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1. Bringing the Submarine Mariana Arc and Backarc Basin to Life for Undergraduates and the Public

   https://doi.org/10.1111/iar.12533  

   Stern, Robert J (January 2024, Island Arc)

   ABSTRACT This paper aims to better teach about submarine arc and backarc basin volcanic and hydrothermal activity using the ~1400 km long Mariana convergent margin as an example. Four US National Oceanographic and Atmospheric Administration (NOAA) expeditions (2004–2016) equipped with a remotely operated vehicle (ROV) have discovered and explored many of submarine volcanoes and associated hydrothermal fields and generated many short (~1 min long) videos about them. Some of these videos would be very useful for teaching about these processes if they were organized and context provided, which is done here. Eighteen short videos about nine sites generated by NOAA are presented and discussed here. These are organized into three categories: volcanic eruptions, magmatic degassing, and hydrothermal activity. Volcanic eruption videos include two about glassy pillow lavas erupted in 2013–2015 and a rare example of a submarine eruption. Four videos about magmatic degassing include an example of sulfur produced by disproportionation of magmatic sulfur dioxide associated with a submarine eruption, two rare examples of molten sulfur lakes, and liquid carbon dioxide venting. Four videos about hydrothermal activity are provided. Suggestions for how this material might be used in the classroom are also given. 

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2. Expedition 376 Scientific Prospectus: Brothers Arc Flux

   https://doi.org/10.14379/iodp.sp.376.2017  

   de Ronde, C.E.J.; Humphris, S.E.; Höfig, T.W. (June 2017, Scientific prospectus)

   null (Ed.)

   Almost two-thirds of Earth’s submarine volcanism is expressed in the ocean basins along the ~65,000 km long mid-ocean-ridge (MOR) system, but the remaining third takes place along intraoceanic arcs and seamounts. The 22,000 km long intraoceanic arc system appears to surpass the MOR system in terms of both the frequency of hydrothermal activity, with several sites per 100 km of arc, and the range in chemical composition of the fluids being discharged. Hydrothermal systems hosted by submarine arc volcanoes differ substantially from those in spreading environments in that they commonly contain a large component of magmatic fluid. Our primary scientific goal is to discover the fundamental underlying processes that develop as a consequence of this difference. A magmatic-hydrothermal signature, coupled with the shallow depths of arc volcanoes and their high-volatile contents, strongly influences the chemistry of fluids and the resulting mineralization and likely has important consequences for the biota associated with these systems. Because of the high metal contents and very acidic fluids, these hydrothermal systems are also thought to be important analogs of many porphyry copper and epithermal gold deposits mined today on land. Drilling at Brothers volcano on the Kermadec arc will provide the missing link (i.e., the third dimension) in our understanding of mineral deposit formation along arcs, the subseafloor architecture of these volcanoes and their related permeability, as well as the relationship between the discharge of magmatic fluids and the deep biosphere. Expedition 376 will drill and log two caldera sites, one on the rim of the caldera and a second inside the caldera, and a third site at the summit of a volcanic cone, located inside the caldera at Brothers. These sites represent discharge zones of geochemically distinct fluids that are variably affected by magmatic volatile input, allowing us to directly address the consequences of magma degassing for metal transport to the seafloor and its effect on the functioning of microbial communities. Drilling will provide access to critical zones dominated by magma degassing and high-temperature hydrothermal circulation over depth intervals regarded as crucial, not only in the development of multiphase mineralizing systems but also in identifying subsurface microbially habitable environments. The specific objectives of Expedition 376 are • To characterize the subvolcano, magma chamber–derived volatile phase to test model-based predictions that this is either a single-phase gas or two-phase brine-vapor; • To determine the subseafloor distribution of base and precious metals and metalloids and the reactions that have taken place along pathways to the seafloor; • To quantify the mechanisms and extent of fluid-rock interaction and consequences for mass transfer of metals and metalloids into the ocean and the role of magmatically derived carbon and sulfur species in mediating these fluxes; and • To assess the diversity, extent, and metabolic pathways of microbial life in an extreme, metal-toxic, and acidic volcanic environment. 

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3. Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume

   https://doi.org/10.1016/j.epsl.2022.117422  

   Labidi, Jabrane; Dottin, James W.; Clog, Matthieu; Hemond, Christophe; Cartigny, Pierre (April 2022, Earth and planetary science letters)

   Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32S, 33S, 34S, 36S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ∼2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ∼ 2,500 mbsl) and 600 ppm S (∼2,000 mbsl). The δ34S values can be accounted for under equilibrium isotope fractionation during degassing, with αgas-melt between 1.0020 and 1.0001 and starting δ34S values between −0.9‰ and +0.6‰. The δ34S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ∼ 1‰ 34S enrichment compared to the Pacific upper mantle. The Δ33S and Δ36S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ33S is statistically different from MORB, by +0.02‰. The Δ33S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33S-36S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn. 

