PACROFI VI - Electronic Program

Chemical Characterisation of Hydrothermal Fluid Inclusions by Laser Ablation-Icp-Mass Spectrometry

SHEPHERD, Thomas J. and CHENERY, Simon R.

British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, U.K.

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Using a new and simple aqueous solution method for the calibration of laser ablation ICP-MS analysis of single inclusions, a wide range of elements can now be quantitatively determined. The standards are easy to prepare and offer an almost unlimited range of fluid compositions. They are also applicable to other laser ablation techniques (e.g. ICP-AES) and are largely independent of the laser wavelength. Extensive evaluation tests have been carried out on synthetic fluid inclusions in halite to determine working detection limits, precision and accuracy as a function of inclusion volume. Data will be presented for Li, Mg, K, Ca, Rb, Sr, Cs, Ba, Mn, Cu, Zn, Pb, B, including results for the direct measurement of Cl/Br ratios. Halogen measurement permits potential sourcing of hydrothermal fluids and allows normalization of analyses to the major chloride anion component. Examples will be given illustrating the use and limitations of laser ICP-MS for the determination of the metal content of hydrothermal fluid inclusions associated with various types of ore deposit (MVT, porphyry Cu-Mo, U-Th-REE). Polyatomic interferences restrict the measurement of certain elements (As, P, S,V) whilst others (Au) appear to be below the detection threshold of the current instrument. Reference will be made to the use a high temperature ablation cell for the analysis of multi-solid, high temperature inclusions.