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Now showing items 1 - 16 of 3276

  • Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets

    Foley, Bradford J.   Smye, Andrew J.  

    Models of thermal evolution, crustal production, and CO2 cycling are used to constrain the prospects for habitability of rocky planets, with Earth-like size and composition, in the stagnant lid regime. Specifically, we determine the conditions under which such planets can maintain rates of CO2 degassing large enough to prevent global surface glaciation but small enough so as not to exceed the upper limit on weathering rates provided by the supply of fresh rock, a situation which would lead to runaway atmospheric CO2 accumulation and an inhospitably hot climate. The models show that stagnant lid planets with initial radiogenic heating rates of 100-250 TW, and with total CO2 budgets ranging from approximate to 10(-2) to 1 times Earth's estimated CO2 budget, can maintain volcanic outgassing rates suitable for habitability for approximate to 1-5 Gyr; larger CO2 budgets result in uninhabitably hot climates, while smaller budgets result in global glaciation. High radiogenic heat production rates favor habitability by sustaining volcanism and CO2 outgassing longer. Thus, the results suggest that plate tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable climate. The model is necessarily highly simplified, as the uncertainties with exoplanet thermal evolution and outgassing are large. Nevertheless, the results provide some first-order guidance for future exoplanet missions, by predicting the age at which habitability becomes unlikely for a stagnant lid planet as a function of initial radiogenic heat budget. This prediction is powerful because both planet heat budget and age can potentially be constrained from stellar observations. Key Words: ExoplanetsHabitabilityStagnant lid tectonicsCarbon cycleVolcanism. Astrobiology 18, 873-896.
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  • The divergent fates of primitive hydrospheric water on Earth and Mars

    Wade, Jon   Dyck, Brendan   Palin, Richard M.   Moore, James D. P.   Smye, Andrew J.  

    Despite active transport into Earth's mantle, water has been present on our planet's surface for most of geological time(1,2). Yet water disappeared from the Martian surface soon after its formation. Although some of the water on Mars was lost to space via photolysis following the collapse of the planet's magnetic field(3-5), the widespread serpentinization of Martian crust(6,7) suggests that metamorphic hydration reactions played a critical part in the sequestration of the crust. Here we quantify the relative volumes of water that could be removed from each planet's surface via the burial and metamorphism of hydrated mafic crusts, and calculate mineral transition-induced bulk-density changes at conditions of elevated pressure and temperature for each. The metamorphic mineral assemblages in relatively FeO-rich Martian lavas can hold about 25 per cent more structurally bound water than those in metamorphosed terrestrial basalts, and can retain it at greater depths within Mars. Our calculations suggest that in excess of 9 per cent by volume of the Martian mantle may contain hydrous mineral species as a consequence of surface reactions, compared to about 4 per cent by volume of Earth's mantle. Furthermore, neither primitive nor evolved hydrated Martian crust show noticeably different bulk densities compared to their anhydrous equivalents, in contrast to hydrous mafic terrestrial crust, which transforms to denser eclogite upon dehydration. This would have allowed efficient overplating and burial of early Martian crust in a stagnant-lid tectonic regime, in which the lithosphere comprised a single tectonic plate, with only the warmer, lower crust involved in mantle convection. This provided an important sink for hydrospheric water and a mechanism for oxidizing the Martian mantle. Conversely, relatively buoyant mafic crust and hotter geothermal gradients on Earth reduced the potential for upper-mantle hydration early in its geological history, leading to water being retained close to its surface, and thus creating conditions conducive for the evolution of complex multicellular life.
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  • Noble gases recycled into the mantle through cold subduction zones

    Smye, Andrew J.   Jackson, Colin R.M.   Konrad-Schmolke, Matthias   Hesse, Marc A.   Parman, Steve W.   Shuster, David L.   Ballentine, Chris J.  

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  • Rutile U-Pb age depth profiling: A continuous record of lithospheric thermal evolution

    Smye, Andrew J.   Stockli, Daniel F.  

    Understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology. Intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history. Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we outline here a method to simultaneously acquire Pb-206/U-238 age and trace element profiles from U-bearing accessory phases. Inversion of Pb-206/U-238 age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships. Thermally-activated volume diffusion of Pb and Zr in rutile is sensitive to the thermal evolution of the mid- to lower-lithosphere. We document the ability of Laser Ablation depth-profiling to simultaneously resolve Pb-206/U-238 age and Zr diffusion profiles in the outer similar to 35 mu m of lower-crustal rutile euhedra from the Ivrea Zone, Southern Alps, with <1.2 mu m depth resolution. Inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from >600 degrees C at similar to 180 Ma followed by a period of slower cooling from similar to 525 degrees C to 450 degrees C. Combined with the topology of Zr diffusion profiles, these data indicate that the Ivrea Zone underwent a brief thermal pulse in the early Jurassic, plausibly associated with hyperextension of the Adriatic margin. Inversion of near-surface Pb-206/U-238 age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere. (C) 2014 Elsevier B.V. All rights reserved.
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  • Rutile U–Pb age depth profiling: A continuous record of lithospheric thermal evolution

    Smye, Andrew J.   Stockli, Daniel F.  

    Understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology. Intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history. Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we outline here a method to simultaneously acquire Pb-206/U-238 age and trace element profiles from U-bearing accessory phases. Inversion of Pb-206/U-238 age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships. Thermally-activated volume diffusion of Pb and Zr in rutile is sensitive to the thermal evolution of the mid- to lower-lithosphere. We document the ability of Laser Ablation depth-profiling to simultaneously resolve Pb-206/U-238 age and Zr diffusion profiles in the outer similar to 35 mu m of lower-crustal rutile euhedra from the Ivrea Zone, Southern Alps, with <1.2 mu m depth resolution. Inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from >600 degrees C at similar to 180 Ma followed by a period of slower cooling from similar to 525 degrees C to 450 degrees C. Combined with the topology of Zr diffusion profiles, these data indicate that the Ivrea Zone underwent a brief thermal pulse in the early Jurassic, plausibly associated with hyperextension of the Adriatic margin. Inversion of near-surface Pb-206/U-238 age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere. (C) 2014 Elsevier B.V. All rights reserved.
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  • The signature of devolatisation: Extraneous 40Ar systematics in high-pressure metamorphic rocks

    Smye, Andrew J.   Warren, Clare J.   Bickle, Mike J.  

    The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar () in metamorphic rocks shows that this assumption is invalid for a significant number of cases. is common in micas that have equilibrated under (ultra-)high pressure () conditions: metasediments from six Phanerozoic terranes yield apparent 40Ar/39Ar phengite ages 50% in excess of the age of peak conditions, whereas cogenetic mafic eclogites yield ages up to 700% older despite lower K2O concentrations. A model is developed that calculates age fractions as a function of variable mica-fluid , bulk K2O and porosity under closed system conditions. Measured concentrations in mafic eclogites are reproduced only when porosities are volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are , reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.
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  • The signature of devolatisation: Extraneous Ar-40 systematics in high-pressure metamorphic rocks

    Smye, Andrew J.   Warren, Clare J.   Bickle, Mike J.  

    The validity of using the Ar-40/Ar-39 system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective Ar-40 concentration of zero. However, the presence of extraneous Ar-40 (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are is common in micas that have equilibrated under (ultra-)high pressure ((U)HP) conditions: metasediments from six Phanerozoic (U)HP terranes yield apparent Ar-40/Ar-39 phengite ages less than or similar to 50% in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to similar to 700% older despite lower K2O concentrations. A model is developed that calculates Are age fractions as a function of variable mica-fluid K-D, bulk K2O and porosity under closed system conditions. Measured Are concentrations in mafic eclogites are reproduced only when porosities are less than or similar to 10(-4) volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are less than or similar to 10(-2), reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated Ar-40 in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle. (c) 2013 Elsevier Ltd. All rights reserved.
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  • Continental growth histories revealed by detrital zircon trace elements:A case study from India

    McKenzie, N. Ryan   Smye, Andrew J.   Hegde, Venkatraman S.   Stockli, Daniel F.  

    Simultaneous acquisition of detrital zircon Pb-Pb ages and trace element abundances from grains collected across the Indian craton, spanning similar to 3 b.y., reveals prominent shifts in Eu/Eu* and light and middle to heavy rare earth element ratios. These shifts correspond to a ca. 3.0-2.2 Ga interval of crustal thickening during Indian craton formation, followed by a period wherein arc magmatism occurred along thinner craton margins from ca. 1.9 to 1.0 Ga, with arc magmatism concentrated along attenuated continental margins after ca. 1.0 Ga. Similar temporal shifts in trace element concentrations are recognized in global whole-rock compilations. We propose that the post-1.0 Ga increase in juvenile magmatism reflects a switch to lateral arc terrane accretion as the primary style of continental growth over the past billion years.
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  • Continental growth histories revealed by detrital zircon trace elements: A case study from India

    McKenzie, N. Ryan   Smye, Andrew J.   Hegde, Venkatraman S.   Stockli, Daniel F.  

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  • U-Pb systematics and trace element characteristics in titanite from a high-pressure mafic granulite

    Marsh, Jeffrey H.   Smye, Andrew J.  

