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East Diamante is a submarine volcano in the southern Mariana arc that is host to a complex caldera ~5 × 10 km (elongated ENE-WSW) that is breached along its northern and southwestern sectors. A large field of barite-sulfide mounds was discovered in June 2009 and revisited in July 2010 with the R/V Natsushima , using the ROV Hyper-Dolphin . The mound field occurs on the northeast flank of a cluster of resurgent dacite domes in the central caldera, near an active black smoker vent field. A 40Ar/39Ar age of 20,000 ± 4000 years was obtained from a dacite sample. The mound field is aligned along a series of fractures and extends for more than 180 m east-west and >120 m north-south. Individual mounds are typically 1 to 3 m tall and 0.5 to 2 m wide, with lengths from about 3 to 8 m. The mounds are dominated by barite + sphalerite layers with the margins of each layer composed of barite with disseminated sulfides. Rare, inactive spires and chimneys sit atop some mounds and also occur as clusters away from the mounds. Iron and Mn oxides are currently forming small (

It is estimated that ~75% of Earth’s volcanism occurs on the seafloor, especially at divergent plate boundaries like the East Pacific Rise and the mid-Atlantic Ridge (Crisp, 1984). These features are known as mid-ocean ridges (MORs), which are large-scale lineaments marking areas of seafloor spreading where lavas are extruded onto the seafloor, and subseafloor dike emplacement is common. A metaphor commonly ascribed to this process is “conveyor belt,” where newly formed oceanic crust, invariably of basaltic composition, moves inexorably away from the axis of the MORs as the forces of plate tectonics take hold. Conversely, at the other end of this conveyor belt are convergent plate boundaries where the oceanic crust, now millions to more than a hundred million years old, is subducted beneath another plate of either oceanic or continental crust. These boundaries dominate regions of the south and western Pacific, and are typically marked by deep trenches on their seaward side, with chains of volcanoes (or volcanic arcs) on their landward side (Fig. 1). These volcanoes sit on top of the overriding plate, typically between ~70 and ~175 km (with a global average of 105 km) above where the subducted plate begins to melt and/or triggers melting of the base of the overriding plate (Syracuse and Abers, 2006). Fig. 1 Map of the central and western Pacific region is a transverse Mercator projection (CM 162° E). Inset shows a map for the Tyrrhenian Sea (part of the Mediterranean Sea), which is a normal Mercator projection. Both maps were created from ETOPO1 data (i.e., a 1 arc-minute global relief model of Earth’s surface that integrates land topography and ocean bathymetry; Amante and Eakins, 2009) downloaded from NGDC (http://www.ngdc.noaa.gov/mgg/global/global.html). 1 = Marsili, 2 = Palinuro, 3 = Bonin Islands, 4 = East Diamante, 5 = SuSu Knolls, 6 = PACMANUS, 7 …

Mercury- and silver-enriched ferromanganese oxide crusts were recovered at water depths of 1,750 to1,300 m from La Victoria knoll, located about 72 km off the coast of northern Baja California. No other ferromanganese precipitate found so far in the modern ocean basins is similarly enriched in Hg and Ag. The precipitates consist of submetallic gray, brecciated, Mn oxide layers overlain by brown earthy, laminated Fe-Mn oxide crusts. Both oxide types are rich in Hg (to 10 ppm) and Ag (to 5.5 ppm). The Mn-rich layers are composed of δMnO 2 , with lesser amounts of 10A and 7A manganates, whereas the Mn phase in the Fe-Mn crusts is solely δMnO 2 . The Fe phase in both layers is X-ray amorphous. Established criteria for distinguishing hydrothermal versus hydrogenetic crusts indicate that the Mn-rich layers are predominantly of low-temperature hydrothermal origin, whereas the Fe-Mn crusts are hydrogenetic, although there is some overlap in the source of chemical components in both types. La Victoria knoll is uplifted continental basement rock with basalt, andesite, and schist cropping out at the surface; the knoll may have an intrusive core. The Hg and Ag were derived from leaching by hydrothermal fluids of organic matter-rich sediments in basins adjacent to La Victoria knoll and, to a lesser extent, from continental basement rocks underlying the knoll and adjacent basins. Both rock types are notably enriched in Ag and Hg. Faults were the main fluid transport pathway, and hydrothermal circulation was driven by high heat flow associated with thinned crust. Other elements derived from the hydrothermal fluids include Tl, Cd, Cr, and Li. The main host for Hg and Ag is FeOOH, although MnO 2 likely hosts some of the Ag. Minor sulfide and barite also may contain small amounts of these metals. Possible analogs in the geologic record for this deposit type are found in the Basin and Range province of the western United States and Mexico. The discovery highlights the fact that fluids circulating along faults in the offshore California borderland are transporting potentially toxic metals (Hg, Ag, Tl, As, Cd, Cr, Pb, and Ni) and depositing them on and just below the ocean floor.

