Radioisotope Systems Research Articles

The Singhbhum Craton (India) records a billion year of continental crust formation and modification

The petrogenesis of continental crust from its ultimate mantle source can be reconstructed from the element abundances and radiogenic isotope compositions of ideally pristine igneous rocks. The initial isotope compositions of igneous rocks provide geochemical constraints on the age, composition and evolution of their source(s). Determining initial isotope ratios for rock samples can be challenging, especially in rocks with a long and protracted thermal history. The Rb-Sr system is highly sensitive to parent-daughter element fractionation during magma differentiation. This makes the Rb-Sr isotope systematics ideal to trace the precursor composition of Archean felsic crust and constrain the time of element fractionation during the formation and subsequent modification of continental crust. Initial isotope compositions can be obtained directly from minerals that strongly prefer the daughter element and effectively exclude the parent element of the radio-isotope system of interest. Apatite, having a near zero Rb/Sr ratio, is ideal for preserving its initial 87Sr/86Sr and zircon records initial 176Hf/177Hf compositions. Combined modelling of Sr and Hf isotope data from granitoids of the Archean Singhbhum Craton, indicates that the older Paleoarchean granitoids, emplaced between 3.53 Ga and 3.44 Ga, were derived from a mafic precursor (∼52–54 wt% SiO2) sourced from a depleted mantle at ∼3.71 Ga. Initial 87Sr/86Sr isotope signatures of matrix apatite and apatite inclusions in zircon from the younger Paleoarchean granitoids (3.4–3.2 Ga) of the Singhbhum Craton indicate these younger granitoids were produced by mixing of magma generated from an older mafic source and partial melts derived from the older granitoids. The combined Sr-Hf isotope modelling links the timing of mantle extraction of the precursor material for Paleoarchean Singhbhum granitoids with a known mafic crust extraction event at ∼3.71 Ga. In combination, the new Sr isotope data from apatite combined with whole rock and zircon Hf isotope data from the literature reveal a ∼1 Ga protracted crustal growth and differentiation history of the nucleus of the Singhbhum Craton. By combining radio-isotope systems like 87Rb-87Sr and 176Lu-176Hf, the petrogenesis of Archean felsic crust from the extraction of mafic material from the mantle to reworking in an orogenic cycle to emplacement can be reconstructed. This approach can be applied to other greenstone-gneiss terranes to quantify the spatio-temporal and compositional evolution of voluminous felsic crust and the formation of cratons in the Archean.

Timescales of faulting through calcite geochronology: A review

Calcite is a common mineral precipitated within upper crustal fault zones and associated fractures, forming veins, mineral cements and fault coatings. It has the potential to be dated using several radioisotopic systems if parent/daughter isotope abundances are suitable for the currently available analytical techniques. U-Pb dating can be most readily achieved using the in-situ laser ablation technique, which has led to a rapid growth area in the U-Pb dating of calcite. The ability to constrain the timing of fault slip hinges critically on the ability to confidently link calcite precipitation to fault movement and/or spatially and temporally associated fracture opening which requires careful microstructural and petrographic documentation. We discuss the varying reliability of different fracture fill types to make these links, and demonstrate that crack-seal-slip fills associated with faults are the most suitable for unambiguously linking calcite growth to phases of fault slip. Previous applications of the U-Pb and U-Th methods to natural examples at a range of temporal and spatial scales are reviewed, in particular their implications for the timescales of faulting and for the rates of fracture-filling. We then highlight the main limitations of the method, and provide a brief commentary on future directions.

Progress, open questions and application prospects of analytical techniques for refractory elements and representative radioisotope systems

