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111.
Seth A. TEITLER Julie M. PAQUE Jeffrey N. CUZZI Robert C. HOGAN 《Meteoritics & planetary science》2010,45(7):1124-1135
Abstract– The variation in sizes of chondrules from one chondrite to the next is thought to be due to some sorting process in the early solar nebula. Hypotheses for the sorting process include chondrule sorting by mass and sorting by some aerodynamic mechanism; one such aerodynamic mechanism is the process of turbulent concentration (TC). We present the results of a series of statistical tests of chondrule data from several different chondrites. The data do not clearly distinguish between various options for the sorting parameter, but we find that the data are inconsistent with being drawn from lognormal or (three‐parameter) Weibull distributions in chondrule radius. We also find that all but one of the chondrule data sets tested are consistent with being drawn from the TC distribution. 相似文献
112.
EC?11481–2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous optical spectroscopy revealed that it is a sdOB star with T eff=41?790 K, log?g=5.84, and He/H = 0.014 by number. We present an on-going spectral analysis by means of non-LTE model-atmosphere techniques based on high-resolution, high-S/N optical (VLT-UVES) and ultraviolet (FUSE, IUE) observations.We are able to reproduce the optical and UV observations simultaneously with a chemically homogeneous NLTE model atmosphere with a significantly higher effective temperature and lower He abundance (T eff=55?000 K, log?g=5.8, and He/H=0.0025 by number). While C, N, and O appear less than 0.15 times solar, the iron-group abundance is strongly enhanced by at least a factor of ten. 相似文献
113.
Migrating magmatism in the northern US Cordillera: in situ U–Pb geochronology of the Idaho batholith
Richard M. Gaschnig Jeffrey D. Vervoort Reed S. Lewis William C. McClelland 《Contributions to Mineralogy and Petrology》2010,159(6):863-883
New in situ laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe U–Pb
geochronology of zircons from the Idaho batholith and spatially overlapping Challis intrusions reveals a series of discrete
magmatic belts of different ages and compositions. Following the accretion of the Blue Mountains province to North America
along the Salmon River suture zone, two compositionally diverse belts of metaluminous plutons formed both adjacent to the
suture and well inboard of it. These were constructed from ~100 to 85 Ma and were followed by a voluminous pulse of peraluminous
magmatism, forming the bulk of the Atlanta lobe and largest fraction of the batholith between ~80 and 67 Ma. Around 70 Ma,
a later and more spatially restricted suite of metaluminous plutons formed around the Bitterroot lobe of the batholith. This
was followed by another pulse of voluminous peraluminous magmatism in the Bitterroot lobe, lasting from ~66 to 54 Ma. The
changes from low volume metaluminous to high volume peraluminous magmatism may reflect a combination of changes in the angle
and segmentation of the subducting Farallon plate and over thickening of the continental lithosphere. All of these features
were then cut by plutons and dikes associated with the Challis volcanic field, lasting from ~51 to 43 Ma. Inherited components
are pervasive in zircons from most phases of the batholith. While Precambrian components are very common, zircons also often
contain cores or mantles that are 5–20 million years older than their rims. This suggests that the early phases of the batholith
were repeatedly cannibalized by subsequent magmas. This also implies that the older suites may have been originally more aerially
extensive than their currently exposed forms. 相似文献
114.
Despite its location on sediment-free basalt, vent fluids from the Main Endeavour Field (MEF) contain chemical species that indicate fluids have interacted with sediments during circulation. We report on the distribution and isotopic abundances of organic compounds (C1-C3 alkanes and alkenes, benzene and toluene) in fluids collected from the Main Endeavour Field (MEF) in July, 2000, to understand the processes that regulate their abundances and characterize fluid sources. Aqueous organic compounds are derived from the thermal alteration of sedimentary organic matter and subsequently undergo further oxidation reactions during fluid flow. Fluid:sediment mass ratios calculated using ΣNH4 concentrations indicate that the sediments are distal to the MEF, resulting in a common reservoir of fluids for all of the vents. Following the generation from sediment alteration, aqueous organic compounds undergo secondary alteration reactions via a stepwise oxidation reaction mechanism. Alkane distributions and isotopic compositions indicate that organic compounds in MEF fluids have undergone a greater extent of alteration as compared to Middle Valley fluids, either due to differences in subsurface redox conditions or the residence time of fluids at subsurface conditions. The distributions of the aromatic compounds benzene and toluene are qualitatively consistent with the subsurface conditions indicated by equilibration of aqueous alkanes and alkanes. However, benzene and toluene do not achieve chemical equilibrium in the subsurface. Methane and CO2 also do not equilibrate chemically or isotopically at reaction zone temperatures, a likely result of an insufficient reaction time after addition of CO2 from magmatic sources during upflow. The organic geochemistry supports the assumption that the sediments with which MEF fluids interact has the same composition as sediments present in Middle Valley itself, and highlight differences in subsurface reaction zone conditions and fluid flow pathways at these two sites. 相似文献
115.
