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doi:10.2204/iodp.proc.301.202.2008

Introduction

Fluid circulation through oceanic crust plays a significant role in global geochemical, thermal, and tectonic processes (review in Fisher, 2005). Integrated Ocean Drilling Program (IODP) Expedition 301 was undertaken to constrain the hydrologic and geochemical processes within a ridge-flank hydrothermal setting by installing two new circulation obviation retrofit kits (CORKs) at IODP Site U1301, upgrading another CORK at Ocean Drilling Program (ODP) Site 1026 (Fisher et al., 2005), and collecting sediment and basalt samples (see the “Expedition 301 Summary” chapter). IODP Site U1301 was selected based on previous investigations in the region (Hasselgren and Clowes, 1995; Davis, Fisher, Firth, et al., 1997; Elderfield et al., 1999) and the proximity to low-temperature hydrothermal springs to the north and south along a buried basement high (Mottl et al., 1998; Becker et al., 2000; Wheat et al., 2000, 2002). The approximate isothermal (~60°C) basement/sediment interface in this region of the Juan de Fuca Ridge (JFR) flank is attributed to vigorous fluid circulation through the uppermost ocean crust (Davis, Fisher, Firth, et al., 1997; Hutnak et al., 2006). In this report we present (1) trace element and Sr isotopic compositions of sediment pore fluids collected during Expedition 301; (2) concentrations of Ge in pore water and CORK fluid from reference sites on the eastern flanks of the JFR, southern East Pacific Rise (EPR), and Cocos Ridge; and (3) bulk sediment composition of the major and several minor elements, including Ge, from IODP Site U1301.

IODP Site U1301 pore fluids were extracted from whole-round sections obtained using the advanced piston corer (APC). Sediments were maintained in a nitrogen atmosphere during sample processing. After preliminary shipboard analysis of these fluids, it was apparent that the improved sampling techniques sufficiently preserved the concentrations of transition metal species (Mottl et al., 2000; Shipboard Scientific Party, 2004). On the basis of this observation, we undertook an extensive chemical analytical program that included many trace elements that were not measured on ODP Leg 168 pore fluids. These measurements were conducted in part to determine whether basal pore fluid concentrations and gradients for these trace elements can be used to predict the basement fluid composition, which is constrained by hydrothermal spring fluids from Baby Bare (Wheat et al., 2002) and borehole fluids from ODP Site 1026 (Wheat et al., 2004). The other topic of interest is the potential for biomediated geochemical exchange between the hydrothermal fluids, basement rocks, and overlying sediments affected by diagenetic processes (Bertine and Turekian, 1973; Morford and Emerson, 1999). Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were measured to assess the extent of hydrothermal fluid–rock interactions because the Sr isotopic compositions of basalt and seawater are well known and distinct (~0.7025 and 0.70918, respectively).

Concentrations of Ge and Ge/Si molar ratios of pore and borehole fluids from other ridge-flank hydrothermal systems and bulk sediment chemical analyses from Site U1301 were conducted to elucidate Ge systematics within the crust. Pore water data from Site U1301 alone do not provide enough constraints for Ge in this setting. Ridge-flank hydrothermal systems could have an impact on Ge concentrations in the oceans and on using Ge as a proxy for paleoceanographic conditions (Wheat and McManus, 2005). These sites and samples include (1) pore and borehole fluid Ge concentrations from ODP Sites 1024 and 1025, which are located 25 and 35 km, respectively, from the JFR axis (Wheat et al., 2003, 2004); (2) borehole fluids from ODP Sites 1026 and 1027, which lie 100 and 105 km from the JFR axis and within kilometers of Site U1301 (Wheat et al., 2003, 2004); (3) pore waters extracted from sediment collected using a gravity core on the Cocos Ridge flank (Fisher et al., 2003; Parsons et al., submitted); and (4) pore waters extracted from sediment collected using a gravity core on the EPR flank (Grevemeyer et al., 2002). Pore waters from the latter two studies were extracted by centrifugation at 1°–4°C and filtered through a 0.45 μm filter. Lastly, bulk sediment chemical analyses were conducted on freeze-dried and ground sediments that were used for shipboard determinations of carbonate and organic carbon content.

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