Thomas Chacko

Further evaluation of the Re-Os geochronometer in organic-rich sedimentary rocks: A test of hydrocarbon maturation effects in the Exshaw Formation, Western Canada Sedimentary Basin

Abstract Re-Os isotopic analyses of a single organic-rich sedimentary rock unit (ORS) of known depositional age, and at three levels of regional hydrocarbon maturity, show that hydrocarbon maturation does not affect the ability the 187Re-187Os chronometer to yield a depositional age for such rocks. We present Re-Os isotope analyses from the Late Devonian Exshaw Formation in the subsurface of the Western Canada Sedimentary Basin, Alberta, and obtain a Re-Os isochron age of 358 ± 10 Ma (2σ, Model 3, λ = 1.666 × 10−11.a−1) for samples ranging from hydrocarbon immature to overmature. This age is within uncertainty of the established absolute age for the Exshaw Formation. Hydrocarbon immature, and mature plus overmature samples show no significant age differences if regressed individually, indicating that hydrocarbon maturation did not greatly disturb the Re-Os isotope system in the Exshaw Formation. As such, we propose that the Re-Os geochronometer may be used as a reliable tool for measuring the depositional ages of ORS regardless of their level of hydrocarbon maturity. We find that minimizing natural variation in the initial 187Os/188Os ratio is more important than avoiding hydrocarbon maturation in obtaining precise Re-Os ages. In particular, the Exshaw Formation appears to contain a nonhydrogenous component of unradiogenic Os, in addition to the hydrogenous Os load. A subset of Exshaw Formation samples with >5% total organic carbon (TOC), which should best reflect the hydrogenous Os load alone, yields a very well-fitted isochron having a depositional age of 358 ± 9 Ma (2σ, λ = 1.666 × 10−11.a−1) with an initial 187Os/188Os ratio of 0.59 ± 0.05 (Model 3, Mean Square of Weighted Deviates (MSWD) = 1.8). The initial 187Os/188Os ratio of this regression may provide an estimate of the Os isotopic composition of local seawater at the time of deposition.

OXYGEN ISOTOPE FRACTIONATIONS IN MUSCOVITE, PHLOGOPITE, AND RUTILE

Oxygen isotope fractionations in laboratory systems have been determined for each of the following minerals relative to calcite: muscovite, phlogopite, fluorophlogopite, and rutile. Statistical mechanical calculations following the method of Kieffer (1982) were fit to the experimental data and then used to extrapolate the experimental results to higher and lower temperatures. The calculations are represented by a series of equations which allow the reduced partition function ratios (β factors) for each of these minerals to be calculated at T > 400 K. These equations can be combined with corresponding equations for calcite, quartz, albite, anorthite, diopside, forsterite, and magnetite (Clayton and Kieffer, 1991) to give a large number of mineral-pair fractionations for use as isotopic thermometers. It was found that the high-frequency vibrations of OH bonds contribute such a small amount of the fractionation factors that they do not introduce significant nonlinearity to plots of Δ vs. T−2. The commonly used calibrations of quartz-muscovite and quartz-biotite fractionations are not in good agreement with the present experimental measurements. This probably reflects disturbance of the rock assemblages on which those calibrations were based, as a consequence of the high diffusivity of oxygen in micas. The experimental quartz-rutile fractionations are in good agreement with some earlier hydrothermal experiments and with an empirically determined calibration. The calculated rutile partition functions of Kieffer (1982) are not consistent with the experimental results, probably due in part to the neglect of the effect of cation mass on the vibrational energies. The large number of mineral systems with measured fractionation factors allows a test of various empirical relationships based on oxygen bond strengths. In general, these relationships are successful for anhydrous silicates, but do not adequately account for the behavior of hydrous minerals or metal oxides.

Role of oceanic plateaus in the initiation of subduction and origin of continental crust

Archean tonalite-trondhjemite-granodiorite (TTG) magmatic suites represent the oldest coherent pieces of felsic continental crust. We present results of long-duration dehydration-melting experiments on amphibolites to show that melting depths of >48 km are required to have sufficient garnet in the residuum for generating the degree of heavy rare earth element (REE) depletion documented in Early Archean TTG. This depth constraint is inconsistent with early crust evolution models that posit melting at the base of oceanic plateaus or oceanic crust to explain the origin of Early Archean continental crust. Alternative explanations for this early crust, e.g., subduction models, are problematic in that they do not readily provide a mechanism for initiation of subduction in a hotter Archean Earth. We hypothesize that intraoceanic subduction systems in the Archean originated due to gravitational instabilities produced by compositional and density contrasts between converging oceanic plateau and normal oceanic lithosphere. Our model differs from earlier models of subduction initiation in that subduction of oceanic lithosphere occurs through the ‘hot’ mantle residuum (protomantle lithosphere) at the base of newly formed oceanic plateau crust. Under a high Archean geothermal gradient, subducted oceanic crust would melt to produce TTG. This model explains the origin of subduction systems, TTG, TTG–mafic and/or ultramafic magma association, stabilization of continental crust, and the broadly coeval formation of cratons and their lithospheric roots.

Circa 2.3‐Ga Magmatism of the Arrowsmith Orogeny, Uranium City Region, Western Churchill Craton, Canada

The western margin of the Churchill craton records a complex history of Paleoproterozoic tectonism. The most prominent of these tectonic events is recorded within the 2.0–1.9‐Ga Taltson‐Thelon magmatic zone. The widespread magmatism and high‐grade metamorphism of this zone, especially in Alberta and Saskatchewan, obscures an older tectonic belt, the ca. 2.4–2.3‐Ga Arrowsmith Orogeny. In the Uranium City region of northwestern Saskatchewan, a suite of Paleoproterozoic granites represent a magmatic product of this orogen. U‐Pb zircon crystallization ages were identified for the Macintosh Bay monzogranite ( \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

Constraints on Rates of Granitic Magma Transport from Epidote Dissolution Kinetics

2330\pm 5

Geochemical and Nd-Pb-O isotope systematics of granites from the Taltson Magmatic Zone, NE Alberta: implications for early Proterozoic tectonics in western Laurentia

\end{document} Ma), the Gunnar monzogranite ( \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

Anton terrane revisited: Late Archean exhumation of a moderate-pressure granulite terrane in the western Slave Province

2321\pm 3