Geochemical Constraints Research Articles

Post-landing major element quantification using SuperCam laser induced breakdown spectroscopy

The SuperCam instrument on the Perseverance Mars 2020 rover uses a pulsed 1064 nm laser to ablate targets at a distance and conduct laser induced breakdown spectroscopy (LIBS) by analyzing the light from the resulting plasma. SuperCam LIBS spectra are preprocessed to remove ambient light, noise, and the continuum signal present in LIBS observations. Prior to quantification, spectra are masked to remove noisier spectrometer regions and spectra are normalized to minimize signal fluctuations and effects of target distance. In some cases, the spectra are also standardized or binned prior to quantification. To determine quantitative elemental compositions of diverse geologic materials at Jezero crater, Mars, we use a suite of 1198 laboratory spectra of 334 well-characterized reference samples. The samples were selected to span a wide range of compositions and include typical silicate rocks, pure minerals (e.g., silicates, sulfates, carbonates, oxides), more unusual compositions (e.g., Mn ore and sodalite), and replicates of the sintered SuperCam calibration targets (SCCTs) onboard the rover. For each major element (SiO2, TiO2, Al2O3, FeOT, MgO, CaO, Na2O, K2O), the database was subdivided into five “folds” with similar distributions of the element of interest. One fold was held out as an independent test set, and the remaining four folds were used to optimize multivariate regression models relating the spectrum to the composition. We considered a variety of models, and selected several for further investigation for each element, based primarily on the root mean squared error of prediction (RMSEP) on the test set, when analyzed at 3 m. In cases with several models of comparable performance at 3 m, we incorporated the SCCT performance at different distances to choose the preferred model. Shortly after landing on Mars and collecting initial spectra of geologic targets, we selected one model per element. Subsequently, with additional data from geologic targets, some models were revised to ensure results that are more consistent with geochemical constraints. The calibration discussed here is a snapshot of an ongoing effort to deliver the most accurate chemical compositions with SuperCam LIBS.

Sedimentological, petrological, and geochemical constraints on the formation of the Beisantai sandstone-type uranium deposit, Junggar Basin, NW China

A sandstone-type uranium deposit has recently been discovered at the bottom of the Neogene Shawan Formation, Beisantai area, southeastern Junggar Basin. In this study, optical and scanning electron microscopic (SEM) observations, whole-rock geochemistry, X-ray fluorescence (XRF) mapping of thin sections, and the electron probe microanalysis (EPMA) of uranium minerals were applied, combined with the sedimentological and tectonic background, to investigate the controlling factors of uranium mineralization. Uranium ores are mainly concentrated on the interface between conglomerate and mudstone both macroscopically and microscopically. Among them, the conglomerate with less interstitial matrix, characterized as high primary porosity and permeability, offers essential passageways for the runoff of groundwater. In contrast, the margin of conglomerate adjacent to the barrier, somewhere with sufficient water–rock interactions, good supply of reducing materials, and efficient adsorbents, is favorable for the precipitation and preservation of uranium minerals. Therefore, the combination of perfect permeable layers and barriers improves the mineralization. From the geochemical data, U is positively correlated with redox-active elements such as S, As, Mo, and V, especially for permeable rocks, suggesting a redox-controlled system. This deposit occurs in a weak reducing reservoir, revealed by the primary green color, and the very low Fe2+/Fe3+ values (0.35–0.77). The much higher enriched Mo (5.89–172.5 ppm, mean = 67.04 ppm) than Se (2–17 ppm, mean = 8.20 ppm) in the redox transition zone suggests that the mineralization is processed during the slow and gentle interlayer oxidation. The pyrites replaced by uranium minerals act as significant reductants in U(Ⅵ) reduction, during which S contributes dominantly over Fe revealed from the increased Fe/S values with the decrease in Fe and S contents in coffinite. Results of EPMA chemical dating reveal that the emplacement of uranium ores started at least from 8 Ma, corresponding to the intensive north–south compression event of the basin since 10 Ma. During the second episode of Himalayan movement, the bottom of the Shawan Formation was gradually denudated and outcropped in the north feasible to the formation of interlayer oxidation zone-type uranium deposit in the Beisantai area. The mineralization is a long-term and continuous process that probably persists till the present.

