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Large Igneous Provinces (LIPs) are unusual volcanic events in which massive amounts of melt (~106 km3) erupt in relatively short time periods (< 106 yrs). Many LIPs were emplaced at the former positions of currently active hot spots and are thought to have formed due to extensive decompression melting of anomalous hot mantle triggered by an initiating mantle plume. Most LIP magmas have interacted extensively with the continental crust, however the location and dimensions of the crustal magmatic plumbing systems and the processes triggering eruptions are debated. Here we present new major and trace element and radiogenic isotope data for 43 individual lava flows from a continuous 1200 m thick stratigraphic profile through the upper, most voluminous part of the Deccan LIP (Bushe to Mahalabaleshwar Formations). Eruption rates for this section are constrained by published paleomagnetic directions and absolute U-Pb ages for zircons from weathered flow tops exposed in the profile. We find four magmatic cycles each lasting ~104 to ~105 yrs during which major and trace element compositions change systematically, which are best explained by repeated sequences of recharge, fractional crystallization and eruption. The lavas at the beginning of each cycle generally have the highest MgO contents, but are the most contaminated (lowest Ce/Pb and Nb/Th), inconsistent with the combined assimilation-fractional crystallization trends usually associated with single magma bodies. Rapid changes of isotopically different contaminants suggest that the magmatic cycles were fed by several different magma reservoirs at different levels in the crust, rather than a single large reservoir.

The monzonitic-granitic intrusion of Maronia in NE Greece hosts an anomalously Re-rich Cu-Mo ± Au porphyry system. The mineralization is dominated by pyrite, molybdenite, chalcopyrite, native Au, and rheniite (ReS2) in quartz-bearing hydrothermal veins associated with potassic, sericitic, and advanced argillic alteration. To decipher the hydrothermal processes leading to the enrichment of Re, Mo, and Au, we used detailed imaging techniques such as BSE-SEM and CL-SEM combined with in-situ trace element analysis of pyrite, molybdenite, and quartz by EPMA and LA-ICP-MS, in-situ δ34S analysis of pyrite by LA-ICP-MS and fluid inclusion microthermometry in quartz from different veins and miarolitic cavities. Hydrothermal quartz CL intensity, textures and Al/Ti and Ge/Ti ratios are characteristic for each vein type and Ti-in-quartz thermometry indicates a temperature decrease from 600°C to 300°C during the porphyry-epithermal transition. Arsenic, Au, Ag, Te, Pb, Cd, and Se contents and ratios of As/Sb, Co/As, Se/Tl, Se/Te and Se/Ge in pyrite record changes in the physicochemical fluid conditions. Extreme Re enrichment in molybdenite at Maronia (⌀=3922 ppm) compared to the global average from porphyry deposits (⌀=779 ppm) is likely controlled by a district-wide Re-rich magmatic source, whereas the enrichment of Re in B-type (⌀=5532 ppm) relative to D-type (⌀=1954 ppm) veins seems to be related to changes in fluid temperature, salinity and fO2. Our multi-mineral trace element approach allows us to constrain the framework of the physicochemical fluid parameters during the porphyry-epithermal transition favorable for the enrichment of Re and related precious elements like Au, Ag and Te at Maronia.

The formation of isolated seamounts distant from active plate boundaries and mantle plumes remains unsolved. The solitary intraplate volcano Vesteris Seamount is located in the Central Greenland Basin and rises ∼3,000 m above the seafloor with a total eruptive volume of ∼800 km3. Here, we present a new high-resolution bathymetry of Vesteris Seamount and a detailed raster terrain analysis, distinguishing cones, irregular volcanic ridges, volcanic debris fans, U-shaped channels and lava flows. The slope angles, ruggedness index and slope direction were combined with backscatter images to aid geologic interpretation. The new data show that the entire structure is a northeast to southwest elongated stellar-shaped seamount with an elongated, narrow summit surrounded by irregular volcanic ridges, separated by volcanic debris fans. Whole-rock geochemical data of 78 lava samples form tight liquid lines of descent with MgO concentrations ranging from 12.6 to 0.1 wt%, implying that all lavas evolved from a similar parental magma composition. Video footage from Remotely Operated Vehicle (ROV) dives shows abundant pyroclastic and hyaloclastite deposits on the summit and on the upper flanks, whereas lavas are restricted to flank cones. The seamount likely formed above a weak zone of the lithosphere possibly related to initial rifting parallel to the nearby Mohns Ridge, but the local stress field increasingly affected the structure of the volcano as it grew larger. Thus, we conclude that the evolution of Vesteris Seamount reflects the transition from deep, regional lithospheric stresses in the older structures to shallower, local stresses within the younger volcanic structures similar to other oceanic intraplate volcanoes. Our study shows how the combination of bathymetric, visual and geochemical data can be used to decipher the geological evolution of oceanic intraplate volcanoes.

Deep-sea and Seabed Mineral resources Research centre, Korean Institute of Ocean Science & Technology, 385 Haeyang-ro, Yeongdo-gu, 49111, Korea(3);Submarine “black smoker” systems and their associated seafloor massive sulfides (SMS) may represent economic resources for future generations. However, the processes leading to spatial variations in the mineralogical and chemical composition of subduction zone-related hydrothermal systems remain poorly constrained. The large submarine caldera of Niuatahi volcano hosts several active hydrothermal vent sites associated with faults at the caldera wall and with young post-caldera volcanic cones, venting vapor-rich and black smoker-type-fluids with temperatures up to 334 °C. We combine bulk sulfide chemistry with in-situ trace element data and S- and Pb isotopes of pyrite, sphalerite, chalcopyrite to decipher key ore-forming processes causing spatial variations in metal(loid) enrichment. We refer these spatial variations within the caldera to a continuum between magmatic fluid-dominated venting at the central cones (high Cu, As, Bi, Te, Au, Sb, δ34S = -10.6 - 2.7 ‰) compared to fluid-rock interaction and seawater mixing at the caldera wall (high Au, Ag, Cd, Pb, δ34S = -0.6 - 6.3 ‰). Lead isotopes of sulfide separates suggest a connected hydrothermal circulation cell and/or similar source rock compositions in the central part of the caldera compared to a discrete one at the caldera wall. We conclude that metal(loid)s from distinct sources (magmatic volatiles vs. host rock leaching) combined with hydrothermal fractionation (e.g., boiling) leads to spatial variations in economically relevant elements (e.g., Te, Au, Ag, Bi, Se, Co) in submarine caldera-hosted hydrothermal systems. This has important implications on exploration of fossil SMS or volcanogenic massive sulfide deposits on land.

