Origin of K-rich diamond-forming immiscible melts and CO2 fluid via partial melting of carbonated pelites at a depth of 180–200 km     

《Gondwana Research》2019

Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO₂ inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K₂O enters potassium feldspar or K₂TiSi₃O₉ wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K_0.90Na_0.10)₂CO₃·60Ca_0.55Mg_0.24Fe_0.21CO₃ + 1.9 mol% SiO₂ + 0.7 mol% TiO₂ + 1.4 mol% Al₂O₃ at the solidus established near 1000 °C. Molecular CO₂ liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO₂ = 57.0/52.4, TiO₂ = 1.8/2.3, Al₂O₃ = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na₂O = 3.2/3.0, K₂O = 10.5/15.2, CO₂ = 12.0/8.0, while carbonatite melt can be approximated as 24(K_0.81Na_0.19)₂CO₃·76Ca_0.59Mg_0.21Fe_0.20CO₃ + 3.0 mol% SiO₂ + 1.6 mol% TiO₂ + 1.4 mol% Al₂O₃. Both melts remain stable to at least 1500 °C coexisting with CO₂ fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K₂O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali‑carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.  相似文献   

Impact of COVID-19 Lockdown on Air Quality in Moscow     

Ginzburg  A. S.  Semenov  V. A.  Semutnikova  E. G.  Aleshina  M. A.  Zakharova  P. V.  Lezina  E. A. 《Doklady Earth Sciences》2020,495(1):862-866

Doklady Earth Sciences - The results of an analysis of changes in the atmospheric air quality in Moscow during the lockdown period and the decline in business activity caused by the COVID-19...  相似文献   

A brief review of ENSO theories and prediction     

Xianghui FANG  Ruihuang XIE 《中国科学:地球科学(英文版)》2020,63(4):476-491

Although the El Ni?o-Southern Oscillation(ENSO) originates and develops in the equatorial Pacific, it has substantial climatic impacts around the globe. Thus, the ability to effectively simulate and predict ENSO one or more seasons in advance is of great societal importance, but this remains a challenging task. The main obstacles are the diversity, complexity,irregularity, and asymmetry of ENSO. The purpose of this article is to organically integrate the understanding of ENSO based on current progress on the physical mechanisms, prediction, and connections between the interannual ENSO phenomenon and physical processes on other time and space scales, and to provide guidance for future studies by extracting specific important questions.  相似文献   

Geological and tectonic evolution of the Transantarctic Mountains,from ancient craton to recent enigma     

《Gondwana Research》2020

The Transantarctic Mountains (TAM) are one of Earth's great mountain belts and are a fundamental physiographic feature of Antarctica. They are continental-scale, traverse a wide range of latitudes, have high relief, contain a significant proportion of exposed rock on the continent, and represent a major arc of environmental and geological transition. Although the modern physiography is largely of Cenozoic origin, this major feature has persisted for hundreds of millions of years since the Neoproterozoic to the modern. Its mere existence as the planet's longest intraplate mountain belt at the transition between a thick stable craton in East Antarctica and a large extensional province in West Antarctica is a continuing enigma. The early and more cryptic tectonic evolution of the TAM includes Mesoarchean and Paleoproterozoic crust formation as part of the Columbia supercontinent, followed by Neoproterozoic rift separation from Laurentia during breakup of Rodinia. Development of an Andean-style Gondwana convergent margin resulted in a long-lived Ross orogenic cycle from the late Neoproterozoic to the early Paleozoic, succeeded by crustal stabilization and widespread denudation during early Gondwana time, and intra-cratonic and foreland-basin sedimentation during late Paleozoic and early Mesozoic development of Pangea. Voluminous mafic volcanism, sill emplacement, and layered igneous intrusion are a primary signature of hotspot-influenced Jurassic extension during Gondwana breakup. The most recent phase of TAM evolution involved tectonic uplift and exhumation related to Cenozoic extension at the inboard edge of the West Antarctic Rift System, accompanied by Neogene to modern glaciation and volcanism related to the McMurdo alkaline volcanic province. Despite the remote location and relative inaccessibility of the TAM, its underlying varied and diachronous geology provides important clues for reconstructing past supercontinents and influences the modern flow patterns of both ice and atmospheric circulation, signifying that the TAM have both continental and global importance through time.  相似文献   

Volatile element chemistry during accretion of the earth     

《Chemie der Erde / Geochemistry》2020,80(1):125594

We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor - melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.  相似文献   

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation     

《Chemie der Erde / Geochemistry》2020,80(1):125535

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.  相似文献   

Regeneration and utilization of nutrients during collapse of a <Emphasis Type="Italic">Mesodinium rubrum</Emphasis> red tide and its influence on phytoplankton species composition     

Yafeng Zhang  Xiuxian Song  Paul J. Harrison  Sheng Liu  Zhiming Yu  Jinjun Kan  Peiyuan Qian  Hao Liu  Kedong Yin 《中国科学:地球科学(英文版)》2018,61(10):1384-1396

