Abstract
Abstract. Recent advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) open new perspectives for quantification of trace metals and metalloids in mineral-hosted fluid inclusions and glass-hosted gas bubbles. This work is devoted to a new method applied to quantify element concentrations (at parts-per-million and weight percent levels) in natural and synthetic fluid inclusions and gas bubbles by using only an external calibrator in cases where internal standardization is unavailable. For example, this method can be applied to calculate element (metal and metalloid) concentrations in carbonic (C–O–H) fluid inclusions and bubbles. The method is devoted to measuring incompatible (with the host mineral and glass) trace elements originally dissolved into the trapped fluid. The method requires precise estimation of the fluid density, the inclusion/bubble volume or average radius, and measurement of the laser ablation crater radius by independent microanalytical techniques as well as accurate data on the concentration of major/minor elements compatible with the host mineral (or host glass). This method, applicable for analyses of hydrous carbonic fluid inclusions and gas bubbles hosted in silicate minerals and glasses, relies on the absence of a matrix effect between fluid, host mineral and daughter phases (silicate, oxide or sulfide) and the external calibrator (e.g., reference silicate glasses) during the LA-ICP-MS analysis, an assumption validated by the use of femtosecond lasers.
Highlights
Development of new methods for a “standardless” quantification of fluid composition is of utmost importance in geochemical research, nowadays, given the continuous evolution of analytical facilities and the difficulties in providing reliable internal calibration
The method that we propose builds on the consideration of element pairs: (i) a first element, termed X, which must be strongly compatible in the mineral hosting the fluid inclusion or gas bubble; a second element, termed Y, which is enriched in the fluid or vapor and must be strongly incompatible in the host mineral
All other elements investigated must belong either to the host-compatible X group or to the host-incompatible Y group, which represents the unknown elements contained in the fluid inclusion or gas bubble. (ii) The method assumes chemical homogeneity of the measured inclusion and of the host mineral in the investigated zone. (iii) An additional requirement is that, during LA-ICP-MS analysis, there are no or only small matrix effects interfering between the investigated fluid–mineral pair and the external calibrator, both of which must be analyzed for the same isotopes under the same analytical conditions
Summary
Development of new methods for a “standardless” quantification of fluid composition is of utmost importance in geochemical research, nowadays, given the continuous evolution of analytical facilities and the difficulties in providing reliable internal calibration. Hanley and Gladney (2011) described quartzhosted, Ni-, Cu-, Pd-, Bi-, Te- and Fe-bearing CO2-rich inclusions from the Roby zone, Lac des Iles Complex (Ontario, Canada), as containing only minor bulk NaCl (0.1 wt %– 1 wt % NaClequiv) They suggested the Na contents in the inclusions and used these values as an internal calibrator to quantify the trace metals in the inclusions. Ferrando et al (2009) assumed a Si concentration and used it as an internal calibrator for inclusions from ultrahigh-pressure rocks of the Brossasco-Isasca Unit of the Southern Dora-Maira Massif without any further correction for the host mineral contribution to the bulk fluid inclusion signals All of these attempts to apply guessed concentrations to fluid composition quantification only produced results plagued by likely high uncertainties (sometimes undiscussed and non-evaluated) and no means to assess them. In order to quantify the trace element concentrations in natural and synthetic low-NaCl carbonic fluid inclusions and gas bubbles, where either no or only minor concentrations of internal standards for the LA-ICP-MS method are available, we have developed the method discussed below
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