Biography:

In the past Paul R. Renne has collaborated on articles with Manoel S. D'Agrella-Filho and Rebecca J. Dorsey. One of their most recent publications is 40Ar/39Ar dating of 1.0–1.1 Ga magnetizations from the Sa˜o Francisco and Kalahari cratons: tectonic implications for Pan-African and Brasiliano mobile belts. Which was published in journal Earth and Planetary Science Letters.

More information about Paul R. Renne research including statistics on their citations can be found on their Copernicus Academic profile page.

Paul R. Renne's Articles: (13)

40Ar/39Ar dating of 1.0–1.1 Ga magnetizations from the Sa˜o Francisco and Kalahari cratons: tectonic implications for Pan-African and Brasiliano mobile belts

AbstractPaleomagnetic poles from 1.1-1.0 Ga dyke swarms in eastern Brazil (Sa˜o Francisco Craton) are compared with 1.0 Ga poles from granulites of the Namaqua Province in southern Africa (Kalahari Graton). The intrusive ages of dykes are estimated from40Ar/39Ar dating of outgassed biotites from baked country rocks. The age of magnetization for the granulites is derived by combining40Ar/39Ar hornblende and biotite dates. When restored to a Mesozoic pre-drift configuration the paleomagnetic poles are in crude spatial agreement but are temporally discordant. To satisfy both paleomagnetic and geochronologic constraints, a reconstruction involving separation of the Sa˜o Francisco and Kalahari cratons is required, indicating that the intervening Pan-African (Brasiliano) mobile belt may record a craton-craton collision.

Research paperPaleomagnetic evidence for the evolution of Meso- to Neo-proterozoic glaciogenic rocks in central-eastern Brazil☆

AbstractPaleomagnetic studies on basic dikes in the eastern São Francisco Craton which have isotopic ages of 1.0–1.1 Ga, define an apparent polar wander path for South America over this time interval. The data indicate that the São Francisco Craton was at paleolatitudes between 40° and 65° at the time of emplacement of these dikes. Neo-Proterozoic sedimentary glaciogenic rocks, the Macaúbas Group, Bebedouro Formation, Ibiá Formation and Carandai Formation, crop out in central-eastern Brazil. An age of about 1.0 Ga has been proposed for these glacial deposits. Paleogeographical reconstructions of South America show a continental movement coherent with paleoenvironmental models proposed for the Macaúbas Group and suggest that the glacial period may have occurred between 1.01 and 1.08 Ga.

Rapid subsidence and stacked Gilbert-type fan deltas, Pliocene Loreto basin, Baja California Sur, Mexico

AbstractPliocene nonmarine to marine sedimentary rocks exposed in the Loreto basin, Baja California Sur, provide a record of syntectonic subsidence and sedimentation in a transform-rift basin that developed along the western margin of the Gulf of California. A thick sequence of twelve Gilbert-type fan deltas, having a total measured thickness of about 615 m, accumulated near the fault-bounded southwestern margin of this basin. Based on stratal geometries and lithofacies associations, sedimentary rocks are divided into Gilbert-delta topset, foreset and bottomset strata, shell beds and background shallow-marine shelf deposits. Topset strata of each Gilbert-type delta cycle are capped by laterally persistent molluscan shell beds containing diverse assemblages of bivalves, pectens, oysters, gastropods and echinoids. These shell beds are interpreted to be condensed intervals that record sediment starvation during abandonment of the fan-delta plain. Delta abandonment may have been caused by large episodic faulting events, which submerged each pre-existing fan-delta plain, substantially slowed detrital input by drowning of alluvial feeder channels, and created new accommodation space for each new Gilbert-type fan delta. Alternatively, it is possible that delta-plain abandonment was caused by upstream avulsions and autocyclic lateral switching of fan-delta lobes during relatively uniform rates of slip along the basin-bounding fault.Two contrasting, plausible basin models are proposed for the Loreto basin: (1) asymmetric subsidence along a high-angle oblique-slip normal fault, producing a classic half-graben basin geometry with vertically stacked Gilbert-type fan deltas; or (2) lateral stacking and horizontal displacement of strata away from a relatively fixed depocenter due to fault movement in the releasing bend of a listric strike-slip fault. We favor the first model because field relations and simple geometric constraints suggest that most of the total measured section represents a true vertical stratigraphic profile. Assuming vertical sediment accumulation and using ages of interbedded tuffs obtained from high-precision 40Ar/ 39Ar dating of plagioclase and biotite, quantitative decompaction and geohistory analysis was carried out for the Loreto basin sequence. Tuff ages range from 2.61 ± 0.01 Ma in the lower part of the basinal sequence to 1.97 ± 0.02 Ma near the top, with two intermediate tuffs dated at 2.46 ± 0.06 and 2.36 ± 0.02 Ma that are separated by 782 m of measured section. Basin subsidence initially took place at moderate rates of 0.43 ± 0.17 mm/yr and accelerated dramatically at 2.46 Ma to 8.1 ± 5.1 mm/yr. This phase of extremely rapid subsidence lasted for only about 100 ka, and it produced much of the total accomodation space and sedimentary thickness in the basin. Accumulation of Gilbert-type fan deltas took place only during the short pulse of very rapid subsidence, between 2.46 and 2.36 Ma. Prior to this time interval, alluvial-fan and shelf-type fan-delta depositional systems prevailed; afterwards no fan deltas of any kind were deposited, and the basin evolved to a slowly subsiding low-energy carbonate shelf setting. This suggests that very rapid subsidence, combined with rapid sediment input, may be required to maintain steep basin-margin slopes and continually create new accommodation space, conditions that seem necessary for the development of thick sequences of stacked Gilbert-type fan deltas.

