INTRODUCTION

Significant chemical differences have been recognized in recent years between hydrothermal vent fluids associated with actively forming sulfide chimneys at mid-ocean ridges (Von Damm, 1995) and backarc basin submarine volcanic sites including PACMANUS in the Manus Basin of northern Papua New Guinea (Binns and Scott, 1993; Gammo et al., 1996; Douville et al., 1999). In order to further understand these differences, particularly the role of subseafloor reactions with contrasted mafic and felsic substrates and possible contributions from introduced magmatic components, sampling deep-seated borehole fluids for comparison with those vented from active seafloor chimneys at PACMANUS was an important although poorly achieved objective of Leg 193. Our intention to develop a tool capable of handling anticipated high temperatures to 350C being unrealized, we used the Water Sampling Temperature Probe (WSTP) (Barnes, 1988) previously employed by the Ocean Drilling Program (ODP) to collect in situ pore fluids from sediments.

Four borehole fluid samples were collected during the leg. Details of the operations and shipboard chemical analyses are provided in Binns, Barriga, Miller, et al. (2002) and are summarized in Table T1. To avoid damage to the tool, we conservatively planned to lower the WSTP to depths where prior logging runs indicated a borehole temperature of 60C, timing the opening of its valve accordingly. This plan was frustrated in Hole 1188B by a blockage close to the seafloor, at which point faulty operation of the inlet valve also apparently prevented complete flushing of deionized water from the sampling coil. Two samples obtained at different times in Hole 1188F were collected at lower than expected temperatures as a consequence of seawater draw-down into the hole. Only the sample from Hole 1189B was collected at a temperature close to the objective (55C), although premature opening of the valve during this operation may have trapped some fluid from higher in the hole.

Shipboard analyses (Binns, Barriga, Miller, et al., 2002) established that all four borehole fluid samples were composed principally of seawater. Elevated Fe, Mn, and Li in the more successful sample from Hole 1189B suggested a hydrothermal component, whereas Mn was also enriched in the near-seabed sample from Hole 1188B. High Ca in the sample from Hole 1189B was interpreted to possibly arise from anhydrite dissolution.

This report presents shore-based chemical and Sr isotopic analyses of subsamples from each operation, taken from the titanium primary sampling coil of the WSTP, filtered through 0.2-m polysulfone membranes, and acidified with 0.1 mL of ultrapure HNO3 per 10 mL of sample. Three blank samples, prepared on board from similarly acidified nanopure water, were also analyzed.

Details of the tectonic setting, volcanology, and sub-seafloor alteration patterns of the PACMANUS hydrothermal field, located along the crest of felsic volcanic Pual Ridge, are provided by the Shipboard Scientific Party (2002a) and elsewhere in this volume. Key points warranting mention here are that Pual Ridge is constructed of relatively thin flows, predominantly dacite and rhyodacite with some andesite, and that at the sites drilled the sequence is pervasively altered to clay-dominated assemblages beneath a thin cap (<10–40 m) of unaltered volcanic rock. Borehole fluids were collected at two sites: Site 1188, located on a zone of low-temperature, diffuse venting (Snowcap site, drilled to 387 meters below seafloor [mbsf] with Hole 1188F), and Site 1189, located at a field of active sulfide chimneys with high-temperature, focused venting (Roman Ruins Site, drilled to 206 mbsf with Hole 1189B). The settings of these and other named sites referred to below are shown in Figure F1.

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