Ar/Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials     

Fernando Barra  Timothy D. Swindle  Randy L. Korotev  Bradley L. Jolliff  Ryan A. Zeigler  Eric Olson 《Geochimica et cosmochimica acta》2006,70(24):6016-6031

Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the ⁴⁰Ar/³⁹Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. ⁴⁰Ar/³⁹Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing.  相似文献   

Feldspathic lunar meteorites Pecora Escarpment 02007 and Dhofar 489: Contamination of the surface of the lunar highlands by post-basin impacts     

Randy L. Korotev  Ryan A. Zeigler  Bradley L. Jolliff 《Geochimica et cosmochimica acta》2006,70(24):5935-5956

PCA (Pecora Escarpment) 02007 and Dhofar 489 are both meteorites from the feldspathic highlands of the Moon. PCA 02007 is a feldspathic breccia consisting of lithified regolith from the lunar surface. It has concentrations of both incompatible and siderophile elements that are at the high end of the ranges for feldspathic lunar meteorites. Dhofar 489 is a feldspathic breccia composed mainly of impact-melted material from an unknown depth beneath the regolith. Concentrations of incompatible and siderophile elements are the lowest among brecciated lunar meteorites. Among 19 known feldspathic lunar meteorites, all of which presumably originate from random locations in the highlands, concentrations of incompatible elements like Sm and Th tend to increase with those of siderophile elements like Ir. Feldspathic meteorites with high concentrations of both suites of elements are usually regolith breccias. Iridium derives mainly from micrometeorites that accumulate in the regolith with duration of surface exposure. Micrometeorites have low concentrations of incompatible elements, however, so the correlation must reflect a three-component system. We postulate that the correlation between Sm and Ir occurs because the surface of the Feldspathic Highlands Terrane has become increasingly contaminated with time in Sm-rich material from the Procellarum KREEP Terrane that has been redistributed across the lunar surface by impacts of moderate-sized, post-basin impacts. The most Sm-rich regolith breccias among feldspathic lunar meteorites are about 3× enriched compared to the most Sm-poor breccias, but this level of enrichment requires only a few percent Sm-rich material typical of the Procellarum KREEP Terrane. The meteorite data suggest that nowhere in the feldspathic highlands are the concentrations of K, rare earths, and Th measured by the Lunar Prospector mission at the surface representative of the underlying “bedrock;” all surfaces covered by old regolith (as opposed to fresh ejecta) are at least slightly contaminated. Dhofar 489 is one of 15 paired lunar-meteorite stones from Oman (total mass of meteorite: 1037 g). On the basis of its unusually high Mg/Fe ratio, the meteorite is likely to have originated from northern feldspathic highlands.  相似文献   

Dip variations of the North American and North Caribbean Plates dominate the tectonic activity of Puerto Rico–Virgin Islands and adjacent areas     

Xiao Xu  G. Randy Keller  Xiaoyu Guo 《Geological Journal》2016,51(6):901-914

The M 7.0 Haiti earthquake of 2010 in the Greater Antilles is a reminder that the northeastern Caribbean is at a high risk for seismic and tsunami hazards. The Greater Antilles consist of the Hispaniola microplate to the west and Puerto Rico–Virgin Islands to the east and are situated between two subduction zones with the Puerto Rico Trench to the north and the Muertos Trough to the south. Although there is no active volcanism on Puerto Rico, earthquake depths and previous seismic tomography results imply that the slabs of Caribbean and North American Plates exist at depth. However, how far the east Muertos Trough subduction of the North Caribbean Plate has extended has not been fully addressed. In addition, the Puerto Rico–Virgin Islands are bounded by extensional regimes to both the west (Mona Rift) and east (Anegada Passage). The cause of the extension is still under debate. In this paper, we use new 3D seismic tomography and gravity data to carry out an integrated study of the geometry of the subducting slabs of the North American and North Caribbean Plates in the Puerto Rico–Virgin Islands area. The results indicate that both slabs have an increase of dip westward, which is strongly controlled by the subduction rollback of the North American Plate. These variations affected the tectonic evolution of the Puerto Rico–Virgin Islands. Thus, the results of this research advance our understanding of the kinematic evolution of the Puerto Rico–Virgin Islands and associated natural hazards. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Strategies for waveform processing in sparker data     

Mathieu J. Duchesne  Gilles Bellefleur  Mike Galbraith  Randy Kolesar  Rick Kuzmiski 《Marine Geophysical Researches》2007,28(2):153-164

