Paulo M. Vasconcelos

In situ radiometric and exposure age dating of the martian surface.

We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of Gale Crater. A K-Ar age of 4.21 ± 0.35 billion years represents a mixture of detrital and authigenic components and confirms the expected antiquity of rocks comprising the crater rim. Cosmic-ray–produced 3He, 21Ne, and 36Ar yield concordant surface exposure ages of 78 ± 30 million years. Surface exposure occurred mainly in the present geomorphic setting rather than during primary erosion and transport. Our observations are consistent with mudstone deposition shortly after the Gale impact or possibly in a later event of rapid erosion and deposition. The mudstone remained buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks exposed by this mechanism may thus offer the best potential for organic biomarker preservation against destruction by cosmic radiation.

Paraná flood basalts: Rapid extrusion hypothesis confirmed by new 40Ar/39Ar results

The duration of volcanism in the Parana continental flood basalt (CFB) province, essential information to assess models of melt generation, eruption rates, continental breakup, and volcanism-driven extinction events, remains unresolved due to conflicting sets of 40Ar/39Ar geochronology data. Some results suggest that the Parana CFB volcanism began at 134.7 ± 1 Ma and lasted <1 m.y. Another set of results reveals an extrusion interval of 11 m.y., from ca. 140 Ma to ca. 129 Ma. To resolve this controversy, we reanalyzed three of the exact hand specimens that previously yielded the oldest and youngest ages in the protracted 11 m.y. range. Our new ages are statistically indistinguishable from each other and the previous 134.7 ± 1 Ma result and indicate a short eruption event. A rapid extrusion for the Parana CFB and the lack of a major cotemporal extinction event challenge proposed direct links between CFB volcanism and mass extinctions and refute petrological models that rely on a protracted extrusion of the Parana CFB.

40Ar/39Ar constraints on a temporal link between gold mineralization, magmatism, and continental margin transtension in the Jiaodong Gold Province, eastern China

The Jiaodong gold province is the largest gold repository in China. Both mineralization and granitoid hosts are spatially related to the crustal‐scale Tan‐Lu strike‐slip fault system, which developed along the Mesozoic continental margin in eastern China. A series of 40Ar/39Ar laser incremental heating analyses of hydrothermal sericite/muscovite from three major gold deposits (Jiaojia, Xincheng, and Wangershan) and igneous biotite from the granodiorite hosts were performed to establish a possible temporal link between gold mineralization, magmatism, and movement along the Tan‐Lu fault zone. Magmatic biotite crystals yield well‐defined and concordant plateau ages between \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration

124.5\pm 0.4

Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation*

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Cenozoic continental weathering and its implications for the palaeoclimate: evidence from 40Ar/39Ar geochronology of supergene K–Mn oxides in Mt Tabor, central Queensland, Australia

124.0\pm 0.4