Many studies of experimental petrology have devoted to partial melting of crustal rocks. In order to provide lithochemical constraints on granite petrogenesis, this paper presents a compilation and ...synthesis of available experimental data for the major element compositions of felsic melts derived from partial melting of natural or synthetic materials in the compositional range of crustal rocks. The experimental melts are categorized into four types according to the species of hydrous minerals in starting materials: (I) amphibole-bearing; (II) amphibole- and biotite-bearing; (III) biotite-bearing; and (IV) biotite- and muscovite-bearing. If dehydration melting takes place at normal crustal conditions (P=5–10kbar, T≤1000°C), experimental melts are rich in SiO2 but poor in MgO+FeOT except those from amphibole-bearing sources. A comprehensive comparison of compositions between experimental melts and starting materials indicates that geochemical fractionation is variable for different major elements and their ratios. Source composition and melting temperature exert stronger controls on the compositional variations of experimental melts than pressure and fluid. By comparing the experimental melts with natural granites, the following insights into granite petrogenesis can be got: (1) while peritectic assemblage entrainment may be the dominant mechanism for the compositional variations of garnet/cordierite-rich S-type granites, fractional crystallization of diverse melts from heterogeneous metasedimentary precursors probably governs the compositional variations of garnet/cordierite-poor S-type granites; (2) relatively K2O-rich mafic to intermediate rocks are appropriate sources for calc-alkaline I-type granites. The compositional variations of calc-alkaline granites are jointly controlled by peritectic assemblage entrainment and subsequent fractional crystallization; (3) while dehydration melting at T>950°C is appropriate for the production of ferroan and alkali-rich granitic melts from intermediate magnesian tonalite or granodiorite, it is also possible for ferroan, alkali–rich and fluorine-rich granitic melts to be produced by dehydration melting of moderately magnesian mica–bearing materials at T≤900°C. Nevertheless, the low-T melts are more peraluminous than the high-T ones. Therefore, the composition of source rocks exerts the first-order control on the composition of granitic melts in closed systems. In addition, the dehydration melting of crustal rocks under different conditions is also responsible for variations in the composition of granites.
•Lithochemical data of experimental melts are grouped based on the species of hydrous minerals in starting materials.•Lithochemical fractionation is significant between experimental melts and starting materials.•Source composition and melting temperature are controlling factors on the lithochemistry of granitic melts.•The experimental results provide lithochemical constraints on granite petrogenesis in closed systems.
The composition of Mars Yoshizaki, Takashi; McDonough, William F.
Geochimica et cosmochimica acta,
03/2020, Volume:
273
Journal Article
Peer reviewed
Open access
Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional ...model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.
Silicate weathering is critical to global carbon cycle and climate change, and has attracted considerable attention in Earth science studies. Chemical weathering intensity evaluation and ...paleoclimatic reconstruction based on siliciclastic sediment composition analysis are popular topics. However, chemical weathering signals are difficult to be accurately extracted from sediment compositions due to multiple, complex processes and factors in sediment source-to-sink systems. To better understand the bias in evaluation of chemical weathering intensity, we conduct Monte Carlo simulations to explore the relationships between sediment compositions and widely-used major elements-based weathering indices, i.e., Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), Weathering Index of Parker (WIP), the modified CIA (CIX) and representative element ratios (i.e., K/Al, Na/Al, Na/K, SiO2/Al2O3). Most chemical weathering indices are dominated by the total clay mineral content in sediments and exhibit remarkable grain size bias from hydrodynamic sorting, manifesting by variations of ca. 20 in CIA, CIW, PIA, WIP and CIX values between simulated sandy (15 wt% clay minerals) and muddy (47 wt% clay minerals) sediments. Clay mineral species, which might be shaped by source lithology, climate and even diagenesis, display noticeable effects to weathering indices in clay mineral-rich sediments. Although the plagioclase/K-feldspar ratios almost have no influence on CIA, the effects of detrital feldspar types and abundances are prominent on most weathering indices. Thus, source lithological bias cannot be overlooked in sediment weathering intensity evaluation under various climatic conditions because of the significant lithology controls on feldspar fertility and clay mineral species. The WIP and SiO2/Al2O3 indices are highly sensitive to quartz contents (e.g., quartz/feldspar ratios), which are closely related to sediment recycling and source lithology. This study, from the numerical simulation perspective, provides quantitative insight into biases in sediment chemical weathering intensity evaluation. Our findings highlight the importance of a comprehensive understanding how the sediment source-to-sink processes and factors influence siliciclastic sediment compositions, textures and chemical weathering indices. We advocate that a systematic analysis (e.g., mineralogy, grain size, provenance, etc.) on targeted sediments or sedimentary rocks is necessary for interpreting sedimentary weathering records.
