VOLATILES IN ALKALINE MAGMATISM

Gases extracted during fusion of volcanic glasses substantiate other sources of evidence that alkaline magmatism is distinguished from calc-alkaline by higher CO2 + SO2 contents, with relatively smaller proportions of H2O. In addition to higher C and S activity, alkaline magmas are characterised by high Cl and F, which are released with alkalis during fusion experiments, in a graphic reminder that alkalis should be regarded as mobile elements in magmatism. The association of C, S and halogens in alkaline magmatism is well shown in some high temperature residua, characterized by assemblages such as calcite-vishnevite. Calcium is an important cation in many aspects of alkaline magmatism and the links with carbonatite further emphasize the key position of C, S, Cl and F throughout and the subordinate role of H2O.