Role of melting process and melt–rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)     

Maria?Rosaria?RennaEmail authorView author&#;s OrcID profile  Riccardo?Tribuzio  Alessio?Sanfilippo  Matthew?Thirlwall 《Contributions to Mineralogy and Petrology》2018,173(4):31

This study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher Ce_N/Sm_N (0.4–0.3 vs. 0.2) and Zr_N/Y_N (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial ε_Nd from +?8.8 to +?8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (ε_Nd at the time of basalt formation?=???5.5 and ??5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin.  相似文献   

Textural evolution of perovskite in the Afrikanda alkaline–ultramafic complex,Kola Peninsula,Russia     

Naomi?J.?PotterEmail author  Matthew?R.?M.?Ferguson  Vadim?S.?Kamenetsky  Anton?R.?Chakhmouradian  Victor?V.?Sharygin  Jay?M.?Thompson  Karsten?Goemann 《Contributions to Mineralogy and Petrology》2018,173(12):100

Perovskite is a common accessory mineral in a variety of mafic and ultramafic rocks, but perovskite deposits are rare and studies of perovskite ore deposits are correspondingly scarce. Perovskite is a key rock-forming mineral and reaches exceptionally high concentrations in olivinites, diverse clinopyroxenites and silicocarbonatites in the Afrikanda alkaline–ultramafic complex (Kola Peninsula, NW Russia). Across these lithologies, we classify perovskite into three types (T1–T3) based on crystal morphology, inclusion abundance, composition, and zonation. Perovskite in olivinites and some clinopyroxenites is represented by fine-grained, equigranular, monomineralic clusters and networks (T1). In contrast, perovskite in other clinopyroxenites and some silicocarbonatites has fine- to coarse-grained interlocked (T2) and massive (T3) textures. Electron backscatter diffraction reveals that some T1 and T2 perovskite grains in the olivinites and clinopyroxenites are composed of multiple subgrains and may represent stages of crystal rotation, coalescence and amalgamation. We propose that in the olivinites and clinopyroxenites, these processes result in the transformation of clusters and networks of fine-grained perovskite crystals (T1) to mosaics of more coarse-grained (T2) and massive perovskite (T3). This interpretation suggests that sub-solidus processes can lead to the development of coarse-grained and massive perovskite. A combination of characteristic features identified in the Afrikanda perovskite (equigranular crystal mosaics, interlocked irregular-shaped grains, and massive zones) is observed in other oxide ore deposits, particularly in layered intrusions of chromitites and intrusion-hosted magnetite deposits and suggests that the same amalgamation processes may be responsible for some of the coarse-grained and massive textures observed in oxide deposits worldwide.  相似文献   

Statistical approaches to the discrimination of mantle- and crust-derived low-Cr garnets using major and trace element data     

Hardman  Matthew F.  Pearson  D. Graham  Stachel  Thomas  Sweeney  Russell J. 《Mineralogy and Petrology》2018,112(2):697-706

Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr₂O₃ < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data.

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The Dihedral Angle and Intersection Processes of a Conjugate Strike‐Slip Fault System in the Tarim Basin,NW China     

WU Guanghui  YUAN Yajuan  HUANG Shaoying  Thomas Matthew Vandyk  XIAO Yang  CAI Quan  LUO Bingxu 《《地质学报》英文版》2018,92(1):74-88

Recent studies, focused on dihedral angles and intersection processes, have increased understandings of conjugate fault mechanisms. We present new 3-D seismic data and microstructural core analysis in a case study of a large conjugate strike-slip fault system from the intracratonic Tarim Basin, NW China. Within our study area, "X" type NE and NW trending faults occur within CambrianOrdovician carbonates. The dihedral angles of these conjugate faults have narrow ranges, 19° to 62° in the Cambrian and 26° to 51° in the Ordovician, and their modes are 42° and 44° respectively. These data are significantly different from the ～60° predicted by the Coulomb fracture criterion. It is concluded that:(1) The dihedral angles of the conjugate faults were not controlled by confining pressure, which was low and associated with shallow burial;(2) As dihedral angles were not controlled by pressure they can be used to determine the shortening direction during faulting;(3) Sequential slip may have played an important role in forming conjugate fault intersections;(4) The conjugate fault system of the Tarim basin initiated as rhombic joints; these subsequently developed into sequentially active "X" type conjugate faults; followed by preferential development of the NW-trending faults; then reactivation of the NE trending faults. This intact rhombic conjugate fault system presents new insights into mechanisms of dihedral angle development, with particular relevance to intracratonic basins.  相似文献   

