Fluid-magmatic interactions at oceanic islands as a possible source for the sodic agpaitic Trend

Experiments on spilite melting at P = 3 kbar and T = 950−850°C indicate that partial melting in the presence of aqueous fluid (P_{H2 O}P_{H_2 O} = 3 kbar) produces a series of melts of normal alkalinity with up to 77 wt % SiO₂. The partial melts derived from spilite in the presence of aqueous fluid generated in the system NaCl-CaCO₃-H₂O have an agpaitic coefficient increasing to a level sufficient for the origin of alkaline granite in the course of fractional crystallization. An increase in the alkalinity of the melt is explained by soda synthesis in the fluid due to an exchange reaction between CaCO₃ and NaCl. In contrast to NaCl, soda in highly soluble in aluminosilicate melts. We discovered that partial melting in the presence of a soda-bearing fluid results in BaO concentrating from a level of 0.05 wt % in the original spilite to 1 wt % in the partial melt. Conceivably, magmatism in the environment of oceanic islands can occur under conditions favorable for the synthesis of soda-bearing fluids. CO₂ that carbonizes the mafic rocks is provided by mantle plumes, whereas seawater penetrating to depths greater than those of intermediate chambers serves as a source of NaCl. The activity of a fluid-magmatic system is able to generate subalkaline sodic rocks, such as benmoreite and pantellerite.