A sparker is a marine seismic impulsive source used for high-resolution seismic surveys. Sparker sources were very popular during the late 1960s and 1970s before being supplanted by small volume airguns. However, in the last 10 years there has been renewed interest in sparker technology because (1) it can be easily deployed at relatively low costs and (2) in certain areas the use of small airguns is restricted for environmental purposes. In this study a sparker source was used to assess the seismic stratigraphy of Quaternary deposits and to image the sediment/bedrock interface. Three different inverse filtering methods were tested (i.e., spiking deconvolution, match-filtering and vertical seismic profile (VSP) deconvolution) to correct the poor shot-to-shot repeatability of the source and to compress its reverberations. Results show that the matched-filter and VSP deconvolution methods, which design and apply one operator for each shot, produced comparable results, whereas the spiking deconvolution that used the same operator on all traces failed to compress the source signature properly.  相似文献   

Response of temperate lakes to drought: a paleolimnological perspective on the landscape position concept using diatom-based reconstructions     

Courtney R. Wigdahl-Perry  Jasmine E. Saros  Jennifer Schmitz  Randy Calcote  James Rusak  Dennis Anderson  Sara Hotchkiss 《Journal of Paleolimnology》2016,55(4):339-356

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Reply to the comment by Zhang et al. on: “First finding of A-type and adakitic magmatism association in Songpan–Garze fold belt, eastern Tibetan Plateau: Implication for lithospheric delamination”     

Zhang Hong-Fei  Randy Parrish  Li Zhang  Wang-Chuan Xu  Hong-Ling Yuan  Shan Gao  Quentin G. Crowley 《Lithos》2008,103(3-4):565-568

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PASSEQ 2006–2008: Passive seismic experiment in Trans-European Suture Zone     

M. Wilde-Piórko  W. H. Geissler  J. Plomerová  M. Grad  V. Babuška  E. Brückl  J. Cyziene  W. Czuba  R. England  E. Gaczyński  R. Gazdova  S. Gregersen  A. Guterch  W. Hanka  E. Hegedűs  B. Heuer  P. Jedlička  J. Lazauskiene  G. Randy Keller  R. Kind  K. Klinge  P. Kolinsky  K. Komminaho  E. Kozlovskaya  F. Krüger  T. Larsen  M. Majdański  J. Málek  G. Motuza  O. Novotný  R. Pietrasiak  Th. Plenefisch  B. Růžek  S. Sliaupa  P. Środa  M. Świeczak  T. Tiira  P. Voss  P. Wiejacz 《Studia Geophysica et Geodaetica》2008,52(3):439-448

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DyeLIF™: A New Direct‐Push Laser‐Induced Fluorescence Sensor System for Chlorinated Solvent DNAPL and Other Non‐Naturally Fluorescing NAPLs          下载免费PDF全文

Murray Einarson  Adrian Fure  Randy St. Germain  Steven Chapman  Beth Parker 《Ground Water Monitoring & Remediation》2018,38(3):28-42

DyeLIF? is a new version of laser‐induced fluorescence (LIF) for high‐resolution three‐dimensional subsurface mapping of nonaqueous phase liquids (NAPLs) in the subsurface. DyeLIF eliminates the requirement that the NAPL contains native fluorophores (such as those that occur in compounds like polynuclear aromatic hydrocarbons [PAHs]) and can therefore be used to detect chlorinated solvents and other nonfluorescing NAPLs that had previously been undetectable with conventional LIF tools. With DyeLIF, an aqueous solution of water and nontoxic hydrophobic dye is continuously injected ahead of the sapphire detection window while the LIF probe is being advanced in the subsurface. If soil containing NAPL is penetrated, the injected dye solvates into the NAPL within a few milliseconds, creating strong fluorescence that is transmitted via fiber‐optic filaments to aboveground optical sensors. A detailed field evaluation of the novel DyeLIF technology was performed at a contaminated industrial site in Lowell, Massachusetts, USA where chlorinated solvent dense nonaqueous phase liquid (DNAPL) persists below the water table in sandy sediments. Continuously cored boreholes were drilled adjacent to 5 of 30 DyeLIF probes that were advanced at that site. The cores were subsampled in high resolution to generate discrete‐depth soil samples as splits at the same depths where DNAPL was detected in the colocated DyeLIF probes. The cores were analyzed above ground using (1) colorimetric screening using hydrophobic dye tests, (2) laboratory extraction and quantitative chemical analysis, (3) “Benchtop” DyeLIF, and (4) volumetric moisture content. Correlation between DyeLIF and aboveground analyses of the soil cores was excellent: 98% agreement with positive DNAPL detections in samples where DNAPL pore saturations were >0.7% (based on quantitative soil analyses) and the ex situ tests. DyeLIF produced the equivalent of one aboveground colorimetric dye test every 0.2 inch (0.5 cm) of probing. With average daily probing of 395 linear feet (120.4 m), this was the equivalent of 12,039 discrete‐depth colorimetric dye tests/day. Because DyeLIF is an in situ measurement, there are no issues with soil core recovery like there would be for conventional ex situ colorimetric dye tests and 100% characterization of the probed intervals is achieved. Tracking the injection rate and pressure of the dye solution provides simultaneous data regarding relative soil permeability, similar to other direct push (DP) hydraulic profiling tools. Conventional LIF is considered the premier DP tool to identify and map NAPL containing PAHs in the subsurface or confirm its absence. While chlorinated solvent DNAPLs at some field sites contain impurities (e.g., solvated greases or oils) that make them detectable with conventional LIF techniques, at other sites, the DNAPL cannot be detected with conventional LIF. At such sites, the injection of a hydrophobic dye ahead of the sapphire window with the DyeLIF system now makes the LIF technology applicable to the many types of NAPLs that were previously invisible using conventional LIF techniques.  相似文献   