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4. Changes in Crater Morphology Associated With Volcanic Activity at Telica Volcano, Nicaragua

   https://doi.org/10.1029/2019GC008889  

   Hanagan, Catherine; La Femina, Peter C.; Rodgers, Mel (July 2020, Geochemistry, Geophysics, Geosystems)

   Abstract Volcanic summit craters are typically noted to form by roof collapse into a depressurized magma chamber or by explosive excavation. Recent examples of effusive activity (e.g., Kilauea Volcano, Hawai'i) allowed specifically for quantification of the collapse process. However, small spatiotemporal morphologic change related to background mass wasting and low‐level explosive activity has not been well quantified in volcanic craters. Telica volcano, Nicaragua, is a persistently restless basaltic‐andesite stratovolcano. Telica's persistent restlessness is caused by a long‐lived magmatic‐hydrothermal system with high‐temperature crater fumaroles and low‐frequency seismicity, punctuated by subdecadal, low‐explosivity (VEI 1–2) phreatic eruptions. We use photographic observations (1994 to 2017) and structure‐from‐motion point cloud construction and differencing (2011 to 2017) to analyze changes at Telica in the context of summit crater formation and eruptive precursors. Crater wall retreat (up to 40 m) spatially correlates with long‐lived high‐temperature fumaroles in the crater walls, whereas eruptions eject material (>5 m) from the crater floor through vent formation and/or clearing. These processes sustain a morphology similar to that of pit craters but without a shallow depressurized magma chamber. Our observations indicate system‐wide sealing prior to eruption by viscous magma in the conduit and eruption of a dome in 2017 and hydrothermal mineralization, not from vent covering talus; though, vent covering talus can redirect the shallow conduit. This study shows promise for photogrammetric techniques in correlating morphologic change with summit crater formation and volcanic activity and the power of long‐term visual observations in understanding active volcanic processes. 

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5. Brothers Arc Flux

   https://doi.org/10.14379/iodp.proc.376.2019  

   de Ronde, C.E.J.; Humphris, S.E.; Höfig, T.W. (July 2019, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Volcanic arcs are the surface expression of magmatic systems that result from subduction of mostly oceanic lithosphere at convergent plate boundaries. Arcs with a submarine component include intraoceanic arcs and island arcs that span almost 22,000 km on Earth’s surface, and the vast majority of them are located in the Pacific region. Hydrothermal systems hosted by submarine arc volcanoes commonly contain a large component of magmatic fluid. This magmatic-hydrothermal signature, coupled with the shallow water depths of arc volcanoes and their high volatile contents, strongly influences the chemistry of the fluids and resulting mineralization and likely has important consequences for the biota associated with these systems. The high metal content and very acidic fluids in these hydrothermal systems are thought to be important analogs to numerous porphyry copper and epithermal gold deposits mined today on land. During International Ocean Discovery Program (IODP) Expedition 376 (5 May–5 July 2018), a series of five sites was drilled on Brothers volcano in the Kermadec arc. The expedition was designed to provide the missing link (i.e., the third dimension) in our understanding of hydrothermal activity and mineral deposit formation at submarine arc volcanoes and the relationship between the discharge of magmatic fluids and the deep biosphere. Brothers volcano hosts two active and distinct hydrothermal systems: one is seawater influenced and the other is affected by magmatic fluids (largely gases). In total, 222.4 m of volcaniclastics and lavas were recovered from the five sites drilled, which include Sites U1527 and U1530 in the Northwest (NW) Caldera seawater-influenced hydrothermal field; Sites U1528 and U1531 in the magmatic fluid-influenced hydrothermal fields of the Upper and Lower Cones, respectively; and Site U1529, located within an area of low crustal magnetization that marks the West (W) Caldera upflow zone on the caldera floor. Downhole logging and borehole fluid sampling were completed at two sites, and two tests of a prototype turbine-driven coring system (designed by the Center for Deep Earth Exploration [CDEX] at Japan Agency for Marine-Earth Science and Technology [JAMSTEC]) for drilling and coring hard rocks were conducted. Core recovered from all five sites consists of dacitic volcaniclastics and lava flows with only limited chemical variability relative to the overall range in composition of dacites in the Kermadec arc. Pervasive alteration with complex and variable mineral assemblages attest to a highly dynamic hydrothermal system. The upper parts of several drill holes at the NW Caldera hydrothermal field are characterized by secondary mineral assemblages of goethite + opal + zeolites that result from low-temperature (<150°C) reaction of rock with seawater. At depth, NW Caldera Site U1527 exhibits a higher temperature (~250°C) secondary mineral assemblage dominated by chlorite + quartz + illite + pyrite. An older mineral assemblage dominated by diaspore + quartz + pyrophyllite + rutile at the bottom of Hole U1530A is indicative of acidic fluids with temperatures of ~230°–320°C. In contrast, the alteration assemblage at Site U1528 on the Upper Cone is dominated by illite + natroalunite + pyrophyllite + quartz + opal + pyrite, which attests to high-temperature reaction of rocks with acid-sulfate fluids derived from degassed magmatic volatiles and the disproportionation of magmatic SO2. These intensely altered rocks exhibit extreme depletion of major cation oxides, such as MgO, K2O, CaO, MnO, and Na2O. Furthermore, very acidic (as low as pH 1.8), relatively hot (≤236°C) fluids collected at 160, 279, and 313 meters below seafloor in Hole U1528D have chemical compositions indicative of magmatic gas input. In addition, preliminary fluid inclusion data provide evidence for involvement of two distinct fluids: phase-separated (modified) seawater and a ~360°C hypersaline brine, which alters the volcanic rock and potentially transports metals in the system. The material and data recovered during Expedition 376 provide new stratigraphic, lithologic, and geochemical constraints on the development and evolution of Brothers volcano and its hydrothermal systems. Insights into the consequences of the different types of fluid–rock reactions for the microbiological ecosystem elucidated by drilling at Brothers volcano await shore-based studies. 

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