Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field     

Valérie Malavergne  Michael J. Toplis  John Jones 《Icarus》2010,206(1):199-1852

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3-6 log units below the iron-wüstite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO₂ being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core-mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe-S-Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe-S liquid, poor in Si; (ii) a middle layer of Fe-Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.  相似文献   

美国内政部四轮战略规划的发展趋势及特点分析     

刘丽  陈丽萍  王威 《国土资源情报》2014,(9):6-13

美国内政部的战略规划是该部施政纲领的集中体现。四轮战略规划的目标从2003年强调资源保护、利用与休闲娱乐的平衡,到2007年增加全球经济、环境、政治和其他因素对内政部战略规划的影响,直至2014年强调资源的娱乐、人文观赏价值,并重视培养下一代的管理责任。内政部战略定位逐渐清晰。资源保护、经济发展、休闲娱乐的相对关系发生变化,从强调三者平衡到强调资源的休闲、文化等景观价值。  相似文献   

Taphonomy of large marine vertebrates in the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale of southern Utah     

《Cretaceous Research》2015

Sediments from the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale were deposited along the western margin of the Western Interior Seaway, in present-day southern Utah. Marine vertebrates from this formation include plesiosaurs, mosasaurs, bony fish, sharks, and turtles. They are concentrated in the lower portion of the Tropic Shale, mostly between Bentonites B and D. Study of the taphonomic condition of these vertebrates has contributed to an understanding of how they were preserved as well as a detailed paleoenvironment for the Tropic Shale. Physical factors played the dominant role in their preservation, with robust and durable skeletal elements, such as teeth and vertebrae, being most common within the shale. Isolated bones and teeth are also relatively common within the formation, while complete and nearly complete skeletons are more rare. Biological factors played a less dominant role, with no evidence of epifaunal or infaunal activity preserved with any of the skeletal remains. In addition, scavenging marks (both bite and gnaw marks) are relatively uncommon, typically only being found on more complete specimens. A signature of post-burial alteration can be recognized as low levels of abrasion, weathering, and compression and high levels of fracturing of vertebrate skeletal material. Slightly higher levels of abrasion and weathering occur to the west, closer to the ancient shoreline, suggesting some pre-burial alteration. The preservation of marine vertebrates in the Tropic Shale suggests a low energy marine environment with some weak bottom currents and low levels of benthic oxygen. The substrate ranged from soft and soupy to firm, with moderate sedimentation rates resulting in relatively rapid burial.  相似文献   

Sequence stratigraphy of the Dakota Formation (Cenomanian), southern Utah: interplay of eustasy and tectonics in a foreland basin     

