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• W2134046539 endingPage "107" @default.
• W2134046539 startingPage "85" @default.
• W2134046539 abstract "Global correlations of Precambrian stratigraphic successions can be hampered by the coarse resolution of biostratigraphic and chemostratigraphic records, and by the scarcity of reliable U–Pb zircon age constraints. The development of the Re (rhenium)–Os (osmium) radioisotope system as an accurate deposition-age geochronometer for organic-rich sedimentary rocks (e.g. black shales) holds great potential for an improved radiometric calibration of the Precambrian rock record. Here, we review Re–Os isotope data obtained for Precambrian black shales and revisit the discrepancy in Re–Os ages for the Neoproterozoic Aralka Formation (central Australia). In addition, we introduce new Re–Os isotope data for the Late Neoproterozoic Doushantuo Formation (South China) that highlights the necessity of a rigorous sampling protocol for depositional age determinations. Improvements in sampling and analytical methodologies have permitted the determination of precise ages (,1%, 2s) from Late Neoproterozoic to Late Archaean shales. Whole-rock digestion using a Cr–H2SO4 solution minimizes the release of detrital Re and Os from shale matrices, and selectively attacks organic matter that hosts hydrogenous Re and Os. The Re–Os system in organic-rich sedimentary rocks appears to be robust during hydrocarbon maturation and up to the onset of lowermost greenschist facies metamorphism, but post-depositional hydrothermal fluid flow can result in scattered Re–Os isotope data. The Re–Os black shale geochronometer should find utility for constraining the age of a diverse range of Precambrian geological phenomena. In addition, the initial Os/Os composition determined from Re–Os isochron regressions serves as a tracer for the Os isotope composition of Precambrian sea water. Accurately determining the depositional ages of sedimentary rocks has proven extremely difficult to accomplish using the conventional long-lived radioisotope systems (e.g. Rb–Sr, Sm–Nd, U–Pb, K–Ar). The U–Pb SHRIMP (sensitive highresolution ion microprobe) dating of detrital minerals such as zircon has proven useful for provenance studies and constraining the maximum depositional age (e.g. Bingen et al. 2005). However, authigenic minerals (e.g. apatite, glauconite, illite, K-feldspar, monazite) generally yield diagenetic ages that are variably younger than the depositional age of the host sedimentary rock. Diagenetic age determinations on authigenic minerals are also hampered by low closure temperatures of the applied radioisotope system, resulting in a high susceptibility to thermal resetting even during relatively low-temperature hydrothermal alteration or metamorphism (Dickin 2005). Diagenetic xenotime is found in a wide variety of siliciclastic and volcaniclastic rocks, and represents a robust U–Pb geochronometer with the potential for resolving complex geological histories within sedimentary basins (Rasmussen 2005). However, U–Pb xenotime dates also reflect the timing of diagenesis rather than deposition. In some cases, Pb/Pb ages from carbonates (e.g. Moorbath et al. 1987; Woodhead et al. 1998; Babinski et al. 2007) and phosphorites (Barfod et al. 2002; Chen et al. 2004) may yield depositional or early diagenetic ages using well-preserved material. However, Pb/Pb ages may be erroneously young or old owing to diagenetic or metamorphic recrystallization and detrital inheritance, respectively, or age information may be lost altogether due to post-depositional mobility of U and Pb (Rasmussen 2005). Currently, the most reliable method of constraining the depositional age of sedimentary rocks is by U–Pb zircon dating of interbedded tuff horizons. In the case of Phanerozoic sedimentary basins, such U–Pb ages can be used to calibrate highresolution Phanerozoic biostratigraphic, chemostratigraphic and magnetostratigraphic records, thereby facilitating regional and global correlations of stratigraphic successions (e.g. Gradstein et al. 2004). However, the generally coarse resolution of these From: CRAIG, J., THUROW, J., THUSU, B., WHITHAM, A. & ABUTARRUMA, Y. (eds) Global Neoproterozoic Petroleum Systems: The Emerging Potential in North Africa. Geological Society, London, Special Publications, 326, 85–107. DOI: 10.1144/SP326.5 0305-8719/09/$15.00 # The Geological Society of London 2009. chronostratigraphic methods for the Precambrian rock record does not currently permit this approach, and, in cases where datable ash beds are absent, the ages of Precambrian sedimentary rocks are generally only poorly constrained by radiometric dates from overlying and underlying volcanic or plutonic rocks, and/or cross-cutting plutonic rocks. The development of the Re–Os radioisotope system as a reliable deposition-age geochronometer for organic-rich sedimentary rocks (ORS; total organic carbon (TOC) 0.5%) like black shales (Ravizza & Turekian 1989; Cohen et al. 1999; Creaser et al. 2002; Selby & Creaser 2003; Kendall et al. 