Plume Generation Zones at the margins of Large Low Shear Velocity Provinces on the core–mantle boundary

Large Igneous Province (LIP) eruption sites of the past 300 My lie vertically above 1% slow shear wave velocity (V_s) contours bounding the African and Pacific Large Low Shear Velocity Provinces (LLSVPs) at the core–mantle boundary (CMB), or in the cases of the Siberian and Columbia River LIPs, bounding one or other of two smaller, Low Shear Velocity Provinces (LSVPs). Steep gradients in V_s at the CMB coincide with those 1% slow contours. The sites of 24 active hotspot volcanoes project down to the same narrowly defined borders of the LLSVPs at the CMB. Plumes that have generated LIPs and major hotspot volcanoes have risen only from the immediate neighbourhoods of the 1% slow V_s contours at the CMB which thus define Plume Generation Zones (PGZs). PGZs projected vertically upward approximately match the + 10 m elevation contour of the geoid showing that the LLSVPs are a dominant control on the positively elevated geoid. Minima in the frequency distribution of shear wave velocities in the lowermost mantle near V_s = ? 1% indicate that regions with more negative velocities, forming ～ 2% of total mantle mass, are likely to be of material compositionally different from the rest of the mantle. Because all LIP eruption sites with ages younger than 300 Ma lie above the borders of LLSVPs or LSVPs at the CMB, PGZ footprints are inferred to have remained in the same places for the past 300 My. Because no plumes have risen from the interior of the LLSVPs and because no lithospheric slabs have penetrated those bodies the volumes of the LLSVPs are inferred to have also remained unchanged for the past 300 My. Because the LLSVPs are the dominant control on the positively elevated areas of the geoid those too must have remained as they now are since 300 Ma. The LLSVPs are not rising buoyant objects but stable features of the deep mantle. LIPs have been erupted throughout the past 2.5 Gy indicating that PGZs comparable to those of the past 0.3 Gy and LLSVPs (of which PGZs mark the margins at the CMB) have also existed for at least that long. LLSVPs could thus form the isolated reservoir invoked by some to explain the distinctive isotopic compositions of terrestrial rocks. PGZs lie at places where the boundaries of: (i) The outer core, (ii) one of the LLSVPs or LSVPs, and (iii) the seismically faster part of the deep mantle meet. Horizontal temperature gradients across the steeply inclined margins to the LLSVPs, the interiors of which are hotter than the surrounding mantle, at the CMB are key controls for the generation of plumes. Near the CMB the association of the high temperature of the outer core with an inclined thermal boundary layer at the margins of LLSVPs facilitates the generation of mantle plumes in the PGZs.     

4D Arctic: A Glimpse into the Structure and Evolution of the Arctic in the Light of New Geophysical Maps,Plate Tectonics and Tomographic Models

Knowledge about the Arctic tectonic structure has changed in the last decade as a large number of new datasets have been collected and systematized. Here, we review the most updated, publicly available Circum-Arctic digital compilations of magnetic and gravity data together with new models of the Arctic’s crust. Available tomographic models have also been scrutinized and evaluated for their potential to reveal the deeper structure of the Arctic region. Although the age and opening mechanisms of the Amerasia Basin are still difficult to establish in detail, interpreted subducted slabs that reside in the High Arctic’s lower mantle point to one or two episodes of subduction that consumed crust of possibly Late Cretaceous–Jurassic age. The origin of major igneous activity during the Cretaceous in the central Arctic (the Alpha–Mendeleev Ridge) and in the proximity of rifted margins (the so-called High Arctic Large Igneous Province—HALIP) is still debated. Models of global plate circuits and the connection with the deep mantle are used here to re-evaluate a possible link between Arctic volcanism and mantle plumes.     

Neoproterozoic hydrothermally altered basaltic rocks from Rajasthan, northwest India: Implications for late Precambrian tectonic evolution of the Aravalli Craton

The isolated volcano-sedimentary sequences of the Punagarh and Sindreth Groups occur along the western flank of the Delhi Fold Belt in northwest India, and include mafic rocks (pillow basalts and dolerite dykes) that are dominantly olivine tholeiites with minor quartz-normative and alkali basalts. Sindreth samples appear to have higher primary TiO₂ and P₂O₅ abundances relative to those from Punagarh. Both suites of mafic rocks show variable, but profound hydrothermal alteration effects, with loss on ignition (LOI) values up to 10.3 wt.%, and extensive secondary minerals including albite, sericite, chlorite and calcite. Despite this, there is excellent preservation of magmatic textures, but there has been extensive albitization of plagioclase phenocrysts, a hallmark of hydrothermal alteration processes in oceanic crust. Supporting evidence for such hydrothermal alteration comes from correlations of LOI abundances with CaO/Na₂O, and evidence for U mobility is apparent on diagrams of Nb/Th vs. Nb/U. Felsic volcanic rocks (rhyolite, dacite) interlayered with the Sindreth basalts yield U–Pb zircon ages (TIMS method) between 761 ± 16 and 767 ± 3 Ma, which we interpret as representing the time of primary magmatic activity. We infer that the volcano-sedimentary rocks of the Punagarh also formed at this time, on the basis of similarities in lithology, stratigraphy, field relations and geochemistry. Intermediate granitoid rocks yield older U–Pb ages of 800 ± 2 and 873 ± 3 Ma, which we correlate with the post-Delhi Supergroup Erinpura Granites. Taken together, the features of the Punagarh and Sindreth Groups are consistent with their formation in a back-arc basin setting. Their coevality with other magmatic systems in NW India (Malani Igneous Suite), the Seychelles and Madagascar, for which a continental arc setting has also been proposed, supports the notion of an extensive convergent margin in western Rodinia at 750–770 Ma.