New multiply-lensed galaxies identified in ACS/NIC3 observations of Cl0024+1654 using an improved mass model     

Adi Zitrin  Tom Broadhurst  Keiichi Umetsu  Dan Coe  Narciso Benítez  Begoña Ascaso  Larry Bradley  Holland Ford  James Jee  Elinor Medezinski  Yoel Rephaeli  Wei Zheng 《Monthly notices of the Royal Astronomical Society》2009,396(4):1985-2002

We present an improved strong-lensing analysis of Cl0024+1654  ( z = 0.39)  using deep Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS)/NIC3 images, based on 33 multiply-lensed images of 11 background galaxies. These are found with a model that assumes mass approximately traces light, with a low-order expansion to allow for flexibility on large scales. The model is constrained initially by the well-known five-image system  ( z = 1.675)  and refined as new multiply-lensed systems are identified using the model. Photometric redshifts of these new systems are then used to constrain better the mass profile by adopting the standard cosmological relation between redshift and lensing distance. Our model requires only six free parameters to describe well all positional and redshift data. The resulting inner mass profile has a slope of  d log  M /d log  r ≃−0.55  , consistent with new weak-lensing measurements where the data overlap, at   r ≃ 200 kpc/ h ₇₀  . The combined profile is well fitted by a high-concentration Navarro, Frenk & White (NFW) mass profile,   C _vir∼ 8.6 ± 1.6  , similar to other well-studied clusters, but larger than predicted with standard Λ cold dark matter (ΛCDM). A well-defined radial critical curve is generated by the model and is clearly observed at   r ≃ 12  arcsec, outlined by elongated images pointing towards the centre of mass. The relative fluxes of the multiply-lensed images are found to agree well with the modelled magnifications, providing an independent consistency check.  相似文献   

Ancient graphite in the Eoarchean quartz-pyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic characterization     

Dominic Papineau  Bradley T. De Gregorio  Marc D. Fries  Stephen J. Mojzsis  Rhonda M. Stroud 《Geochimica et cosmochimica acta》2010,74(20):5862-5883

Because all known Eoarchean (>3.65 Ga) volcano-sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation, the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate. To determine the biogenicity of carbon in graphite in such rocks, carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon. Here we report the petrological and spectroscopic characteristics of carbonaceous material, typically associated with individual apatite grains, but also with various other minerals including calcite, in a >3.83 Ga granulite-facies ferruginous quartz-pyroxene unit (Qp rock) from the island of Akilia in southern West Greenland. In thin sections of the fine-grained parts of Akilia Qp rock sample G91-26, mapped apatites were found to be associated with graphite in about 20% of the occurrences. Raman spectra of this carbonaceous material had strong G-band and small D-band absorptions indicative of crystalline graphite. Three apatite-associated graphites were found to contain curled graphite structures, identified by an anomalously intense second-order D-band (or 2D-band) Raman mode. These structures are similar to graphite whiskers or cones documented to form at high temperatures. Raman spectra of apatite-associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 °C, which are consistent with granulite-facies metamorphic conditions. Three graphite targets extracted by focused ion beam (FIB) methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and Raman spectroscopy. TEM analyses of graphite in these FIB sections showed a (0 0 0 2) interplanar spacing between 3.41 and 3.64 Å for apatite-associated graphite, which is larger than the spacing of pure graphite (3.35 Å) and may be caused by the presence of non-carbon heteroatoms in interlayer sites. Samples analyzed by synchrotron-based scanning transmission X-ray microscopy (STXM) also confirmed the presence of crystalline graphite, but abundances of N and O heteroatoms were below detection limit for this method. Graphite in the Akilia Qp rock was also found to occur in complex polyphase mineral assemblages of hornblende ± calcite ± sulfides ± magnetite that point to high-temperature precipitation from carbon-bearing fluids. These complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite-facies metamorphic event at 2.7 Ga. Several observations point to graphitization from high-temperature fluid-deposition for some of the Akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite, but ambiguities remain for the origin of this carbon.  相似文献   

Isotopic anomalies in organic nanoglobules from Comet 81P/Wild 2: Comparison to Murchison nanoglobules and isotopic anomalies induced in terrestrial organics by electron irradiation     

Bradley T. De Gregorio  Rhonda M. Stroud 《Geochimica et cosmochimica acta》2010,74(15):4454-1135

