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The Re-Os geochronometer constrains the timing of petroleum formation and provides an oil-source isotopic correlation tool for improved geologic knowledge and exploration success. However, the effects of secondary processes that may sometimes complicate interpretations are not well understood. This paper discusses Re-Os systematics in a petroleum system that experienced extensive asphaltene precipitation upon mixing of different oil phases. The Solveig oil field (formerly Luno II), Utsira high, Norwegian North Sea, comprises three structural compartments that capture different stages of the mixing process. In the central compartment, an Re-Os age of circa 40 Ma for whole-rock extracts sampling the residual (asphaltene-rich and biodegraded) oil below the oil–water contact is consistent with burial models for Paleocene–Miocene generation. A 10-m-thick, asphaltene-rich (65 wt. %) tar mat zone precipitated upon mixing of at least two components does not yield meaningful ages. The Re-Os isochrons for the free-flowing crude oil and whole-rock extracts from the oil leg above the tar mat zone indicate circa 10 to 0 Ma ages for the younger oil phase, supporting independent estimates for recent oil influx. Decreasing 187Os/188Os from maltenes to asphaltenes in oil from the southeastern compartment are highly unusual and provide geochemical evidence for recent mixing, consistent with the strong asphaltene-in-oil gradient. Very low and remarkably uniform Re/Os ratios for all petroleum phases, combined with relatively low and uniform 187Os/188Os ratios, strongly support isotopic overprint of the Re-Os system, likely during water–oil interaction. Despite this overprint, Re-Os ages retain information on primary events, whereas initial 187Os/188Os ratios are strongly affected.

The temporal coincidence between the Late Permian mass extinction (LPME) and the emplacement of Siberian Trap basalts suggests a causal link between the two events. Here, we discuss stratigraphic changes of organic and inorganic (including isotopic) geochemical properties of marine sediments across the Permian–Triassic boundary (PTB) in the Hovea-3 core, Western Australia, a key PTB section in the southern Neo-Tethys ocean. These data are compared with published data from the Meishan section, southern China, and from the Opal Creek section in western Canada, providing a view of Tethys and Panthalassa changes at the PTB. Trace metal and N-isotopic data, together with organic matter properties suggest that anoxic conditions were established prior to the LPME, intensified close to the LPME, and continued with photic-zone euxinia into the Early Triassic. For the Hovea-3 section, Re-Os ages confirm Changhsingian (253.5 ± 1.4 Ma) deposition of the dated interval sampled immediately below the stratigraphic level characterized by major lithological and isotopic changes. Evaluation of Re-Os, N, and Hg elemental and isotopic data for Hovea-3 suggests that anoxic conditions in the latest Permian were generally unrelated to direct magmatic contributions. A major increase in the initial Os isotopic ratio of Lower Triassic shales suggest an ~8× increase in the Early Triassic continental runoff, based on moderately conservative assumptions for end-members contributing Os to the Permian–Triassic ocean. Comparison to other PTB sections confirms a global signal of increasing Re/Os ratios in the Late Permian, and major and long-lived changes in the isotopic composition of the post-extinction ocean. A distinct peak in Hg concentrations carrying a volcanic isotopic signature, also identified in other PTB sections, likely represents a major pulse of Siberian Trap volcanism. This Hg peak in the Hovea-3 section, however, is detected above the stratigraphic level containing multiple other widely recognized and more permanent geochemical changes. Therefore, direct volcanic inputs to the Permian–Triassic Ocean likely post-date the LPME in this Western Australian section.