Pt-Os、Re-Os和Hf-W同位素体系的母体和子体Pt、Os、Re、Hf和W以及其它铂族元素（PGE）化学性质上都是难熔的，我们将这些元素和相关同位素称为难熔元素和难熔元素放射性同位素体系。难熔元素及其放射性同位素体系不仅可以直接测定金属矿床、油气藏和沉积岩系的年龄，而且还可以有效示踪地球深部的核幔反应等过程，这是其它地球化学工具不可替代的。本文评述了当前Pt-Os、Re-Os和Hf-W同位素体系和PGE元素分析技术的研究进展，指出：（1）目前Re-Os同位素定年误差仍较大，迫切需要优化改进分析技术，大幅提高Re-Os同位素定年的准确度和成功率；（2） Pt-Re-Os和W同位素的分析技术需要简化繁琐的化学分离流程和提高仪器分析的效率，同时探索建立新的化学分离和仪器分析方法，进一步提高分析精度，实现对更低Pt、Re、Os和W含量样品的准确分析，才能更有效地示踪地球深部物质循环过程；（3） PGE的元素分析技术尚需改进，分析精度仍需要进一步提升。我们预期随着进一步提升难熔元素和代表性放射性同位素的分析精度，显著降低同位素定年误差，其将在核-幔、幔-幔和壳-幔物质循环过程示踪、金属矿床精细定年和成因机制、复杂含油气系统的定年和示踪以及中元古代环境变化等研究中得到更广泛的应用。

Timing of Martian core formation from models of Hf–W evolution coupled with N-body simulations

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars.We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ∼5 million years of solar system formation to satisfy the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

An assessment of sanidine from the Fire Clay tonstein as a Carboniferous 40Ar/39Ar monitor standard and for inter-method comparison to U-Pb zircon geochronology

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of synthetic and natural reference materials for both 40Ar/39Ar and U-Pb techniques that permit both inter-laboratory and inter-technique comparisons. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein provides a potential natural Paleozoic reference for these two widely used radioisotopic systems. Here we report results for both radioisotopic systems, examining the suitability of this tonstein as a geochronologic reference. Sanidine crystals from the Fire Clay and co-irradiated monitors from eight irradiation positions were divided into eleven 40Ar/39Ar experiments. Single-grain sanidine 40Ar/39Ar analyses (n = 263) of the simplest 9 experiments have internal 2σ uncertainties at the ±1 Myr level (±0.3%), with a range of dates between ~315 and ~317 Ma (~1% precision), similar to the observed dispersion in the Fish Canyon sanidine monitor dates. Forty-one U-Pb analyses by the CA-ID-TIMS method on carefully selected single Fire Clay tonstein zircons have produced 206Pb/238U dates with an average 2σ precision of ±0.23 Myr (0.14%). Our Fire Clay preferred mean 40Ar/39Ar date of 315.36 ± 0.10 Ma (±1.10 Ma: fully propagated 2σ uncertainty, relative to a Fish Canyon age of 28.201 Ma) is consistent with our weighted mean 206Pb/238U zircon date of 314.629 ± 0.039 Ma (±0.35 Ma: fully propagated 2σ uncertainty; n = 27). The good single-crystal reproducibility of the sanidine data and the overall consistency between the two chronometers suggest that the tonstein holds promise as a Paleozoic age reference material.

The Pb isotope evolution of Bulk Silicate Earth: Constraints from its accretion and early differentiation history

Constraining the evolution of Pb isotopes in the bulk silicate Earth (BSE) is hampered due to the lack of a direct determination of Earth’s U/Pb and initial Pb isotope composition. All estimates of these parameters are strongly model dependent and most Pb evolution models start with a meteoritic source, i.e., the primordial Pb composition determined in troilite from the Canyon Diablo iron meteorite. During the condensation of the elements in the solar nebula, accretion of the Earth, and its subsequent chemical evolution, the U/Pb was modified. Different models make different assumptions about the timing and extent of this U-Pb fractionation during Earth’s chemical evolution that cannot always be related to known global geological processes at the time of this modification. This study explores geochemical constraints that can be related to known geological processes to derive an internally consistent model for the evolution of the U-Th-Pb systematics of the silicate Earth.Lead is chalcophile, moderately volatile, and as a result strongly depleted in the BSE compared to primitive meteorites. Any process affecting the abundance and isotope composition of Pb in Earth throughout its early history has to be consistent with the abundance of elements with similar chemical and physical properties in the same reservoir. The abundances of refractory to moderately and highly volatile elements in the BSE imply that the proto Earth was highly depleted in volatile elements, and therefore evolved with a very high U/Pb (238U/204Pb = µ ≥ 100) prior to collision with the Moon-forming giant impactor. This impactor had close to chondritic abundances of moderately to highly volatile elements and delivered most of Earth’s volatile elements, including the Pb budget. Addition of this volatile-rich component caused oxidation of Earth’s mantle and allowed effective transfer of Pb into the core via sulfide melt segregation. Sequestration of Pb into the core therefore accounts for the high µBSE, which has affected ca. 53% of Earth’s Pb budget. In order to account for the present-day Pb isotope composition of BSE, the giant impact must have occurred at 69 ± 10 Myr after the beginning of the solar system. Using this point in time, a model-derived µ-value and the corresponding initial Pb isotope composition of BSE, a single stage Pb isotope evolution curve can be deduced. The result is a model evolution curve for BSE in 208Pb-207Pb-206Pb-204Pb-isotope space that is fully consistent with geochemical constraints on Earth’s accretionary sequence and differentiation history. This Pb-evolution model may act as a reference frame to trace the silicate Earth’s differentiation into crust and mantle reservoirs, similar to the CHUR reference line used for other radio-isotope systems. It also highlights the long-standing Th/U paradox of the ancient Earth.