Nanoglobules are a form of organic matter found in interplanetary dust particles and primitive meteorites and are commonly associated with ¹⁵N and D isotopic anomalies that are suggestive of interstellar processes. We report the discovery of two isotopically-anomalous organic globules from the Stardust collection of particles from Comet 81P/Wild 2 and compare them with nanoglobules from the Murchison CM2 meteorite. One globule from Stardust Cometary Track 80 contains highly aromatic organic matter and a large ¹⁵N anomaly (δ¹⁵N = 1120‰). Associated, non-globular, organic matter from this track is less enriched in ¹⁵N and contains a mixture of aromatic and oxidized carbon similar to bulk insoluble organic material (IOM) from primitive meteorites. The second globule, from Cometary Track 2, contains non-aromatic organic matter with abundant nitrile (CN) and carboxyl (COOH) functional groups. It is significantly enriched in D (δD = 1000‰) but has a terrestrial ¹⁵N/¹⁴N ratio. Experiments indicate that similar D enrichments, unaccompanied by ¹⁵N fractionation, can be reproduced in the laboratory by electron irradiation of epoxy or cyanoacrylate. Thus, a terrestrial origin for this globule cannot be ruled out, and, conversely, exposure to high-energy electron irradiation in space may be an important factor in producing D anomalies in organic materials. For comparison, we report two Murchison globules: one with a large ¹⁵N enrichment and highly aromatic chemistry analogous to the Track 80 globule and the other only moderately enriched in ¹⁵N with IOM-like chemistry. The observation of organic globules in Comet 81P/Wild 2 indicates that comets likely sampled the same reservoirs of organic matter as did the chondrite parent bodies. The observed isotopic anomalies in the globules are most likely preserved signatures of low temperature (<10 K) chemistry in the interstellar medium or perhaps the outer regions of the solar nebula. In other extraterrestrial samples, D isotopic anomalies, but not those of ¹⁵N, may be explained in part by exposure to ionizing electron radiation.  相似文献   

Highly N-enriched chondritic clasts in the CB/CH-like meteorite Isheyevo     

L. Bonal  G.R. Huss  K. Nagashima  J.P. Bradley 《Geochimica et cosmochimica acta》2010,74(22):6590-6609

The metal-rich carbonaceous chondrites (CB and CH) have the highest whole-rock ¹⁵N-enrichments (δ¹⁵N up to 1500‰) among planetary materials. They are also characterized by the absence of interchondrule fine-grained matrix. The only fine-grained material is present as lithic clasts, which experienced extensive aqueous alteration in contrast to the surrounding high-temperature components (chondrules, refractory inclusions, metal grains). Hence, the clasts are foreign objects that were incorporated at a late stage into the final parent body of Isheyevo. Their origin is poorly constrained. Based on mineralogy, petrography, and thermal processing of the aromatic carbonaceous component, different types of clasts have been previously identified in the CB/CH-like chondrite Isheyevo. Here, we focus on the rare lithic clasts characterized by the presence of anhydrous silicates (chondrules, chondrule fragments, and CAIs). Their mineralogy and oxygen isotopic compositions reveal them to be micro-chondrules, fragments of chondrules, and refractory inclusions related to those in the Isheyevo host, suggesting accretion in the same region. In contrast to previously studied IDPs or primitive chondritic matrices, the fine-grained material in the clasts we studied is highly and rather uniformly enriched in heavy nitrogen, with bulk δ¹⁵N values ranging between 1000‰ and 1300‰. It is also characterized by the presence of numerous ¹⁵N hotspots (δ¹⁵N ranging from 1400‰ to 4000‰). No bulk (δD <-240‰) or localized deuterium enrichments were observed. These clasts have the highest bulk enrichment in heavy nitrogen measured to date in a fine-grained material. They represent a unique material, of asteroidal or cometary origin, in our collection of cosmomaterials. We show that they were ¹⁵N-enriched before their incorporation in the final parent body of Isheyevo. They experienced an extensive aqueous alteration that most likely played a role in redistributing ¹⁵N over the whole fine-grained material and may have significantly modified its initial hydrogen isotopic composition. Based on a review of isotopic fractionation models, we conclude that the nitrogen isotopic fractionation process, its timing, and its location are still poorly constrained. The ¹⁵N-rich clasts may represent the surviving original carrier of the ¹⁵N anomaly in Isheyevo whole-rock.  相似文献   

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway     

Bradley R. Hacker  Torgeir B. Andersen  Scott Johnston  Andrew R.C. Kylander-Clark  Emily M. Peterman  Emily O. Walsh  David Young 《Tectonophysics》2010,480(1-4):149-171

