Mineralogical and Geochemical Characteristics of Hydrothermal Minerals Collected from Hydrothermal Vent Fields in the Southern Mariana Spreading Center

Abstract

Seafloor hydrothermal mineralization of four active hydrothermal fields (Snail, Yamanaka, Archaean and Pika sites) in the Southern Mariana Trough was investigated to clarify the mineralogical and geochemical characteristics of the hydrothermal minerals. The Snail site and the Yamanaka site are located on the crest of the backarc spreading ridge (on-axis). The Pika site sits atop an off-axial volcano, 4.9 km distant from the spreading axis, while the Archaean site sits on the flank of the spreading ridge, about 1.5 km distant from the axis. In the four hydrothermal sites, chimney and mound sulfides are composed mainly of pyrite, marcasite, sphalerite and chalcopyrite. Conduits and interstices of these sulfide minerals are filled with late barite and anhydrite. Mineralizations of off-axial hydrothermal fields have mineralogical and geochemical signatures (Fe-rich and low f S2 condition) similar to those of mineralizations at sediment-free mid-ocean ridge settings. By contrast, mineralizations of on-axial sites show distinctly Zn and Pb-rich (high f S2 condition) signatures similar to those found in arc or rifting settings. Sulfur isotopic compositions of collected sulfide minerals show a similar diversity with a mid-ocean ridge range for the off-axial sites and an island-arc range for the on-axial sites. These isotopic signatures could be explained by varying proportions of magmatic sulfur leached from the underlying volcanic rocks and reduced seawater sulfates. Keywords

Taphonomy of microorganisms and microbial microtextures at sulfidic hydrothermal vents: A case study from the Roman Ruins black smokers, Eastern Manus Basin

“…The pyrite studied here shows strong similarities in terms of micro‐mineralogy and microstructure—concentric to laminated corrugated pyrite composed of fibrous nanocrystals aligned normal or in radial orientation (Figures 5e‐g and 10a)—compared to previous reports of pyrite formed from deep‐marine venting of acidic and Fe‐ and H 2 S‐rich hydrothermal fluids into the water column (e.g. Ikehata et al, 2015; Lehrmann et al, 2015; Xu & Scott, 2005). In such environments of mixing with cold, oxic seawater, the (localized) attainment of (super‐) saturation of pyrite in mixed solution is not necessarily controlled by temperature and the availability of reduced Fe and S (H 2 S), but also by pH and redox conditions (and hence sulfur speciation); parameters that generally influence the crystals morphology and aggregation pattern of pyrite (Barrie et al, 2009; Graham & Ohmoto, 1994; Murowchick & Barnes, 1987; Ohfuji & Rickard, 2005; among others).…”

“…The pyrite studied here shows strong similarities in terms of umn (e.g. Ikehata et al, 2015;Lehrmann et al, 2015;Xu & Scott, 2005). In such environments of mixing with cold, oxic seawater, the (localized) attainment of (super-) saturation of pyrite in mixed solution is not necessarily controlled by temperature and the availability of reduced Fe and S (H 2 S), but also by pH and redox conditions (and hence sulfur speciation); parameters that generally influence the crystals morphology and aggregation pattern of pyrite (Barrie et al, 2009;Graham & Ohmoto, 1994;Murowchick & Barnes, 1987;Ohfuji & Rickard, 2005; among others).…”

Potential for biogeochemical cycling of sulfur, iron and carbon within massive sulfide deposits below the seafloor

“…Pyrite was the dominant component of the deeper subsamples (1.36–2.67 mbsf) of the SMS deposits at either the Pika or Archaean site, except BMS9A, which is consistent with previous reports (Kakegawa et al ., ; Ikehata et al ., in press; Ishibashi et al ., in press). Concentration of O 2 in penetrating seawater might decrease via microbial respiration before it reached the deeper regions.…”

“…Anhydrite, which is a typical component of active sulfide chimneys, is dissolved from sulfide chimneys after hydrothermal activity ceases (Haymon and Kastner, ). In fact, anhydrite has been detected in the active chimneys, but rarely in the inactive chimneys of the SMT (Ikehata et al ., in press). Furthermore, anhydrite has not been detected in the other parts of the sulfide core samples of BMS03 and BMS09 (Ishibashi et al ., in press) or in our subsamples.…”

Geological interpretation of volcanism and segmentation of the Mariana back‐arc spreading center between 12.7°N and 18.3°N

“…Early investigations of the Mariana region were driven by the discovery of hydrothermal activity in the central back‐arc during a series of Alvin dives in 1987, which located the Alice Springs (including Central Peak), Burke (including Snail Pits and Anemone Heaven), and Illium vent fields near 18°12'N, and the Central Trough vent field near 18°2'N [ Craig et al ., ; Campbell et al ., ; Hessler et al ., ; Hessler and Martin , ; Hessler and Lonsdale , , Gamo and Shipboard Scientific Party , ; Fujikura et al ., ; Ishibashi et al ., ]. Venting was subsequently discovered in the southern back‐arc near 13°N, beginning with the Forecast Seamount vent field in 1992 [ Johnson et al ., ; Gamo and Shipboard Scientific Party , ], the 13°N Ridge low‐temperature vent field in 1999 [ Mitsuzawa et al ., ; Masuda et al ., ], the Snail (also known as the “Fryer” site), Yamanaka (also known as the “Y” site), Pika, and Archean (3030–2980 mbsl) vent fields in 2003, and the Urashima vent field in 2010 [ Wheat et al ., ; Ishibashi et al ., ; Urabe et al ., ; Baker et al ., ; Kakegawa et al ., ; Yoshikawa et al ., ; Nakamura et al ., ; Ikehata et al ., ]. However, the regional geological context of the different vent sites was poorly understood.…”