An in situ investigation of the preservation and alteration of presolar silicates in the Miller Range 07687 chondrite     

Laura B. Seifert  Pierre Haenecour  Tarunika Ramprasad  Adrian J. Brearley  Thomas J. Zega 《Meteoritics & planetary science》2023,58(3):360-382

Dust grains that formed around ancient stars and in stellar explosions seeded the early solar protoplanetary disk. While most of such presolar grains were destroyed during solar system formation, a fraction of such grains were preserved in primitive materials such as meteorites. These grains can provide constraints on stellar origins and secondary processing such as aqueous alteration and thermal metamorphism on their parent asteroids. Here, we report on the nature of aqueous alteration in the Miller Range (MIL) 07687 chondrite through the analysis of four presolar silicates and their surrounding material. The grains occur in the Fe-rich and Fe-poor lithologies, reflecting relatively altered and unaltered material, respectively. The O-isotopic compositions of two grains, one each from the Fe-rich and Fe-poor matrix, are consistent with formation in the circumstellar envelopes of low-mass Asymptotic Giant Branch (AGB)/Red Giant Branch (RGB) stars. The other two grains, also one each from the Fe-rich and Fe-poor matrix, have O-isotopic compositions consistent with formation in the ejecta of type-II supernovae (SNe). The grains derived from AGB/RGB stars include two polycrystalline pyroxene grains that contain Fe-rich rims. The SNe grains include a polycrystalline Ca-bearing pyroxene and a polycrystalline assemblage consistent with a mixture of olivine and pyroxene. Ferrihydrite is observed in all focused ion beam sections, consistent with parent-body aqueous alteration of the fine-grained matrix under oxidizing conditions. The Fe-rich rims around presolar silicates in this study are consistent with Fe-diffusion into the grains resulting from early-stage hydrothermal alteration, but such alteration was not extensive enough to lead to isotopic equilibration with the surrounding matrix.  相似文献   

<Superscript>10</Superscript>Be, <Superscript>18</Superscript>O and radiogenic isotopic constraints on the origin of adakitic signatures: a case study from Solander and Little Solander Islands,New Zealand     

Fiona?V.?FoleyEmail author  Simon?Turner  Tracy?Rushmer  John?T.?Caulfield  Nathan?R.?Daczko  Paul?Bierman  Matthew?Robertson  Craig?D.?Barrie  Adrian?J.?Boyce 《Contributions to Mineralogy and Petrology》2014,168(3):1048

Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands, south of mainland New Zealand, are porphyritic trachyandesites and andesites (58.20–62.19 wt% SiO₂) with phenocrysts of amphibole, plagioclase and biotite. The Solander and Little Solander rocks are incompatible element enriched (e.g. Sr ~931–2,270 ppm, Ba ~619–798 ppm, Th ~8.7–21.4 ppm and La ~24.3–97.2 ppm) with MORB-like Sr and Nd isotopic signatures. Isotopically similar quench-textured enclaves reflect mixing with intermediate (basaltic-andesite) magmas. The Solander rocks have geochemical affinities with adakites (e.g. high Sr/Y and low Y), whose origin is often attributed to partial melting of subducted oceanic crust. Solander sits on isotopically distinct continental crust, thus excluding partial melting of the lower crust in the genesis of the magmas. Furthermore, the incompatible element enrichments of the Solander rocks are inconsistent with partial melting of newly underplated mafic lower crust; reproduction of their major element compositions would require unrealistically high degrees of partial melting. A similar argument precludes partial melting of the subducting oceanic crust and the inability to match the observed trace element patterns in the presence of residual garnet or plagioclase. Alternatively, an enriched end member of depleted MORB mantle source is inferred from Sr, Nd and Pb isotopic compositions, trace element enrichments and εHf ? 0 CHUR in detrital zircons, sourced from the volcanics. ¹⁰Be and Sr, Nd and Pb isotopic systematics are inconsistent with significant sediment involvement in the source region. The trace element enrichments and MORB-like Sr and Nd isotopic characteristics of the Solander rocks require a strong fractionation mechanism to impart the high incompatible element concentrations and subduction-related (e.g. high LILE/HFSE) geochemical signatures of the Solander magmas. Trace element modelling shows that this can be achieved by very low degrees of melting of a peridotitic source enriched by the addition of a slab-derived melt. Subsequent open-system fractionation, involving a key role for mafic magma recharge, resulted in the evolved andesitic adakites.  相似文献   

Glacial and periglacial buzzsaws: fitting mechanisms to metaphors     

Adrian M. Hall  Johan Kleman 《Quaternary Research》2014

The buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier Equilibrium Line Altitudes (ELAs), irrespective of uplift rates, rock type or pre-existing topography. Here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and ELAs, and whether the glacial system can produce low-relief surfaces or limit summit heights. Plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited, pre-glacial relief. Summit heights may correlate with ELAs but no causal link need be implied as summit erosion rates are low, cirque headwalls may not directly abut summits and, on passive margins, cirques are cut into pre-existing mountain topography. Any simple links between ELAs and glacial erosion break down on passive margins due to topographic forcing of ice-sheet growth, and to the km-scale vertical swaths through which ELAs have shifted through the Quaternary. Glaciers destroy rather than create low-relief rock surfaces through the innate tendency for ice flow to be faster, thicker and warmer along valleys. The glacial buzzsaw cuts down.  相似文献   

