RESULTS

Analyses of the shipboard blank samples (Table T1) were below or close to detection limits for most elements. Minor traces of Ni, Zn, Rb, Sr, Y, Mo, Cd, Sb, REE, Pb, and Bi were measured. In no case did the level require application of a correction for the acid spike to those borehole fluids where presence of a hydrothermal component is apparent.

Allowing a density of 1.025 g/mL for seawater, agreement between the shore-based analyses (here cited gravimetrically) and shipboard analyses (cited volumetrically by Binns, Barriga, Miller, et al., 2002) of the borehole fluids is excellent for Na, Mg, and S and good for K, Ca, B, Sr, and Li. However, the new results for Mn, Fe, and Ba are distinctly lower.

The compositions and 87Sr/86Sr ratios of the two borehole samples from 107 and 207 mbsf in Hole 1188F (CSIRO numbers 142725 and 142726) are essentially identical with seawater, except for modest enrichments in Mn, Fe, Co, Zn, Cd, light REE, Pb, Bi, and Th—elements (apart from Th) conceivably contributed by a tiny proportion of hydrothermal fluid (1%). The lower Na, Mg, and S contents cited here, as well as the shipboard salinity measurement and Cl analysis (506 mMol/L) for the borehole sample at 3 mbsf in Hole 1188B (CSIRO 142724) indicate that some deionized water remained in the WSTP coil. Using Na and Cl contents as a guide, both elements of comparable abundance in both seawater and hydrothermal fluids, the dilution is estimated at 8%. This near-seabed sample and also the higher-temperature borehole fluid from 130 mbsf in Hole 1189B (CSIRO 142727) display a number of elemental enrichments potentially attributable to hydrothermal components, which we assessed by applying corrections for entrained seawater (Table T2). Both also show significantly lower bulk 87Sr/86Sr ratios than seawater (Table T1).

The composition used for seawater (Table T2) is based on four hydrocast samples, analyzed using the same methods, all collected near 1500 meters below sea level (mbsl) over Pual Ridge, northeast of PACMANUS, from background positions outside the detectable particulate plume. This seawater composition differs significantly from literature values only by its higher Pb, an element that appears to have been dispersed beyond the particulate plume at PACMANUS, but for which any adjustment makes no relevant change to our end-member calculations. Literature values were used for the REE (Mitra et al., 1994) because like Ga, Ge, As, and Te these were below detection by ICP-MS in the Pual Ridge background seawaters.

The proportions of seawater in borehole samples from 3 mbsf in Hole 1188B (CSIRO 142724) and from 130 mbsf in Hole 1189B (CSIRO 142727) were assessed by assuming this component contained all the bulk Mg and that hydrothermal fluids contain zero Mg (see Von Damm, 1995). Taking into account also the estimated deionized water dilution in the former, their calculated hydrothermal end-member components (Mg = 0) are 2% and 12%, respectively, the first figure being subject to greater error. End-member compositions computed by subtracting seawater contributions and recalculating to 100% are presented in Table T2 for elements adequately detected. The calculations yield negative end-member values for U, reflecting lower contents than seawater in the raw analyses of these two samples. We are especially confident regarding the latter, so an additional extractive factor discussed below appears involved.

Table T2 also lists, for comparison, similar calculations applied to analyses (Douville, 1999) of fluids collected by the Shinkai-6500 from two chimney vents at Satanic Mills (Fig. F1) during the 1995 ManusFlux cruise (Auzende et al., 1996). Bach et al. (2004) argue the case for using Mg = 0 to calculate these PACMANUS end-members, rather than Mg = 20 mM as proposed by Douville et al. (1999). For many elements including the alkalis, the calculated end-member composition for the 55C borehole fluid at 130 mbsf in Hole 1189B (CSIRO 142727) falls within or close to the range of the chimney vent fluid end-members. In comparison with the vent fluids, highly enriched elements in the borehole fluid include total S, Ca, Cd, and REE; moderately enriched elements include Sr and Sb; and depleted elements include Al, Mn, Fe, Mo, and Pb.

Although subject to greater error arising from the estimated dilution by deionized water, the calculated end-member for near-seabed borehole fluid at 3 mbsf in Hole 1188B (CSIRO 142724) is broadly similar that of the sample from 130 mbsf in Hole 1189B (CSIRO 142727), but Mn, Zn, Mo, Cd, Sb, and La are distinctly higher, whereas K and Fe are lower. In addition, Co, Ni, and Th are higher, but no data are available for these elements in the chimney vent fluids.

A chondrite-normalized REE plot (Fig. F2) for end-member fluid from 130 mbsf in Hole 1189B (CSIRO 142727) shows progressive relative depletion in heavy REE but lacks any significant Eu anomaly. In both this and the overall increased REE abundances, this end-member composition differs significantly from the end-member chimney vent fluids at Satanic Mills, which are comparatively depleted in light REE and have distinctly positive Eu anomalies (Fig. F2). Douville et al. (1999) attribute depletion of light REE in the vent fluids to subsurface precipitation of barite. This mineral is conspicuously rare in Leg 193 drill core from below Roman Ruins and Snowcap, but Satanic Mills remains untested in that respect because Hole 1191A at that site did not penetrate beyond a >20 m capping of unaltered dacite.

The partial end-member REE plot (Fig. F2) for the borehole fluid from 3 mbsf in Hole 1188B (CSIRO 142724) shows highly enriched La and a negative Ce anomaly. Raw values for La and Ce for the two borehole fluids from 107 and 207 mbsf in Hole 1188F (CSIRO 142725 and 142726) (Table T1) are higher than seawater, and the ratios suggest they share a negative Ce anomaly.

Using the proportions of seawater and apparent hydrothermal end-member, their measured bulk 87Sr/86Sr, and the original and computed end-member contents of Sr, we estimated 87Sr/86Sr for the end-member component of borehole fluids at 3 mbsf in Hole 1188B (CSIRO 142724) and 130 mbsf in Hole 1189B (CSIRO 142727) and also of the chimney vent fluids (Table T2). The borehole fluid end-member ratios are comparable (0.7064 and 0.7062), but they are more radiogenic than those of the chimney vent fluids (0.7050 and 0.7054). The former fall within the range measured for vein anhydrite from deeper cores at Sites 1188 and 1189 (Roberts et al., 2003); anhydrites closer to the seabed have ratios nearer seawater. All four borehole and chimney end-member ratios are higher than unaltered lavas dredged from Pual Ridge (0.70359; n = 13) and an altered dacite cropping out at Snowcap (0.70470) (CSIRO data).

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