Petrological and geochemical constraints on the petrogenesis of granitoids in the Gonghe geothermal basin, western Qinling (China)

The Gonghe geothermal basin is situated in the westernmost part of the western Qinling orogen and possesses the supreme potentials to explore and develop hot dry rock (HDR) geothermal resources in China. The basal rock of HDR reservoir in Gonghe basin is composed of granitoids. In this contribution, in order to constrain the formation mechanism of the Gonghe HDR, granitic samples from outcropping have been collected and drilling core samples data from the archives also been compiled. The studied results show that the obtained zircon U–Pb ages of 255.1 ± 1.9 Ma and 253.9 ± 2.6 Ma for granodiorite and monzogranite, respectively. All granitoids from both outcropping and drilling core have the similar rock associations and their major and trace elements possess the consistent evolved patterns, such as relative enrichment of LILEs (e.g., K, Rb), depletion of HFSEs (e.g., Nb, Ti), and negative Eu anomalies (0.32–0.66), indicating that they share the same magma sources and evolutional processes. And they have the affinity of I-type granites. They were derived from the mixture of partial melting of metabasaltic rocks in the lower crust and a small amount of mantle-derived magma and formed in a subduction-related setting relative to the southward subduction of the Zongwulong oceanic crust beneath the west Qinling terrane. The heat production values of rocks around the Gonghe basin were calculated and all basal granitic rocks ranges from 0.35 to 8.51 μW/m3, yielding an arithmetic mean value of 1.97 μW/m3, indicating that the radioactive heat generation capacity of granitoids in the Gonghe basin is slightly lower. The HDR reservoir in the Gonghe basin was probably due to an allied thermal effect of the radiogenic heat production in the thickening continental crust and the heat contributed by a deep magma chamber below the basin. The early Triassic granitoids with a higher thermal conductivity serve as the conductive medium of the geothermal heat; whilst the sedimentary rocks marked by lower thermal conductivity above the granitic batholiths act as the reservoir cap. This contribution provides a relatively rational understanding for the mechanism of the HDR reservoir in Gonghe geothermal basin from the petrological and geochemical perspectives, which is conducive to the geothermal potential assessment and estimation, and to-be implementation of the Enhanced Geothermal System demonstration project in China.

Geochemical constraints on the origin and evolution of the volcanic Rooiberg Group, Bushveld Large Igneous Province, South Africa

This study combines a new bulk-rock dataset of major and trace element compositions with an existing database for the Rooiberg Group, the latter representing one of the largest felsic volcanic provinces in the world and recording the earliest magmatic activity associated with the Bushveld Large Igneous Province (LIP), South Africa. The Rooiberg Group comprises four formations including, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formations. The Dullstroom Formation consists of a differentiated suite ranging from basalt to rhyolite, whereas the three upper formations are composed principally of rhyolite. Intrusion of the mafic–ultramafic rocks of the Rustenburg Layered Suite (RLS) physically divided the Dullstroom Formation into a lower portion beneath the RLS consisting mainly of basalts with minor felsic volcanics, and an upper section above the RLS dominated by dacite and rhyolite. Bulk-rock geochemical data combined with quantitative trace element modelling indicate that the low-Ti and high-Ti basalts and dacite-rhyolites of the Dullstroom Formation collectively represent an advanced degree of partial melting of a mafic, lower crustal source genetically linked to a Mg-rich magma referred to as the B1 parental magma of the Lower Zone and lower Critical Zone of the RLS and comprising mainly pyroxenite, norite and peridotite. In contrast, the silica-rich Damwal, Kwaggasnek and Schrikkloof Formations of the Rooiberg Group are the products of a fractionally crystallised melt derived from a different source rock to that for the Dullstroom suite, likely of intermediate composition. This study also suggests that the low-Ti and high-Ti basalts of the Dullstroom Formation were derived from a common source, and that the variation in the Ti content is due to the crystallisation of phenocrystic, Ti-rich magnetite present only in the high-Ti basalts, whereas the Ti-poor magnetite of the low-Ti basalts precipitated from a slurry dominated by interstitial liquid.