•Clay mineral content is the primary control of most sediment weathering indices.•Effect of clay mineral types on the indices is noteworthy in fine-grained sediments.•Detrital feldspar species and abundance have prominent effects on most indices.•Source lithology and sediment grain size may produce bias in weathering evaluation.•Bias analysis is necessary for interpreting sedimentary weathering records.
Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. ...Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments. Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(Mg + Fe) in equilibrium with Fo sub(90) olivine. Both sets of corrected compositions show large variations in Na sub(2)O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al sub(90) and Fe sub(90). These new data are not in agreement with the presentation of Niu & O'Hara (Journal of Petrology 49, 633-664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for high- and low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 200 degree C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slow-spreading ridges display a characteristic chemical signature of elevated Na sub(90) and Al sub(90) and lowered Si sub(90) relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si sub(90) should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle-melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K sub(90). Such variations do not correlate with the other major elements, showing that major element and trace element (and isotope) heterogeneity reflect different processes. This supports the model of movement of low-degree melts for the creation of trace element and isotope mantle heterogeneity, and is inconsistent with large variations in the amount of recycled crust in most ocean ridge mantle sources.
By coordinating the design and distribution of global climate model simulations of the past, current, and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the ...foundational elements of climate science. However, the need to address an ever-expanding range of scientific questions arising from more and more research communities has made it necessary to revise the organization of CMIP. After a long and wide community consultation, a new and more federated structure has been put in place. It consists of three major elements: (1) a handful of common experiments, the DECK (Diagnostic, Evaluation and Characterization of Klima) and CMIP historical simulations (1850–near present) that will maintain continuity and help document basic characteristics of models across different phases of CMIP; (2) common standards, coordination, infrastructure, and documentation that will facilitate the distribution of model outputs and the characterization of the model ensemble; and (3) an ensemble of CMIP-Endorsed Model Intercomparison Projects (MIPs) that will be specific to a particular phase of CMIP (now CMIP6) and that will build on the DECK and CMIP historical simulations to address a large range of specific questions and fill the scientific gaps of the previous CMIP phases. The DECK and CMIP historical simulations, together with the use of CMIP data standards, will be the entry cards for models participating in CMIP. Participation in CMIP6-Endorsed MIPs by individual modelling groups will be at their own discretion and will depend on their scientific interests and priorities. With the Grand Science Challenges of the World Climate Research Programme (WCRP) as its scientific backdrop, CMIP6 will address three broad questions: – How does the Earth system respond to forcing? – What are the origins and consequences of systematic model biases? – How can we assess future climate changes given internal climate variability, predictability, and uncertainties in scenarios? This CMIP6 overview paper presents the background and rationale for the new structure of CMIP, provides a detailed description of the DECK and CMIP6 historical simulations, and includes a brief introduction to the 21 CMIP6-Endorsed MIPs.
Coal, containing all the elements that are present in nature and more than 200 minerals, has a complex chemical structure, making it one of the most complex geological materials. Inorganic matter in ...coal includes minerals (in which element concentration may vary from trace to major), non-crystalline mineraloids, and elements with non-mineral associations such as those occurring in pore waters, organically bound, or in an organic association. Understanding the modes of occurrence of elements in coal is important because, theoretically, they provide useful information on peat deposition, diagenesis and epigenesis of coal, coal-hosted basin formation, and the regional geological background or evolution. Practically, the modes of occurrence of elements play a significant role in affecting coal mining, coal preparation, coal combustion, and coal utilization, and in exerting adverse effects on both the environment and human health. The modes of occurrence of critical elements in coal and coal ash are key factors for designing the method and technology required for extracting critical metals from coal or coal ash. In this paper, the following aspects are reviewed, including the modes of occurrence of 73 elements and rare gases that occur in coal (with the exceptions of organically associated C, H, O, and N), the definition of modes of occurrence and their practical and academic significance, analytical methods for determining modes of occurrence of elements in coal and their advantages and limitations, and reported modes of occurrence of elements in coal and their likely associations.