Dynamics of soil organic carbon following land-use change: insights from stable C-isotope analysis in black soil of Northeast China     

Chenglong Tu  Congqiang Liu  Timothy A. Quine  Matthew William Jones  Taoze Liu  Longbo Li  Wenjing Liu 《中国地球化学学报》2018,37(5):746-757

Intensive soil tillage is a significant factor in soil organic matter decline in cultivated soils. Both cultivation abandonment and foregoing tillage have been encouraged in the past 30 years to reduce greenhouse gas emissions and soil erosion. However, the dynamic processes of soil organic carbon (SOC) in areas of either continuous cultivation or abandonment remain unclear and inconsistent. Our aims were to assess and model the dynamic processes of SOC under continuous tillage and after cultivation abandonment in the black soil of Northeast China. Soil profiles were collected of cultivated or abandoned land with cultivation history of 0–100 years. An isotope mass balance equation was used to calculate the proportion of SOC derived from corn debris (C₄) and from natural vegetation (C₃) to deduce the dynamic process. Approximately 40% of SOC in the natural surface soil (0–10 cm) was eroded in the first 5 years of cultivation, increasing to about 75% within 40 years, before a slow recovery. C₄ above 30 cm soil depth increased by 4.5%–5% or 0.11–0.12 g·kg^?1 on average per year under continuous cultivation, while it decreased by approximately 0.34% annually in the surface soil after cultivation abandonment. The increase in the percentage of C₄ was fitted to a linear equation with given intercepts in the upper 30 cm of soil in cultivated land. A significant relationship between the change of C₄ and time was found only in the surface soil after abandonment of cultivation. These results demonstrate the loss and accumulation of corn-derived SOC in surface black soil of Northeast China under continuous tillage or cultivation abandonment.  相似文献   

Analysis of hydrological data with correlation matrices: technical implementation and possible applications     

Johannes Christoph Haas  Matthew Switanek  Steffen Birk 《Environmental Earth Sciences》2018,77(8):310

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On the evolution of terrestrial planets:Bi-stability,stochastic effects,and the non-uniqueness of tectonic states     

Matthew B.Weller  Adrian Lenardic 《地学前缘(英文版)》2018,9(1):91-102

The Earth is the only body in the solar system for which significant observational constraints are accessible to such a degree that they can be used to discriminate between competing models of Earth's tectonic evolution.It is a natural tendency to use observations of the Earth to inform more general models of planetary evolution.However,our understating of Earth's evolution is far from complete.In recent years,there has been growing geodynamic and geochemical evidence that suggests that plate tectonics may not have operated on the early Earth,with both the timing of its onset and the length of its activity far from certain.Recently,the potential of tectonic bi-stability(multiple stable,energetically allowed solutions)has been shown to be dynamically viable,both from analytical analysis and through numeric experiments in two and three dimensions.This indicates that multiple tectonic modes may operate on a single planetary body at different times within its temporal evolution.It also allows for the potential that feedback mechanisms between the internal dynamics and surface processes(e.g.,surface temperature changes driven by long term climate evolution),acting at different thermal evolution times,can cause terrestrial worlds to alternate between multiple tectonic states over giga-year timescales.The implication within this framework is that terrestrial planets have the potential to migrate through tectonic regimes at similar‘thermal evolution times'(e.g.,points were they have a similar bulk mantle temperature and energies),but at very different'temporal times'(time since planetary formation).It can be further shown that identical planets at similar stages of their evolution may exhibit different tectonic regimes due to random variations.Here,we will discuss constraints on the tectonic evolution of the Earth and present a novel framework of planetary evolution that moves toward probabilistic arguments based on general physical principals,as opposed to particular rheologies,and incorporates the potential of tectonic regime transitions and multiple tectonics states being viable at equivalent physical and chemical conditions.  相似文献   

