Pb and Sr concentrations and isotopic compositions in prehistoric North American teeth: A methodological study

We present Pb and Sr isotopic compositions and elemental concentrations for dentin and enamel from 28 human teeth from 10 locations in the southwestern United States. These teeth represent Native American communities that were functioning between 300 BCE and 1200 CE. We wished to assess whether Pb isotopic data can extend the interpretations made from Sr isotopic data alone, and to determine the analytical requirements for doing Sr and Pb analyses from single samples. Pb concentrations in our samples are between 0.06 and 48 ppm, with Sr concentrations between 10 and 3300 ppm. [superscript 87]Sr/[superscript 86]Sr compositions lie between 0.708 and 0.712. Pb isotopic compositions are more variable, with [superscript 206]Pb/[superscript 204]Pb ranging between 17.9 and 21.7. Isotopic compositions and elemental concentrations for both Sr and Pb can be determined on samples smaller than 1 mg, but there is real heterogeneity between samples of this size. Neither concentrations nor isotopic compositions are reproducible to within analytical uncertainty in replicate aliquots from individual teeth. We cannot determine whether this variability is an in-vivo or a diagenetic effect. Combined Pb and Sr isotopic data discriminate most locations, and the inclusion of Pb data provides a stronger basis for archeological interpretations. Our data suggest that Pb is more susceptible to post-mortem exchange than Sr. Background isotopic data are needed to characterize Pb and Sr sources in the study areas. Pb concentration data for enamel samples indicate a blood Pb load of 0.1–0.2 μg Pb/dL for preindustrial Native Americans

Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post–Snowball Earth hothouse.Agouron Institut

The nature of transition from adakitic to non-adakitic magmatism in a slab window setting: A synthesis from the eastern Pontides, NE Turkey

The eastern Pontides orogenic belt provides a window into continental arc magmatism in the Alpine–Himalayan belt. The late Mesozoic–Cenozoic geodynamic evolution of this belt remains controversial. Here we focus on the nature of the transition from the adakitic to non-adakitic magmatism in the Kale area of Gumushane region in NE Turkey where this transition is best preserved. The adakitic lithologies comprise porphyries and hyaloclastites. The porphyries are represented by biotite-rich andesites, hornblende-rich andesite and dacite. The hayaloclastites represent the final stage of adakitic activity and they were generated by eruption/intrusion of adakitic andesitic magma into soft carbonate mud. The non-adakitic lithologies include basaltic-andesitic volcanic and associated pyroclastic rocks. Both rock groups are cutting by basaltic dikes representing the final stage of the Cenozoic magmatism in the study area. We report zircon U-Pb ages of 48.71 ± 0.74 Ma for the adakitic rocks, and 44.68 ± 0.84 Ma for the non-adakitic type, suggesting that there is no significant time gap during the transition from adakitic to non-adakitic magmatism. We evaluate the origin, magma processes and tectonic setting of the magmatism in the southern part of the eastern Pontides orogenic belt. Our results have important bearing on the late Mesozoic–Cenozoic geodynamic evolution of the eastern Mediterranean region

U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge

New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected ²⁰⁶Pb/²³⁸U dates of 13.528±0.101 to 13.353±0.057Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1-20μm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected ²⁰⁶Pb/²³⁸U dates for the inclusion-rich zircons range from 13.598±0.012 to 13.503±0.018Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction,206Pb/238U dates range from 13.341±0.162 to 12.993±0.055Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges. Key words: zircon; xenotime; hydrothermal; Mid-Atlantic Ridge; Vema; mid-ocean ridgeNational Science Foundation (U.S.) (Grant OCE‐0727914)National Science Foundation (U.S.) (Grant OCE‐0960892)National Science Foundation (U.S.) (Grant EAR‐1250522

Origin of clay minerals in Early Eocene volcanic paleosols on King George Island, Maritime Antarctica

The paleoclimate during the Early Eocene in Maritime Antarctica is characterized by cool conditions without a pronounced dry season. Soils formed on volcanic material under such climate conditions in modern analogue environments are usually Andosols rich in nanocrystalline minerals without pedogenic smectite. The paleosols formed on volcanic material on King Georges Island are covered by basalts, dated by 6 new 40Ar/39Ar datings to 51-48 Ma, and are rich in smectite. A pedogenic origin of the smectites would suggest a semi-arid rather than a wet non-seasonal humid paleoclimate. To investigate the origin of the smectites in these paleosols we used X-ray diffraction and microscopic techniques. Minor mineralogical changes between the volcanic parent material and the paleosols and a homogenous distribution of smectites throughout the paleosol horizons indicate that these smectites were mainly inherited from the pyroclastic parent material, which was altered prior to surficial weathering. Nevertheless, the mineralogical properties, such as degree of crystallinity and octahedral site occupancy, of these smectites were modified during the ancient soil formation. Our findings highlight that trioctahedral smectites were a product of deuteric alteration of pyroclastic rocks and were progressively transformed to dioctahedral smectites during weathering in a soil environment on King George Island