Fluid inclusions as hosts for noble gases

Within the mantle. He in fluid inclusions is more likely to be preserved in place than is He in the mineral lattice. He diffusion coefficients in clinopyroxenes and amphibole (two major U-bearing minerals) are 10 cmVsec at ISOO C (e.g., Lippolt and Weigel 

Inter-mineral differences. Systematic differences between He concentrations in coexisting mineral phases have been observed (Fig. 1). Olivines in samples from E Africa and SE Australia were generally found to contain less He than coexisting clinopyroxenes 

Trapping pressures. A common observation in fluid inclusion studies is that inelusions reeord maximum trapping pressures equivalent to depths that are mueh shallower than the souree region of the xenoliths (Roedder 1984 Andersen and Neumann 

There are several observations that can be considered in evaluating the circumstances in which the C02-rich fluids in xenoliths were introduced and whether the host magma was the source of volatiles. 

In order to introduce inclusions, sufficient shear stress must be applied to the xenoliths within the magma to create fractures. It is not clear under what circumstances this can be done without destroying the xenolith. CO2 bubbles within the magma also must be available on the xenolith surfaces to enter propagating cracks. It is more likely that in many cases the fracturing and fluid introduction occur prior to xenolith entrainment, either immediately before xenolith entrainment, or by an earlier sequence of fluid invasion of the xenolith source region. 

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