Petrogenesis of Cenozoic Basaltic Rocks from the Leiqiong Area, South China: Evidence from Geochemical Constraints

Petrogenesis of Cenozoic Basaltic Rocks from the Leiqiong Area, South China: Evidence from Geochemical Constraints

Geochemical constraints on the formation of chondrules: Implication from Os and Fe isotopes and HSE abundances in metals from CR chondrites

CR chondrites are suitable for understanding the genetic linkage between metals and chondrules due to the unique characteristics of the coexisting metal phases with chondrules. Metal grains are found in three different locations of CR chondrites; chondrule interior (“interior grain”), chondrule surficial shells (“margin grain”), and the matrix (“isolated grain”). Here we report the abundances of highly siderophile elements (HSEs) and major elements in three types of metals (interior, margin, and isolated grains) from three CR chondrites (NWA 801, NWA 7184, and Dhofar 1432) by using femtosecond LA-ICP-MS (fs LA-ICP-MS) and EPMA. Additionally, we report the isotopic compositions of Os and Fe in the metals by using micro-milling sampling coupled with N-TIMS and MC-ICP-MS. The CR metals have variations in 187Os/188Os and δ57Fe values ranging from 0.1193 to 0.1314 and from −1.05 to +0.25, respectively. HSE abundances, except for Pd and Au, in the three types of metals increase as the abundance of Ir increases. A possible explanation for the variations of HSE abundances within and among grains, 187Os/188Os values within each grain, and δ57Fe values among grains, is the condensation of liquid metal from a gaseous reservoir followed by fractional crystallization. Most of the CR metals have negative δ57Fe values, suggesting that Fe in metal phases might have formed by condensation prior to Fe condensation in silicate phases. The chondrules and three types of metal grains in CR chondrites are believed to have formed contemporaneously in the same region. The existence of large isolated metals in matrix and compound chondrules might be the result of collision and merging of the metal and silicate droplets.

New geochemical constraints on the development of active continental margin in Southeast China during the Middle Permian and its tectonic implications

Two competing hypotheses that passive or active continental margins developed in South China during the Late Paleozoic are still under debate. Previous studies were mainly focused on the remnant magmatic rocks, but little attention has been paid to the contemporary sediments deposited in the back-arc basin. Therefore, a deeper knowledge of the tectonic link between the continental arc and regional basins is not only vital for understanding their tectono-sedimentary evolution, but also provide the potential to evaluate these hypotheses. Here two Middle Permian sections of bedded chert-mudstone sequence (Gufeng Formation) and an overlying siliciclastic succession (Yinping Formation) from the Lower Yangtze region, were analyzed to understand their provenance, tectonic and depositional setting, and to provide new constraints on the Late Paleozoic tectonics of the South China. Discrimination of tectonic setting indicates that the Middle Permian sedimental source rocks of the Lower Yangtze basin probably originated in a transitional setting from continental island arc to active continental margin, consistent with the development of an active margin magmatic arc along Southeast China during that time. Provenance analysis also showed that, the Gufeng Formation was mainly derived from an Andesitic arc source with mixed mafic and felsic source, whereas the Yinping Formation received detritus from a more felsic source. This provenance switching could reflect a possible tectonic transition related to the continental arcs’ evolution that from Andean-type normal/high-angle subduction to flat-slab subduction, which weakened the arc volcanism and intensified the bedrock uplift in South China. The depositional environments of the Lower Yangtze basin also evolved from an open deep ocean environment to a continental margin setting during the deposition of Gufeng Formation, and then transformed into a coastal/neritic shallow water setting during the deposition of Yinping Formation. This sedimentary evolution can be attributed to the tectonic transition during the Middle Permian.