Petroleum systems are inherently complex, comprising multiple components and processes, which variably interact in space and time to form petroleum accumulations. Improved understanding of these complex systems requires a holistic approach of integrating diverse geological, geophysical and organic and inorganic geochemical data. Here, we present an applied and comprehensive Re-Os geochronological and geochemical study on the Brynhild petroleum system from the Norwegian North Sea. Essential components of this system include shale source rocks of the Mandal and Farsund Formations, reservoir sandstones of the Ula Formation and their bitumen extracts, and produced Brynhild crude oil. Rock-Eval, trace metal and Re-Os data reveal the excellent oil-generation potential of the shaley source rocks (high contents of organic matter containing large amounts of type I kerogens) and constrain their depositional conditions (anoxic bottom waters in a high-productivity regime driven by enhanced supply of land-derived nutrients). Seven Re-Os shale ages between ~146 and ~134 Ma are consistent with existing Volgian–Ryazanian age estimates and constrain the initial 187Os/188Os of the shales to ~0.52 in the lowermost Farsund Formation and ~0.75 in the uppermost Mandal Formation. Solvent-extracted reservoir rocks of the Ula Formation contain insoluble organic matter with type III–type IV kerogens likely supplied by erosion of older (a precise 179 Ma Re-Os age with unrealistically low initial 187Os/188Os) coal-bearing sediments or organic-rich shales. The Brynhild crude oil has relatively low 187Os/188Os and a generation age of ~44 Ma based on an asphaltene-only Re-Os isochron. Maltenes from the Brynhild crude oil are isotopically affected by interaction with organic-rich source rocks that represent the seal for the Brynhild oil field. Compared with the crude oil, bitumen from Ula Formation sandstones have contrasting Re-Os properties: high 187Os/188Os ratios and young generation ages between 8 and 0 Ma based on asphaltene-bitumen-maltene Re-Os triplets, asphaltene-only Re-Os regressions, and Re Os model ages for components with extremely high 187Re/188Os. The 44 Ma and 8–0 Ma Re-Os ages agree well with regional burial models that indicate oil generation from ~44 Ma to present day. A genetic link between crude oil and bitumen is indicated by their similar alkane patterns, V and Ni contents, and V/Ni and pristane/phytane ratios. Modeling of 187Os/188Os for individual organic components through time shows that shales of the Mandal Formation developed 187Os/188Os ratios that match those of the crude oil and bitumen at ~44 Ma and ~8 Ma, respectively, and therefore confirm the source for both hydrocarbon phases. We suggest that bitumen formed during in-reservoir mixing of a ~44 Ma old early mature, asphaltene-poor oil (Brynhild crude oil) with an ~8 Ma old asphaltene-rich oil.

Strandings of various types of bitumen along the coast of southern Australia are long known. Among these, brittle, angular lumps termed ‘asphaltites’ are possibly sourced from Cretaceous source rocks linked to an oceanic anoxic event (OAE), but the exact source remains unclear. The unusual chemical composition of these asphaltites and their survival during transport and shoreline stranding suggest that they formed by nearby submarine seepage of asphaltene-rich crude oils. Here, we provide a detailed organic and inorganic geochemical characterization of asphaltites to constrain their origin and age. High-pressure hydropyrolysis (HyPy) of asphaltene fractions from ten asphaltites released similar assemblages of macromolecularly bound compounds, suggesting a common source for all asphaltites. Comprehensive gas chromatography–time-of-flight mass spectrometry (GC×GC-TOFMS) was used to compare these asphaltene-derived compounds with the maltene fractions, while compound specific isotope analysis (CSIA) was used to compare δ13C and δ2H of n-alkanes and isoprenoids. A large offset between the δ2H of the n-alkanes and isoprenoids suggests oil generation and expulsion at low thermal maturity. The mean concentrations of isorenieratane and chlorobactane, carotenoid derivatives indicative of photic zone euxinia (PZE), in the asphaltites were 8.8 ± 0.8 SEM µg g−1 and 1.4 ± 0.1 SEM µg g−1, respectively. A mean Aryl Isoprenoid Ratio of 0.75 (SD = 0.17) is accompanied by Pr/Ph of ∼1.2. These features strongly support persistent PZE conditions at the level expected for an OAE. Trace metal contents of the asphaltites, including low selenium and high vanadium concentrations, also support anoxic conditions. Rhenium-osmium (Re-Os) analyses constrain the age of asphaltite generation to 103 ± 22 Ma, with a relatively low initial 187Os/188Os ratio of 0.44 ± 0.18. Integrating local geologic knowledge with organic and inorganic geochemistry and Re-Os isotopic results, we identify a Cretaceous unit associated with OAE1a (∼125 Ma) as the most likely source of the asphaltites. Alternative scenarios involving source rocks deposited during OAE1b (∼112 Ma) are possible, but require rapid burial of organic-rich sediments to reach required maturation levels in a shorter time.

