A model for plate tectonic evolution of mantle layers

In plate tectonic theory, lithosphere that descends into the mantle has a largely derivative composition, because it is produced as a refractory residue by partial melting, and cannot be resorbed readily by the parent mantle. We suggest that lithosphere sinks through the asthenosphere, or outer mantle, and accumulates progressively beneath to form an accretionary mesosphere, or inner mantle. According to this model, there is an irreversible physicochemical evolution of the mantle and its layers. We make the key assumption that the rate at which mass has been transferred from the lithosphere to the mesosphere is proportional to the rate of radiogenic heat production. Calculations of mass transfer with time demonstrate that the entire mass of the present mesosphere could have been produced in geologically reasonable times (3 x 10(9) to 4.5 x 10(9) years). The model is consistent with the generation of the continental crust during the last 3 x 1O(9) years and predicts an end to plate tectonic behavior within the next 10(9) years.     

Ages of pacific deep-sea basalts, and spreading of the sea floor

Potassium-argon determinations of age from whole-rock samples of tholeiitic basalts, dredged from the crest of the East Pacific Rise and from the flanks of three seamounts at varying distances from the crest, show that the crest is younger than 1 million years and that age does not correlate with distance from the crest. Our data, however, do not necessarily oppose the general concept of spreading of the ocean floor.     

Early miocene subglacial basalts, the East antarctic ice sheet, and uplift of the transantarctic mountains

Subglacially erupted volcanic rocks from Mount Early and Sheridan Bluff, Antarctica, yield whole-rock potassium-argon dates and argon-40/argon-39 release spectra of Early Miocene age. Field associations suggest the existence of the East Antarctic ice sheet and significant uplift of the Transantarctic Mountains by that time.     

Volcanic production rates: comparison of oceanic ridges, islands, and the columbia plateau basalts

New potassium-argon age data from the Columbia Plateau suggest a basalt production rate of 108 cubic meters per year during a middle Miocene volcanic episode. This is two to three times the production rate in some oceanic islands, and about four to six times the production rate in spreading mid-oceanic ridge systems.     

Eclogites, pyroxene geotherm, and layered mantle convection

Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000 degrees and 1250 degrees C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 10(9) years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth.     

Grain size--dependent alteration and the magnetization of oceanic basalts

Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts.     

Fissure basalts and ocean-floor spreading on the East pacific rise

A basalt pavement outcrops almost continuously in a band along the crestal region of the East Pacific Rise from about 14 degrees S to 6 degrees S, that is, for more than 800 ikilometers; the outcrop may well extend beyond the above limits along the axis of the rise. The basalt band is generally between 40 and 60 kilometers wide and is replaced laterally by sediment. The lavas are fresh, "oceanic tholeiites" which were emplaced less than I million years ago by fissure eruptions. These findings, can be explained by the hypothesis of ocean-floor spreading; the basalts are the expression of material originating from the mantle and rising through fissures along the axis of the ridge. The absence of an axial rift valley on the East Pacific Rise may be explained by the fact that large volumes of lava are being outpoured along its crest.     

Petrogenesis of lunar basalts and the internal constitution and origin of the moon

Petrographic and electron-microprobe studies combined with high pressure-temperature investigations of phase relationships in average Apollo 11 basalt and possible source material show that the lower parts of maria may be composed of eclogite (density 3.74 grams per cubic centimeter), thus explaining the existence of mascons. The Apollo 11 basalt was probably formed at depths of 200 to 400 kilometers by a small degree of partial melting from pyroxenitic source material [FeO/(FeO + MgO) = 0.25, A1(2)O(3) 4 percent, CaO 3 percent]. This composition may be representative of the lunar interior and yields the observed mean lunar density and moment of inertia. Present data are in conflict with fission, binary planet, and capture hypotheses of lunar origin but are consistent with Ringwood's (1966) precipitation hypothesis.     