New geochronologic and geochemical constraints for the Pedro Pereira metavolcanosedimentary sequence: Evidence for a 2.77 Ga oxygen oasis record in the São Francisco-Congo paleocontinent

Exposed within the reworked basement of the Araçuaí orogen, eastern Brazil, the Pedro Pereira metavolcanosedimentary sequence (PPMS) comprises a lower unit dominated by metaultramafic/mafic rocks, an intermediate unit dominated by metarhyolites and a predominant upper unit of acid metavolcanites associated with chemical and clastic metasedimentary rocks. This sequence has been defined and investigated in the ’80s when it was considered as a Mesoarchean greenstone belt. Since then, few studies have addressed these rocks, and due to the lack of reliable geochronological data, a Paleoproterozoic age has also been suggested for the sequence. To reevaluate the age and evolution of the unit, new whole-rock geochemical and zircon U–Pb (LA-ICPMS) geochronologic data were obtained for the iron formation and acid metavolcanic rocks, respectively. When normalized to post-Archean Australian Shale standard, BIF samples can be divided into three groups. Group 1 BIF samples are marked by significant enrichment of the LREE, very high ∑REE + Y, positive La anomaly, extreme negative Ce anomaly, negative Eu anomaly, and subchondritic Y/Ho ratio. In contrast, Group 2 BIFs are marked by the enrichment of HREE, low ∑REE + Y, positive La anomaly, negative Eu anomaly, slight negative Ce anomaly, and predominantly super- and subchondritic Y/Ho ratios. The massive iron formation (MIF) group shows very high middle-to-heavy REE + Y enrichment, strong negative Ce anomaly, negative Eu anomaly, very high ∑REE + Y, and variable Y/Ho ratios. The modification of the iron formation by post-depositional hydrothermal alteration might have disturbed the original basin water signature. However, Group 2 is more compatible with pure chemical precipitates and its slight negative anomalies suggest an oxic environment. A PPMS metarhyolite from the upper portion of the sequence interlayered with the BIFs yielded the Concordia Age of 2771 ± 8.4 Ma. This age rules out a Paleoproterozoic origin for the PPMS, as suggested/considered by recent papers. The results imply that PPMS possibly represents one of the oldest records of oxygenation previous to the GOE in the geologic archives and could provide information to understand the early evolution of localized pockets of oxygen before the Great Oxidation Event.

Initial Cenozoic magmatic activity in East Africa: new geochemical constraints on magma distribution within the Eocene continental flood basalt province

Abstract The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centred on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly alkaline small-volume Akobo Basalt (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric–mantle metasomes; (2) the primitive and spatially restricted Amaro Basalt (45.2–39.8 Ma), representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly; and (3) the spatially extensive Gamo–Makonnen magmatic unit (38–28 Ma), representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere and resulted in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 myr preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia that is associated with the East African Rift.

Origin of dolomites in oolitic carbonates of the Middle Jurassic Dorgali Formation, eastern Sardinia, Italy: Petrographic and geochemical constraints

The Bathonian-Callovian (Middle Jurassic) Dorgali Formation in eastern Sardinia (Italy) consists of dolomitized oolitic grainstone and records a complex diagenetic history. Petrographic examinations revealed three main phases of replacive (D1, D2) and fracture-filling (D3) dolomites with crystal-size ranges of <4–50 μm (dolomicrite to dolomicrosparite D1), 50–200 μm (eu-to subhedral dolomite D2), and 50–400 μm (sub-to anhedral crystals of dolomite D3). The early D1 exhibits dull luminescence (CL) under cathodoluminoscope, whereas the later D2 shows dull to zoned CL, and D3 consistently zoned CL. The near-micritic grainsize, low Sr contents (136 ± 43 ppm) and estimated δ18O values of the parent dolomitizing fluids (−1.3‰ to −0.9‰ VSMOW) suggest an early-diagenesis origin for D1 at low temperatures of near-surface conditions. In contrast, the larger crystal size and microthermometric measurements of primary two-phase fluid inclusions (Th = 101.7 ± 8.0 °C) reveal that D2 precipitated during later stages at higher temperatures of mid-to deeper burial settings, which is supported by the high calculated δ18O values of D2 parent dolomitizing fluids (10.0‰–10.7‰ VSMOW). Petrographic examinations suggest that the vug- and fracture-filling D3 and coarse calcite cement (C3, up to 2 mm) precipitated at deeper burial conditions, which is consistent with the low δ18O (−5.2 ± 1.0‰ VPDB) and δ13C (−5.1 ± 2.6‰ VPDB) compositions of C3. The similarity in shale-normalized REESN patterns, strong negative Ce anomalies and lack of Eu anomalies, suggest a seawater origin for the parent dolomitizing fluids.