Reconciliation of the excess 176Hf conundrum in meteorites: Recent disturbances of the Lu-Hf and Sm-Nd isotope systematics

The long-lived 176Lu-176Hf and 147Sm-143Nd radioisotope systems are commonly used chronometers, but when applied to meteorites, they can reveal disturbances. Specifically, Lu-Hf isochrons commonly yield dates up to ∼300Myr older than the solar system and varying initial 176Hf/177Hf values. We investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites. Meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible. Minerals scatter away from isochron regressions for both meteorite classes, with low-Hf phases such as plagioclase and olivine typically being most displaced above (or left of) reference isochrons. Relatively Hf-rich pyroxene is less disturbed but still to the point of steepening Lu-Hf errorchrons. Using our Lu-Hf and Sm-Nd data, we tested various Hf and Lu redistribution scenarios and found that decoupling of Lu/Hf from 176Hf/177Hf must postdate the accumulation of significant radiogenic 176Hf. Therefore early irradiation or diffusion cannot explain the excess 176Hf. Instead, disturbed meteorite isochrons are more likely caused by terrestrial weathering, contamination, or common laboratory procedures. The partial dissolution of phosphate minerals may predominantly remove rare earth elements including Lu, leaving relatively immobile and radiogenic Hf behind. Robust Lu-Hf (and improved Sm-Nd) meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic Hf and silicate-hosted terrestrial contaminants without disturbing parent-daughter ratios.

Growth of mica porphyroblasts under low-grade metamorphism – A Taiwanese case using in-situ40Ar/39Ar laser microprobe dating

Mica porphyroblasts, a common metamorphic microstructure, are analyzed in the slate belt of northern Taiwan where large fish-like growths are found within a meta-pyroclastics. With constraints on the time-temperature history from deposition through peak metamorphic state to exhumation, in-situ40Ar/39Ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm-scale mica porphyroblasts grown on a pressure-solution seam. Because the peak metamorphic temperature and the porphyroblast formation condition (∼250 °C) remained far below the closure temperature of the K-Ar radioisotope system in muscovite, and the absence of muscovite in the mafic protolith, the dating results likely document the growth of the mica porphyroblast fabrics. The syn-kinematic nature of the analyzed porphyroblasts is confirmed by the ∼6 to ∼2.5 Ma growth ages, suggesting that the host rock was continuously deformed during the earlier two-thirds of the Taiwan Orogeny. The pattern of fiber growth, in contrast to outward-decreasing ages normally observed in peripheral recrystallization, appears random and resembles void fills in boudin openings. We postulate that syntaxial crack-seal following tensile micro-boudinage, along with slips on sub-grain boundaries, as a viable mechanism for the development of mica porphyroblasts and fish especially in lower-grade metamorphic rocks.

Nuclear Forensic Science: Analysis of Nuclear Material Out of Regulatory Control

Nuclear forensic science seeks to identify the origin of nuclear materials found outside regulatory control. It is increasingly recognized as an integral part of a robust nuclear security program. This review highlights areas of active, evolving research in nuclear forensics, with a focus on analytical techniques commonly employed in Earth and planetary sciences. Applications of nuclear forensics to uranium ore concentrates (UOCs) are discussed first. UOCs have become an attractive target for nuclear forensic researchers because of the richness in impurities compared to materials produced later in the fuel cycle. The development of chronometric methods for age dating nuclear materials is then discussed, with an emphasis on improvements in accuracy that have been gained from measurements of multiple radioisotopic systems. Finally, papers that report on casework are reviewed, to provide a window into current scientific practice.

