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101.
Tarryn Aucamp Geoffrey H. Howarth Chad J. Peel Gelu Costin James M. D. Day Petrus le Roux James M. Scott Ansgar Greshake Rainer Bartoschewitz 《Meteoritics & planetary science》2023,58(11):1654-1676
The Dar al Gani (DaG) olivine-phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk-rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi-stage formation model for the DaG shergottites and other 1.1 Ma ejection-paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re-equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth. 相似文献
102.
Silicon isotope fractionation during magmatic differentiation 总被引:3,自引:0,他引:3
Paul S. Savage R. Bastian Georg Kevin W. Burton Alex N. Halliday 《Geochimica et cosmochimica acta》2011,75(20):6124-6139
The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ30Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO2 content of ∼60 wt.%, the predicted δ30Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle. 相似文献
103.
Timothy O. Lawrence Chad T. Ritter Janos Gergely 《Geotechnical and Geological Engineering》2011,29(5):919-926
Over fifty percent of our nation’s 560,000-plus bridges are obsolete or structurally deficient. To upgrade or replace these
structures would require a serious budget. In order to shorten this list of bridges and to improve their ratings, state DOTs
are investigating new repair techniques and better performance evaluation tools in order to implement the new drive to sustainability
within the infrastructure. The research presented herein focused on the evaluation of current US bridge analysis and rating
procedures for the North Carolina Department of Transportation (NCDOT). To evaluate these procedures, several bridges were
rated with commercially available software and a newly developed spreadsheet program. In parallel, the bridges were also load
tested for comparison, from which, it was concluded that the NCDOT’s bridge rating software directly follows the latest AASHTO
requirements. The experimental results, however, also proved that the bridges had significant strength reserves associated
to several factors, such as, girder/deck composite action, impact and distribution factors, material strength, and contribution
of non-structural elements. In this research, a simplified bridge testing protocol was developed from which it was concluded,
that a relatively simple instrumentation setup could be effective in the load rating of bridges through testing. The data
gathered through these instruments provide vital performance data for normal traffic conditions, as well as during extreme
loading conditions and evacuations, which could lead to less restrictive load ratings and removal of the structurally deficient
classification, yielding a more sustainable structure. 相似文献
104.
Chad S. Lane Sally P. Horn Claudia I. Mora Kenneth H. Orvis David B. Finkelstein 《Journal of Paleolimnology》2011,45(3):323-338
Continuous terrestrial records of paleoclimate and paleovegetation that extend to the late Pleistocene are rare for the circum-Caribbean
uplands. In this study we analyzed the bulk and compound-specific carbon isotope composition of lake sediments spanning this
period from Lago de las Morrenas 1 (LM1), a glacial lake in the highlands of southern Costa Rica, for evidence of climate
and vegetation changes that may not have been apparent in previous analyses. The stable carbon isotope ratios of n-alkanes typically derived from terrestrial plants (δ13CC27–C33) indicate an increased abundance of C4 plant taxa during the late Pleistocene and earliest Holocene that may be related to decreased atmospheric carbon dioxide
concentrations, increased aridity, or habitat availability. These n-alkane isotope ratios also provide evidence of more arid conditions during the early and late Holocene, and more mesic conditions
during the middle Holocene, a pattern prevalent in other paleoclimate records from the region that is thought to be related
to millennial-scale dynamics of the intertropical convergence zone (ITCZ). The sensitivity of the LM1 paleorecord to trade
wind dynamics provides further support for the role of millennial-scale shifts in ITCZ dynamics in driving neotropical environmental
change, and indicates that the effects of ITCZ migration were not limited to the lowlands. 相似文献
105.
106.
N. Epchtein B. De Batz E. Copet P. Fouqué F. Lacombe T. Le Bertre G. Mamon D. Rouan D. Tiphène W. B. Burton E. Deul H. Habing J. Borsenberger M. Dennefeld A. Omont J. C. Renault B. Rocca Volmerange S. Kimeswenger I. Appenzeller R. Bender T. Forveille F. Garzon J. Hron P. Persi M. Ferrari-Toniolo I. Vauglin 《Astrophysics and Space Science》1994,217(1-2):3-9
The DENIS project is the first attempt to carry out a complete digitized survey of the southern sky in the range 1–2.5µm. The main objectives of the programme and the main specifications of the camera and of the data processing stream are briefly outlined 相似文献
107.
108.
109.
Cecilia Eriksson Harry Burton Stuart Fitch Martin Schulz John van den Hoff 《Marine pollution bulletin》2013,66(1-2):199-208
The worlds’ oceans contain a large but unknown amount of plastic debris. We made daily collections of marine debris stranded at two sub-Antarctic islands to establish (a) physical causes of strandings, and (b) a sampling protocol to better estimate the oceans’ plastic loading. Accumulation rates at some beaches were dependent on tide and onshore winds. Most of the 6389 items collected were plastic (Macquarie 95%, Heard 94%) and discarded or lost fishing gear comprised 22% of those plastic items. Stalked barnacles (Lepas spp.) were a regular attachment on Macquarie debris but not at Heard Island. The daily accumulation rate of plastic debris on Macquarie Island was an order of magnitude higher than that estimated from monthly surveys during the same 4 months in the previous 5 years. This finding suggests that estimates of the oceans’ plastic loading are an order of magnitude too low. 相似文献
110.
Stars form in magnetized molecular clouds composed primarily of neutral gas with a trace amount of ions. We present a semi-implicit strategy for incorporating the equations that describe the coupled ion and neutral two-fluid equations, with a full energy equation, into the RIEMANN code that uses a TR-BDF2 algorithm to stably handle the stiffness of the source terms. We demonstrate that the numerical implementation works through the use of a suite of test problems that we catalog here. We show that reproducing the analytic dispersion analysis for the propagation of waves in a two-fluid plasma is an especially strong code test. We also present a two-fluid analogue of the Noh wall-shock problem and demonstrate the performance of the code on the Wardle instability. We also present a novel blast wave test, showing that the results reduce to the single fluid results under strong coupling, yet differing considerably when the coupling is weak. These test problems demonstrate that the numerical implementation can accurately capture the dissipation rate of waves and reproduce the structure of a C-shock. 相似文献