Implications of pre-eruptive magmatic histories of zircons for U–Pb geochronology of silicic extrusions

AbstractZircons in silicic magmas begin to crystallize 10's to 100's of thousands of years (ka) prior to their eruption. U/Pb and U/Th dating for zircons from 32 young extrusions indicate a range of mean pre-eruption ages from ∼ 33 to 247 ka, with a median of 92 ka. Explosive large volume (widespread) eruptions show a narrower range and a younger median of 70 ka. These results indicate that the difference between initial zircon growth and eruption age measured by other (e.g., 40Ar/39Ar) techniques is in excess of the achievable analytical uncertainty for eruptions < 30 million years (Ma), and likely for eruptions of up to 300 Ma. Protracted pre-eruption zircon growth is well established by an increasing number of studies on magma process time scales, but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high-precision U/Pb data. Until intrinsic properties of zircons or their host deposits enable a priori estimates of pre-eruption age bias, it is prudent to expand the uncertainties attending all U/Pb zircon ages interpreted as deposition ages (in e.g., chronostratigraphic studies) by + 0/− 200 ka.

Argon diffusion in pyroxenes: Implications for thermochronometry and mantle degassing

AbstractThe rate at which argon diffuses in pyroxenes is relevant to several problems in the Earth and planetary sciences. Pyroxenes are among the most abundant minerals in the Earth's upper mantle and are important hosts for potassium and radiogenic 40Ar (40Ar*). Additionally, pyroxenes commonly contain > 25% of the total 40Ar* within meteorites, and therefore hold great potential for quantifying the timing, duration, and thermal conditions of events experienced by meteorites and their parent bodies. We conducted detailed, feedback-controlled laser heating diffusion experiments using synthetically produced 37Ar in gem-quality clinopyroxene (CPX) and orthopyroxene (OPX) crystals. Stepwise heating between 850 and 1350 °C yielded linear Arrhenius arrays from which activation energies (Ea) of 379.2 ± 4.1 and 371.0 ± 6.0 kJ/mol and pre-exponential factors (Do) of (1.36 × 10− 4)− 6.15 × 10− 5+ 1.12 × 10− 4 and (5.73 × 10− 2)− 2.25 × 10− 2+ 3.72 × 10− 2 were determined for CPX and OPX, respectively; these Ea's are an order of magnitude higher than previously reported values obtained with a different experimental approach (Watson et al., 2007) and indicate that Ar diffusion within pyroxenes is strongly temperature dependent. At high temperatures Ar diffusion in pyroxenes is rapid, which indicates that diffusive 40Ar distributions observed within meteoritic pyroxenes may provide information about brief, high-temperature shock heating events. In contrast, subsequent low-T conditions should minimally influence these concentration gradients. The experimental Ar diffusion kinetics in OPX and CPX correspond to closure temperatures of 600–700 and 700–800 °C, respectively (for 10 °C/Ma cooling). These results suggest that diffusive equilibration through 0.1–0.5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures (~ 1300 °C), and is therefore unlikely to inhibit mantle degassing.