The Oligocene shoshonitic Maronia pluton in NE Greece is part of an age-progressive belt of calc-alkaline intrusions and associated with a porphyry Cu-Mo +/- Re +/- Au mineralisation. We present new data on mineral and whole rock compositions as well as Sr, Nd and Pb isotopes of the monzodioritic to granitic rocks from Maronia, including the mineralised microgranite, which crosscuts the main intrusion. The age progression of the plutons in NE Greece follows the southward-directed trench retreat implying that the magmas formed during subduction of continental lithosphere after the closure of the Pindos ocean. The shoshonitic magmas of Maronia were generated by hydrous melting of a phlogopite-bearing mantle that was enriched by 3-5% of slab-derived sediment melts. A lower degree of melting controls the higher K, LILE and incompatible element contents of the shoshonitic magmas in comparison to the neighbouring calc-alkaline magmas. The mafic shoshonitic rocks inNE Greece contain much higher P2O5 contents than the associated calc-alkaline magmas, which probably reflects subducted phosphate-rich sediments in their source. Constant Sr- and Nd-isotopes with increasing SiO2 contents imply minimal crustal assimilation and that felsic magmas including the mineralised porphyritic granite were generated by fractional crystallisation of mafic magmas. Amphibole and apatite analyses provide evidence for the intrusion of S-poor, Cl-rich magmas at 5 to 6 km depths, followed by rapid uplift and exsolution of magmatic fluids. Late granitic dikes at Maronia indicate that volatiles, sulphur and metals became enriched during the late stages of the magma evolution. (C) 2020 Elsevier B.V. All rights reserved.

Boiling is a crucial process triggering ore formation in magmatic-hydrothermal systems and controlling the enrichment of precious and rare metal(loid)s in epithermal-porphyry mineralizations. Steep physicochemical gradients during boiling of hydrothermal fluids at shallow water depths caused metal(loid) precipitation along a 3 km long Pb-Zn-Ag vein system on Milos Island in the South Aegean Volcanic Arc. We present new trace element and Pb, Sr, and S isotope data from sulfides providing insights into the diversity of mineralization processes in shallow-marine hydrothermal systems. Lead and Sr isotope compositions of sulfides and sulfates reflect the mixing of fluids that reacted with metamorphic basement and the volcanic host rocks, whereas some of the S were derived from seawater. Investigation of mineralized samples along the Kondaros-Vani fault zone revealed distinct chemical variations that represent a vertical profile through the boiling zone of a hydrothermal system. Boiling during fluid ascent at decreasing temperatures (230-150 degrees C) and sulfur fugacities triggered the precipitation of sulfides rich in Zn, Pb, Fe, Cu, Ag, Sb, and As, resulting in increasing Pb, Ag, and Sb contents with decreasing depth. A pyrite group with high Tl/Cu and low As/Sb ratios, as well as delta S-3(4)VCDT values reaching as low as -6 parts per thousand, is interpreted as precipitating from high-Cl liquids that underwent vigorous boiling at deeper crustal levels subsequent to tectonic faulting. Condensation of vapor (high Hg, Bi, and As contents) and mixing with seawater (high Mo contents) in the shallow-marine subseafloor sequences at Vani caused Hg-Bi-Mo-As-rich mineralization similar to the active hydrothermal system at Milos.

The oldest volcanic rocks exposed on Malekula Island, now belonging to the New Hebrides Island Arc, formed in the Upper Eocene to Middle Miocene Vitiaz Island Arc, Southwest Pacific. They are thought to have formed contemporaneously with Fiji and the Izu-Bonin-Mariana (IBM) arc during westward subduction initiation of the Pacific beneath the Indo-Australian Plate [e.g., 1]. To test this hypothesis with regard to the mantle source compositions and contributions from the subducting slab, we provide major- and trace element data combined with Hf, Nd, and Pb isotopes for twenty-seven volcanic rocks of Malekula Island. Our results show that Malekula lavas display similar magma types, i.e., boninite-series rocks, island arc basalts, and MORB-type tholeiites, to the earliest volcanic rocks of Fiji and the IBM arc resembling the sequential stratigraphy of the IBM system [2], rather than the interlayered stratigraphy of early arc rocks on Fiji [3]. Moreover, Malekula lavas display a change in Hf-Nd isotope composition from isotopically ‘Indian’, similar to the IBM arc [4, 5], to mainly ‘Pacific’, like on Fiji [3]. We interpret this progressive change in mantle source composition to reflect the propagation of ‘Pacific’ South Fiji Basin spreading into the Vitiaz Arc. Hence, the Malekula lava succession provides a link between subduction initiation in the Northwest and Southwest Pacific. [1] Hall (2002) J Asian Earth Sci 20. [2] Ishizuka et al. (2011) EPSL 306. [3] Todd et al. (2012) EPSL 335-336. [4] Reagan et al. (2010) G311(3). [5] Li et al. (2019) EPSL 518.