A new approach to cosmogenic corrections in 40Ar/39Ar chronometry: Implications for the ages of Martian meteorites

Anomalously old 40Ar/39Ar ages are commonly obtained from Shergottites and are generally attributed to uncertainties regarding the isotopic composition of the trapped component and/or the presence of excess 40Ar. Old ages can also be obtained if inaccurate corrections for cosmogenic 36Ar are applied. Current methods for making the cosmogenic correction require simplifying assumptions regarding the spatial homogeneity of target elements for cosmogenic production and the distribution of cosmogenic nuclides relative to trapped and reactor-derived Ar isotopes. To mitigate uncertainties arising from these assumptions, a new cosmogenic correction approach utilizing the exposure age determined on an un-irradiated aliquot and step-wise production rate estimates that account for spatial variations in Ca and K is described. Data obtained from NWA 4468 and an unofficial pairing of NWA 2975, which yield anomalously old ages when corrected for cosmogenic 36Ar using conventional techniques, are used to illustrate the efficacy of this new approach. For these samples, anomalous age determinations are rectified solely by the improved cosmogenic correction technique described herein. Ages of 188±17 and 184±17Ma are obtained for NWA 4468 and NWA 2975, respectively, both of which are indistinguishable from ages obtained by other radioisotopic systems. For other Shergottites that have multiple trapped components, have experienced diffusive loss of Ar, or contain excess Ar, more accurate cosmogenic corrections may aid in the interpretation of anomalous ages. The trapped 40Ar/36Ar ratios inferred from inverse isochron diagrams obtained from NWA 4468 and NWA 2975 are significantly lower than the Martian atmospheric value, and may represent upper mantle or crustal components.

Behavior of Re and Os during contact between an aqueous solution and oil: Consequences for the application of the Re–Os geochronometer to petroleum

Several recent studies have raised the exciting possibility that oils can be dated using the Re–Os radioisotope system. However the exact nature of the events dated by this technique often remains unclear. Geochronologic interpretation of Re–Os data for oils is hampered by our limited knowledge of how these metals behave in petroleum systems. In particular, it is difficult to understand how isotopic homogenization, an essential prerequisite for the development of an isochronal relationship, can be achieved at the scale of a petroleum basin. The mechanisms capable of fractionating the Re/Os ratio in a suite of oils are also poorly understood. For this reason, we have performed an experimental study aimed at investigating the behavior of Re and Os during a particularly widespread phenomenon in petroleum systems, the interaction of formation waters with oils during migration. Contact experiments between natural oils and aqueous solutions enriched in Re and/or Os were carried out for varying lengths of time (6h to 5months), at different temperatures (25–150°C), over a wide range of metal concentrations in the enriched solution (0.001–100μg/g for Re; 1 and 10ng/g for Os). In addition, the effect of oil composition on Re–Os exchange at the water–oil interface was examined by testing two oils with very different properties.All of our results demonstrate that Re and Os are transferred massively and very rapidly from the aqueous solution to the organic phase. This is true regardless of temperature or oil composition. It is also true for a very wide range of metal concentrations in the aqueous solution, up to an apparent saturation level that exceeds natural concentrations in oils by several orders of magnitude. Given the efficiency of Re and Os transfer from water to oil demonstrated here, and assuming that our findings are applicable to natural conditions, water/oil ratios of only about 250 would be needed to explain the Re and Os contents of most oils, based on the measured concentrations of Re and Os in groundwaters. Thus transfer of Re and Os from formation waters could plausibly provide a source for these metals in oils. Waters from a given basin may have roughly homogeneous Os isotopic compositions, approximating the average value of the source rocks. On the other hand, variable Re/Os ratios might be expected to develop, as a result of interaction with various phases (e.g. Fe–Mn oxides or authigenic pyrite) encountered by the water as it flows through the reservoir. If homogeneous 187Os/188Os ratios coupled with variable Re/Os ratios are transferred to the oils with which the waters come in contact, the isotopic geochronometer could be reset.

