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  • 1
    ISSN: 1365-3091
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have 87Sr/86Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe-rich (up to 5·0 mol%) and are characterized by increasing 87Sr/86Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age).These geochemical variations appear principally to result from changes in pore-water chemistry during diagenesis. The increase in 87Sr/86 Sr and Sr content of the cements is most likely due to interaction between pore waters and 87 Sr-rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock 87Sr/86 Sr signatures for the mudrocks are 〉 0·716). Pore-water Fe2+ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak.The low δ18 O value of some calcite cements (-11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid-Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid-Neocomian rifting.
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  • 2
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    In:  [Poster] In: AGU Chapman Conference on Submarine Volcanism, 29.01.-03.02.2017, Hobart, Tasmania, Australia .
    Publication Date: 2017-11-28
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
    Publication Date: 2018-09-05
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  • 4
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    In:  [Talk] In: Chapman Conference on Magnetic Storms, 12.-16.02.1996, Pasadena, California .
    Publication Date: 2018-09-05
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  • 5
    Publication Date: 2019-02-01
    Description: The 4·0–3·6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry-style copper mineralization. This study uses petrographic, petrological, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence for progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3·5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas. Mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite, testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.
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  • 6
    Publication Date: 2018-10-30
    Description: Highlights • New 40Ar/39Ar age and geochemical (major, trace element, Sr-Nd-Pb-Hf isotope) data are presented from the Walvis Ridge, belonging to the Tristan-Gough hotspot track in the South Atlantic. • The entire Tristan-Gough hotspot system, including Walvis Ridge, display a spatially continuous age progression. • The Gough-type component is the dominant geochemical flavor of the Tristan-Gough plume and has also been identified in the Discovery and Shona hotspot systems. • The geochemical heterogeneity in the South Atlantic DUPAL region can be reproduced by mixing of Gough-type enriched mantle with continental crust and a FOZO/PREMA-like component. • The HIMU-type alkalic lavas on the Walvis Ridge and older part of Shona hotspot track are ∼30 Ma younger in age than the EMI-type primarily tholeiitic basement lavas at a given locality. Abstract Long-lived spatial geochemical zonation of the Tristan-Gough and Discovery hotspot tracks and temporal variations from EMI-type basement to HIMU-type late-stage volcanism at the Walvis Ridge and Shona hotspot tracks point to a complex evolution and multiple source areas for the South Atlantic hotspots. Here we report 40Ar/39Ar age and geochemical (major and trace element, Sr-Nd-Pb-Hf isotope) data for samples from 16 new sites on the Walvis Ridge. This aseismic ridge is the oldest submarine expression of the Tristan-Gough mantle plume and represents the initial reference locality of the EMI end member in the South Atlantic Ocean. The EMI-type lavas display an excellent age progressive trend of ∼31 mm/a along the entire Tristan-Gough hotspot track, indicating constant plate motion over a relatively stationary melt anomaly over the last ∼115 Ma. The Gough-type EMI composition of the Tristan-Gough hotspot track is the dominant composition on the 〉70 Ma part of the Walvis Ridge, the Etendeka and Parana flood basalts, and along the Gough sub-track, extending from DSDP Site 525A on the SW Walvis Ridge to Gough Island, whereas Tristan-type EMI dominates on the Tristan Track, extending from DSDP Sites 527 and 528 to Tristan da Cunha Island. Gough-type EMI is also the dominant composition of the northern Discovery and Shona hotspot tracks, suggesting that these hotspots tap a common source reservoir. The continuous EMI-type supply over ≥132 Ma, coupled with high 3He/4He (〉10 RA), points to a deep-seated reservoir for this mantle material. The Tristan and Southern Discovery EMI-type flavors can be reproduced by mixing of the Gough-type component with (1) FOZO/PREMA to produce Tristan-type lavas, and (2) marine sediments or upper continental crust to generate the Southern Discovery-type composition. South Atlantic hotspots with EMI-type compositions overlie the margin (1 % ∂Vs velocity contour) of the African Large Low Shear Velocity Province (LLSVP), which may promote the emergence of geochemical “zonation”. The St. Helena HIMU-type volcanism, however, is located above internal portions of the LLSVP, possibly reflecting a layered LLSVP.
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  • 7
    Publication Date: 2019-01-07
    Description: Highlights • The youngest known (2 Ma) volcanically-active subduction system. • Exceptionally diverse range of magma compositions coeval and spatially juxtaposed. • Mixing of an upwelling asthenospheric mantle melt and a slab melt. • Modern example of an immature subduction system building its proto forearc. • Modern analog of the environment where SSZ ophiolites lithosphere forms. Abstract The development of ideas leading to a greater understanding of subduction initiation is limited by the scarcity of present-day examples. Furthermore, the few examples identified so far unfortunately provide few insights into the nature of magmatism at the inception of subduction. Here we report new observations from the Matthew and Hunter (M&H) subduction zone, a very young subduction zone located in the South-West Pacific. Tectonics of the area show it is younger than 2 Ma, making the M&H the youngest known volcanically-active subduction system and hence providing unique insights into the earliest stages of subduction initiation. Volcanism in this area comprises an exceptionally diverse range of contemporaneously erupting magma compositions which are spatially juxtaposed. Pb isotopic compositions and abundance of LILE and REE strongly suggest melting of upwelling asthenospheric mantle (Indian MORB) and subducted oceanic crust (Pacific MORB of the South Fiji Basin) and the mixing of these two components. Volcanism occurs much closer to the trench compared to volcanism in more mature subduction zones. We demonstrate that the M&H subduction zone is a modern example of an immature subduction system at the stage of pre-arc, near-trench magmatism. It is not yet building an arc but what we propose to call a Subduction Initiation Terrane (SITER). Today, the proto-forearc of the M&H subduction zone is a collage of these SITERs, coeval back-arc domains and remnants of pre-existing terranes including old Vitiaz Arc crust. The M&H area represents a modern analog of a Supra Subduction Zone setting where potentially a majority of ophiolites have formed their crustal and lithospheric components. Present-day magmatism in the M&H area therefore provides clues to understanding unforeseen distribution of contrasted magmatic rock types in fossil forearcs, whether they are at the front of mature subduction zones or in ophiolites.
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  • 8
    Publication Date: 2019-01-10
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
    Publication Date: 2019-01-11
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  • 10
    Publication Date: 2019-01-11
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