A prominent subduction-related magmatic arc hosting significant mineralization formed in SE Europe during the Late Cretaceous. Previous studies on major magmatic centers and ore deposits suggested that this belt formed through southwards retreat of a subducting Neotethys oceanic slab. However, the timing and the petrologic characteristics of magmatic products from the less mineralized eastern portions of this belt remain largely unknown. The complete lack of radiometric ages and limited geochemical characterization of this magmatism adds considerable uncertainty on existing large-scale geodynamic reconstructions focused on the Late Cretaceous magmatism in SE Europe. Here, we address this question by studying little known Cretaceous lavas and sub-volcanic bodies from the Eastern Balkan, Bulgaria. Major and trace element contents of these volumetrically limited rocks show a clear subduction signature. The most primitive rocks are high-Al basalts, which further differentiated into andesites and dacites via fractional crystallization in relatively small magma chambers. The mineral chemistry and assemblages constrain magmatic conditions prior to crystallization to pressures of 3–7 kb, temperatures of 900–1020 °C, water contents of ~4–7 wt% and high oxygen fugacities. Phenocryst features like reverse zonation of clinopyroxene and amphibole and sieve and patchy textures of plagioclase suggest magma mixing processes. Initial eHf values of Cretaceous zircons (+2 to −2) and inherited, mainly Variscan and older zircons (+3 to −11) provide clear evidence for assimilation of crustal lithologies by mantle-derived Late Cretaceous magmas. Amphibole phenocrysts from an andesite and a dacite give 40Ar/39Ar plateau ages of 94.67 ± 0.40 Ma and 94.56 ± 0.40 Ma, respectively. These dates are the oldest recorded in the entire Late Cretaceous magmatic belt and constrain the onset of the subduction magmatism to the later parts of the Cenomanian stage. Regional correlations based on these results reveal that processes of slab retreat were active also in the eastern part of the magmatic arc. Further, these results outline a clear temporal along-arc trend of progressively younger initiation of the arc magmatism from east to west.

The National Institute of Science and Technology (NIST) research material (RM) 8505 is a Venezuelan crude oil certified for its vanadium concentration. It has recently been proposed as a matrix-matched standard for interlaboratory comparison of Re-Os isotopic analyses while serving as a benchmark for petroleum-based Re-Os method development. Here, the NIST RM 8505 oil is characterized using a multi-solvent asphaltene precipitation, permitting interlaboratory Re-Os data comparison and evaluation of techniques used to derive Re-Os isochron ages. We present a detailed discussion of analytical blank sources, critical evaluation of analytical uncertainties, method improvements and data corrections. Additionally, we provide a comprehensive discussion on the effect of data population size and analytical uncertainty on isochron statistics and the robustness of isochron ages. In addition to NIST RM 8505 (2.43 ± 0.15 ng/g Re, 30.0 ± 0.7 pg/g Os), we provide the first Re-Os data for NIST SRM 1634c (1.73 ± 0.01 ng/g Re, 32.5 ± 0.8 pg/g Os) and EnviroMAT SCP Science (39 ± 1 pg/g Re, 1.01 ± 0.02 pg/g Os), two other commercially available hydrocarbon trace metal standards. The application of several asphaltene (ASPH) precipitations using different n-alkane solvents (pentane, heptane and decane) for the NIST RM 8505 significantly improved the spread of data in the 187Re/188Os-187Os/188Os space. Multi-solvent asphaltene precipitations yield an asphaltene-only Model 1 isochron (75 ± 24 Ma) and the maltene-crude oil-asphaltene triplets yield a more precise Model 1 isochron age of 78.5 ± 2.3 Ma with a 187Os/188Osinitial of 0.877 ± 0.021. Our age is considerably older compared to a previously reported Model 1 age of 62.7 ± 5.7 Ma based on C7 ASPH-crude oil-MALT triplets ( Liu and Selby, 2017 ) and considerably younger than the Model 1 age of 98.4 ± 9.5 Ma based on ASPH separates ( Liu et al., 2019 ). The analytical reasons leading to these age differences are discussed in this paper. The geological context for the NIST 8505 oil is unknown, therefore we use trace metal data (Ni, V, Fe, Mo, etc.) to derive plausible constraints for the origin of this oil. The NIST 8505 oil shows trace metal data similar to published data for oils from the Urdaneta district, the La Paz, Mara and Lagunillas oil fields and source rocks of the La Luna Formation in the Maracaibo Basin, Venezuela. In this geological context, our ~79 Ma age for the NIST 8505 oil implies a Late Cretaceous early hydrocarbon generation event from older Upper Cretaceous source rocks and overlaps with the youngest age of potential source rocks in the Maracaibo Basin. However, the age of large-scale oil generation in the Maracaibo Basin is significantly younger, lasting from the Oligocene to the Pliocene.

