Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three ...components, is defined as “mixed sedimentary rocks”. The Permian Lucaogou Formation (P2l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.
•Lacustrine mixed fine-grained sedimentary rocks have three sources: extrabasinal, intrabasinal and pyroclastic component.•Coupled chemostratigraphic-petrographic analyses for depositional conditions of the mixed fine-grained sedimentary rocks.•The effect of sedimentary environments on lithofacies and OM accumulation variation.•High TOC content and high porosity reservoir interbedded layers are benefit for the development of tight oil.
The high-altitude Tibetan Plateau (TP) is a potential substantial source of dust that could have major effects on the global dust cycle and lead to environmental change. Aeolian loess on the TP is a ...valuable archive for studying the history of dust activities. Identification of TP dust sources will deepen understanding of dust transport and atmospheric circulation patterns and their environmental impacts. However, consensus on the source of TP loess has not been reached, which hinders understanding of dust transport and paleoenvironmental explanations. In this study, topsoil and loess samples from Northwest (NW) China and eastern TP were used to systematically investigate the spatial variability in geochemical composition, and the element characteristics of the TP loess were used to identify its dust source. Compared with samples from NW China, the eastern TP topsoil and loess were relatively enriched in TiO2, MnO, Cr, Co, Y, Zr, Nb, La, Ce, and Hf, but relatively depleted in Na2O, MgO, CaO, Sr, and Pb. The geochemical composition and source analyses indicated that the dust from the Qaidam Basin, Hexi Corridor, and Tarim Basin in NW China was not the main contributing source to the TP loess. However, the elemental characteristics of the TP loess were broadly consistent, and element contents were similar to those of major and trace elements in surface sediments on the TP. Thus, the eastern TP loess was sourced primarily from the TP interior, revealing the TP as an important dust source in Asia.
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•Geochemical elements were characterized in NW China and eastern Tibetan Plateau.•Dust from NW China deserts was not the main contributor to Tibetan Plateau loess.•Element ratios indicated Tibetan loess mainly sourced from the plateau interior.•The Tibetan Plateau is an important dust source in Asia.
The majority of arc magmas are highly evolved due to differentiation within the lithosphere or crust. Some studies have suggested a relationship between crustal thickness and magmatic ...differentiation, but the exact nature of this relationship is unclear. Here, we examine the interplay of crustal thickness and magmatic differentiation using a global geochemical dataset compiled from active volcanic arcs and elevation as a proxy for crustal thickness. With increasing crustal thickness, average arc magma compositions become more silicic (andesitic) and enriched in incompatible elements, indicating that on average, arc magmas in thick crust are more evolved, which can be easily explained by the longer transit and cooling times of magmas traversing thick arc lithosphere and crust. As crustal thickness increases, arc magmas show higher degrees of iron depletion at a given MgO content, indicating that arc magmas saturate earlier in magnetite when traversing thick crust. This suggests that differentiation within thick crust occurs under more oxidizing conditions and that the origin of oxidation is due to intracrustal processes (contamination or recharge) or the role of thick crust in modulating melting degree in the mantle wedge. We also show that although arc magmas are on average more silicic in thick crust, the most silicic magmas (>70 wt.% SiO2) are paradoxically found in thin crust settings, where average compositions are low in silica (basaltic). We suggest that extreme residual magmas, such as those exceeding 70 wt.% SiO2, are preferentially extracted from shallow crustal magma bodies than from deep-seated magma bodies, the latter more commonly found in regions of thick crust. We suggest that this may be because the convective lifespan of crustal magma bodies is limited by conductive cooling through the overlying crustal lid and that magma bodies in thick crust cool more slowly than in thin crust. When the crust is thin, cooling is rapid, preventing residual magmas from being extracted; in the rare case that residual magmas can be extracted, they represent the very last melt fractions, which are highly silicic. When the crust is thick, cooling is slow, so intermediate melt fractions can readily segregate and erupt to the surface, where they cool and crystallize before highly silicic residual melts can be generated.
•Arc lavas tend towards more evolved compositions with increasing crustal thickness.•Lavas become more calc-alkaline, or Fe-depleted, with increasing elevation.•Crustal thickness effects composition by modifying relative melt cooling and segregation times.•Relative cooling and segregation times determine composition of erupted melts.•Intermediate compositions may preferentially form in thickened and reprocessed crust.