    Simultaneous acquisition of U-Pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic, high pressure granulite using LA-ICP-MS split-stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high-temperature, melt-present environments. Although the titanite studied here have a comparatively low initial Pb (Pb-0) component (Pb-0/Pb*), the Pb-0 is highly radiogenic relative to model crustal values, indicating inheritance from U-bearing accessory minerals consumed in the melt/titanite-forming reactions. Additionally, titanite crystals typically exhibit core-rim decreases in Pb-0/Pb*, as defined by Pb-204/Pb-206, calculated Pb-206(0)/Pb-206(T), and uncorrected Pb-206/U-238 spot date profiles. Near the margins this is clearly dominated by local U-enrichment, but in the uniformly low-U interiors outwardly decreasing Pb-0/Pb* appears to reflect decreasing Pb-0 concentrations during growth. The positive correlation among Pb-0 and Sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation, Given the high crystallization temperatures (> 800 degrees C) and likely slow cooling rates (similar to 5 degrees C), our data support slow Pb diffusivity in titanite, even at high temperature conditions, as has been proposed in a number of recent studies. Along the outer similar to 50-100 mu m, U, Th, Zr, and REE concentrations are variably elevated relative to the crystal interiors, with profiles taking one of two forms: 1) sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge, or 2) gradually increasing to highest concentrations nearest the crystal edge. High-contrast BSE imaging shows that the former profiles are associated with similar to 1-2 mu m wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is (or formerly was) in contact with matrix feldspar (i.e. crystallized melt), and are inferred to represent trace-element- enriched dissolution-precipitation reaction fronts. The latter profiles are associated with diffuse, locally wispy, brightness gradients adjacent to resorbed crystal boundaries, subgrain boundaries, or thicker bright bands formed in contact with matrix hornblende crystals, and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro-porosity pathways at some point in the recrystallization process.
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  • Characterising the U–Th–Pb systematics of allanite by ID and LA-ICPMS: Implications for geochronology

    Smye, Andrew J.   Roberts, Nick M.W.   Condon, Daniel J.   Horstwood, Matthew S.A.   Parrish, Randall R.  

    Allanite has the potential to be a useful chronometer of crustal evolution, forming in response to a wide spectrum of metamorphic and magmatic conditions and incorporating weight-percent concentrations of LREE, Th and U. Despite its growing use in in situ U-Th-Pb geochronology, allanite reference materials lack sufficient U-Th-Pb isotopic characterisation and little is known concerning the response of U-Th-Pb systematics of allanite to hydrothermal alteration and self-irradiation. This contribution presents the results of a combined ID-TIMS and LA-ICPMS U-Th-Pb study on a suite of five allanite crystals, spanning similar to 2.6 Ga and including three commonly-used allanite reference materials: the Siss, Bona and Tara allanites. Siss and Bona allanites preserve an inherited ca. 1 Ga Pb component, consistent with the presence of xenocrystic allanite cores or the presence of zircon micro-inclusions. Tara allanite yields concordant U-Pb ages (407-430 Ma), but is affected by Th/ U fractionation, likely caused by hydrothermal alteration. Additionally, the tendency for Th to become mobilised post-crystallisation is further evidenced by two Precambrian allanite megacrysts, LE40010 (ca. 2.8 Ga) and LE2808 (ca. 1.1 Ga), that both exhibit discordant Th/Pb analyses, linked to the formation of thorite micro-inclusions along hydration pathways. Self-irradiation dose versus discordance relationships show that a percolation threshold is present in allanite at cumulative dose values close to 3 x 10(17) alpha-decay g(-1), an order of magnitude smaller than zircon. Collectively, the presence of common-Pb and excess-Pb-206, its susceptibility to incur Th/U fractionation and hydrothermal Pb-loss complicates the use of allanite as a geochronometer. These factors explain dispersion of similar to 4% in the isotopic compositions of Siss and Tara allanites measured by LA-ICPMS, providing a fundamental limit on the accuracy of the allanite chronometer using these reference materials. (C) 2014 Elsevier Ltd. All rights reserved.
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  • Episodic heating of continental lower crust during extension:A thermal modeling investigation of the Ivrea-Verbano Zone

    Smye, Andrew J.   Lavier, Luc L.   Zack, Thomas   Stockli, Daniel F.  

    Rheology of the continental lower crust plays an integral role in governing the style of continental extension. Temperature-dependent creep deformation in the lower crust decreases lithospheric strength and promotes coupling of deformation in the brittle crust and upper mantle; however, few constraints exist concerning the thermal evolution of extending lower crust. Here, we present a high-temperature thermochronological investigation of the Ivrea-Verbano Zone archetypal continental lower crust that was attenuated during opening of the Alpine Tethys oceanic basin. Rutile U-Pb dates collected from three samples spanning the bottom similar to 2-km of the crustal section are between similar to 160 and similar to 180 Ma, and exhibit near-rim zonations in Zr, Hf, Nb and Ta. Thermal-kinematic modeling of these data, combined with existing rutile U-Pb dates, show that the base of the Ivrea-Verbano Zone experienced heating on two timescales: conductive heating over similar to 10 Myr, associated with thinning of the lithospheric mantle, and advective heating over <0.1 Myr, associated with high-temperature infiltration of fluids during crustal exhumation. These constraints match the thermal predictions of geodynamic models that predict high-magnitude thinning of lithospheric mantle during the early stages of extension. Conductive heating of lower crust directly preceded mantle exhumation and crustal excision, suggesting that thermal weakening of the lithosphere promotes focusing of extensional strain. (C) 2019 Elsevier B.V. All rights reserved.
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  • Andrew J. Roger

    Andrew J. Roger  

    Interview with Andrew Roger, who studies the early evolutionary history of eukaryotes at Dalhousie University.
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  • Andrew J. Roger

    Roger, Andrew J.  

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  • Andrew J. Nathan

    Nathan, Andrew J.  

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  • J. Andrew Ross,

    Andrew M. Thomson  

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