Hao Cheng S. Andrew DuFrane Jeffrey D. Vervoort Eizo Nakamura Qiuli Li Zuyi Zhou 《Lithos》2010,117(1-4):82-98
Low-temperature and high-pressure eclogites with an oceanic affinity in the western part of the Dabie orogen have been investigated with combined Lu–Hf and U–Pb geochronology. These eclogites formed over a range of temperatures (482–565 °C and 1.9–2.2 GPa). Three eclogites, which were sampled from the Gaoqiao country, yielded Lu–Hf ages of 240.7 ± 1.2 Ma, 243.3 ± 4.1 Ma and 238.3 ± 1.2 Ma, with a corresponding lower-intercept U–Pb zircon age of 232 ± 26 Ma. Despite the well-preserved prograde major- and trace-element zoning in garnets, these Lu–Hf ages mostly reflect the high-pressure eclogite-facies metamorphism instead of representing the early phase of garnet growth due to the occurrence of omphacite inclusions from core to rim and the shell effect. An upper-intercept zircon U–Pb age of 765 ± 24 Ma is defined for the Gaoqiao eclogite, which is consistent with the weighted-mean age of 768 ± 21 Ma for the country gneiss. However, the gneiss has not been subjected to successive high-pressure metamorphism. The new Triassic ages are likely an estimate of the involvement of oceanic fragments in the continental subduction. 相似文献
116.
117.
Jérôme Gattacceca Francis M. McCubbin Jeffrey N. Grossman Devin L. Schrader Nancy L. Chabot Massimo D'Orazio Cyrena Goodrich Ansgar Greshake Juliane Gross Katherine Helen Joy Mutsumi Komatsu Bingkui Miao 《Meteoritics & planetary science》2023,58(6):901-904
Meteoritical Bulletin 111 contains the 3094 meteorites approved by the Nomenclature Committee of the Meteoritical Society in 2022. It includes 11 falls (Antonin, Botohilitano, Cranfield, Golden, Great Salt Lake, Longde, Msied, Ponggo, Qiquanhu, Tiglit, Traspena), with 2533 ordinary chondrites, 165 HED, 123 carbonaceous chondrites (including 4 ungrouped), 82 lunar meteorites, 28 Rumuruti chondrites, 27 iron meteorites, 23 ureilites, 22 mesosiderites, 22 Martian meteorites, 21 primitive achondrites (one ungrouped), 17 ungrouped achondrites, 13 pallasites, 7 enstatite achondrites, 6 enstatite chondrites, and 5 angrites. Of the meteorites classified in 2022, 1787 were from Antarctica, 1078 from Africa, 180 from South America, 34 from Asia, 6 from North America, 4 from Europe, and 1 from Oceania. 相似文献
118.
119.
In September 2011 and March 2012, benthic nutrient fluxes were measured in the San Francisco Bay Delta, across a gradient from above the confluence of the Sacramento and San Joaquin Rivers to Suisun Bay. Dark and illuminated core incubation techniques were used to measure rates of denitrification, nutrient fluxes (phosphate, ammonium, nitrate), and oxygen fluxes. While benthic nutrient fluxes have been assessed at several sites in northern San Francisco Bay, such data across a Delta–Bay transect have not previously been determined. Average September rates of DIN (nitrate, nitrite, ammonium) flux were net positive across all sites, while March DIN flux indicated net uptake of DIN at some sites. Denitrification rates based on the N2/Ar ratio approach were between 0.6 and 1.0 mmol m?2 day?1, similar to other mesotrophic estuarine sediments. Coupled nitrification–denitrification was the dominant denitrification pathway in September, with higher overlying water nitrate concentrations in March resulting in denitrification driven by nitrate flux into the sediments. Estimated benthic microalgal productivity was variable and surprisingly high in Delta sediments and may represent a major source of labile carbon to this ecosystem. Variable N/P stoichiometry was observed in these sediments, with deviations from Redfield driven by processes such as denitrification, variable light/dark uptake of nutrients by microalgae, and adsorption of soluble reactive phosphorus. 相似文献
120.
Christian Koeberl Jack B. Hartung Michael J. Kunk Jeffrey Klein Jun-Ichi Matsuda Keisuke Nagao Wolf Uwe Reimold Dieter Storzer 《Meteoritics & planetary science》1993,28(2):204-212
Abstract The well-preserved 2.5 km diameter Roter Kamm impact crater is located in the Namib desert in Namibia. The impact has occurred in Precambrian granitic and granodioritic orthogneisses of the 1200–900 Ma old Namaqualand Metamorphic Complex which were partly covered by Gariep metasediments; the granites are invaded by quartz veins and quartz-feldspar-pegmatites. Previous geological field evidence suggested a crater age of about 5–10 Ma. In order to constrain this age, we selected a set of basement rocks (granites, granodiorites) exposed at the crater rim and studied the Rb-Sr, K-Ar, 40Ar-39Ar, and 10Be-26Al isotopic systems as well as apatite fission track ages of these samples. The Rb-Sr isotopic systematics confirm the derivation of these samples from the Namaqualand basement (age about 1.29 Ga), which underwent Damaran orogenesis at about 650 Ma. No basement rocks with Rb-Sr ages younger than about 410 Ma were identified. The K-Ar ages of pseudotachylite and melt breccia samples show that these samples are dominated by incompletely degassed fragments of basement rocks, with some retaining their original metamorphic ages of about 470 Ma. The apatite fission track ages range from 20–28 Ma, which may be interpreted as an extension of the 25 Ma Burdigalian peneplanation event, or as incomplete resetting of the apatite fission tracks during the impact event. The 10Be and 26Al exposure age of a quartz sample isolated from a quartz-pegmatite was found to be 150 ka; it is likely that the exposure of the sample began after material covering it had been removed by erosion 150 ka ago. Two glassy fractions extracted from a rim granite were dated by 40Ar-39Ar analysis. One sample gives practically a plateau age of 3.7 ± 0.3 Ma, while the other gives a minimum age of 3.6 Ma. The best available age estimate for the Roter Kamm crater is therefore 3.7 ± 0.3 Ma. 相似文献