Orogenic gold and geologic time: a global synthesis

The oxygenation of the Earth's atmosphere is thought to have occurred in two steps near the beginning and the end of the Proterozoic eon, around 2,500 to 550 million years ago. The oxidation state of the ocean between these two steps and the timing of deep ocean oxygenation, however, remain poorly known. Scott et al. now use molybdenum and total organic carbon data from black shales to track the redox state of the ocean at this time. Molybdenum is an essential participant in nutrient cycling, and its availability is highly sensitive to Earth's redox state. The results provide a new narrative for the historical texture of Earth's oxygenation, and will be of relevance for the study of the events that presaged the appearance of animals on Earth. Molybdenum and total organic carbon data from black shales is used to gain insights into the redox state of the ocean. The data suggests mild oxidative weathering of the continents before ∼2,200 Myr ago, but weathering becomes more persistent and vigorous at ∼2,150 Myr ago, 200 million years after the initial rise in atmospheric oxygen. Limited availability of molybdenum after 1,800 Myr ago may have acted as a negative nutrient feedback limiting the spatial and temporal extent of sulphidic conditions. Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth’s atmosphere. Geochemical data1,2,3,4,5,6 suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient7 sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen1,2. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms10. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

Tracing the stepwise oxygenation of the Proterozoic ocean

Tectonic evolution of the Qinling orogen, China: Review and synthesis

▪ Abstract The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic 187Os/188Os, indicating long-term i...

THE Re-Os ISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 ± 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O 2 in the environment more than 50 million years before the start of the Great Oxidation Event.

http://science.sciencemag.org/content/sci/317/5846/1903.full.pdf

A Whiff of Oxygen Before the Great Oxidation Event

The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS2) is now demonstrated to be remarkably robust, surviving intense deformation and high-grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of 187Re parent and 187Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID-NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP-MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and 40Ar/39Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid-state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point-in-time markers for age comparisons.

The remarkable Re-Os chronometer in molybdenite : how and why it works

Two Re-Os dating reference material molybdenites were prepared. Molybdenite JDC and molybdenite HLP are from a carbonate vein-type molybdenum-(lead)-uranium deposit in the Jinduicheng-Huanglongpu area of Shaanxi province, China. The samples proved to be homogeneous, based on the coefficient of variation of analytical results and an analysis of variance test. The sampling weight was 0.1 g for JDC and 0.025 g for HLP. An isotope dilution method was used for the determination of Re and Os. Sample decomposition and pre-concentration of Re and Os prior to measurement were accomplished using a variety of methods: acid digestion, alkali fusion, ion exchange and solvent extraction. Negative thermal ionisation mass spectrometry and inductively coupled plasma-mass spectrometry were used for the determination of Re and 187Os concentration and isotope ratios. The certified values include the contents of Re and Os and the model ages. For HLP, the Re content was 283.8 ± 6.2 μg g−1, 187Os was 659 ± 14 ng g−1 and the Re-Os model age was 221.4 ± 5.6 Ma. For JDC, the Re content was 17.39 ± 0.32 μg g−1, 187Os was 25.46 ± 0.60 ng g−1 and the Re-Os model age was 139.6 ± 3.8 Ma. Uncertainties for both certified reference materials are stated at the 95% level of confidence. Three laboratories (from three countries: PR. China, USA, Sweden) joined in the certification programme. These certified reference materials are primarily useful for Re-Os dating of molybdenite, sulfides, black shale, etc.

Preparation and certification of Re-Os dating reference materials: Molybdenites HLP and JDC

Isotope dilution with a modified alkali fusion procedure and negative thermal ion mass spectrometry yields highly precise and accurate Re-Os ages for molybdenite from two well-studied molybdenite deposits in the East Qinling molybdenum belt, China. Individual Re-Os ages carry a 2Sigma precision of + or - 0.40 to 0.57 percent which includes a 0.31 percent uncertainty in the 187 Re decay constant. For the unusual carbonatite-hosted Mo-Pb deposit at Huanglongpu, the weighted average of seven analyses yields an age of 221.5 + or - 0.3 (0.15%) Ma. The weighted average of two analyses of molybdenite from a porphyry Mo deposit at Jinduicheng, about 10 km to the southwest, yields an age of 138.4 + or - 0.5 (0.39%) Ma. These data provide uncertainties an order of magnitude less than previous Re-Os ages. Molybdenite Re-Os ages are slightly older than ages obtained by other isotopic methods for genetically related host-rock and vein material. It appears that the direct dating of sulfide, rather than altered host and vein material, may be critical to acquiring the correct age for mineralization.The East Qinling molybdenum belt is part of a larger east-west-trending zone that marks the suture between two major cratonic blocks. Consequently, the belt was a site for Early-Middle Triassic compression (Indosinian orogeny) followed by Jurassic-Cretaceous extension (Yenshanian orogeny). We suggest that the Huanglongpu and Jinduicheng deposits provide an analogue for processes that may have been important in generating major molybdenum deposits in the Colorado mineral belt. In Colorado, Late Cretaceous (Laramide) compression-related, alkalic magmatism was followed by Tertiary (Rio Grande) extension-related, granitic magmatism and the development of major Climax-type porphyry Mo deposits. In particular, the Jinduicheng deposit appears to be a nearly perfect match for Climax-type mineralization in Colorado. In contrast, the older Huanglongpu deposit may record a mechanism whereby molybdenum is concentrated in the lower crust. In both the Qinling molybdenum belt and the Colorado mineral belt, a time gap of about 50 to 80 m.y. separates alkalic magmatism and exceptionally evolved granitic magmatism.

R. J. Markey

Papers

The remarkable Re-Os chronometer in molybdenite : how and why it works

Preparation and certification of Re-Os dating reference materials: Molybdenites HLP and JDC

Highly precise and accurate Re-Os ages for molybdenite from the East Qinling molybdenum belt, shaanxi Province, China

Primitive Os and 2316 Ma age for marine shale: implications for Paleoproterozoic glacial events and the rise of atmospheric oxygen

Highly precise Re-Os dating for molybdenite using alkaline fusion and NTIMS.