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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.

The Re-Os geochronometer can be applied to date petroleum, which is the end product of complex natural processes. Improved interpretation of natural Re-Os datasets requires identification of key controlling factors for Re-Os systematics and their isotopic imprint in oils. Here, we focus on the role of water in (re)setting the Re-Os geochronometer in oils, as formation waters and oils often coexist in basinal systems. To quantify Re-Os systematics during water-oil interaction, we performed a series of equilibration experiments between different oils and dilute aqueous solutions doped with Re and Os. The investigated parameters include the oil composition, water to oil ratio, interaction time and the initial Re-Os content in the aqueous solution. In experiments performed across a high concentration gradient, the oils fully homogenized with respect to their Os isotopic composition but had variable 187Re/188Os ratios. This indicates that the Re-Os geochronometer has been reset during water-oil interaction. However, in experiments investigating partial isotopic overprint, the crude oil and its maltene and asphaltene fractions preserved their isochron-derived age. Thus, the Re-Os radiometric clock of a single crude oil sample remains intact, even during partial equilibration of the 187Os/188Os ratio. These experimental findings provide a new foundation for interpretation of Re-Os systematics in natural water-hydrocarbon systems.

Precise and accurate measurements of the isotopic composition of rhenium (Re) in geologic samples are of paramount importance for the successful application of the Re-Os geochronometer. Two methods for chemical isolation of Re are used in the geochronology community – column chemistry using anion exchange resin and solvent-solvent extraction using a sodium hydroxide-acetone mixture. Despite their ubiquitous application, a systematic comparison of these two methods is missing. Here we compare Re-isotopic data on a variety of samples (sulfides, hydrocarbons, shales, and in-house standard solutions) that were processed by both chemical methods. Measured 185Re/187Re ratios for both methods have similar precision and overlap within the propagated long-term analytical reproducibility of ~1‰ for samples that yield moderate to strong intensities (>50 mV on the weaker Re mass). In contrast, measured 185Re/187Re ratios for most samples that yield less intense Re signals (

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.

Re–Os isochron ages for black shales of the Hekkingen Formation in the Barents Sea constrain the onset ( 157.7 ± 1.3 Ma ) and termination ( 138.8 ± 1.0 Ma ), and thereby indicate a long duration (∼19 Myr) of widespread Jurassic–Cretaceous anoxia in the Arctic. Integration of these new Re–Os ages with published radiometric ages, ammonite biostratigraphy and geomagnetic polarity chrons shows shorter late Oxfordian–late Kimmeridgian and longer Berriasian stages relative to estimates in the 2012 and 2016 Geological Time Scales. Late Jurassic anoxia was likely the result of warming climate due to high atmospheric CO2 levels from increased oceanic crust production. Rising temperatures enhanced weathering and nutrient supply, increased productivity, and slowed ocean circulation before a sea-level rise brought anoxic waters onto continental shelves. Assessment of new and published Os- and Sr-isotopic data suggests that prolonged oceanic anoxia required a sustained CO2 source from fast spreading rates and/or longer subduction zones and spreading ridges to balance large burial of carbon in voluminous Upper Jurassic and Lower Cretaceous black shales.

Re-Os geochronology of black shales provides absolute time pins in sedimentary successions and Os isotopic constraints on paleo-environmental conditions. Recognition and reconstruction of pre-faulting chronology within a visually monotonous shale section are critical to interpretation of the paleo-environmental record, and have relevance for fluid migration routes and basin modelling. Re-Os isochrons for five core intervals spanning 14 m of black shale from the Hekkingen Formation on the northern margin of the Hammerfest Basin document within-unit reverse faulting, which is corroborated by detailed petrography, geochemical profiles, and the Os isotope record. The reverse fault is coincident with a beef-like carbonate vein. The five core intervals provide six isochrons with increasingly younger depositional ages of 153.3 + 4.1/− 1.7 Ma to 148.6 + 3.0/− 4.1 Ma, with increasingly higher initial 187 Os/ 188 Os ratios (0.475 + 0.025/− 0.060 to 0.593 + 0.134/− 0.098), consistent with original stratigraphic position. Our age data and inferred sedimentation rates suggest a vertical displacement of several meters across the fault. We suggest such compressional features are the distal (i.e., down-dip) components of gravity gliding structures. The rocks of the Hekkingen Formation at the studied locality record a period of water column euxinia. With the geochemical data in the correct stratigraphic order, we identify two periods of increased sulfide deposition with distinct trace-metal characteristics that reflect differences in the sharpness of the chemocline. These sulfide-producing events create complexity in cross-plots traditionally used to infer oceanographic conditions. Stratigraphically-corrected Os-isotope profiles allow us to correlate this stratigraphic interval of the Hekkingen Formation from the northern margin of the Hammerfest Basin with other sections in the Barents Sea and offshore Norway.

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.

Lithos, 154, ISSN:0024-4937