High-biomass red tides occur frequently in some semi-enclosed bays of Hong Kong where ambient nutrients are not high enough to support such a high phytoplankton biomass. These high-biomass red tides release massive inorganic nutrients into local waters during their collapse. We hypothesized that the massive inorganic nutrients released from the collapse of red tides would fuel growth of other phytoplankton species. This could influence phytoplankton species composition. We tested the hypothesis using a red tide event caused by Mesodinium rubrum (M. rubrum) in a semi-enclosed bay, Port Shelter. The red tide patch had a cell density as high as 5.0×10⁵ cells L^?1, and high chlorophyll a (63.71 μg L^?1). Ambient inorganic nutrients (nitrate: \(\rm{NO}_3^-\), ammonium: \(\rm{NH}_4^+\), phosphate: \(\rm{PO}_4^{3-}\), silicate: \(\rm{SiO}_4^{3-}\)) were low both in the red tide patch and the non-red-tide patch (clear waters outside the red tide patch). Nutrient addition experiments were conducted by adding all the inorganic nutrients to water samples from the two patches followed by incubation for 9 days. The results showed that the addition of inorganic nutrients did not sustain high M. rubrum cell density, which collapsed after day 1, and did not drive M. rubrum in the non-red-tide patch sample to the same high-cell density in the red tide patch sample. This confirmed that nutrients were not the driving factor for the formation of this red tide event, or for its collapse. The death of M. rubrum after day 1 released high concentrations of \(\rm{NO}_3^-\), \(\rm{PO}_4^{3-}\), \(\rm{SiO}_4^{3-}\), \(\rm{NH}_4^+\), and urea. Bacterial abundance and heterotrophic activity increased, reaching the highest on day 3 or 4, and decreased as cell density of M. rubrum declined. The released nutrients stimulated growth of diatoms, such as Chaetoceros affinis var. circinalis, Thalassiothrix frauenfeldii, and Nitzschia sp., particularly with additions of \(\rm{SiO}_4^{3-}\) treatments, and other species. These results demonstrated that initiation of M. rubrum red tides in the bay was not directly driven by nutrients. However, the massive inorganic nutrients released from the collapse of the red tide could induce a second bloom in low-ambient nutrient water, influencing phytoplankton species composition.  相似文献   

Extracting water-related features using reflectance data and principal component analysis of Landsat images     

Boglárka Balázs  Tibor Bíró  Gareth Dyke  Sudhir Kumar Singh 《水文科学杂志》2018,63(2):269-284

This study aimed to map water features using a Landsat image rather than traditional land cover. We involved the original bands, spectral indices and principal components (PCs) of a principal component analysis (PCA) as input data, and performed random forest (RF) and support vector machine (SVM) classification with water, saturated soil and non-water categories. The aim was to compare the efficiency of the results based on various input data. Original bands provided 93% overall accuracy (OA) and bands 4–5–7 were the most informative in this analysis. Except for MNDWI (modified normalized differenced water index, with 98% OA), the performance of all water indices was between 60 and 70% (OA). The PCA-based approach conducted on the original bands resulted in the most accurate identification of all classes (with only 1% error in the case of water bodies). We therefore show that both water bodies and saturated soils can be identified successfully using this approach.  相似文献   

Preface to Special Issue ‘Insights on the vertebrate paleontology of the Neuquén Group,Argentina’     

《Cretaceous Research》2018

  相似文献   

A model‐based data‐driven dictionary learning for seismic data representation          下载免费PDF全文

Can Evren Yarman  Rajiv Kumar  James Rickett 《Geophysical Prospecting》2018,66(1):98-123

Planar waves events recorded in a seismic array can be represented as lines in the Fourier domain. However, in the real world, seismic events usually have curvature or amplitude variability, which means that their Fourier transforms are no longer strictly linear but rather occupy conic regions of the Fourier domain that are narrow at low frequencies but broaden at high frequencies where the effect of curvature becomes more pronounced. One can consider these regions as localised “signal cones”. In this work, we consider a space–time variable signal cone to model the seismic data. The variability of the signal cone is obtained through scaling, slanting, and translation of the kernel for cone‐limited (C‐limited) functions (functions whose Fourier transform lives within a cone) or C‐Gaussian function (a multivariate function whose Fourier transform decays exponentially with respect to slowness and frequency), which constitutes our dictionary. We find a discrete number of scaling, slanting, and translation parameters from a continuum by optimally matching the data. This is a non‐linear optimisation problem, which we address by a fixed‐point method that utilises a variable projection method with ?₁ constraints on the linear parameters and bound constraints on the non‐linear parameters. We observe that slow decay and oscillatory behaviour of the kernel for C‐limited functions constitute bottlenecks for the optimisation problem, which we partially overcome by the C‐Gaussian function. We demonstrate our method through an interpolation example. We present the interpolation result using the estimated parameters obtained from the proposed method and compare it with those obtained using sparsity‐promoting curvelet decomposition, matching pursuit Fourier interpolation, and sparsity‐promoting plane‐wave decomposition methods.  相似文献