Seawater recharge into oceanic crust: IODP Exp 327 Site U1363 Grizzly Bare outcrop. (17th June 2013)

• Record Type: Journal Article
• Authors: Wheat, C. Geoffrey; Hulme, Samuel M.; Fisher, Andrew T.; Orcutt, Beth N.; Becker, Keir
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 14:Number 6(2013)
• Page Start: 1957
• Page End: 1972
• Publication Date: 2013-06-17
• Subjects: ridge flank -- hydrothermal -- outcrop -- chemical exchange -- low temperature -- pore water; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/ggge.20131
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: [1] Systematic differences in sediment thermal and pore water chemical profiles from Integrated Ocean Drilling Program Site U1363 document mixing and reaction within the basaltic crust adjacent to Grizzly Bare outcrop, a site of hydrothermal recharge into 3.6 My‐old basaltic crust. A transect of seven holes was drilled ~50 m to ~750 m away from the base of the outcrop. Temperatures at the sediment‐basement interface increase from ~6°C to >30°C with increasing distance from the outcrop, and heat flow is suppressed within several hundred meters from the outcrop. Calculated fluid compositions at the sediment‐basement interface are generally explained by mixing between bottom seawater and altered crustal basement fluids, with a composition similar but not identical to fluids from seeps at Baby Bare outcrop, located ~45 km to the northeast. Reactions within upper basement and overlying sediment affect a variety of ions (Mn, Fe, Mo, Si, PO4 3‐, V, and U) and δ 13 DIC, indicating a diagenetic influence and diffusive exchange with overlying sediment pore waters. The apparent 14 C age of basal pore fluids is much older than bottom seawater. Collectively, these results are consistent with seawater recharge at Grizzly Bare outcrop; however, there are strong gradients in fluid composition within 50 m of the outcrop, providing evidence for complex flow paths and vigorous mixing of young, recently recharged seawater with much older, more reacted basement fluid. The proximity of these altered fluids to the edge of the outcrop raises the possibility for fluid seepage from the outcrop in addition to seawater recharge. Key Points: Warm, highly altered fluids lie 50 meters from Grizzly Bare outcrop Fluid flow within upper basaltic crust is complex and compartmentalized Redox gradients near zones of seawater inflow affect microbial populations