Age and geochemistry of the Newania dolomite carbonatites, India: implications for the source of primary carbonatite magma

The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accepted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using 147Sm–143Nd, 207Pb–206Pb and 40Ar–39Ar dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr–Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at ~1,473 Ma and parts of it were affected by a thermal event at ~904 Ma. The older 207Pb–206Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261–273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signatures of these rocks, such as its dolomite–magnesite–ankerite–Cr-rich magnetite–magnesio-arfvedsonite–pyrochlore assemblage, mantle like δ13C and δ18O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (≥80) and have similar major element oxide contents as that of primary carbonatite melts experimentally produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite melt and that it was derived from a carbonate bearing mantle. The Sr–Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.

Effect of the progressive precipitation of petroleum asphaltenes on the Re–Os radioisotope system

Recent studies (e.g., Selby and Creaser, 2005) have raised the intriguing possibility that the evolution of oil can be dated using the Re–Os radioisotope system. However, in order to understand which specific events are being dated, it is essential to determine the processes that could create the range in the parent–daughter ratio necessary for the use of the geochronometer. One particularly important process in the evolution of petroleum is the sequential and progressive precipitation of asphaltenes during oil migration, mixing and reservoir filling. As asphaltenes are known to contain the near totality of the Re and Os present in oil, this event could potentially lead to the required fractionation of Re/Os ratios. Sequential asphaltene precipitation was performed in the laboratory with the aim of assessing the effect of progressive precipitation on relative and absolute Re and Os contents. The experiments were performed on two well preserved oils with distinct properties: one was a medium density, sulfur-poor crude oil containing about 7% asphaltenes; the other a heavy, sulfur-rich oil containing about 20% asphaltenes. Bulk asphaltenes were extracted from each oil and dissolved in an organic solvent (dichloromethane). Starting with these bulk fractions, sequential precipitation into fractions of decreasing polarity was provoked by adding increasing quantities of precipitant (n-heptane or n-pentane). For both oils, the Re and Os concentrations of the successive asphaltene fractions decreased steadily during the progressive precipitation. However the Re/Os ratio remained constant, except at very high degrees of asphaltene precipitation. This suggests that natural asphaltene precipitation during the evolution of hydrocarbons in a petroleum system has very little effect on the Re/Os ratio. Thus asphaltene precipitation during the evolution of hydrocarbons in a petroleum system or during oil recovery is unlikely to perturb the Re–Os geochronometer. Conversely, asphaltene precipitation is probably not the process responsible for the Re/Os fractionation required to develop an isochron.The similarity of the concentration profiles with progressive precipitation obtained for Re and Os suggests that these metals have similar binding properties in asphaltenes. In contrast, V and Ni concentration profiles obtained during these experiments display a different character. Unlike Re and Os, which are hosted by the most polar asphaltenes, V and Ni are preferentially bound to asphaltenes of intermediate polarity. This suggests that Re and Os are not hosted by the same organic entities that carry V and Ni (e.g., metallo-porphyrins).

Radon concentration of outdoor air: measured by an ionization chamber for radioisotope monitoring system at radioisotope institute

Gas-flow ionization chambers for radioisotope (RI) monitoring systems at RI institutes throughout Japan are commonly used to measure RIs which leak from the RI institutes. Before the Japan’s 2011 Tohoku earthquake [11 March 2011, moment magnitude (M w) 9.0], ionization current data measured with a gas-flow ionization chamber at the RI institute of Fukushima Medical University were found to change. The question we must raise is whether the variation ionization current can be considered to the variation of outdoor radon concentration. The conversion factors (from ionization current to radon concentration in air) of the gas-flow ionization chamber can be obtained by measuring four levels of radon concentration (outdoor air, indoor air, high level and radon-free gas) with an AlphaGUARD monitor and the chamber itself. The two gas-flow ionization chambers consist of the air intake and terminal exhaust duct of the RI institute. It was found that the radon concentration in the exhaust air was the same as that in the air intake. This study provided evidence that variations of outdoor radon concentration could be determined using gas-flow ionization chambers for RI monitoring systems.

Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite

Research Article| October 01, 2010 Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite John M. Hora; John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian R. Jicha; Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian L. Beard; Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Clark M. Johnson; Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Shan de Silva; Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Morgan Salisbury Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Publisher: Geological Society of America Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (10): 923–926. https://doi.org/10.1130/G31064.1 Article history Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John M. Hora, Brad S. Singer, Brian R. Jicha, Brian L. Beard, Clark M. Johnson, Shan de Silva, Morgan Salisbury; Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite. Geology 2010;; 38 (10): 923–926. doi: https://doi.org/10.1130/G31064.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra–marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an “infinite reservoir” such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re–Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re–Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re–Os ages of 1361 ± 21 Ma (2 σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2 σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105–119 ppm) and degree of pyritization [DOP] values (0.90–0.92) implying quantitative conversion of molybdate (MoO 4 2−) to thiomolybdate (MoS 4 2−) in overlying bottom waters. The average δ 97/95Mo (0.72 ± 0.10‰, 2 σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ 97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H 2S] aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H 2S] aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30–70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively. The Re–Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re–Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2 σ, n = 21, MSWD = 85, Model 3). This age is younger than U–Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re–Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb–Zn–Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re–Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41–58 ppm), but are characterized by variable δ 97/95Mo (0.3–0.8‰) and DOP (0.57–0.92). The lightest δ 97/95Mo values occur near the base of the black shale unit. Based on the Re–Os systematics, hydrothermal fluids have probably overprinted the authigenic δ 97/95Mo signature in those shales. However, the heaviest δ 97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ 97/95Mo at 1.73 Ga.

Short-lived radionuclides as monitors of early crust–mantle differentiation on the terrestrial planets

The kinetic energy from large impacts, the gravitational energy released by core formation, and the heat provided by the decay of short-lived radioactive isotopes all drive extensive melting and chemical differentiation of silicate planets/planetesimals during and shortly after their formation. This early differentiation is best preserved on small objects such as the parent bodies of the eucrite and angrite meteorites where silicate melts were produced within 3 million years of solar system formation. The W isotopic composition of some iron meteorites testifies to core segregation on small planetesimals within as little as one million years or less of solar system formation. On larger objects, such as the Moon, Mars and Earth, the evidence for early differentiation provided by long-lived radioisotope systems has been variably overprinted by the continuing differentiation of these objects, but a clear signature of extensive early planet-scale differentiation is preserved in a variety of short-lived radioisotope systems, particularly, I–Pu–Xe, Hf–W and 146Sm– 142Nd. All these systems suggest that global differentiation of planetesimals and the terrestrial planets occurred during the first hundred million years of solar system history. This early processing of the Moon, Mars and Earth, may have fundamentally affected the evolution of these planets and their current internal compositional structure.

187 Re- 187 Os geochronology of Precambrian organic-rich sedimentary rocks

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 187 Re (rhenium)– 187 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 (&lt;1%, 2σ) from Late Neoproterozoic to Late Archaean shales. Whole-rock digestion using a Cr VI –H 2 SO 4 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 187 Os/ 188 Os composition determined from Re–Os isochron regressions serves as a tracer for the Os isotope composition of Precambrian sea water.

Analysis of Electric Propulsion System for Exploration of Saturn

Exploration of the outer planets has experienced new interest with the launch of the Cassini and the New Horizons Missions. At the present time, new technologies are under study for the better use of electric propulsion system in deep space missions. In the present paper, the method of the transporting trajectory is used to study this problem. This approximated method for the flight optimization with power‐limited low thrust is based on the linearization of the motion of a spacecraft near a keplerian orbit that is close to the transfer trajectory. With the goal of maximizing the mass to be delivered in Saturn, several transfers were studied using nuclear, radioisotopic and solar electric propulsion systems.

Applications of PGE Radioisotope Systems in Geo- and Cosmochemistry

The platinum-group elements contain three radioisotope systems that have been used in many and varied ways in geo- and cosmochemistry. Unique chronological applications include dating the formation of such diverse materials as sulfides, gold, organic-rich sediments, iron meteorites, and sulfide inclusions in diamonds. These systems also serve as isotope tracers for processes such as continental erosion, the deposition of extraterrestrial materials on Earth's surface, crust-mantle differentiation, recycling of subducted crust into the mantle, core-mantle exchange, and volatile-element depletion of planets and planetesimals. Although these systems have been in use for only a short time, the discoveries they have provided bode well for their incorporation as staples in the geochemical toolbox.