Rare gases in cyclosilicates and cogenetic minerals

Abstract

The component and isotopic composition of the rare gases in beryls, cogenetic pegmatite minerals, and metamorphic cordierites has been measured. Beryls contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases. Beryls contain the most pristine samples of the rare gases in pegmatite volatile phases but have imposed significant elemental fractionations during crystallization. Both the excess radiogenic and nonradiogenic components are progressively depleted in the heavy gases relative to the light gases. Other pegmetite minerals contain rare gases with much smaller excess radiogenic components and apparently did not sample the same source as did the beryls. The other minerals exhibit varying but mineral‐specific fractionation patterns for the nonradiogenic rare gases. Cordierites also contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases. In addition, cordierites show a strong, consistent fractionation of both gas components. The fractionation pattern is significantly different from that found in beryls and is characterized by an enrichment of Xe relative to the Ar, though both gases are depleted relative to He. The cyclosilicates can serve as probes of the rare gases present in the volatile phases when the cyclosilicates crystallized. The cyclosilicates contain a record of the rare gases present in geologically interesting environments over a wide range of time and physical conditions.

Noble Gas Reactivity in Planetary Interiors

“…These minerals are not occurrences of noble gas reactivity at depth, but are essential carriers of noble gas recycling via subduction of lithospheric plates [36,37], from the surface reservoirs (atmosphere, oceans) to the deep Earth where reactions may take place. Natural occurrences of He and Ar in cyclo-silicates are abnormally high [38]. He, Ne and Ar occupy ring sites in amphibole [37], a mineral formed in the altered oceanic crust, and experimentally measured solubilities are up to four orders of magnitude higher than for other silicates at similar run P (0.17 GPa).…”

Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: Insights from the Rum layered intrusion, Scotland

“…3d). Of the three possible K-bearing crustal sources of the excess 40 Ar (Archean gneisses, Torridonian sandstones and Mesozoic sediments), it is worth noting that the Torridonian sediments locally contain tourmaline, which is known to host significant 40 Ar (several orders of magnitude greater than co-genetic crustal minerals; Saito et al, 1984). Together with the fact that many of the Torridonian sandstones are arkosic (K-rich), they represent at least one potential source for the high 40 Ar concentrations observed in the RLS rocks.…”