A lattice Boltzmann model for noble gas diffusion in solids: The importance of domain shape and diffusive anisotropy and implications for thermochronometry

AbstractThermochronometry based on radiogenic noble gases is critically dependent upon accurate knowledge of the kinetics of diffusion. With few exceptions, complex natural crystals are represented by ideal geometries such as infinite sheets, infinite cylinders, or spheres, and diffusivity is assumed to be isotropic. However, the physical boundaries of crystals generally do not conform to ideal geometries and diffusion within some crystals is known to be anisotropic. Our failure to incorporate such complexities into diffusive models leads to inaccuracies in both thermal histories and diffusion parameters calculated from fractional release data. To address these shortcomings we developed a code based on the lattice Boltzmann (LB) method to model diffusion from complex 3D geometries having isotropic, temperature-independent anisotropic, and temperature-dependent anisotropic diffusivity. In this paper we outline the theoretical basis for the LB code and highlight several advantages of this model relative to more traditional finite difference approaches. The LB code, along with existing analytical solutions for diffusion from simple geometries, is used to investigate the affect of intrinsic crystallographic features (e.g., crystal topology and diffusion anisotropy) on calculated diffusion parameters and a novel method for approximating thermal histories from crystals with complex topologies and diffusive anisotropy is presented.

Quantifying interference of krypton produced from neutron irradiation of inclusion-hosted and lattice-coordinated bromine with 40Ar/39Ar geochronology

AbstractVarious interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed with the significant ones being routinely corrected for in 40Ar/39Ar geochronology. Interference of double charged 80Kr (80Kr++) with 40Ar has not yet been considered. Significant amounts of 80Kr are produced during neutron irradiation through the reaction 79Br(n,β−)80Kr. While previous workers reported a computed fission spectrum averaged cross section of ∼48 mb—compared to ∼113 mb for 39K(n,p)39Ar—we determined a ∼33-fold higher production rate of 80Kr from Br compared to 39Ar from K in the CLICIT facility of the OSU reactor. Low-K, high-Br phases, e.g., some amphiboles, and fluid or melt inclusion rich samples are prone to 80Kr++ interference biasing 40Ar/39Ar dates in the ‰ to % range. 80Kr++ resembles excess 40Ar in step-heating experiments. The interference can be corrected for by using the parallel reaction 81Br(n,β−)82Kr.

Design, construction, and characterization of a compact DD neutron generator designed for 40Ar/39Ar geochronology

AbstractA next-generation, high-flux DD neutron generator has been designed, commissioned, and characterized, and is now operational in a new facility at the University of California Berkeley. The generator, originally designed for 40Ar/39Ar dating of geological materials, has since served numerous additional applications, including medical isotope production studies, with others planned for the near future. In this work, we present an overview of the High Flux Neutron Generator (HFNG) which includes a variety of simulations, analytical models, and experimental validation of results. Extensive analysis was performed in order to characterize the neutron yield, flux, and energy distribution at specific locations where samples may be loaded for irradiation. A notable design feature of the HFNG is the possibility for sample irradiation internal to the cathode, just 8 mm away from the neutron production site, thus maximizing the neutron flux (n/cm2/s). The generator’s maximum neutron flux at this irradiation position is 2.58 × 107 n/cm2/s ± 5% (approximately 3 × 108 n/s total yield) as measured via activation of small natural indium foils. However, future development is aimed at achieving an order of magnitude increase in flux. Additionally, the deuterium ion beam optics were optimized by simulations for various extraction configurations in order to achieve a uniform neutron flux distribution and an acceptable heat load. Finally, experiments were performed in order to benchmark the modeling and characterization of the HFNG.

The role of tephra studies in African paleoanthropology as exemplified by the Sidi Hakoma Tuff