Siliciclastic sediments owe their origin mainly to land sources. New discriminant-function-based major-element diagrams for the tectonic discrimination of siliciclastic sediments from three main ...tectonic settings: island or continental arc, continental rift, and collision, have been constructed for the tectonic discrimination of high-silica (SiO2)adj=63%–95% and low-silica rocks (SiO2)adj=35%–63%, where (SiO2)adj refers to the SiO2 value obtained after volatile-free adjustment of the ten major-elements to 100wt.%. These diagrams are based on worldwide examples of Neogene–Quaternary siliciclastic sediments from known tectonic settings, loge-ratio transformation of ten major-elements with SiO2 as the common denominator, and linear discriminant analysis of the loge-transformed ratio data. The success rates of these diagrams as judged from the original data varied from 84.5% to 93.6%. These diagrams were successfully tested on Neogene to Quaternary rocks not included in the original database. These discriminant diagrams were also successfully applied on older high-silica and low-silica sandstones and shales of Paleoarchean–Ediacaran age. Finally, these diagrams were shown to be useful against chemical changes related to analytical errors, weathering, recycling and post-depositional processes.
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•We present the first multi-dimensional diagrams for silicic sedimentary rocks.•These are the first diagrams for discrimination of three tectonic settings.•These diagrams are based on coherent statistical treatment of compositional data.•Only major elements are required for their use in deciphering tectonic setting.•The existing diagrams do not perform well and should be replaced by these new ones.
Chemical compositions of Fe-Mg biotite have been used to understand the petrogenesis of metamorphic and igneous rocks. However, biotite is affected by sub-solidus hydrothermal alteration, ...metamorphism, and chemical exchange with other common coexisting phases such as garnet and muscovite. Therefore, the interpretation of igneous and metamorphic processes using biotite compositions is not always straightforward. Here we compare biotite compositions in igneous rocks, meta-igneous rocks, and meta-sedimentary rocks from localities in northeast (Dehnow, Khalaj, Khajeh Morad) and central (Jandaq and Airekan) Iran, with similar rock types in the global GEOROC database and from other localities, in order to constrain associated petrogenetic classification schemes. We find important compositional contrasts in biotite associated with muscovite and/or garnet (in both igneous and metamorphic rocks), suggesting careful use of common discrimination schemes. For example, magmatic biotite associated with garnet and/or muscovite (i.e., Bt + Ms, Bt + Ms + Grt, Bt + Grt) is often enriched in Al and depleted in Fe, Mg, and Ti, likely due to crystallization prior to muscovite but synchronous with or following garnet crystallization. Metamorphic biotites in garnet- and/or muscovite-bearing rocks tend to be enriched in Ti, Fe, and Mg and depleted in Al. The contrasting compositional behavior of magmatic and metamorphic biotites also poses problems for garnet-biotite, biotite-muscovite, and Ti-in-biotite thermometers. Our analysis indicates that biotite rare earth and trace element concentrations are strongly influenced by co-existing garnet and muscovite. When magmatic biotite crystallization occurs with muscovite and garnet, HREE concentrations respectively decrease and increase.
•Biotites of igneous and metamorphic rocks show important compositional contrasts.•Associated garnet and muscovite affect major, REE, and trace elements of biotite composition.•Biotite is not a recommended thermometer or petrogenetic indicator for all igneous rock types.
The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building ...blocks-volcanic and plutonic rocks-and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.
Sector-zoned clinopyroxene is common in igneous rocks, but has been overlooked in the study of magmatic processes. Whilst concentric zoning is commonly used as a record of physicochemical changes in ...the melt feeding crystal growth, clinopyroxene is also highly sensitive to crystallisation kinetics. In sector-zoned crystals, the fidelity of compositional changes as recorders of magma history is dubious and the interplay between thermodynamic and kinetic controls remains poorly understood. Here we combine electron probe and laser ablation micro-chemical maps of titanaugite crystals from Mt. Etna (Sicily, Italy) to explore the origin of sector zoning at the major and trace element levels, and its implications for the interpretation of magmatic histories. Elemental maps afford the possibility to revisit sector zoning from a spatially controlled perspective. The most striking observation is a clear decoupling of elements into sectors vs. concentric zones within single crystals. Most notably, Al-Ti enrichments and Si-Mg depletions in the prism sectors {1 0 0}, {1 1 0} and {0 1 0} relative to the hourglass (or basal) sectors {−1 1 1} correlate with enrichments in rare earth elements and highly charged high field strength elements due to cation exchanges driven by kinetic effects. In contrast, transition metals (Cr, Ni, Sc) show little partitioning into sectors and strong enrichments in concentric zones following resorbed surfaces, interpreted as evidence of mafic recharge and magma mixing. Our results document that kinetic partitioning has minor effects on the compositional variations of cations with low charge relative to the ideal charge/radius of the structural site they occupy in the clinopyroxene lattice. We suggest that this may be due to a lower efficiency in charge balance mechanisms compared to highly charged cations. It follows that compatible metals such as Cr can be considered trustworthy recorders of mafic intrusions and eruption triggers even in sector-zoned crystals. We also observe that in alkaline systems where clinopyroxene crystallisation takes place at near-equilibrium conditions, sector zoning should have little effect on Na-Ca partitioning and in turn, on the application of experimentally calibrated thermobarometers. Our data show that whilst non-sector-zoned crystals form under relatively stagnant conditions, sector zoning develops in response to low degrees of undercooling, such as during slow magma ascent. Thus, we propose that the chemistry of sector-zoned crystals can provide information on magma history, eruption triggers, and possibly ascent rates.
