Robert A. Creaser

A Whiff of Oxygen Before the Great Oxidation Event

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 O2 in the environment more than 50 million years before the start of the Great Oxidation Event.

Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode

Oceanic anoxic events (OAEs) were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen-deficient bottom waters. OAE2, occurring at the Cenomanian/Turonian boundary (about 93.5 Myr ago), is the most widespread and best defined OAE of the mid-Cretaceous. Although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios, the actual trigger mechanism, corresponding closely to the onset of these episodes of increased carbon sequestration, has not been clearly identified. It has been postulated that large-scale magmatic activity initially triggered OAE2 (refs 4, 5), but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of OAE2 has not yet been found. Here we report seawater osmium isotope ratios in organic-rich sediments from two distant sites. We find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of OAE2. Using a simple two-component mixing equation, we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin, a ∼30–50-fold increase relative to pre-OAE conditions. Furthermore, the magmatic osmium isotope signal appears slightly before the OAE2—as indicated by carbon isotope ratios—suggesting a time-lag of up to ∼23 kyr between magmatism and the onset of significant organic carbon burial, which may reflect the reaction time of the global ocean system. Our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of OAE2, which may have triggered the subsequent deposition of large amounts of organic matter.

Osmium Recycling in Subduction Zones

Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.

Isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans: Implications for secular curves

Isotopic analysis of conodonts and their host limestones sampled between two regionally extensive, altered volcanic ash layers in eastern Laurentia shows that a 454 Ma epeiric sea maintained large lateral differences in Nd and C isotope compositions. This is consistent with inferred temperature-salinity‐defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments. Because the majority of old marine fossils and sedimentary rocks are known from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system.

Re-Os geochronology of postglacial black shales in Australia: Constraints on the timing of “Sturtian” glaciation

New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ± 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ± 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685–750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the “Sturtian” ice age was markedly diachronous, and/or there was more than one “Sturtian”-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than “snowball” or “slushball” Earth ice ages.

Re-Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods

Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re–Os date obtained. To minimize the influence of any nonhydrogenous Re and Os, we evaluate analysis of isolated organic matter from the whole rock, together with whole rock analysis using a CrO3–H2SO4 digestion medium instead of inverse aqua regia, for a black shale unit of the Exshaw Formation, Canada. This unit previously returned a whole rock Re–Os date of 358±10 Ma (Model 3) [Geochim. Cosmochim. Acta (2002)] using inverse aqua regia dissolution. Organic matter isolated from the whole rock matrix using the HF–BF3 technique [Org. Geochem. 20 (1993) 249] yields scattered data and a Re–Os date of 449±220 Ma (Model 3, MSWD=616). The organic matter analyses show similar 187Os/188Os values, but significantly lower 187Re/188Os values in comparison to the whole rock analyses. We show that the Re–Os systematics of organic matter are altered during chemical isolation, and as such we suggest that the HF–BF3 method should not be used for Re–Os analysis of organic matter. Whole rock Re–Os analysis using a CrO3–H2SO4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method, and the Re–Os data identify two distinct initial 187Os/188Os values for the sample set. Separate regressions of these data yield precise dates [366.1±9.6, MSWD=2.2 and 363.4±5.6 Ma, MSWD=1.6 (Model 3)], which are indistinguishable from the age constraints for this formation (363.4±0.4 Ma, U–Pb monazite). Comparison of the Re–Os dates obtained from aqua regia and CrO3–H2SO4 methods suggests that the former may contain nonhydrogenous Re and Os, whereas the CrO3–H2SO4 method dominantly liberates hydrogenous Re–Os from organic matter, allowing for better stratigraphic age determinations and evaluation of the Os isotope composition of seawater.

U–Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array

This study reports U–Pb geochronological data for zircon obtained by laser ablation-multi-collector-ICP-MS using a new collector block design that includes three ion counters and twelve Faraday buckets. The collector configuration allows for simultaneous detection of ion signals from mass 238U to 203Tl, an important factor for the achievement of highly precise and reproducible Pb–Pb and Pb–U ratios. The main advantage of the multiple ion counting system is the capability to readily measure low Pb ion signals ( 50) of zircon grains.