Understanding processes of sediment bleaching in glacial settings using a portable OSL reader     

Georgina E. King  David C. W. Sanderson  Ruth A. J. Robinson  Adrian A. Finch 《Boreas: An International Journal of Quaternary Research》2014,43(4):955-972

Analysis of a high‐resolution suite of modern glacial sediments from Jostedalen, southern Norway, using a portable optically stimulated luminescence (OSL) reader, provides insights into the processes of sediment bleaching in glacial environments at the catchment scale. High‐magnitude, low‐frequency processes result in the least effective sediment bleaching, whereas low‐magnitude, high‐frequency events provide greater bleaching opportunities. Changes in sediment bleaching can also be identified at the scale of individual bar features: tails of braid‐bars and side‐attached bar deposits have the lowest portable reader signal intensities, as well as the smallest conventional OSL residual doses. In addition to improving our understanding of the processes of sediment bleaching, portable reader investigations can also facilitate more rapid and comprehensive modern analogue investigations, which are commonly used to confirm that the OSL signals of modern glacial sediments are well bleached.  相似文献   

Comparison of sedimentation-compression curves obtained by nuclear and gravimetric methods for Wilkinson Basin,Gulf of Maine,sediments     

Sung Kwun Chough  Adrian F Richards 《Marine Geology》1974,17(4):249-261

Accurate measurements of bulk density are important in the construction of sedimentation-compression curves of water-saturated, fine-grained cohesive sediments for consolidation studies. The bulk density of five cores was measured nondestructively on a laboratory nuclear transmission densitometer and the water content, expressed as void ratio, was derived from an empirical relationship between bulk density and water content. Bulk density and water content measured destructively was determined every 10 cm for a comparison with the nuclear data. Agreement between nuclear and gravimetric methods was within ± 0.02 g/cm³ for bulk density ranging from 1.2 to 1.4 g/cm³. Sedimentation-compression curves made using both methods were in generally good agreement.In the Wilkinson Basin, void ratio gradually decreases with increasing effective overburden pressure. This indicates that consolidation by dewatering the surficial sediments is normal and that the environmental conditions during deposition remained relatively constant.  相似文献   

Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination   总被引：7，自引：5，他引：7  

Adrian Jäggi  R. Dach  O. Montenbruck  U. Hugentobler  H. Bock  G. Beutler 《Journal of Geodesy》2009,83(12):1145-1162

Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved from about 10 to 6 mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO positions.  相似文献   

Compositions and microstructures of CB sulfides: Implications for the thermal history of the CB chondrite parent body          下载免费PDF全文

Poorna Srinivasan  Rhian H. Jones  Adrian J. Brearley 《Meteoritics & planetary science》2017,52(10):2193-2219

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low‐Cr sulfides composed of troilite, troilite‐containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock‐melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ?600 °C, and irregular Fe‐FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.  相似文献   

The electromagnetic properties of aperture-synthesis telescopes shaped as Reuleaux triangles     

Adrian Webster 《Monthly notices of the Royal Astronomical Society》2004,353(4):1304-1310

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分支河流体系河网形态演变规律——以格尔木河流扇为例     

王浩楠  张昌民  孙家惠  张祥辉  徐清海  刘洪平  Adrian John Hartley 《地质论评》2023,69(6):2023060016-2023060016

为了探索分支河流体系河网分布规律,为创建储层建模训练图像提供知识库信息。利用Google Earth、Global Mapper、91卫图助手等现代地理信息软件,系统测量了现代格尔木河流扇表面河道分叉特征,应用Horton定律和分形方法建立了河流扇的河网形态模型。结果表明：①格尔木河流扇从顶点到入湖段共识别出26级河道,882个节点和2162个河道段,泉线上、下的河道表现出不同的河网形态;②泉线上部共发育64个节点,每级河道数目增长是上一级河道的RB=1.8900倍,每级河道平均长度是下一级河道的RL=1.0095倍,即河网形态特征参数RB=1.8900和RL=1.0095;泉线下部共发育818个节点,每级河道数目增长是上一级河道的RB=1.0279倍,每级河道平均长度是下一级河道的RL=0.9899倍,即河网形态特征参数是RB=1.0279和RL=0.9899;③河网形态特征参数受坡度影响变化较大,坡度较大则河网形态特征参数较大,坡度较小则河网形态特征参数较小。河网形态特征参数定量表征促进了分支河流体系沉积学发展,为开展陆相盆地储层建模和砂体预测提供了新的知识库信息  相似文献   

Time variable Earth’s gravity field from SLR satellites   总被引：4，自引：4，他引：0  

Krzysztof Sośnica  Adrian Jäggi  Ulrich Meyer  Daniela Thaller  Gerhard Beutler  Daniel Arnold  Rolf Dach 《Journal of Geodesy》2015,89(10):945-960

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