Mantle wedge enrichment beneath southern Tibet during the late stage (100–45 Ma) of oceanic subduction: Geochemical constraints from mantle-derived intrusions

The Miocene K-rich mafic rocks in the Lhasa Terrane of the southern Tibetan Plateau are considered to be related to either the subduction of the Neo-Tethyan oceanic slab or Indian continental slab. In order to investigate how subduction of the Neo-Tethyan oceanic slab has geochemically affected the mantle beneath southern Tibet, we obtained geochronological and geochemical data for the late Early Cretaceous Cangguo gabbros (97 Ma) and early Eocene Langka diorites (51 Ma) in southern Tibet. Although both the Cangguo gabbros and Langka diorites are characterized by arc-like trace element patterns, with enrichment in light rare earth and large-ion lithophile elements (e.g., Ba, Pb, and U), depletion in high-field-strength elements (e.g., Nb and Ta), and depleted Sr–Nd–Hf isotopic compositions, there are some other major geochemical differences (e.g., Ba/La and Th/La ratios) between these two suites of rocks. These geochemical characteristics of the Cangguo gabbros and Langka diorites indicate they were produced by melting of the depleted mid-ocean ridge mantle (DMM) that was enriched by oceanic sediment-derived fluids and melts, respectively. Our data and previous studies suggest that dehydration and melting of subducted Neo-Tethyan oceanic sediments beneath the Lhasa Terrane occurred in the late stage (100–45 Ma) of Neo-Tethyan oceanic slab subduction, which may have enriched the mantle beneath the Lhasa Terrane. However, the mantle beneath the Lhasa Terrane that was enriched by the subduction of oceanic slab during 100–45 Ma is still isotopically more depleted than the source of Miocene post-collisional K-rich mafic rocks in the Lhasa Terrane. This indicates that the enriched component in the mantle source of the K-rich mafic rocks cannot be simply attributed to materials derived from the Neo-Tethyan oceanic slab, and other additional materials are required.

Large igneous provinces of the Amazonian Craton and their metallogenic potential in Proterozoic times

Abstract This paper overviews the Proterozoic large igneous provinces (LIPs) of the Amazonian Craton, characterized by large volumes of extrusive and intrusive magmatic rocks. We reassess the geologic, geochronological and geochemical information to stablish three intracontinental felsic volcanic–plutonic igneous belts (i.e. SLIPs), namely: Orocaima (1.98–1.96 Ga), Uatumã (1.88–1.87 Ga) and Alta Floresta (1.80–1.79 Ga). The Avanavero LIP (1.79–1.78 Ga), as well as the Rincón del Tigre-Huanchaca LIP (1.11 Ga) are also revisited. The relationships of these events to intraplate settings through time and space are apparent. We examine the main characteristics of each magmatic event in light of the U–Pb zircon and baddeleyite ages and coupled isotopic–geochemical constraints, the geodynamic significance and metallogenetic potential. The Uatumã and Alta Floresta SLIPs host the most important mineral resources within the Amazonian Craton. Global barcode matches of the Proterozoic SLIP/LIP events of Amazonia are also addressed, as well as their possible links with geological timescale periods: the Orosirian, Statherian and Stenian boundaries. We also evaluate the available palaeomagnetic data to address issues related to the barcode match of such SLIP/LIP events in the context of supercontinent cycles.