David Uli&#;ný 《Sedimentology》1999,46(5):807-836

The Dakota Formation in southern Utah (Kaiparowits Plateau region) is a succession of fluvial through shallow-marine facies formed during the initial phase of filling of the Cretaceous foreland basin of the Sevier orogen. It records a number of relative sea-level fluctuations of different frequency and magnitude, controlled by both tectonic and eustatic processes during the Early to Late Cenomanian. The Dakota Formation is divided into eight units separated by regionally correlatable surfaces that formed in response to relative sea-level fluctuations. Units 1–6B represent, from bottom to top, valley-filling deposits of braided streams (unit 1), alluvial plain with anastomosed to meandering streams (2), tide-influenced fluvial and tide-dominated estuarine systems (3A and 3B), offshore to wave-dominated shoreface (4, 5 and 6A) and an estuarine incised valley fill (6A and 6B). The onset of flexural subsidence and deposition in the foredeep was preceded by eastward tilting of the basement strata, probably caused by forebulge migration during the Early Cretaceous, which resulted in the incision of a westward-deepening predepositional relief. The basal fluvial deposits of the Dakota Formation, filling the relief, reflect the onset of flexural subsidence and, possibly, a eustatic sea-level rise. Throughout the deposition of the Dakota Formation, flexure controlled the long-term, regional subsidence rate. Locally, reactivation of basement faults caused additional subsidence or minor uplift. Owing to a generally low subsidence rate, differential compaction locally influenced the degree of preservation of the Dakota units. Eustasy is believed to have been the main control on the high-frequency relative sea-level changes recorded in the Dakota. All surfaces that separate individual Dakota units are flooding surfaces, most of which are superimposed on sequence boundaries. Therefore, with the exception of unit 6B and, possibly, 3B, most of the Dakota units are interpreted as depositional sequences. Fluvial strata of units 1 and 2 are interpreted as low-frequency sequences; the coal zones at the base and within unit 2 may represent a response to higher frequency flooding events. Units 3A to 6B are interpreted as having formed in response to high-frequency relative sea-level fluctuations. Shallow-marine units 4, 5 and 6A, interpreted as parasequences by earlier authors, can be divided into facies-based systems tracts and show signs of subaerial exposure at their boundaries, which allows interpretation as high-frequency sequences. In general, the Dakota units are good examples of high-frequency sequences that can be misinterpreted as parasequences, especially in distal facies or in places where signs of subaerial erosion are missing or have been removed by subsequent transgressive erosion. Both low- and high-frequency sequences represented by the Dakota units are stacked in an overall retrogradational pattern, which reflects a long-term relative sea-level rise, punctuated by brief periods of relative sea-level fall. There is a relatively major fall near the end of the M. mosbyense Zone, whereas the base of the Tropic shale is characterized by a major flooding event at the base of the S. gracile Zone. A similar record of Cenomanian relative sea-level change in other regions, e.g. Europe or northern Africa, suggests that both high- and low-frequency relative sea-level changes were governed by eustasy. The high-frequency relative sea-level fluctuations of ≈100 kyr periodicity and ≈10–20 m magnitude, similar to those recorded in other Cenomanian successions in North America and Central Europe, were probably related to Milankovitch-band, climate-driven eustasy. Either minor glacioeustatic fluctuations, superimposed on the overall greenhouse climate of the mid-Cretaceous, or mechanisms, such as the fluctuations in groundwater volume on continents or thermal expansion and contraction of sea water, could have controlled the high-frequency eustatic fluctuations.  相似文献   

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潘晖  张建国  杨冬梅  李美 《大地测量与地球动力学》2014,34(4):83-87

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1970—2000年青藏高原内流区冰川储量变化的初探          下载免费PDF全文

崔志勇  李志伟  李佳  汪长城  周文明 《地球物理学报》2014,57(5):1440-1450

青藏高原内流区特殊的地理位置和自然环境使得对本区域冰川体积变化的研究相比中国西部其他冰川发育区难度大很多.而作为本区域重要水源补给,同时又是区域气候变化重要指示器的冰川,其储量的变化又是迫切需要获知的信息.在此前提下,本文基于RS、GIS技术平台提取了本区域冰川面积变化数据,并结合冰川目录数据、本区域冰川面积变化的研究资料,采用冰川体积与面积之间的统计关系模型,探讨了本区域冰川自1970年至2000年的体积变化.结果表明在这30年间,区域内冰川体积大约减少了36.25亿m³,相比整个高亚洲区域的冰川退缩速度,本区域冰川的退缩表现的更缓慢.然后,对内流区6个二级流域的冰川体积变化做了详细的对比分析,发现整个内流区冰川体积变化有较大的区域差别：其中5Z3流域冰川体积的退缩率最大,年均退缩率达-11.19%,而变化最小的5Z1流域年均变化率仅为-0.79%.各二级流域在体积变化方面的相同点是,体积退缩率均大于面积退缩率.通过对冰川面积、体积变化特点及其原因的分析,认为气候变化的区域差异是该区域内各二级流域冰川变化存在差异的主要原因.最后根据本文得到的结果,讨论了冰川体积变化对本区域的生态环境造成的影响.  相似文献   

