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1.
  • Deegan, Frances, et al. (author)
  • Magma-Shale Interaction in Large Igneous Provinces : Implications for Climate Warming and Sulfide Genesis
  • 2022
  • In: Journal of Petrology. - : Oxford University Press. - 0022-3530 .- 1460-2415. ; 63:9
  • Journal article (peer-reviewed)abstract
    • Large igneous provinces (LIPs) whose magma plumbing systems intersect sedimentary basins are linked to upheavals of Earths carbon and sulfur cycles and thus climate and life history. However, the underlying mechanistic links between these phenomena are elusive. We address this knowledge gap through short time-scale petrological experiments (1200 degrees C and 150 MPa) that explore interaction between basaltic melt and carbonaceous shale (mudstone) using starting materials from the Canadian High Arctic LIP and the Sverdrup Basin in which it intrudes. Here we show that entrainment of shale xenoliths in basaltic melt causes shale to shatter due to incipient thermal stress and devolatilization, which accelerates assimilation by increasing reactive surface area. Shale assimilation therefore facilitates transfer of sediment-derived volatile elements to LIP magma plumbing systems, whereupon carbon dominates the vapor phase while sulfur is partitioned into sulfide melt droplets. This study reveals that although carbon and sulfur are efficiently mobilized as a consequence of shale assimilation, sulfides can sequester sulfuran important climate cooling agentthus enhancing net emissions of climate warming greenhouse gases by shale-intersecting LIPs.
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2.
  • Bedard, Jean H., et al. (author)
  • Basaltic sills emplaced in organic-rich sedimentary rocks : Consequences for organic matter maturation and Cretaceous paleo-climate
  • 2024
  • In: Geological Society of America Bulletin. - : Geological Society of London. - 0016-7606 .- 1943-2674. ; 136:5-6, s. 1982-2006
  • Journal article (peer-reviewed)abstract
    • Many continental large igneous provinces coincide with climate perturbations and mass extinctions. When basaltic plumbing systems traverse carbon-rich sedimentary rocks, large volumes of greenhouse gases may be generated. We document how intrusive sills of the Mesozoic High Arctic Large Igneous Province affected surrounding fine-grained, organic-rich siliciclastic rocks of the Sverdrup Basin in the Canadian Arctic Archipelago. Petrographic and X-ray diffraction data from samples located near sills show the presence of high-temperature metamorphic phases (diopside, andalusite, garnet, and cordierite). Raman thermometry on organic matter yields peak temperatures of 385-400 degrees C near sill contacts, tailing off to far-field temperatures of <= 230 degrees C. Samples located >20 m from sills show no systematic change in vitrinite reflectance and have a VRo eq% value of similar to 2.5%, which indicates a temperature of similar to 210 degrees C. The finite element thermal modeling tool SUTRAHEAT was applied to the 17-m-thick Hare Sill, emplaced at 3 km depth at 1105 degrees C. SUTRAHEAT results show that contact-proximal rocks attain temperatures of >700 degrees C for a brief period (similar to 1 year). By 5 years, the Hare Sill is completely solidified (<730 degrees C), and the temperature anomaly collapses rapidly thereafter as the thermal pulse propagates outward. By 10 years, all rocks within 10 m of the Hare Sill are between 450 degrees C and 400 degrees C, rocks at 20 m from the contact attain 200 degrees C, yet far-field temperatures (>50 m) have barely changed. When multiple sills are emplaced between 4 km and 6 km depth, all rocks between sills reach similar to 250 degrees C after 100 years, showing that it is possible to raise regional-scale background temperatures by similar to 150 degrees C for the observed High Arctic Large Igneous Province sill density. Vitrinite reflectance data and pyrolysis results, together with SILLi thermal modeling, indicate that much of the hydrocarbon-generating potential was eliminated by High Arctic Large Igneous Province intrusions. The SILLi model yields similar to 20 tonnes/m(2) of organic equivalent CO2 (all carbon gas is reported as CO2) from the Hare Sill alone when emplaced into Murray Harbour Formation rocks with 5.7 wt% organic carbon, and similar to 226 tonnes/m(2) by emplacement of multiple sills throughout the 2-km-thick Blaa Mountain Group with 3 wt% organic carbon. On a basin scale, this yields a total of similar to 2550 Gt CO2 from the Hare Sill, with similar to 13,000 Gt CO2 being generated by the multiple sill scenario, similar to estimates from other large igneous provinces. Much of the Blaa Mountain Group rocks now have organic carbon contents of <1 wt%, which is consistent with large volumes of carbon-species gas having been generated, likely a mixture of CO2, CH4, and other species. However, organic-rich Murray Harbour Formation rocks show no obvious reduction in organic carbon content toward the Hare Sill intrusive contacts, which suggests that not all of the carbon was lost from the sedimentary package hosting High Arctic Large Igneous Province magmas. We suggest that some of the gas generated by contact metamorphism failed to drain out for lack of high-permeability conduits, and then back-reacted to form calcite cements and pyrobitumen during cooling.