The Taurides region of Turkey is host to a number of important bauxite, Al-rich laterite, and Mn deposits. The most important bauxite deposits, Dog ˘ankuzu and Mortas¸, are karst-related, unconformity-type deposits in Upper Cretaceous limestone. The bottom contact of the bauxite ore is undulatory, and bauxite fills depressions and sinkholes in the footwall limestone, whereas its top surface is concordant with the hanging-wall limestone. The thickness of the bauxite varies from 1 to 40 m and consists of bohmite, hematite, pyrite, marcasite, anatase, diaspore, gypsum, kaolinite, and smectite. The strata-bound, sulfide- and sulfate-bearing, low-grade lower part of the bauxite ore bed contains pyrite pseudomorphs after hematite and is deep red in outcrop owing to supergene oxidation. The lower part of the bauxite body contains local intercalations of calcareous conglomerate that formed in fault-controlled depressions and sinkholes. Bauxite ore is overlain by fine-grained Fe sulfidebearing and calcareous claystone and argillaceous limestone, which are in turn overlain by massive, compact limestone of Santonian age. That 50-m-thick limestone is in turn overlain by well-bedded bioclastic limestone of Campanian or Maastrichtian age, rich with rudist fossils. Fracture fillings in the bauxite orebody are up to 1 m thick and consist of bluish-gray-green pyrite and marcasite (20%) with bohmite, diaspore, and anatase. These sulfide veins crosscut and offset the strata-bound sulfide zones. Sulfur for the sulfides was derived from the bacterial reduction of seawater sulfate, and Fe was derived from alteration of oxides in the bauxite. Iron sulfides do not occur within either the immediately underlying or overlying limestone. The platform limestone and shale that host the bauxite deposits formed at a passive margin of the Tethys Ocean. Extensive vegetation developed on land as the result of a humid climate, thereby creating thick and acidic soils and enhancing the transport of large amounts of organic matter to the ocean. Alteration of the organic matter provided CO2 that contributed to formation of a relatively 12 C-rich marine footwall limestone. Relative sea-level fall resulted from strike-slip faulting associated with closure of the ocean and local uplift of the passive margin. That uplift resulted in karstification and bauxite formation in topographic lows, as repre

Siliceous and calcareous biogenic rocks host pervasive, although volumetrically minor, authigenic barite on Lefkas Island, Greece. The barite composes up to 10 percent of the carbonate-free fraction of samples. In decreasing order of abundance, authigenic barite occurs as infilled foraminifera and radiolarian tests, disseminated grains and aggregates in porcelanite and siliceous limestone, replaced siliceous and calcareous biogenic tests, and thin laminae and lenses. The laminae form anastomosing networks that were probably originally rich in organic matter. Barite abundance shows a good correlation with the abundance of skeletal biogenic components. Biogenic silica diagenesis produced opal-CT and clinoptilolite, which compose the cement of porcelanite, replaced radiolarians, diatoms, and echinoderms and filled in some foraminifera tests. Subsequent silica diagenesis produced chalcedony at deeper stratigraphic levels, which also filled in pore space and calcareous microfossils in siliceous limestones and formed chert beds. Textural evidence shows that barite mineralization postdated opal-CT formation and preceded chalcedony formation. Barite precipitated in microenvironments including microfossil chambers, fecal pellets, and areas rich in organic matter. The source of the barium was probably from the decomposition of organic matter derived from plankton and bacteria and from the diagenesis of silica and carbonate tests. The source of the sulfate ion was seawater; possibly additional sulfate was produced and utilized in microenvironments from degradation of organic matter.