Radar observations and a physical model of contact binary Asteroid 4486 Mithra     

Marina Brozovic  Lance A.M. Benner  Steven J. Ostro  Jon D. Giorgini  Jean-Luc Margot  Randy Rose 《Icarus》2010,208(1):207-220

Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) delay-Doppler radar images obtained in July and August of 2000 reveal that 4486 Mithra is an irregular, significantly bifurcated object, with a central valley ∼380 m deep and a long axis potentially exceeding 2 km. With its bimodal appearance, Mithra is a strong candidate for a contact binary asteroid. Sequences of Goldstone images spanning up to 3 h per day show very little rotation and establish that Mithra is an unusually slow rotator. We used Goldstone and Arecibo data to estimate Mithra’s 3D shape and spin state. We obtain prograde (λ = 337°, β = 19°) and retrograde (λ = 154°, β = −19°) models that give comparable fits, have very similar shapes roughly resembling an hourglass, and have a rotation period of 67.5 ± 6.0 h. The dimensions of these two models are very similar; for the prograde solution the maximum dimensions are X = 2.35 ± 0.15 km, Y = 1.65 ± 0.10 km, Z = 1.44 ± 0.10 km. Dynamical analysis of our models suggests that in the past, Mithra most likely went through a period of even slower rotation with its obliquity close to 90°. The spin rate is predicted to be increasing due to thermal torque (YORP), while the obliquity, which is currently +68° and +106° for the prograde and retrograde models, respectively, is predicted to move away from 90°.  相似文献   

Mineralogy, geochemistry, and Ar-Ar geochronology of lunar granulitic breccia Northwest Africa 3163 and paired stones: Comparisons with Apollo samples     

Jillian A. Hudgins  Randy L. Korotev 《Geochimica et cosmochimica acta》2011,75(10):2865-2881

The lunar meteorites Northwest Africa (NWA) 3163, 4881, and 4483 are paired stones classified as granulitic breccias. At 2.4 kg, these three stones constitute one of the largest known lunar meteorite masses. Here we describe the petrography, mineralogy, and chemistry of NWA 3163, 4881, and 4483, and present ⁴⁰Ar-³⁹Ar data for two of the meteorites. Two-pyroxene thermometry indicates that the rocks equilibrated at 1050 ± 50 °C, which represents the high-temperature, low-pressure event that generated their characteristic recrystallization textures and reset their Ar systematics. Stepped-heating, in situ infrared laser microprobe ⁴⁰Ar-³⁹Ar geochronology yields a mean age of 3327 ± 29 Ma for NWA 3163, and a more disturbed release spectrum for NWA 4881. NWA 4881 shows an upward-trending pattern, suggesting that it may have had a ⁴⁰Ar-³⁹Ar age of >3.0 Ga, but that it was partially reset at ∼2.6 Ga. NWA 3163 et al. exhibit shock effects, including maskelynitized plagioclase, shock veins, and melt pockets, which are absent in the Apollo granulitic breccias. Although the Apollo and meteorite samples are texturally similar and have comparable bulk compositions and equilibration temperatures, their trace and siderophile element contents point to distinct parental lithologies derived from different regions of the Moon. Based on mineralogical and geochemical differences between the Apollo and meteorite samples, we conclude that the parent rock(s) of the paired NWA meteorites came from an area outside the Imbrium region and that they underwent high-temperature (granulite event) metamorphism long after the Late Heavy Bombardment.  相似文献