2004) has the potential to alleviate the problems associated with radiometric calibration of the Precambrian sedimentary rock record. Improvements in sampling and analytical methodologies, combined with the high precision of isotope dilution–negative thermal ionization mass spectrometry (ID-NTIMS: Creaser et al. 1991; Volkening et al. 1991; Walczyk et al. 1991), have made it possible to obtain a Re–Os age for black shale with a precision of better than +1% (2s), with the absolute uncertainty comparable in some cases to the uncertainties on U–Pb ages derived from SHRIMP or laser ablation MC-ICP-MS (multicollectorinductively coupled plasma-mass spectrometry) analyses of zircons from tuffaceous beds (e.g. Kendall et al. 2004, 2006; Selby & Creaser 2005a; Anbar et al. 2007; Creaser & Stasiuk 2007). Recently, the Re–Os ORS geochronometer has been successfully applied to studies regarding geological timescale calibration (Devonian– Mississippian boundary: Selby & Creaser 2005a), the timing of Proterozoic glaciation (Hannah et al. 2004; Kendall et al. 2004, 2006; Azmy et al. 2008), the Earth’s early history of atmosphere and ocean oxygenation (Hannah et al. 2004; Anbar et al. 2007), and sedimentary basin analysis (Hannah et al. 2006; Creaser & Stasiuk 2007; Kendall et al. 2009a). In addition, the initial Os/Os value (IOs) from Re–Os isochron regressions has served as a tracer for the Os isotope composition of Phanerozoic sea water (Cohen et al. 1999, 2004; Creaser et al. 2002; Selby & Creaser 2003; Cohen 2004; Cohen & Coe 2007; Selby 2007). Here, we review recent applications of Re–Os geochronology to Precambrian ORS, and demonstrate how careful sampling and analytical methodologies are essential for precise and accurate depositional age determinations. The Re–Os system in ORS: a deposition-age geochronometer and sea-water Os isotope tracer In addition to being siderophilic and chalcophilic (e.g. Shirey & Walker 1998), Re and Os are also organophilic and redox-sensitive (Ravizza et al. 1991; Ravizza & Turekian 1992; Colodner et al. 1993; Crusius et al. 1996; Levasseur et al. 1998; Selby & Creaser 2003, 2005b; Selby et al. 2005, 2007a). This geochemical behaviour of Re and Os in Earth surface environments enables the Re–Os isotope system to be used as a deposition-age geochronometer for ORS and a tracer of palaeo-sea-water Os isotope composition. Under oxidizing atmospheric conditions, Re is transported to the oceans primarily by rivers (as the highly soluble perrhenate anion ReO4 : Colodner et al. 1993). Rhenium is removed rapidly from reducing pore waters at centimetres to tens of centimetres below the sediment–water interface, and is sequestered into suboxic, anoxic and euxinic sediments (Colodner et al. 1993; Crusius et al. 1996; Morford & Emerson 1999; Nameroff et al. 2002; Sundby et al. 2004; Morford et al. 2005). Removal of Re from pore waters occurs by reductive capture (ReO4 2 is reduced to Re: Colodner et al. 1993), the rate of which is controlled by slow precipitation kinetics (Crusius & Thomson 2000; Sundby et al. 2004). The Os/Os isotopic compositionofpresentday sea water is almost homogenous (c. 1.06: Sharma et al. 1997; Levasseur et al. 1998; Burton et al. 1999; Woodhouse et al. 1999; PeuckerEhrenbrink & Ravizza 2000), consistent with an Os sea-water residence time of approximately 10 years (Oxburgh 1998, 2001; Levasseur et al. 1999). The dominant source of present-day seawater Os (c. 70–80%) is from the weathering of the upper continental crust (McDaniel et al. 2004). Average Os/Os for the currently eroding upper continental crust and riverine inputs are between 1.0 and 1.4 (Esser & Turekian 1993; Peucker-Ehrenbrink & Jahn 2001; Hattori et al. 2003) and approximately 1.5 (with an uncertainty of .20%: Levasseur et al. 1999), respectively. The remainder of the present-day marine Os budget is derived from extraterrestrial cosmic dust (Peucker-Ehrenbrink 1996) and the lowand hightemperature hydrothermal alteration of oceanic crust and peridotites (Ravizza et al. 1996; Sharma et al. 2000, 2007; Cave et al. 2003). Both sources contribute unradiogenic Os/Os to sea water (0.126–0.130: Becker et al. 2001; Meisel et al. 1996, 2001; Walker et al. 2002a, b). Dissolved Os is probably present in sea water as an octavalent oxyanion (e.g. HOsO5 , H3OsO6 ). Osmium is removed into organic-rich sediments in direct association with organic matter (Levasseur et al. 1998) and/or is rapidly removed to organic-rich sediments first as Os (IV), and then is further reduced to Os (III) by organic complexation (Yamashita et al. 2007). Osmium removal from sea water into reducing sediments may occur below the depth of Re enrichment (Poirer 2006). B. KENDALL ET AL. 86" @default.
• W2134046539 created "2016-06-24" @default.
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• W2134046539 date "2009-01-01" @default.
• W2134046539 modified "2023-10-16" @default.
• W2134046539 title "¹⁸⁷ Re- ¹⁸⁷ Os geochronology of Precambrian organic-rich sedimentary rocks" @default.
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