Abstract
Abstract Drilling during International Ocean Discovery Program (IODP) Expedition 370 at Site C0023 encountered instances of hydrothermal mineralization from 775 to 1121 m below seafloor. Fluid inclusion homogenization temperatures measured on barite veins within this interval indicate precipitation from fluids with temperatures up to 220 °C, and salinities ten times higher than interstitial water (16–25 c.f. 2.8–3.6 wt % NaCl). Patches of stratabound mineralization (rhodochrosite, calcite, barite and anhydride) are largely confined to the vicinity of veins and have vertical thicknesses and extents that can be explained by precipitation within the thermal aureoles of veins. Thermal maturities measured on petroleum biomarkers in underthrust sediments record a rise from pre-oil window to early oil window thermal maturities. Basin models show that increases in thermal maturity can be explained by burial metamorphism, and kinetic-based calculations suggest that hot fluids would only have had a minimal effect on hydrocarbon generation. However, the movement of hot fluids still has geochemical implications and creates a complex thermal history where both short-duration, localized heating within fracture zones at hot temperatures (~200 °C for less than 1 yr) and heating during burial over long durations (80–110 °C for 0.5–2 Myrs) need to be considered.
Highlights
The seaward end of the Nankai Accretionary Complex is one of the localities where it has been suggested that catagenesis in hot sediments may provide electron donors to support deep microbial communities (Horsfield et al, 2006)
Basin models show that increases in thermal maturity can be explained by burial metamorphism, and kinetic-based calculations suggest that hot fluids would only have had a minimal effect on hydrocarbon generation
A distinctive difference between the characterizations of Sites 1174 (Underwood et al, 1993) and C0023 was the recognition of hydrothermal mineralization between 775 and 1121 mbsf at Site C0023
Summary
The seaward end of the Nankai Accretionary Complex is one of the localities where it has been suggested that catagenesis (thermal breaking of carbon-carbon bonds) in hot sediments may provide electron donors to support deep microbial communities (Horsfield et al, 2006). Within the Nankai Accretionary Complex and some similar hot sedimentary settings, indicators of the presence of microbial communities including cell counts have been observed to “revive” as temperatures approach conditions under which organic substrates can ∗. These two authors contributed on this manuscript. Received 24 June 2019; Received in revised form 7 October 2019; Accepted 8 October 2019. Shown are formation tops for the Upper (green) and Lower (purple) Shikoku Basin formations, and the base of the décollement zone. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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