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3.
  • Bedard, Jean H., et al. (author)
  • Geochemical Systematics of High Arctic Large Igneous Province Continental Tholeiites from Canada-Evidence for Progressive Crustal Contamination in the Plumbing System
  • 2021
  • In: Journal of Petrology. - : Oxford University Press. - 0022-3530 .- 1460-2415. ; 62:9
  • Journal article (peer-reviewed)abstract
    • Cretaceous High Arctic large igneous province (HALIP) sub-alkaline magmatic rocks in Canada are mostly evolved (MgO 2-7 wt%), sparsely plagioclase + clinopyroxene +/- olivine-phyric tholeiitic basalts. There were two main HALIP continental flood basalt (CFB) eruption episodes: 135-120 Ma (Isachsen Fm.) and 105-90 Ma (Strand Fiord Fm.), both associated with cogenetic doleritic sills and dykes. Building on a large modern database, 16 HALIP tholeiite types are defined and grouped into genetic series using Ce vs Sm/Yb-NMORB distributions. Comparison with model melting curves implies that higher-Sm/Yb HALIP basalt types record low-degree melting of garnet-bearing mantle sources. More voluminous intermediate- and low-Sm/Yb HALIP basalt types separated from the mantle at shallower levels after further extensive melting in the spinel-peridotite field. Within a given Sm/Yb range, increases in incompatible elements such as Ce are coupled with progressive clockwise rotation of normalized incompatible trace element profiles. Trace element modeling implies this cannot be due to closed-system fractional crystallization but requires progressive and ubiquitous incorporation of a component resembling continental crust. The fractionation models imply that low-Sm/Yb HALIP basalts (similar to 7 wt% MgO) initially crystallized olivine gabbro assemblages, with lower-MgO basalts successively crystallizing gabbro and ilmenite-gabbro assemblages. In contrast, higher-Sm/Yb basalts fractionated more clinopyroxene and ilmenite, but extensive plagioclase fractionation is still required to explain developing negative Sr-Eu anomalies. Backfractionation models require about 40% addition of olivine to bring the most primitive HALIP basalts (similar to 7% MgO) into equilibrium with Fo(89) mantle. Inverse fractionation-assimilation modeling shrinks the CFB signature, making decontaminated model parental melts more similar to enriched mid-ocean ridge basalt. The progressive increase of the contamination signature within each HALIP tholeiitic differentiation series is not consistent with models involving derivation of HALIP basalts from a mantle source previously enriched by subduction. Strong interaction of basalt with Sverdrup Basin sedimentary rocks may cause localized over-enrichment in K-Rb-Th-U, but cannot explain strong Ba enrichment in the absence of concomitant K-Rb-Th-U enrichment. The localized Ba enrichment could reflect either a Ba-rich lithospheric mantle component that is strongly manifested in the coeval HALIP alkaline suites, or syn- to post-emplacement fluid-mediated transfer from Ba-rich host rocks.
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4.
  • Bedard, Jean H., et al. (author)
  • High Arctic Large Igneous Province Alkaline Rocks in Canada : Evidence for Multiple Mantle Components
  • 2021
  • In: Journal of Petrology. - : Oxford University Press. - 0022-3530 .- 1460-2415. ; 62:9
  • Journal article (peer-reviewed)abstract
    • The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135-90 Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (similar to 96 Ma) scatter around major fault and basement structures. They are represented by the newly defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanic rocks. The younger alkaline group is represented by the Wootton Intrusive Complex (92.2-92.7 Ma), and the Audhild Bay Suite (83-73 Ma), both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba-K-Zr-Ti that are nearly orthogonal to predicted fractionation- or melting-related variations, which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet Iherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5-Ba-Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba-Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low Sr-87/Sr-86(i). These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare earth element (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar-Hassel rocks, whereas other samples trend towards possible High-P2O5 + Zr (PZr) and High-P2O5 + K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases such as apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1) could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K-Rb-U-Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provides little support for a ubiquitous fossil sedimentary subduction-zone component in the HALIP mantle source.
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  • Result 1-4 of 4

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