Overall, the modes of occurrence of elements in coal are classified into inorganic, organic, and intimate organic associations. Although there are common modes of occurrence of many elements in coal, there are many exceptions and most, if not all, elements have multiple modes of occurrence. Each mode of occurrence of an element may also show different levels of confidence, namely, certain, probable, possible, doubtful, unlikely, and may occur in coal with different frequencies, namely abundant, common, uncommon, rare, and unlikely. For each element, the authors present concluding comments viewpoints on the modes of occurrence of almost each element in coal that is listed in the old literature.
The different modes of occurrence for each element in different coals depend on the geological conditions of coal formation, and do not necessarily indicate inconsistency in the reported results. However, due to limitations of the analytical methods used, some data relating the modes of occurrence of elements in coal are not convincing, and in some cases are invalid or even misleading. Overall, while precisely determining the concentrations of many elements in coal is not difficult, determination of the modes of occurrence of some elements, particularly those with low concentrations and high volatility, is still a challenge. Although analytical methods certainly play critical roles in determining the modes of occurrence of elements in coal, in-depth understanding of the nature of the coal and host-rocks and the geological background of coal formation is very useful in investigating when and how these modes of occurrence of elements were formed.
The Miaodao Archipelago, located in the southern Bohai Strait, serves as a major route for the exchange of energy and materials between the Yellow and Bohai seas and is distinguished by a complicated ...hydrodynamic environment and provenance. Based on the grain size and elemental data of 107 surface sediment samples from the archipelago waters, and 58 coastal erosional sediment and bedrock samples, this study investigated the factors influencing the geochemical elemental distribution, discusses the provenance of the surface sediments and relative contributions, as well as proposes a sediment source–sink model for the region. The distribution of major and trace elements in the surface sediments, typically characterized by “high in the north, low in the south” patterning (except for SiO2, Ba, and Sr), with their contents primarily controlled by the input of terrestrial materials. The surface sediments in the study area are separated into the geochemical subdivisions of north and south. The main provenances of the study area include the Yellow River, the nearshore Miaodao Archipelago, and the north side of nearshore Penglai, whose sediments were transported under the combined action of the Shandong Peninsula's tidal current, waves, and longshore current. Deposition occurred under the capturing action of the archipelago. The relative contributions of the three provenance areas were 74.7%, 15.5%, and 9.8%, respectively; the relative contribution of coastal erosion reaches 1/3 of the river. Studies have demonstrated that the contribution of coastal erosion provenances to sea surface sediments is non-negligible, and in fact, it may even be quite important; thus, in the context of decreasing riverine inputs to the sea and enhanced coastal erosion processes, the contribution of coastal erosion requires special attention.
•In the context of decreasing riverine inputs to the sea, the contribution of coastal erosion requires special attention.•The relative contribution of coastal erosion to sediment sources reaches 1/3rd of the river.•The sea area of Miaodao Archipelago is both an important source and a sink of sediment.
Coal, one of the most complex geological materials, consists of organic and mineral matter, the latter including crystalline minerals, non-crystalline mineraloids, and elements with non-mineral ...associations. Overall, the modes of occurrence of elements in coal are classified into organic, mineral, and intimate organic associations, the latter including those adsorbed on to the surface of organics, dissolved in pore waters, and hosted in very fine-grained minerals (sub-micro- or nano-minerals) encased in or shielded by the organic matter of coal. Mineral associations, defined as elements associated with minerals are straightforward; however, confusion about organic and intimate organic associations of elements usually arise in the literature. Understanding organic, mineral, and intimate organic associations of elements is important not only because non-mineral elements and, to a lesser extent, elements associated with fine-grained minerals, play a significant role in affecting the utilization of coal, but also such modes of occurrence of elements provide useful geochemical information on coal formation and coal-bearing basin evolution. With a few exceptions (such as Cd, Nb, Ta, Zr, and Hf), most elements determined in coal, particularly in low-rank coal, have varying-degrees of organic association. In this paper, we review the definition of associations of non-mineral elements in coal, as well as their methods of determination, and then review the associations of selected elements including environmentally-sensitive (e.g., S, As, U, and Hg) and critical elements, the latter of which drive some of the significant advancements in technology and energy efficiency in the world today (e.g., rare earth elements and Y, Ge, and U), and some major elements (e.g., Ca, Mg, Fe, Al, and Ti) that largely occur in non-mineral forms in low-rank coals.
