CLINOPYROXENE CHEMISTRY OF THE HIGH-POTASSIUM SUITE FROM THE ALBAN HILLS, ITALY

Chemical data on clinopyroxene phenocrysts in twenty-four lava samples from the Alban Hills (Roman comagmatic region) show coexistence, within the same rock, of two core-rim evolution trends: diopside-salite and salite-diopside, respectively. The Alban volcanics can be divided in two groups depending on which type of core predominates. Geochemical mixing tests, conducted with elements showing a different degree of incompatibility, such as Ce, Sr, Th, La, Ta, and Hf, show no evidence of a mixing process which might be responsible for coexistence of both diopside and salite clinopyroxenes within the same lava. Taking into account the results of known experiments on the influence of water on clinopyroxene composition in potassium rich lavas, the reverse zoning trend observed and the consequent predominance of salite cores in some lavas are ascribed to the effect of volatiles, and particularly to water. Changes of water pressure may also be responsible for phenocryst corrosion and salitic clinopyroxene replacement by olivine, phlogopite and titaniferous magnetite. The prominent role of water in the Alban Hills magma evolution is also supported by the high fluorine content found in the Alban products, which enhances water solubility in the magmas, by the frequent occurrence of mica in the rock groundmass and, finally, by the explosive character of Alban volcanism. © 1988 Springer-Verlag.