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources,
a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns
for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade
arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators
of slab-fluids such as (Sr/P)
n
and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle
source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification
is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most
Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and
isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass
of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization
in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher
than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ
18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding
involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope
compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile
arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. 相似文献
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