The geochemical record for the Permian–Triassic boundary in northern latitudes is essential to evaluation of global changes associated with the most profound extinction of life on Earth. We present inorganic and organic geochemical data, and Re–Os isotope systematics in a critical stratigraphic interval of pre- and post-extinction Upper Permian–Lower Triassic sediments from Opal Creek, western Canada (paleolatitude of ∼30°N). We document significant and long-lived changes in Panthalassa seawater chemistry that were initiated during the first of four magmatic or meteoritic inputs to Late Permian seawater, evidenced by notable decreases of Os isotopic ratios upsection. Geochemical signals indicate establishment of anoxic bottom waters shortly after regional transgression reinitiated sedimentation in the Late Permian. Euxinic signals are most prominent in the Upper Permian sediments with low organic carbon and high sulfur contents, and gradually wane in the Lower Triassic. The observed features may have been generated in a strongly euxinic ocean in which high bacterioplankton productivity sustained prolific microbial sulfate reduction in the sediment and/or water column, providing hydrogen sulfide to form pyrite. This scenario requires nearly complete anaerobic decomposition of predominantly labile marine organic matter (OM) without the necessity for a complete collapse of primary marine productivity. Similar geochemical variations could have been achieved by widespread oxidation of methane by sulfate reducers after a methanogenic burst in the Late Permian. Both scenarios could have provided similar kill mechanisms for the latest Permian mass extinction. Despite the moderate thermal maturity of the section, OM in all studied samples is dominantly terrestrial and/or continentally derived, recycled and refractory ancient OM. We argue that, as such, the quantity of the OM in the section mainly reflects changes in terrestrial vegetation and/or weathering, and not in marine productivity. At Opal Creek, a pyrite layer and

Re and Os isotopes in shales are increasingly used for precise and accurate geochronology and for tracking rates of continental weathering in the geologic past. Moreover, trace element compositions of organic-rich shales reveal paleoredox conditions and properties of shale organic matter (OM) useful for lithostratigraphic correlations. Chemical weathering alters the original shale composition and hence may compromise geologic interpretations based on geochemical data. Here we compare Re–Os isotopes, trace elements and OM properties in correlative shales from outcrops and nearby drillcore to quantify the effects of weathering. We define chemical parameters that distinguish weathered from pristine shale in samples that are macroscopically identical. Black shale from the Ravnefjeld Formation in East Greenland yields disturbed Re–Os isotope systematics for outcrop samples, and highly precise Re–Os isochron ages for drillcore. Geochemical data show that Re, Os and common Os are hosted in the organic matter rather than sulfides. Re and Os both show mobility during weathering. Combined use of Rock-Eval pyrolysis indices and sulfur contents documents oxidation of both OM and pyrite, thereby providing a valuable chemical criterion to assess invisible shale weathering. Further studies on chemically diverse shales will test the widespread application of these criteria as weathering indicators. Systematic differences in major and trace element content and kerogen quality between weathered and fresh shale samples are used to characterize and quantify weathering effects. The character of the OM in shales is most telling. Ravnefjeld data are compared with data from time correlative shales from the mid-Norwegian shelf [Georgiev et al., 2011. Hot acidic Late Permian seas stifle life in record time. Earth Planet. Sci. Lett., 310, 389–400] to present a strategy for identification of Re–Os isochroneity in advance of Re–Os analytical work. Our recent compilation and comparison of Upper Permian shales from widely separated sequences on Pangea's margins reveal exceptionally high Re/Os ratios on a global scale. We suggest Upper Permian shales may have significantly influenced the Cenozoic (e.g. Eocene) 187Os/188Os ratio of seawater on their exposure and weathering.

Lithos, 154, ISSN:0024-4937

Journal of Petrology, 50 (10), ISSN:0022-3530, ISSN:1460-2415

The Upper Cretaceous Yambol–Burgas region of the Eastern Srednogorie continental arc is characterized by unusually large volumes of mafic shoshonitic and ultra-K magmatism represented by high-Mg cumulitic rocks, and nepheline-normative ankaramites, absarokites and shoshonitic (high-alumina) basalts. The cumulitic rocks consist of phenocrysts of clinopyroxene and olivine (Fo91–85) with inclusions of spinel (Cr#75–78). Olivine high-Fo cores and clinopyroxene host glassy melt inclusions which have nepheline-normative compositions with ankaramitic CaO/Al2O3 ratios (> 1), low SiO2 (47.4–50.0 wt.%), high MgO (7.5–10.8 wt.%), high total alkalis and shoshonitic K2O/Na2O ratios (> 1). Electron microprobe and LA-ICPMS analyses demonstrate that parental magma was a high-Ca primitive mantle melt. Melt inclusions from the less Fo85 rim approach absarokite composition with CaO/Al2O3 ratios

Geochimica et Cosmochimica Acta, 70 (18 / Supplement), ISSN:0016-7037, ISSN:1872-9533