Partitioning noble gases

Vapor-melt partitioning. Noble gas solubility in basaltic melt decreases with increasing atomic mass, and is directly related to the atomic radius of the gas (Jambon et al. 1986 Lux 1987 Broadhurst et al. 1992 Shibata et al. 1998). At 1400°C the experimentally determined values for mid-ocean ridge basalt are 56, 25, 5.9, 3.0 and... 

There are no noble gas partitioning data for amphiboles. Given the multiplicity of cation and anion sites in this mineral it seems likely that Uru will be higher than in other silicate minerals. 

There are no noble gas partition coefficients for phlogopite. The large size and low mean charge of the large X-site (Zg )) suggest that noble gases could be readily incorporated into phlogopite. 

Partition (Distribution) Coefficients In describing the partitioning of a trace element among coexisting phases, we frequently use a partition (distribution) coefficient for a given element, defined as a concentration ratio C2/Cj. Here C is concentration, and the subscripts identify the phases often the normalizing phase is some convenient reservoir, such as a silicate melt, with which several other phases may equilibrate. For noble gases, it is often most convenient to normalize to a gas phase. If the concentrations are expressed in the same units, the distribution coefficient is dimensionless. It is conventional to cite noble gas concentrations in condensed phases in cm3 STP/g, however, and to describe the gas phase by partial... 

The crystal-melt partition coefficient KD = CJCh where Cs is concentration in a solid and Q is concentration in coexisting liquid, is a key parameter in trace element studies of igneous systems. A noble gas crystal-melt partition coefficient is the ratio of the gas solubilities considered here. As seen in Table 2.3, solubilities have now been reported for a variety of melt compositions, but solubility data are still very scarce for solids in general. 

Hiyagon and Ozima (1986) employed a laboratory approach of measuring crystal-melt partition coefficients. They measured noble gas concentration in olivine crystals and basalt melts, which were synthesized at 1370-1300°C under an atmospheric pressure, and also at 1360-1050°C under high pressure (0.2-1.5 GPa), of noble gas mixture. From these experimental results, they obtained ranges for noble gas partition coefficients XHe = 0.07, XNe = 0.006-0.08, KM = 0.05-0.15, KXe = 0.3. These partition coefficients are much larger than the values obtained by Marty and Lussiez (1993) and also these of common incompatible elements such as U (-0.002) or K (0.0002 - 0.008) between olivine and basalt melt (e.g. Henderson, 1982). 

A common difficulty in these partition experiments, with the use of either natural or synthesized samples, is in achieving perfect separation of the melt from the crystal phase for determining the noble gas content. Even a very small amount of glass (melt) contamination in crystal would increase the partition coefficient considerably, since noble gasses are much more enriched in glass. To circumvent this difficulty, Broad-hurst et al. (1990, 1992) prepared natural minerals and synthetic silicate melts that... 

Matsuda et al. (1993) studied noble gas partition between iron melt and silicate melt at 1550°C for pressures ranging from 5 to lOOkbar. Noble gas partition coefficients thus determined showed a systematic decrease with increasing pressure (Figure 2.7), from about 5 x 10 2 at 5kbar to about 5 x 10 4 at lOOkbar. The results may... 

Unlike elemental concentrations, isotopic compositions are only affected a little by chemical differentiation processes. Mass-dependent isotopic fractionations can arise in chemical partitioning (cf. Section 2.9), of course, but on the scale of interest in the present context, plausible fractionation effects are small, especially at the high temperatures prevalent in the mantle. We can thus be much more confident that a noble gas isotopic composition measured in a mantle-derived sample is indeed characteristic of its mantle source. Representative mantle ranges for selected isotopic ratios are presented in Table 6.2. 

Even if we accept that noble gases are mostly partitioned in magma and then effectively transported to the surface, we must further examine whether or not noble gases can be readily released from magma into the atmosphere once magma reaches the surface. Effective noble gas degassing mechanism from magma may be due to bubble... 

Matsuda, I, Sudo, M., Ozima, M., Ito, K., Ohtaka, O., Ito, E. (1993) Noble gas partitioning between metal and silicate under high pressures. Science, 259, 788-90. 

Shibata, T., Takahashi, E., Ozima, M. (1994) Noble gas partition between basaltic melt and olivine crystals at high pressure. In Noble Gas Geochemistry and Cosmochemistry, J. Matsuda, Ed., pp. 343-54. Tokyo Terra Scientific Publishing Co. 

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