Fluid inclusions are small samples of fluid trapped in a host mineral during growth or later during healing of fluid-filled fractures. They provide samples of the broad spectrum of fluids that have interacted with the earth's crust and upper mantle over geological time. Fluid inclusion research helps elucidate the role of fluid phases in sedimentary, diagenetic, metamorphic and magmatic processes. In particular, fluid inclusions provide a window on the origin and evolution of ore-forming fluids and ultimately on ore genesis. The steady development, since 1987, of Nuclear Microprobe methods for in situ, non-destructive fluid inclusion analysis has resulted in techniques for standardless, quantitative analysis of individual fluid inclusions in minerals. These techniques offer detection sensitivities as low as 20-50 ppm in the fluid using PIXE, in a short analysis time, and provide simultaneous multi-element data. PIXE is complemented by gamma-ray analysis using PIGE, which provides a simultaneous light element (Li, Be, B, F, Na, Mg, Al) detection capability, and by H recoil analysis using ERDA.

PIXE Analysis of Fluid Inclusions A unique technique, developed at the CSIRO, enables individual fluid inclusions as small as 5 µm in diameter, to be analyzed non-destructively. The technique uses a beam of 3 MeV protons from the CSIRO-GEMOC Nuclear Microprobe, focussed to a 1.3 µm spot-size, to penetrate the host mineral and excite X-rays from elements within the fluid. Using a model of X-ray production within each inclusion, the result is a quantitative measure of the composition of the original trapped fluid. The proton beam is scanned over the area of a fluid inclusion. This not only provides an even dose distribution needed for analysis, but also enables the imaging of the element distribution throughout the inclusion. PIXE microanalysis of fluid inclusions provides a direct tool for the study of fluid chemistry and ore-forming processes and the development of exploration methods.

Imaging Copper Partitioning into the Vapour Phase The imaging capabilities of the new CSIRO-GEMOC Nuclear Microprobe can be applied to image the internal contents of fluid inclusions. One example are coexisting brine and vapour inclusions from the Batu Hijau Cu-Au deposit, Indonesia. The images clearly display a classic example of strong partitioning of Cu into the vapour phase (Ryan et al., 2001). The images at right (of 14 elements in all) show Cu in one brine and one vapour inclusion. Both show clear images of Cu in solution (at room temperature) around a central vapour bubble (b), and also a large cubic halite daughter (h) in the brine, and a chalcopyrite daughter (cp) in the vapour. The concentration of Cu is 0.15 wt% and 2.5 wt%, in the infered homogeneous brine and vapour, respectively, indicating very strong partitioning of Cu into the vapour phase. These data provide a vivid demonstration of the strong copper partitioning effect first observed and quantified using PIXE at the CSIRO in 1992 (Heinrich et al., 1993). Note that solid phases outside the inclusion can be recognized and avoided using this PIXE imaging approach. That is, they are not included in the integration performed for quantitative analysis.

For further details, see: Notes on fluids inclusion samples for NMP analysis. Analysis of hydrothermal fluids in copper-gold deposits. More on the PIXE method for microanalysis of fluid inclusions. Imaging the associated sulfide minerals.

For further information contact: Dr Jamie Laird or Dr Chris Ryan +61-3-8344 8375 +61-8-6436 8586 CSIRO Earth Science and Resource Engineering c/o School of Physics, University of Melbourne, VIC 3010, Australia	CSIRO Earth Science and Resource Engineering

Further Reading

References

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