Correction to: A quantified study of segmentation techniques on synthetic geological XRM and FIB-SEM images     

Matthew Andrew 《Computational Geosciences》2018,22(6):1513-1513

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The Science and Policy of the Verified Carbon Standard Methodology for Tidal Wetland and Seagrass Restoration     

Brian A. Needelman  Igino M. Emmer  Stephen Emmett-Mattox  Stephen Crooks  J. Patrick Megonigal  Doug Myers  Matthew P. J. Oreska  Karen McGlathery 《Estuaries and Coasts》2018,41(8):2159-2171

The restoration of tidal wetland and seagrass systems has the potential for significant greenhouse gas benefits, but project-level accounting procedures have not been available at an international scale. In this paper, we describe the Verified Carbon Standard Methodology for Tidal Wetland and Seagrass Restoration, which provides greenhouse gas accounting procedures for marsh, mangrove, tidal forested wetland, and seagrasses systems across a diversity of geomorphic conditions and restoration techniques. We discuss and critique the essential science and policy elements of the methodology and underlying knowledge gaps. We developed a method for estimating mineral-protected (recalcitrant) allochthonous carbon in tidal wetland systems using field-collected soils data and literature-derived default values of the recalcitrant carbon that accompanies mineral deposition. We provided default values for methane emissions from polyhaline soils but did not provide default values for freshwater, oligohaline, and mesohaline soils due to high variability of emissions in these systems. Additional topics covered are soil carbon sequestration default values, soil carbon fate following erosion, avoided losses in organic and mineral soils, nitrous oxide emissions, soil profile sampling methods, sample size, prescribed fire, additionality, and leakage. Knowledge gaps that limit the application of the methodology include the estimation of CH₄ emissions from fresh and brackish tidal wetlands, lack of validation of our approach for the estimation of recalcitrant allochthonous carbon, understanding of carbon oxidation rates following drainage of mineral tidal wetland soils, estimation of the effects of prescribed fire on soil carbon stocks, and the analysis of additionality for projects outside of the USA.  相似文献   

Investigating the Functional Role of an Artificial Reef Within an Estuarine Seascape: a Case Study of Yellowfin Bream (<Emphasis Type="Italic">Acanthopagrus australis</Emphasis>)     

Matthew D. Taylor  Alistair Becker  Michael B. Lowry 《Estuaries and Coasts》2018,41(6):1782-1792

Estuaries contain mosaic habitats which support fish across different life stages. Artificial reefs represent a form of habitat enhancement which can provide additional structure for fishes and improve fishing opportunities, but the role of artificial reefs within the broader estuarine seascape has not been extensively studied. We used a VEMCO Positioning System (VPS) to monitor the fine-scale movements of yellowfin bream (Acanthopagrus australis, referred to as Bream), an estuarine predator and important recreational species. Fish were implanted with acoustic tags with accelerometer sensors (to measure relative fish activity), and their movements monitored on an artificial reef and adjacent habitats. Elevated activity patterns during crepuscular periods indicated that foraging was likely occurring over a large seagrass bed adjacent to the artificial reef system. Alternatively, lower activity was observed when fish were on the artificial reef, which may reflect the role of this habitat as a refuge, or that alternative foraging strategies were being employed. All fish exhibited a high degree of fidelity to the artificial reef on which they were tagged, and there was minimal movement among other reef groups within the array. There was extensive overlap in space use contours for smaller fish on the seagrass edge, but no overlap for larger fish that also tended to forage further afield. These findings have implications for the way in which artificial reefs support fish production, especially the importance of connectivity with other key habitats within the estuarine seascape.  相似文献