Chlorine, Sulfur, and Water in Magmas and Oceans

Abstract

The concentrations of chlorine, sulfur, and water in basaltic and andesitic glasses trapped in large crystals are commonly 5 to 20 times as high as those found in vesicular glasses and bulk lavas. Chlorine and sulfur concentrations in trapped glasses range from 100 to 2,200 ppm and from 50 to 2,800 ppm, respectively. Vesicular glasses contain 100 to 600 ppm chlorine and 50 to 400 ppm sulfur. The chlorine-rich trapped glasses tend to have high water contents, from 1 to 7 percent by weight. In general, the basaltic trapped glasses have as high or higher concentrations of chlorine, sulfur, and water as do the andesitic glasses. For individual eruptions, water-rich magmas (Mount Shasta, California) lose an appreciable fraction of their chlorine prior to extrusion, whereas water-poor magmas (Kilauea, Hawaii) lose little if any chlorine, even after extrusion. Apparently, water acts as a carrier gas and distills chlorine out of magmas as it boils away. The behavior of sulfur is complex and appears to reflect pre-eruption saturation with respect to a sulfide melt and strong loss of sulfur as SO 2 during eruption (the pre-eruption fugacity of SO 2 is about 60 atm for Kilauean fractionated basalt). The potential chlorine, sulfur, and water contents of andesitic magmas have been estimated by assuming derivation from basaltic precursors and extrapolating the volatile contents of related basalts along lines of constant ratios to K 2 O to 1 percent K 2 O. The potential volatile concentrations are: 0.08 to 0.66 percent Cl, 0.10 to 0.67 percent S, and about 5 to 15 percent H 2 O. Assuming that the estimated rate of production of continental crust in Cenozoic island arcs has been constant throughout earth history, the total amounts of Cl, H 2 O, and continental crust produced are within factors of 0.3 to 4 times the presently existing masses of these substances in the surface reservoir of sea water, sediment, and crust. The amount of sulfur produced is larger than that in the reservoir. Sulfur probably is less efficiently transferred into the surface reservoir than are chlorine and water. The data can be reconciled with models of recycling of volatiles through the mantle, but the near coincidence of residence times for chlorine, water, and crust in the surface reservoir with the age of the earth is not explained. Also, the equality between the C1/H 2 O ratio of oceanic tholeiite, Kilauean basalt, and H 2 O-rich Mount Shasta basalts with the same ratio for the surface reservoir is consistent with the idea of igneous derivation and permanent storage of Cl and H 2 O in the surface reservoir.