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doi:10.2204/iodp.proc.301.202.2008

Results

Results are presented in Tables T1, T2, T3, T4, and T5 and Figures F2, F3, and F4. At IODP Site U1301, all of the trace elements that were analyzed in the pore fluids, with the exception of U, were mobilized within the sediment column to concentrations above those in bottom seawater (shown as 0 depth in Fig. F2). Transition metals and REEs exhibited maximum values in the upper to middle sediment column as a result of redox conditions produced during diagenesis (Haley et al., 2004). Cr, Cs, and Cd concentrations peaked sharply in the lower zone of sulfate reduction at 170.1 m (Cr = 10.4 nmol/kg, Cs = 14.6 nmol/kg, and Cd = 7700 pmol/kg). A majority of the trace element gradients in basal sediments approach calculated concentrations at the sediment/basaltic interface that are close to concentrations in Baby Bare Spring fluids, which represent pristine formation fluid compositions (Wheat et al., 2002). Concentrations of Rb and Cs in basal sediments are clearly lower than predicted basement concentrations (Rb: 222 vs. 1120 nmol/kg; Cs: 0.95 vs. 5.3 nmol/kg), which could be attributed to continued alteration of the basement fluids associated with the exchange of the major constituents K and Na with Ca (Elderfield et al., 1999). Basal pore fluid concentrations of Ni (133 nmol/kg), Zn (290 nmol/kg), Mo (435 nmol/kg), Ba (800 nmol/kg), and La (119 pmol/kg) are significantly above those predicted by Baby Bare Spring compositions. This could be a result of steep concentration gradients near the basement, resulting in a larger uncertainty, as observed in the Mn data (Shipboard Scientific Party, 2004). In cases where there is no discernable gradient near the basement (e.g., Mo; Fig. F2), the overestimation of predicted values can be attributed to increases in basement fluid concentrations along this transect, similar to those recorded for the major and minor species (Wheat et al., 2000).

The Sr isotopic compositions of IODP Site U1301 pore waters range from near seawater values in the uppermost sediments (0.709138) to more evolved compositions with a clear basaltic ⁸⁷Sr/⁸⁶Sr signature deeper in the sedimentary section. The ⁸⁷Sr/⁸⁶Sr profile is similar in shape to that of nearby ODP Hole 1027B (Mottl et al., 2000) in crust of similar age and upper basement temperature (~3.6 Ma and 63°C) (Davis, Fisher, Firth, et al., 1997), with a local ⁸⁷Sr/⁸⁶Sr ratio maximum (0.708163) at 78.7 meters below seafloor (mbsf) and a local minimum (0.707609) between 190 and 240 mbsf. The lowermost pore water sample has an ⁸⁷Sr/⁸⁶Sr ratio of 0.707609, similar to that of Baby Bare Spring fluids (~0.70745) (Butterfield et al., 2001) and ODP Hole 1026B basement fluid (~0.70739) (Elderfield et al., 1999). The similar Sr isotopic composition of in situ basement fluid at ODP Site 1026 and basement fluids venting at Baby Bare to the IODP Site U1301 near-basement pore fluid is consistent with near-basement pore fluids being representative of upper basement ridge-flank hydrothermal fluids. This result is consistent with conclusions derived from Sr isotopic and trace element compositions of basement-hosted calcium carbonate veins and near-basement pore fluids from ODP Sites 1025–1028 and 1032 (Coggon et al., 2004).

Concentrations of inorganic Ge dissolved in pore fluids often mirror Ge/Si molar ratios (Figs. F2, F3). Maximum values of both in the pore fluids occur in IODP Hole U1301C, with 4800 pmol/kg Ge and 12.3 Ge/Si (μmol/mol). In IODP Hole U1301C, both values increased in the basal sediments, whereas the opposite trend existed at ODP Sites 1024 and 1025. Despite this apparent contradiction, borehole fluids at all three sites contained Ge/Si ratios many factors above any ratio in the pore fluids. Of particular interest is the relatively low concentrations of Ge in the ODP Site 1024 CORK fluids, yet Ge/Si molar ratios are nearly four times greater than any of the pore fluid ratios. The highest values for dissolved Ge concentrations and Ge/Si molar ratios both occur in the basement fluids recovered from CORKs (22,300 pmol/kg at ODP Site 1026 and 68 μmol/mol at ODP Site 1027). Pore fluid Ge concentrations and Ge/Si molar ratios in gravity cores from the southern EPR flank were minimally altered from bottom seawater with the lowest values measured from any of the locations (46 pmol/kg and 0.26 μmol/mol). Pore waters from the flanks of the Cocos Ridge display Ge and Ge/Si patterns similar to those observed in surficial sediments at ODP Sites 1024 and 1025.

Solid-phase sediment compositions are presented in Table T5 and Figure F4. The most striking characteristic of the sediment chemistry is a carbonate- and Mn-rich layer at 184.1 mbsf, corresponding to minima in Si (82,100 mg/kg), Mg (8,100 mg/kg), Fe (19,600 mg/kg), Al (28,700 mg/kg), Ti (1,530 mg/kg), and Ge (0.215 mg/kg). Mn shows a clear local maximum at 184.1 mbsf, but the greatest enrichment (5830 mg/kg) occurs in the basal sediments at 243.3 mbsf. Ge/Si ratios do not appear to be as closely correlated with variations in Si content.

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