Crustal recycling, mantle dehydration, and the thermal evolution of Mars     

A. Morschhauser  D. Breuer 《Icarus》2011,212(2):541-400

We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650 K, preferably a primordial crustal thickness of 30 km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500 Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core-mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100 ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14 m thick global surface layer, suggesting that volcanic outgassing of H₂O could have significantly influenced the early martian climate and increased the planet’s habitability.  相似文献   

Planetary core size: A seismological approach     

Martin Knapmeyer 《Planetary and Space Science》2011,59(10):1062-1068

A key parameter for understanding the geodynamics of a terrestrial planet is the size of its core. Numerical evaluation of 28 different interior structure models of Mercury, Venus, Earth, the Moon, and Mars suggests that there is an almost linear relationship between the core radius and the extent of the seismic P-wave core shadow. A scaling law is derived from a simple mantle density and velocity model that permits the interpretation of respective seismic measurements on terrestrial planetary bodies.  相似文献   

High-resolution calcareous nannofossil biostratigraphy of the Santonian/Campanian Stage boundary,Western Interior Basin,USA     

《Cretaceous Research》2017

The base of the Campanian Stage does not have a ratified Global Stratotype Section and Point (GSSP); however, several potential boundary markers have been proposed including the base of the Scaphites leei III ammonite Zone and the base of the paleomagnetic Chron C33r. Calcareous nannofossil assemblages from the Smoky Hill Member of the Niobrara Formation in the central Western Interior Seaway, USA were analyzed from two localities to determine relevant biohorizons and their relationships to these potential boundary markers. In a previous study, the Aristocrat Angus 12-8 core (Colorado) was astrochronologically dated and constrained using macrofossil zonations and radiometric ages. The Smoky Hill Member type area (Kansas) provides an expanded interval with good to excellent nannofossil preservation.Five biohorizons are useful for recognition of the Santonian/Campanian transition within the Smoky Hill Member type area, and three are useful in the Aristocrat Angus 12-8 core. The first occurrences (FOs) of Aspidolithus parcus parcus and Aspidolithus parcus constrictus, as well as the last occurrences (LOs) of Zeugrhabdotus moulladei, Helicolithus trabeculatus specimens larger than 7 μm, and Zeugrhabdotus biperforatus are in close stratigraphic proximity to the base of the Scaphites leei III Zone and the base of Chron C33r.  相似文献   

Charophytes from the Upper Berriasian of the Western Interior Basin of the United States     

《Cretaceous Research》2013

A charophyte flora from the Upper Berriasian is described from the Lakota (Black Hills, South Dakota) and Cedar Mountain formations (San Rafael Swell, Utah) of the Western Interior Basin, United States of America. Whereas the latter is dominated by monotonous assemblages of the clavatoracean Nodosoclavator bradleyi (Harris, 1939), found in temporary lakes within palustrine facies, the flora of the Lakota Formation consists of more varied assemblages of the clavatoraceans N. bradleyi (Harris, 1939), Clavator grovesii grovesii Harris 1939, C. bilateralis Peck 1957 and early characeans (Mesochara sp. or Tolypella sp.). This flora was found in deposits related to permanent lakes in fluvial floodplains, i.e. lacustrine marls and limestones that do not show any evidence of subaerial exposure. To date, little is known about C. bilateralis, we provide a new definition on the basis of its particular structure, which shows lateral bract-cell units with a pinnate arrangement. This species appears to be endemic to North America and stratigraphically limited to the earliest Cretaceous. C. grovesii grovesii is part of a long-lasting charophyte lineage, which until now was considered to have originated in the Central Tethyan Archipelago (Europe) during the Early Berriasian, about 145 Ma, and limited to Eurasia until most of the Early Cretaceous. The new data presented here suggest that during the Berriasian this species was very broadly distributed, comprising North America, Europe and China.  相似文献