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.  相似文献   

天然石棉矿床地质学及其在公共卫生政策中的应用(英文)          下载免费PDF全文

Bradley S. Van Gosen 《中国地质》2010,37(3):704-711

20世纪,越来越多的证据使人们意识到,吸入含石棉的灰尘会使从事与石棉有关的几种职业(如石棉采矿、造船和石棉产品组装)的工人患上几种严重的呼吸道疾病(石棉沉着病,肺癌和mesotheliuma)。为此已经制定了若干规定,以控制工人们在特殊的制造、采矿和其他工作地点与石棉尘埃的接触。比较间接的是控制和管理"天然产生的石棉"(NOA),这一问题近年来也引起了管理机构、健康机构和居民团体的注意。NOA包括在自然状态下原地找到的被描述为石棉的矿物,例如在基岩或土壤中的这种矿物。NOA之所以引起关注,是因为如果含石棉的岩石受到自然侵蚀或人类活动(例如修路、城市开挖、农业、采矿、压碎和碾磨)的影响,就有可能暴露并变成空中尘埃的微小纤维。天然石棉矿床的规模差异很大,从薄的四散的细脉一直到大的矿体。它们的地理分布与地质条件直接有关。石棉产出的地质背景是含镁丰富的主岩,这些岩石受到低压低温的变质作用蚀变而成。已知的容矿主岩包括变质的和已被交代的超基性岩(尤其是蛇纹岩)、一些基性火成岩、变质白云岩和变质的铁建造。石棉形状的角闪石在几种热液蚀变型的碱性火成侵入体中也能作为副矿物存在。石棉形成于可预见和可鉴别的地质环境。这种认识是一种信息,而这种信息使公众健康机构可以执行有情报根据的屏蔽和管理规划。第一步是在区域的或国家规模的尺度上确定和描述已知的(报道过的)石棉矿床。第二步是对容纳已知石棉矿的单位进行填图,以划定可能的其他石棉矿化的范围。这种地质研究使有关机构对遇到石棉的可能性做出规划,而尽量节省在不太可能遇到石棉矿的地区的管理费用。对石棉地质条件的认识可以事先应用于岩石和土壤的挖掘项目,而不管挖掘的规模有多大。这种对石棉管理的科学探索能有助于缓解对挖掘所发现的自然石棉矿床的连续不断地做出反应。因此,通过对很可能含有石棉矿床的地区做填图,规划者可开发出一些控制尘埃的程序,这些程序适用于工人和附近居民最容易暴露于空气携带的石棉的地区。  相似文献   

Observations of metallic species in Mercury’s exosphere     

Rosemary M. Killen  Andrew E. Potter  E. Todd Bradley  Carrie M. Anderson 《Icarus》2010,209(1):75-87

From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury’s exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury’s exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K.  相似文献   

Monte Carlo modeling of sodium in Mercury’s exosphere during the first two MESSENGER flybys     

Matthew H. Burger  Rosemary M. Killen  E. Todd Bradley  Menelaos Sarantos  Nelly Mouawad 《Icarus》2010,209(1):63-74

We present a Monte Carlo model of the distribution of neutral sodium in Mercury’s exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to ∼10⁶-10⁷ cm⁻² s⁻¹, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.  相似文献   

Water ice cloud formation on Mars is more difficult than presumed: Laboratory studies of ice nucleation on surrogate materials     

Laura T. Iraci  Bruce D. Phebus  Bradley M. Stone  Anthony Colaprete 《Icarus》2010,210(2):985-991

The role of water ice clouds in the martian water cycle and climate depends on cloud properties such as particle size and number distribution. These properties, in turn, depend on heterogeneous nucleation parameters which are poorly understood. Here we report laboratory experiments performed under martian temperature and water partial pressure conditions (158–185 K, 9 × 10⁻⁷–1 × 10⁻⁴ Torr H₂O) to determine the critical saturation ratio for ice onset, S_crit, as a function of temperature and dust composition. Using infrared spectroscopy to monitor ice nucleation and growth, we find a significant barrier to ice formation, with a pronounced temperature dependence. Even on clay minerals which show uptake of non-crystalline water before ice nucleation, we find a saturation ratio of 2.5 or more (RH_ice > 250%) is needed to begin ice growth at temperatures near 160 K. These results could lead to changes of four orders of magnitude in the nucleation rate relative to the presumptions used currently in Mars microphysical models, which commonly set the contact parameter, m, to a single value of 0.95. Our results range from m = 0.84 to m = 0.98. For ice nucleation on Arizona Test Dust, the temperature dependence is described by m = 0.0046 * T_nucl + 0.1085, while m = 0.0055 * T_nucl + 0.0003 on a smectite-rich clay sample. Our findings suggest that cloud formation will be more difficult than previously thought, potentially leading to areas of increased near-surface humidity but generally drier conditions in the atmosphere of Mars, overall.  相似文献   

Preface   总被引：1，自引：0，他引：1  

Raymond S. Bradley 《Climatic change》1999,43(1):1-2

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