We report Li isotope composition (δ7Li) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a ...continental scale. The δ7Li in the dissolved load (+1.2‰ to +32‰) is fractionated toward heavy values compared to the inferred bedrock (−1‰ to 5‰) and the suspended sediments (−6.8‰ to −0.5‰) as a result of the preferential incorporation of 6Li into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved δ7Li (+1.2‰ to +18‰). Correlations between dissolved δ7Li and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the δ7Lidiss across the entire Amazon basin. A “batch” steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load (αsec-dis) of 0.983±0.002. Two types of supply-limited weathering regimes can be identified for the lowlands: “clearwaters” with dominant incorporation of Li in secondary minerals, and “black waters” (e.g., Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low δ7Li. Apart from the black waters, the δ7Li of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest δ7Li corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher δ7Li compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved δ7Li along the course of these rivers. Unlike other rivers, the dissolved δ7Li in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor αsec-dis of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved δ7Li and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean δ7Li and its use as a paleo weathering proxy.
Herein, an improved wavelength-dispersive X-ray fluorescence (WD-XRF) analytical technique for determining the major elements (including SiO2, TiO2, Al2O3, TFe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) ...in precious geological samples, particularly extraterrestrial basaltic rock samples, was developed. For WD-XRF analysis, undersized (approximately 11 mm in diameter) glass disks were prepared by fusing 10 mg of a powdered sample with 350 mg of lithium borate flux at a sample-to-flux ratio of 1:35. Specialized PtAu crucibles and molds were crafted for glass disk preparation to facilitate automatic fluxer mounting. This procedure enabled the automatic preparation of undersized glass disks, with repeatability between sample batches. Calibration curves were constructed for 30 certified reference materials (CRMs), including felsic, intermediate, mafic, and ultramafic igneous rocks. Regular-sized CRM disks with large sample sizes were prepared at the same sample-to-flux ratio as the test samples to avoid issues caused by CRM heterogeneity. The influence of various factors, including glass disk thickness and homogeneity, releasing agent amount, and interference between lines (Br Lα with Al Kα, Rh Lγ2 and Rh Lγ3 with K Kα, and K Kβ with Ca Kα), on analytical accuracy was investigated. Two homogeneous synthetic reference glass samples (ARM-2 and ARM-3) were pulverized and used to verify the analytical results for small samples using the WD-XRF method, which could eliminate the uncertainty caused by sample homogeneity. Subsequently, three CRMs (JB-1b, JA-3, and JR-1) were used to validate the accuracy and precision of the method. Finally, two lunar meteorite samples (NWA4898 and NWA4734) were assayed by the proposed method, and comparison with inductively coupled plasma optical emission spectroscopy results confirmed the reliability of the method. The proposed small sample protocol, which involves the destructive pretreatment of precious and limited extraterrestrial basaltic samples, has great application potential for precious samples such as lunar soil samples.
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•A facile, accurate method for analyzing major elements in 10 mg precious samples is proposed.•The calibration curves are validated from 150 mg CRMs with the same dilution ratio.•Homogeneous glass samples and heterogeneous CRMs are used.•The reliability and robustness of the method is confirmed.•Special PtAu crucibles and molds are devised for automatic glass disk preparation.
Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through ...geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle.
When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean–continent transition.
Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.
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•Review of >900 geochemical analyses of serpentinites worldwide•Identification of geochemical criteria to distinguish serpentinite protoliths•Discussion of the possible origins of subducted serpentinites•Serpentinites are one of the major components of the Earth's budget for fluid-mobile elements.
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