Scanning Microscopy


Laser induced combustion provides a powerful new technique for sulfur isotope measurements in natural sulfides. A high power continuous laser, focused through a modified microscope system onto a sulfide mineral target, produces highly localized heating. The laser beam is focused to a spot 25 μm in diameter at the sample surface. In the presence of low pressure oxygen, temperatures at the centre of the spot are sufficiently high to produce localized oxidation. Resultant SO2 is transferred to a gas source isotope ratio mass spectrometer, where the 𝛿34S can be measured to a precision better than 0.25‰. Sulfur isotopes are fractionated during the combustion by an amount which is reproducible, mineral specific and favours the lighter isotopes. The system is calibrated for given sulphide minerals and specified operating conditions. Laser combustion analysis of sulfur isotopes in pyrite "chimneys" from the Silvermines ore deposit in Ireland, illustrates the benefits of 100 μm spatial resolution possible with this technique.

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