Sampling and analysis of hydrothermal vent fluids

Hydrothermal fluid samples from the sulfide structures at Longqi-1 vent area were collected using pressurized, piston-driven sampling devices, constructed entirely of titanium. These devices make use of compressed nitrogen to maintain each sample at seafloor pressure before and during sampling, and are similar in design and concept to IGT samplers developed in ref. ⁵³. Immediately after arrival of the samplers on board, aliquots were taken for pH, and measurement of the dissolved gases (H₂, H₂S, CH₄) were undertaken. For a detailed description of on-board analytical methods see refs. ^28,53. Additional sub-samples were preserved for shore-based analysis of major dissolved anions/cations and trace metals. The trace metal aliquot was immediately acidified with analytical grade HCl (Optima). A fraction of this sub-sample was then diluted 50-fold and utilized for analysis of aqueous silica. Both fractions were stored in pre-weighed and acid cleaned high-density polyethylene bottles. Precipitates that formed in the titanium samplers were collected on a 0.45-μm nylon filter and subsequently re-dissolved in HCl/HNO₃ (Ultrex). Based on the total volume of the particular sampler used, the amount of metals measured in the precipitate was recombined with metals that remained in solution to obtain a complete metal inventory for the vent fluid samples. Essentially, this process only affected Fe, Cu, and Zn. For vent fluid samples above 300 °C, the precipitates account for 90% of the reported concentrations of Cu and Zn, but less than 10% of the Fe²⁸. The shore-based analysis of all major dissolved cations/anions was conducted using ICP-OES and/or ion chromatography, while trace metals were determined by ICP-MS. The analytical uncertainties (2σ) is ±2% for Cl (Ion chromatography), ±10% for H₂, CH₄ (gas chromatography–TCD) and H₂S (CuCl₂-precipitation/H₂O₂ reduction). Other elements are measured with ICP-MS, and 2σ uncertainties are ±2% for major species and ±5% for minor species²⁷. Detection limits by ICP-MS are approximately an order of magnitude below reported concentrations. The hydrogen and oxygen isotopes of hydrothermal vent fluids were measured at the University of Arizona using a Thermo Finnigan Delta XP isotope ratio mass spectrometer. All samples were measured at least in duplicate and results are principally given in the standard delta notation in per mil (‰) vs. VSMOW according to δ¹⁸O (D) [‰] = (R_sample/R_reference−R_reference) × 1000 fluid samples⁵⁴. The endmember composition of hydrothermal fluids was calculated using a least-squares regression of the individual components versus Mg with extrapolation to Mg zero and including the respective seawater values. Vent fluid chemistry is summarized in Supplementary Tables ¹ and ².