New geochronological and geochemical constraints on petrogenesis and tectonic setting of the Loe-Shilman carbonatite complex, Northwest Pakistan

The Loe-Shilman carbonatite complex in northwest Pakistan comprises carbonatite and ultrapotassic silicate rocks. The age and environment of emplacement of these rocks have long been controversial, and therefore, impeded our understanding on the regional tectonic setting. New whole-rock geochemistry, Sr-Nd isotopic and zircon petrochronological data provide age and petrogenetic information on the carbonatite and ultrapotassic silicate rocks of the Loe-Shilman complex. Zircon U/Pb data yield 206Pb/238U weighted mean ages of 90.6 ± 1.0 Ma (n = 33) for the carbonatite and 89.3 ± 1.0 Ma (n = 30) for the ultrapotassic rocks, which indicates that the two lithologies were emplaced simultaneously during a single tectonic/thermal event. The Nd isotope data of the ultrapotassic rocks is homogenous (εNd(i) = −2.5 to −2.9) and overlap εNd(i) of the carbonatite (−2.6 to −2.8), indicating derivation from a common, enriched mantle source. Despite this temporal overlap, whole-rock geochemical data, however, indicates that generation of both rock types by fractional crystallization or liquid immiscibility is unlikely and that the carbonatite and ultrapotassic rocks are actually products of two independent melts generated by partial melting of an enriched sub-continental lithospheric mantle (SCLM) source at the same time. Trace element data further indicate that the ultrapotassic rocks are products of fractional crystallization of a melt produced by a low degree partial melting of an enriched mantle source, within the garnet stability field. Moreover, the ultrapotassic rocks and carbonatites show association with anorogenic and within-plate plume type tectonic settings, coeval with juxtaposition of the northwestern margin of the Indian plate over the Réunion hotspot mantle-plume during Late Cretaceous time.

Geochemical constraints of hydrothermal alteration of dolostones: An example of Lower Cretaceous Qamchuqa Formation, Kurdistan Region, northern Iraq

This paper integrates major, minor, trace and rare-earth element (REE) data into C-, O-, Sr-isotopes, fluid-inclusions microthermometry of replacive matrix dolomite and vug/fracture-filling saddle dolomite and calcite of the Lower Cretaceous Qamchuqe Formation, the Kurditan Region of Iraq, in order to get further insights into the origin and geochemical evolution of their parent diagenetic fluids. Multiple generations of dolomite have been observed. These dolomites constitute replacive matrix dolomite and saddle dolomite cement, dominantly poorly ordered, non-ferroan, non-stoichiometric, with an average of 59.4 and 60 mol% CaCO3, respectively, and have low Sr, Fe and Mn concentrations. Replacive matrix dolomites display fairly comparable REE patterns to saddle dolomite, but with light REE (LREE) enrichment, and moderate low (∑REEs) contents (18.9–1111.5 and 16.6–593.4 ppb, respectively). Saddle dolomite has moderately positive Eu anomalies while replacive matrix dolomite has slightly less positive Eu anomalies and negative Dy, Ho and Yb anomalies. Both dolomites are characterized by slightly radiogenic 87Sr/86Sr ratios ranging from 0.707702 to 0.707839, overlapping, wide-ranging and negative δ18O values from −10.43 to −4.47 and −12.87 to −7.79 ‰VPDB, and δ13C values ranging from 0.38 to 3.57 and −3.45 to 3.11‰VPDB, respectively. The primary fluid inclusions microthermometry of replacive matrix and saddle dolomites yield homogenization temperatures (Th) ranging from 71.3 to 228.3 °C and 82–188 °C, respectively, and salinity values ranging between 14.4 to 25.4 and 15.6 to 27.9 wt% NaCl eq., respectively. These data suggest multiple hydrothermal fluid fluxes during and post Zagros orogeny. The geochemical data also show that these fluids derived from similar source of mixed hydrothermal and marine fluids, which caused the dolomitization of host rock and precipitation of dolomite cements.

