CI-BEARING AMPHIBOLE IN THE SALTON-SEA GEOTHERMAL SYSTEM, CALIFORNIA

Calcic amphiboles with up to 2.7 wt.% Cl occur in metasandstones, metabasites and veins at depths between 3,100 to 3,180 m and temperatures in excess of 350-degrees-C in the State 2-14 well of the Salton Sea geothermal system (California). These amphiboles were formed by reactions involving high-salinity geothermal fluids, with 15.4 to 19.7 wt.% total dissolved Cl. Coexisting phases include quartz, plagioclase, K-feldspar, epidote, clinopyroxene, apatite, and titanite. The Cl-bearing amphiboles range in composition from hastingsitic (Cl > 1 wt.%) to actinolitic (Cl < 0.5 wt.%). Texturally complex intergrowths of actinolitic and hastingsitic amphiboles occur at depths greater than 3,140 m, suggesting a miscibility gap between the two amphiboles. Measured compositional variations suggest a crystal-chemical control on the Cl content in the calcic amphiboles: (1) the chlorine content of amphiboles increases with increasing edenite substitution { [A](Na,K)[4]Al square -1Si-1}; (2) the maximum observed Cl content of an amphibole increases with increasing X(Fe2+) value. Comparison of Cl content in amphiboles from the Salton Sea geothermal system, submarine metabasites, skarns, high-grade metamorphic rocks and igneous rocks implies that a main factor in controlling Cl-for-OH substitution in amphibole is different in low- and high-chlorinity environments. In low-chlorinity environments, the Cl content of an amphibole increases with increasing chlorinity of the coexisting fluid, and is defined by partitioning of Cl between the two phases, as well as the crystal-chemical constraints imposed by (Na+K), Fe, and Al substitution. On the other hand, amphiboles coexisting with the Salton Sea and more saline fluids are enriched in Cl; Cl content strongly depends on X(Fe2+) and the edenite content of the amphiboles. They may achieve a maximum Cl content, and the extent of Cl-for-OH substitution is crystal-chemically controlled.