AbstractBeginning in the 1960s, geological and paleoanthropological exploration of the Ethiopian rift system’s basins have led to the discovery and assembly of the most comprehensive record of human biological and technological change during the last 6 million years. The hominid fossils, including partial skeletons, were primarily discovered in the Afar Rift, the Main Ethiopian Rift, and in the Omo Basin of the broadly rifted zone of SW Ethiopia. The paleoanthropological research areas within the SW Afar Rift that have yielded many diverse hominid species and the oldest stone tools are, from north to south, Woranso-Mille (aff. Ardipithecus and Au. afarensis), Hadar (Au. afarensis, Homo sp.), Dikika (Au. afarensis), Gona (Ar. kadabba, Ar. ramidus, H. erectus, and oldest stone tools), Middle Awash (Ar. kadabba, Ar. ramidus, Au. anamensis, Au. afarensis, Au. garhi, H. erectus, H. rhodesiensis, H. sapiens idaltu, and the oldest paleo-butchery locality), and Galili (Au. afarensis). Additional hominid remains were discovered at Melka Kunture on the banks of the Awash River near its source along the western margin of the central part of the Main Ethiopian Rift (H. erectus), at Konso (H. erectus and A. boisei), and at the southern end of the MER, and in the Omo Basin (Au. anamensis, Au. afarensis, Au. aethiopicus, Au. boisei, H. habilis, and H. erectus).Distal and sometimes proximal tephra units interbedded within fossilifeous sedimentary deposits have become key elements in this work by providing chronological and correlative control and depositional contexts. Several regional tephra markers have been identified within the northern half of the eastern African rift valley in Ethiopia and Kenya, and in marine sediments of the Gulf of Aden Rift and the NW Indian Ocean. Out of the many regional tephra stratigraphic markers that range in age from the early Pliocene (3.97 Ma) to the late Pleistocene (0.16 Ma), the Sidi Hakoma Tuff (SHT) has been more widely identified and thoroughly characterized than any of the others.An age of 3.446 ± 0.041 Ma was determined on the SHT according to the most recent calibration, and it is the only regional stratigraphic marker whose source has been traced to a buried caldera in the central sector of the Main Ethiopian Rift. This paper describes new SHT occurrences and presents chemical and chronological results in the context of a broader review of the importance of this key marker. Moreover, the geographic distributions, probable dispersal mechanisms, and importance of regional tephra units in determining the tectonic and sedimentological processes in the different rift basins of the eastern African rift valleys are considered.

Research PaperArchaeological age constraints from extrusion ages of obsidian: Examples from the Middle Awash, Ethiopia

AbstractExtrusion ages of archaeological obsidian, especially as determined by the 40Ar/39Ar method, can provide reliable maximum ages for tool manufacture. In at least one case in the Middle Awash of Ethiopia, freshly extruded obsidian was used for tool making, resulting in useful maximum ages for site occupation. Hydration resulting in mobility of K and/or Ar in glass, and recoil artifacts produced by neutron irradiation, fatally affect most glass shards from volcanic ashes. The much lower surface area to volume ratio of most archaeological obsidian, however, indicates that the affected areas can be manually removed prior to analysis and the recoil and hydration problems can be easily overcome. A more important issue in dating obsidian is that of possible mass-dependent kinetic isotope fractionation during or subsequent to quenching of volcanic glasses. This is evidenced in some cases by sub-atmospheric initial 40Ar/36Ar ratios, and more generally in sub-atmospheric 38Ar/36Ar. Resulting bias can be avoided through the use of isochron ages, which do not entail the assumption of an initial value of 40Ar/36Ar as is required for plateau ages. Since step heating of glasses often yields limited variability in 40Ar:39Ar:36Ar (and therefore little spread on isochrons), another approach is to use an average value for initial 40Ar/36Ar, with concomitantly larger uncertainty than is associated with atmospheric 40Ar/36Ar, when calculating a plateau age. The 38Ar/36Ar of an un-irradiated subset of our samples validates the inference of kinetic fractionation, and potentially provides a basis for determining initial 40Ar/36Ar in samples that fail to yield isochrons, but only in samples lacking magmatic excess 40Ar. These approaches allow us to reliably apply the 40Ar/39Ar method to volcanic glasses, which has resulted in maximum ages for archaeological sites that are not amenable to traditional geochronological methods. 40Ar/39Ar geochronology can also provide information on the geological provenance of the raw material used for tool making, especially when combined with geochemical data.

Short CommunicationData reporting norms for 40Ar/39Ar geochronology

AbstractData reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses).

Research paperIntercalibration and age of the Alder Creek sanidine 40Ar/39Ar standard

Highlights•Alder Creek sanidine (ACs) is calibrated to astronomically dated Miocene tuffs.•These provide stepwise and direct intercalibration with Fish Canyon sanidine (FCs).•New data yield a weighted mean intercalibration factor RFCsACs=0.041702±0.000014(σ).•Combining with published data yields an interlaboratory RFCsACs=0.041707±0.000011.•These results yield a weighted mean age of tACs = 1.1848 ± 0.0006 Ma (±0.05%).

Research paperHigh-precision 40Ar/39Ar dating of pleistocene tuffs and temporal anchoring of the Matuyama-Brunhes boundary

AbstractHigh-precision 40Ar/39Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) (40Ar/39Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision 40Ar/39Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale.

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