Effect of an algal amendment on the microbial conversion of coal to methane at different sulfate concentrations from the Powder River Basin, USA

Biogenic methane is estimated to account for one-fifth of the natural gas worldwide and there is great interest in controlling methane from different sources. Biogenic coalbed methane (CBM) production relies on syntrophic associations between fermentative bacteria and methanogenic archaea to anaerobically degrade recalcitrant coal and produce methanogenic substrates. However, very little is known about how differences in geochemistry, hydrology, and microbial community composition influence subsurface carbon utilization and CBM production. The addition of an amendment consisting of microalgal biomass has previously been shown to increase CBM production while providing the possibility of a closed-loop fossil system where waste (production water) is used to grow algae to ultimately produce energy (methane). However, the efficiency of enhancing CBM production under different redox conditions remains unresolved. In this study, we focused on the U.S. Geological Survey's Birney test site (Montana, USA) that has nine wells vertically accessing four coal seams with varying geochemistry (low and high sulfate (SO42−)) and methane production rates. We used organic matter (OM) in the form of algal biomass to discern the effect of this amendment on OM degradation and microbially enhanced CBM production potential under different geochemical constraints. We tracked changes in community composition, OM composition, organic carbon (OC) concentration, methane production, and nutrients in batch systems over six months. Methane production was detected only in microcosms from low SO42− wells (168 to 800 μg methane per gram of coal). The OC consumption varied across time for all wells and the variation was greatest for the low SO42− wells. Different groups of syntrophic bacteria were associated with net‑carbon consuming microcosms, and specifically Syntrophorhabdus was identified with several different statistical methods as a potentially important coal degrader. Results from this study provide insight into potential coal-degraders, the compositional changes in some of the different OM fractions, and trends in carbon consumption related to methane production across coal seams along the vertical SO42− gradient.

Timing of Martian core formation from models of Hf–W evolution coupled with N-body simulations

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars.We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ∼5 million years of solar system formation to satisfy the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

Rutile and Chlorite Geochemistry Constraints on the Formation of the Tuwu Porphyry Cu Deposit, Eastern Tianshan and Its Exploration Significance

The chemical composition of rutile has been used as an indicator in magmatic and metamorphic-related diagenetic systems, but rarely in porphyry-style ore systems. The Tuwu deposit (557 Mt at 0.58% Cu) is a large porphyry-style Cu mineralization in Eastern Tianshan, Xinjiang, with typical disseminated, stockwork mineralized veins hosted in tonalite and diorite porphyry, and to a lesser extent in volcanic rocks of the Qi’eshan Group. We first present determination of rutile minerals coupled with chlorite identified in mineralized porphyries from Tuwu to reveal their geochemical features, thus providing new insights into the ore-forming processes and metal exploration. Petrographic and BSE observations show that the rutile generally occurs as large crystals (30 to 80 µm), in association with hydrothermal quartz, chlorite, pyrite, and chalcopyrite. The rutile grains display V, Fe, and Sn enrichment and flat LREE-MREE patterns, indicating a hydrothermal origin. Titanium in rutile (TiO2) is suggested to be sourced from the breakdown and re-equilibration of primary magmatic biotite and Ti-magnetite, and substituted by Sn4+, high field strength elements (HFSE; e.g., Zr4+ and Hf4+), and minor Mo4+ under hydrothermal conditions. The extremely low Mo values (average 30 ppm) in rutile may be due to rutile formation postdating that of Mo sulfides (MoS2) formation in hydrothermal fluids. Chlorite analyses imply that the ore-forming fluids of the main stage were weakly oxidized (logfO2 = −28.5 to −22.1) and of intermediate temperatures (308 to 372 °C), consistent with previous fluid inclusion studies. In addition, Zr-in-rutile geothermometer yields overestimated temperatures (&gt;430 °C) as excess Zr is incorporated into rutile, which is likely caused by fast crystal growth or post crystallization modification by F-Cl-bearing fluid. Thus, application of this geothermometer to magmatic-hydrothermal ore systems is questionable. Based on the comparison of rutile characteristics of porphyry Cu with other types of ore deposits and barren rocks, we suggest that porphyry Cu-related rutile typically has larger grain size, is enriched in V (average 3408 ppm, compared to &lt;1500 ppm of barren rocks) and to a lesser extent in W and Sn (average 121 and 196 ppm, respectively), and has elevated Cr + V/Nb + Ta ratios. These distinctive signatures can be used as critical indicators of porphyry-style Cu mineralization and may serve as a valuable tool in mineral exploration.