•The modes of occurrence of elements in coal are classified into organic, mineral, and intimate organic associations•The intimate organic associations include those adsorbed on to the surface of organics, dissolved in pore waters, and hosted in very fine-grained minerals.•Most elements determined in coal have varying-degrees of organic association.
This research reveals the depositional environment can affect the organic matter enrichment. The major elements and trace elements are important indexes to reconstruct the depositional environment of ...Ordovician subsalt marine formations in the central-east Ordos Basin, combining organic parameters with inorganic parameters. Based on the microfacies, the study area is divided into gypsum-bearing dolomite flat, dolomite flat, gypsum-salt depression and limy dolomite flat. The higher Ti/Al ratios and the characteristics of trace elements show that there were detrital inputs in the sedimentation process. When the amounts of detrital inputs are large enough, they can obviously adsorb the elements and dilute the organic matter. Therefore, gypsum-bearing dolomite flat and dolomite flat have lower organic matter accumulation with higher detrital input. Lower P/Ti and Baexcess indicate the deficiency of paleoproductivity. Sr/Ba is higher than 0.6, indicating marine-continental transitional and marine environment. Even the paleosalinity is higher, previous researches have demonstrated the descending sea levels and strong hydrodynamic conditions are unfavorable for organic matter preservation. Lower paleoclimate index “C” and higher Sr/Cu illustrate arid and hot conditions. The value of V/(V + Ni) suggests that the limy dolomite flat is under the oxidizing conditions, and the three other environments are reductive to a certain extent. The Mo-U covariation shows that the water body is under strongly restricted conditions. These conditions are not conducive to biological growth and the accumulation of organic matter, causing low ancient productivity. The partial correlation analysis shows: the organic matter enrichment is affected by multiple factors; paleoproductivity, paleosalinity and paleoclimate have greater influence on the results of organic matter enrichment, and detrital input and oxidation conditions can decrease the paleoproductivity, indicating multiple factors are interrelated and interact with each other.
•Major and trace elements are used to reconstructs the depositional environment of four types microfacies.•Paleoproductivity, paleosalinity and paleoclimate have greater influence on the organic matter enrichment.•Depositional environment has direction relationship with low paleoproductivity, causing low organic matter enrichment.
•Garnet schists from the South Beishan Orogen experienced high-grade metamorphism at ca. 900Ma.•~900Ma metamorphism is coeval with extensive continental arc formation in the SBOB and CTA.•The SBOB ...and CTA occupied in the periphery of the Rodinia during the final assembly stage.
An early extensive Neoproterozoic (ca. 900Ma) continental magmatic arc system covering hundreds of kilometers has been reported to occur in the South Beishan Orogenic Belt (SBOB) and the Central Tianshan (CTA) in the southern Central Asian Orogenic Belt (CAOB). However, evidence for coeval high-grade metamorphism and thus the formation of an accretionary orogen in the framework of Rodinia is ambiguous or absent. This study provides new petrological, geochemical and geochronological data for garnet-bearing schists (quartz+garnet+biotite+plagioclase±muscovite) from the SBOB in order to constrain its Neoproterozoic metamorphic history. The metamorphic zircon rims are either unzoned or display sector zoning in CL-images and reveal REE patterns with flat HREE patterns and negative Eu anomalies, which are interpreted to be in chemical equilibrium with garnet and plagioclase. The zircon U-Pb dating yields concordant U-Pb ages of 900±3Ma, 897±2Ma and 898±4Ma for the metamorphic zircon rims. The inherited detrital zircon cores of one sample display a concordant U-Pb age of 1397±5Ma that is consistent with the timing of formation for the extensive Mesoproterozoic continental arc in the SBOB and CTA. Based on phase equilibrium geothermobarometry and average P-T thermobarometric calculations, minimum amphibolite-facies P-T conditions are estimated to be >600°C at pressure >0.6GPa, which is thought to have been overprinted by subsequent Paleozoic metamorphism. However, the Ti-in-zircon thermometer still reveals temperatures of up to 840°C using the composition of metamorphic zircon rims, suggesting former ca. 900Ma granulite-facies peak metamorphic temperatures. The combined petrological and geochronological evidence in conjunction with the continental affinity of the regional metamorphic rocks suggests that the SBOB and the eastern CTA experienced an early Neoproterozoic accretionary orogenesis during the final assembly stage of Rodinia.
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