2.3.2 Isotopes analysis
Approximately 2 L of water was subsampled in a beaker and acidified to pH < 2 with distilled 6 M HCl to remove HCO3- or CO32-, and BaSO4 was precipitated by adding 10% BaCl2 (aq.) to the beaker and left to stand overnight. The gypsum sample was ground to a powder, which was dissolved in 10% NaCl (aq.) for 24 hours and then filtered through a 0.45-μm Millipore membrane of cellulose acetate. The filtrates were acidified to pH< 2 with 25% HCl (aq.), and BaSO4 was precipitated by adding excessive 10% BaCl2 (aq.) to the beaker and left to stand overnight (Goldberg, Poulton & Strauss, 2005; Kampschulte, Bruckschen & Strauss, 2001 ). Then, the BaSO4 precipitated from the gypsum sample, and the water samples were both filtered and collected on a 0.45-μm Millipore membrane of cellulose acetate. The precipitate was continuously flushed with ultrapure water until the Cl- concentrations could be ignored in the filtrates (tested with AgNO3 reagent). Finally, the precipitate was dried for 24 hours at 60 ℃.
The BaSO4 from each sample was divided into two portions, and the sulfur and oxygen isotopes were measured by using a continuous flow isotope mass spectrometer (CF-IRMS, CV Instrument Corp., UK). One portion was placed in a tin cup and burned and decomposed to SO2 in an elemental analyzer (Euro EA 3000, Euro Vector S.P.A., Italy) at 1,000 °C and was then passed into IRMS (IsoPrime) by a helium gas stream to measure the value of δ34SSO4. The other portion was wrapped tightly in a silver cup, was burned and lysed to CO in the same analyzer at 1,280 °C; then, the product was passed into IRMS (IsoPrime) by a helium gas stream to measure the δ18OSO4 value. The resulting values of δ34SSO4and δ18OSO4 were compared with the Vienna Canyon Diablo Troilite (V-CDT) standard and the Vienna Standard Mean Ocean Water (V-SMOW) standard, respectively. The international sulfur standard, NBS-127, was measured as an internal standard correction. The precision of the δ34SSO4 and δ18OSO4 analyses was estimated to be better than ±0.2‰ and ±0.5‰, respectively.