HR: 0800h
AN: V41D-1480    [Abstracts]
TI: In Situ Analysis of 34S/32S by Laser Ablation MC-ICP-MS
AU: * Craddock, P R
EM: pcraddock@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02536 United States
AU: Ball, L
EM: lball@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02536 United States
AU: Rouxel, O
EM: orouxel@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02536 United States
AU: Bach, W
EM: wbach@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02536 United States
AB: We present results from the development of a novel in situ approach to measurement of ³⁴S/³²S ratios in anhydrite (CaSO₄). Sulfur isotopes have been used extensively to trace the sources of S in hydrothermal vent deposits (e.g., S from volcanic rocks, reduced seawater sulfate, reactions with sediments and contributions from magmatic S) and to infer the primary conditions controlling the genesis of hydrothermal deposits. Conventionally, S isotopes are determined via gas-source mass spectrometry using SO₂ or SF₆ as the S source. Such techniques are time-consuming and require extensive sample preparation stages. Important textural information and spatial resolution is lost following dissolution of sulfide and sulfate minerals. For hydrothermal systems in which changes of the physiochemical conditions of fluid circulation and mineral precipitation are rapid, only an in situ approach will allow the resolution of the signatures of variable sources and physiochemical conditions contained within individual sulfate and sulfide crystals over spatial scales of tens of microns. To fully characterize the complexity of S behavior (and of other elements) in hydrothermal systems, requires an in situ approach to isotope measurement. By optimizing coupled laser ablation and multi-collector inductively coupled plasma mass spectrometry (ThermoElectron Neptune LA-MC-ICP-MS) we have developed a method for precise measurements of ³⁴S/³²S ratios in anhydrite. A New Wave UP213 (quad Nd YAG 213 nm) laser is used as the ablation source with helium as the sample carrier gas.
The accuracy of S isotope ratios that can be achieved by ICP-MS measurements is limited by two principal artifacts; instrumental mass bias and spectral interferences on masses (m/z) 32, 33, 34 and 36. Mass bias is corrected by employing "sample-standard bracketing" techniques, in which the mass bias of the unknown sample is interpolated between the known biases of adjacent S standard analyses. To bracket laser analyses, we use an in-house solution standard (S_AlfaAesar = + 2.9 ‰ _V-CDT), for which the ³⁴S/³²S ratio was previously determined using NIST standards. Major spectral interferences for S (e.g., O₂) are eliminated by applying a mass resolution greater than m/Žm = 4000, which is achieved in medium resolution mode. The effects from matrix differences between solution and mineral phases have been investigated and shown to be within the analytical error of the method (± 0.2 ‰).
This method has several advantages versus conventional approaches; (1) the extremely high temperatures achieved by a plasma source enables a higher ionization yield for S and gives high precision, (2) sample measurement is rapid, (3) the need for exhaustive sample preparation is alleviated and (4) a spatial resolution of the order of 100 æm is achieved, enabling identification of small-scale heterogeneities contained within hydrothermal precipitates.
We present preliminary data from analyses of anhydrite recovered from drilling of the active Pacmanus hydrothermal system, Papua New Guinea. The results indicate subtle changes in the S isotope ratios recorded in individual anhydrite crystals that are likely related to multiple sources of sulfur or complex sulfur cycling within the subseafloor of this hydrothermal system. In addition to characterizing anhydrite from hydrothermal systems, this method can be used to explore in situ isotopic variations of multiple sulfate minerals from a variety of environments (e.g., evaporite deposits).

DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4825 Geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005