Journal: Geology

Abbreviation

Geology

Publisher

Geological Society of America

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ISSN

0091-7613
1943-2682

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Publications 1 - 10 of 180
  • Spatial variations in catchment-averaged denudation rates from normal fault footwalls
    Item type: Journal Article
    Densmore, Alexander L.; Hetzel, Ralf; Ivy-Ochs, Susan; et al. (2009)
    Geology
  • A common origin of carbonatite magmas
    Item type: Journal Article
    Weidendorfer, Daniel; Schmidt, Max W.; Mattsson, Hannes B. (2017)
    Geology
    The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ∼48% calcite, ∼12% apatite, and ∼2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.
  • Recovery of the forest ecosystem in the tropical lowlands of northern Guatemala after disintegration of Classic Maya polities
    Item type: Journal Article
    Mueller, Andreas D.; Islebe, Gerald A.; Anselmetti, Flavio S.; et al. (2010)
    Geology
  • Mafic dikes displacing Witwatersrand gold reefs
    Item type: Journal Article
    Meier, Dimitri L.; Heinrich, Christoph A.; Watts, Mark A. (2009)
    Geology
  • Sulfate-dominant apatite in the deep arc crust indicates that high oxidation state promotes copper fluxing in arcs
    Item type: Journal Article
    Nathwani, Chetan Lalitkumar; Giovanini, Emanuel; Bachmann, Olivier; et al. (2026)
    Geology
    Sulfide saturation during arc magma evolution depletes ascending magmas in chalcophile elements (e.g., Cu) and promotes metal recycling into the mantle. Although sulfide saturation is posited to be delayed by high oxygen fugacity (ƒO2) in the deep crust of arcs, the timing and controls on sulfide saturation in arcs remain disputed. Here, we report new constraints on S speciation in the deep arc crust using measurements of the relative abundance of S species (S2–, S4+, and S6+) in apatite in deep crustal arc cumulates from the Sierra Valle Fértil–La Huerta (Argentina). Sulfide is rare in primitive cumulates (<0.01%), only becoming abundant in evolved cumulates (Mg# <0.65), indicating that sulfide saturation was delayed in the magmatic evolution. Apatite yields consistently high S6+/ΣS (0.8–1.0), giving an fO2 of 1.4 ± 0.4 log units above the fayalite-magnetite-quartz (FMQ) redox buffer. Delayed sulfide saturation was therefore the result of the parental magma having elevated fO2 acquired from its mantle source. We use modeling to show that magma differentiation at FMQ+1.4 would be sufficient to delay sulfide saturation and produce mafic to intermediate melts with elevated Cu concentrations. Oxidized, hydrous intermediate magmas may therefore play a key role in delivering elevated sulfur and Cu fluxes to magmatic-hydrothermal systems and the atmosphere, contributing to the Cu deficit of modern continental crust.
  • Globally significant mass of terrestrial organic carbon efficiently transported by canyon-flushing turbidity currents
    Item type: Journal Article
    Baker, Megan L.; Hage, Sophie; Talling, Peter J.; et al. (2024)
    Geology
    Burial of organic carbon in marine sediments is a long-term sink of atmospheric CO2, and submarine turbidity currents are volumetrically the most important sediment transport process on Earth. Yet the processes, amounts, and efficiency of organic carbon transfer by turbidity currents through submarine canyons to the deep sea are poorly documented and understood. We present an organic carbon budget for the submarine Congo Canyon, offshore West Africa, constrained with time-lapse bathymetry, sediment cores, and flow monitoring, including the effects of two >1000-km-runout canyon-flushing turbidity currents. In one year, flows eroded an estimated 6.09 +/- 2.70 Mt of previously buried terrestrial organic carbon in the canyon, primarily from fine-grained and vegetation-rich muddy sand facies with high organic carbon contents (as high as 11%). The age and composition of organic carbon in the Congo Canyon is comparable to those in the Congo River, indicating that transfer is efficient. Over the whole canyon-channel system, we extrapolate that 43 +/- 15 Mt of organic carbon was eroded and transported to the deep (>5 km) sea, equivalent to 22% of the annual global particulate organic carbon export from rivers to oceans and 54%-108% of the predicted annual terrestrial organic carbon burial in the oceans. Canyon-flushing turbidity currents carried a globally significant mass of terrestrial organic carbon down just one submarine canyon in a single year, indicating their importance for redistribution and delivery of organic carbon to the deep sea.
  • Mode of continental breakup of marginal seas
    Item type: Journal Article
    Mohn, Geoffroy; Ringenbach, Jean-Claude; Nirrengarten, Michael; et al. (2022)
    Geology
    We investigated the continent-ocean transition (COT) structure of three main marginal seas in the western Pacific Ocean (South China Sea, Coral Sea, and Woodlark Basin) to determine the tectono-magmatic processes acting during continental breakup. The COT formed from the activity of a low-angle normal fault system localizing deformation during final rifting. Extension was contemporaneous with magmatic activity, including volcanic edifices, dikes, and sills in the distalmost parts of these basins. The COT shows a sharp juxtaposition in space and time of continental crust against igneous oceanic crust, and its overall structure differs from that of magma-poor or magma-rich passive-margin archetypes. We propose that this mode of breakup is characteristic of marginal seas due to the high extension rates imposed by kinematic forces of nearby subduction zones. Revealed in the context of marginal seas, this mode of breakup and the resulting COT structures highlight the underestimated diversity of continental breakup mechanisms.
  • Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz
    Item type: Journal Article
    Lepper, Kenneth; Fisher, Timothy G.; Hajdas, Irka; et al. (2007)
    Geology
  • Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits
    Item type: Journal Article
    Heinrich, Christoph A.; Driesner, Thomas; Stefánsson, Andri; et al. (2004)
    Geology
  • Extreme delta O-18 signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga
    Item type: Journal Article
    Vezinet, Adrien; Thomassot, Emilie; Pearson, D. Graham; et al. (2019)
    Geology
Publications 1 - 10 of 180