Metagenomic Insights Into the Microbial Iron Cycle of Subseafloor Habitats

Microbial iron cycling influences the flux of major nutrients in the environment (e.g., through the adsorptive capacity of iron oxides) and includes biotically induced iron oxidation and reduction processes. The ecological extent of microbial iron cycling is not well understood, even with increased sequencing efforts, in part due to limitations in gene annotation pipelines and limitations in experimental studies linking phenotype to genotype. This is particularly true for the marine subseafloor, which remains undersampled, but represents the largest contiguous habitat on Earth. To address this limitation, we used FeGenie, a database and bioinformatics tool that identifies microbial iron cycling genes and enables the development of testable hypotheses on the biogeochemical cycling of iron. Herein, we survey the microbial iron cycle in diverse subseafloor habitats, including sediment-buried crustal aquifers, as well as surficial and deep sediments. We inferred the genetic potential for iron redox cycling in 32 of the 46 metagenomes included in our analysis, demonstrating the prevalence of these activities across underexplored subseafloor ecosystems. We show that while some processes (e.g., iron uptake and storage, siderophore transport potential, and iron gene regulation) are near-universal, others (e.g., iron reduction/oxidation, siderophore synthesis, and magnetosome formation) are dependent on local redox and nutrient status. Additionally, we detected niche-specific differences in strategies used for dissimilatory iron reduction, suggesting that geochemical constraints likely play an important role in dictating the dominant mechanisms for iron cycling. Overall, our survey advances the known distribution, magnitude, and potential ecological impact of microbe-mediated iron cycling and utilization in sub-benthic ecosystems.

Geochemical Constraints on the Structure of the Earth's Deep Mantle and the Origin of the LLSVPs

AbstractGeophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.

Magma‐Mush Interactions in the Lower Oceanic Crust: Insights From Atlantis Bank Layered Series (Southwest Indian Ridge)

AbstractMagma migration and differentiation processes are key to understanding the development and evolution of oceanic magma reservoirs. To provide new quantitative geochemical constraints on these processes, we applied a high‐resolution approach to study an interlayered section of the lower oceanic crust sampled at Atlantis Bank, on the (ultra)slow‐spreading Southwest Indian Ridge. The section is characterized by sharp grain‐size layering between fine‐ and coarse‐grained olivine gabbros that is representative of other layered structures described at the Atlantis Bank oceanic core complex. The textures and fabrics of the layers and the nature of their contacts indicate formation by intrusion of a magma (i.e., crystal‐bearing) into an almost solidified coarse‐grained mush. Petrographic observations and in situ incompatible trace element signatures indicate that the fine‐ and coarse‐grained layers record reactive porous migration of melts. Widespread reactive porous flow occurred prior to intrusion within the coarse‐grained gabbro, producing mineral compositions enriched in incompatible elements. The intrusive fine‐grained lithology records a late stage event of localized reactive melt percolation in cm‐scale structures, which lead to strong light rare earth elements depletion relative to heavy rare earth elements. In addition, we highlight the occurrence of interactions at the contacts between layers and partial modification in compositions of the intruded lithology. This layered section likely represents a contact between two larger magma bodies emplaced within the lower crust during accretion, where the type of melt migration (intrusion or porous flow) and the modalities of melt percolation (widespread or localized) strongly govern the composition of the crustal lithologies.