Absolute timing of sulfide and gold mineralization: A comparison of Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska

New Re-Os molybdenite dates from two lode gold deposits of the Tintina Gold Belt, Alaska, provide direct timing constraints for sulfide and gold mineralization. At Fort Knox, the Re-Os molybdenite date is identical to the U-Pb zircon age for the host intrusion, supporting an intrusive-related origin for the deposit. However, 40Ar/39Ar dates from hydrothermal and igneous mica are considerably younger. At the Pogo deposit, Re-Os molybdenite dates are also much older than 40Ar/39Ar dates from hydrothermal mica, but dissimilar to the age of local granites. These age relationships indicate that the Re-Os molybdenite method records the timing of sulfide and gold mineralization, whereas much younger 40Ar/39Ar dates are affected by post-ore thermal events, slow cooling, and/or systemic analytical effects. The results of this study complement a growing body of evidence to indicate that the Re-Os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems.

Further evaluation of the Re-Os geochronometer in organic-rich sedimentary rocks: A test of hydrocarbon maturation effects in the Exshaw Formation, Western Canada Sedimentary Basin

Abstract Re-Os isotopic analyses of a single organic-rich sedimentary rock unit (ORS) of known depositional age, and at three levels of regional hydrocarbon maturity, show that hydrocarbon maturation does not affect the ability the 187Re-187Os chronometer to yield a depositional age for such rocks. We present Re-Os isotope analyses from the Late Devonian Exshaw Formation in the subsurface of the Western Canada Sedimentary Basin, Alberta, and obtain a Re-Os isochron age of 358 ± 10 Ma (2σ, Model 3, λ = 1.666 × 10−11.a−1) for samples ranging from hydrocarbon immature to overmature. This age is within uncertainty of the established absolute age for the Exshaw Formation. Hydrocarbon immature, and mature plus overmature samples show no significant age differences if regressed individually, indicating that hydrocarbon maturation did not greatly disturb the Re-Os isotope system in the Exshaw Formation. As such, we propose that the Re-Os geochronometer may be used as a reliable tool for measuring the depositional ages of ORS regardless of their level of hydrocarbon maturity. We find that minimizing natural variation in the initial 187Os/188Os ratio is more important than avoiding hydrocarbon maturation in obtaining precise Re-Os ages. In particular, the Exshaw Formation appears to contain a nonhydrogenous component of unradiogenic Os, in addition to the hydrogenous Os load. A subset of Exshaw Formation samples with >5% total organic carbon (TOC), which should best reflect the hydrogenous Os load alone, yields a very well-fitted isochron having a depositional age of 358 ± 9 Ma (2σ, λ = 1.666 × 10−11.a−1) with an initial 187Os/188Os ratio of 0.59 ± 0.05 (Model 3, Mean Square of Weighted Deviates (MSWD) = 1.8). The initial 187Os/188Os ratio of this regression may provide an estimate of the Os isotopic composition of local seawater at the time of deposition.

Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies

Pristine conodonts (CA1 4 IS), inarticulate brachiopods, and conulariids, all from a single hand sample of Ordovician limestone, define a co-varying trend of *‘Sr/ “Sr and Sr concentration. Most of the apatitic fossils have *‘Sr/ *6Sr ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a general susceptibility of biogenic apatites to post-depositional Sr exchange. ‘Ihe largest isotopic shifts were measured in inarticulate brachiopods and conulariids, and deduced for conodont basal body material. Conodont crown material exhibits the smallest effects. The Sr exchange effects are strongly dependent on differences in apatite composition, as revealed by contrasting Ca/P ratios. Although conodont crown material (with low Ca/P ratios) is less prone to isotopic disturbance relative to other types of coexisting apatite fossils, high resolution X-ray mapping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. In contrast to Sr, all coexisting fossil apatites have identical initial 143Nd/ l”Nd ratios over a wide range of Nd concentration. No relationship between *‘Sr/ 86Sr and ‘43Nd/ l”Nd was observed despite a pronounced antithetic pattern of Sr and Nd distribution both between the fossil types, and within individual conodonts containing preserved basal body material. In agreement with earlier studies, it is concluded that the bulk of the Nd in fossil apatites is from seawater that originally overlay the depositional site.