METHODS

Chemical analyses (Table T1) were performed by inductively coupled plasma–atomic emission spectrometry (ICP-AES) at Commonwealth Scientific and Industrial Research Organisation (CSIRO), Sydney, Australia, and by inductively coupled plasma–mass spectrometry (ICP-MS) at University of Technology, Sydney, Australia, using Spectroflame and PerkinElmer instruments, respectively. For ICP-AES measurements, conducted in duplicate, the borehole fluids and shipboard blank samples were introduced undiluted and at various dilutions to optimize the concentration range and minimize corrections for spectral overlaps and suppressive interferences. Aqueous calibration standards were used, and instrumental blanks were computed using ultrapure deionized water.

Measurements by ICP-MS were conducted twice on different days with similar results and, for the borehole samples, a third time under conditions providing increased sensitivity, particularly for rare earth elements (REE). The shipboard blank samples were run undiluted, whereas borehole fluids were diluted x30 to help minimize interferences (which nevertheless prevented determination of Sc, V, Cu, and As by this method). Calibration and blank solutions were prepared from ultrapure deionized water and were acidified to a pH equivalent to that of the samples. Internal reference spikes (Re and Rh) were added to samples and calibration solutions.

Detection limits based on count statistics and respective dilutions are listed in Table T1. These are considered variously conservative, and some analyses below the nominal detection limits are cited as meaningful. Data are not provided where our prior experience with saline groundwaters indicates that the use of matrix-unmatched aqueous standards is not appropriate for analysis of seawater.

For strontium isotope measurements, Sr was separated directly from the borehole fluids by cation exchange chromatography with AG50W-X8 resin and loaded with H3PO4 onto Ta filaments. Isotope ratios were measured on a VG 354 thermal ionization mass spectrometer at CSIRO. Measured blanks were negligible. Raw data from 54 ratio determinations were filtered using a 2 rejection criterion. Internal precisions calculated as two standard errors of the mean are cited in Table T1. Twelve simultaneous ratio measurements of the U.S. National Bureau of Standards Standard Reference Material 987 standard averaged 0.710231 (2 = 0.0023%).

Chemical analyses of other CSIRO samples used in comparative discussions below were obtained using the same methods. These include a filtered vent fluid collected at a Tsukushi chimney in 1998 by C.J. Yeats (BIOACCESS cruise, Shinkai-2000 Dive 1066), samples from within the particulate plume above PACMANUS collected in standard Niskin bottles during hydrocasts at three sites between the Satanic Mills and Roman Ruins chimney fields (BINATANG and BISMARCK cruises of the Franklin in 2000 and 2002, respectively, and the DaeYang-02 cruise of the Onnuri in 2002; unpublished CSIRO reports) and vent funnel (VUNL) samples from three warm seeps on the diffusely venting Snowcap field (PACMANUS-IV and BINATANG cruises of Franklin in 1997 and 2002, respectively; unpublished CSIRO reports). The VUNLs were inverted tetragonal prisms, 1.5 m across on their open base and with narrow openings at the top, constructed of stainless steel (mild steel lined with plastic in the first version), and fitted with an altimeter, a temperature sensor, and a small Niskin bottle. With favorable weather conditions, the VUNL was moved around and gently "pogoed" to bottom until a temperature anomaly was noted, at which point the Niskin bottle was triggered after allowing 15–20 min to stabilize. Temperatures during collection ranged from 0.1 to 0.5C above ambient.

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