An experiment-based model for the petrogenesis of high-alumina basalts

To understand magmatism at convergent margins, one must know the origin of their characteristic, plagioclase-rich, high-alumina basalts (HABs). Wet melting experiments on basalts at 3 kilobars yielded high-alumina liquids and a coexisting mineral assemblage with little or no plagioclase. An isothermal pressure drop to 1 kilobar caused 20 to 30 percent plagioclase crystallization in these melts, while mafic minerals underwent limited crystallization or even resorption. These results suggest that hydrous (>/=4 percent H(2)O) HAB liquids, presumably formed by fractionation of a hydrous basalt at depth, will precipitate voluminous plagioclase as pressure drops during ascent and eruption. Plagioclase accumulation is not necessarily required to explain the petrogenesis of plagioclase-rich HAB.     

Short-lived chemical heterogeneities in the archean mantle with implications for mantle convection

The neodymium isotope and samarium-neodymium systematics of 2.7-billion-year-old mantle-derived magmas indicate that the lifetime of chemical heterogeneities was much shorter in the Archean mantle than in the modern mantle. Isotopic evidence is compatible with a Rayleigh number 100 times larger and convection 10 times faster in the Late Archean compared with the present-day mantle. Modern plate tectonics thus may be an improbable analog for the Archean. Chemical heterogeneities in the mantle may originate upon magma migration and mineralogical phase changes rather than by recycling of oceanic and continental crust.     

我国水肥耦合研究热点及趋势探析

利用CiteSpace软件绘制作者、机构、关键词等知识图谱,对中国学术网络出版数据库(CNKI)上发表的水肥耦合文献进行分析和评论。研究发现,研究机构、学者之间的联系和合作较少。研究热点冬小麦、果实品质和枸杞为我国水肥耦合研究的主要研究动向。     

区块链农业应用现状与技术热点

以德温特系列数据库为主要数据来源,利用文献计量学方法和CiteSpace可视化工具从技术发展历程、技术研发力量分布、技术热点与前沿等方面对近十年区块链农业应用研究的发明专利进行全面分析。结果表明：区块链农业应用研究已经引起世界各国,特别是区块链技术强国的高度重视,但目前尚处于起步阶段;中国在区块链农业应用研发强度上较为领先,但创新性不足,研发主体以企业为主;商业应用模式、数据处理与数据传输、安全管理等相关研究是区块链农业应用研究热点,数据库应用和软件技术相关研究是区块链农业应用研究前沿。在这种形式下,我国应重视技术成果质量、把握技术前沿、追求技术创新与农业领域应用深度融合,提高我国区块链农业应用的全球影响力。     

基于社会网络分析法的全民阅读研究热点探析

通过社会网络分析法(SNA)对近10年(2005年-2015年)来关于"全民阅读"研究的984篇重要期刊论文的关键词共现网络进行了分析,结果显示相关研究成果呈现8个主要研究热点.进而对这8个热点进行了进一步考察,发现10年来,随着国家对"全民阅读"愈来愈重视,中国学者纷纷从不同的角度、不同层面对该主题进行了基础理论研究和案例实证研究,并取得喜人成果.但相较于欧美文化大国,还存在相当大的差距,需要各相关部门机构、科学研究者以及人民大众共同努力,以开创"全民阅读"研究新局面.     

Iron isotope fractionation and the oxygen fugacity of the mantle

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (delta57/54Fe) of mantle spinels. Spinel delta57/54Fe values correlate with relative oxygen fugacity, Fe3+/sigmaFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling.     

国内社会化阅读研究热点追溯与分析

[目的/意义]阅读数字化背景下社会化阅读的研究热点分析,为后续研究提供参考.[方法/过程]以CNKI数据库作为数据来源,利用Spss、Citespace、Bibexcle软件进行共词分析和聚类分析,梳理国内社会化阅读为主题研究热点的成熟度,并制作战略坐标图.[结果/结论]国内社会化阅读仍处于发展阶段,有相对核心且成熟的...