Oxygen and strontium isotope studies of K-rich volcanic rocks from the Alban Hills, Italy

Abstract

18O/16O and 87Sr/86Sr ratios and major and trace element contents were measured for 33 leucite-bearing lavas from the Alban Hills, located just south of Rome. Petrologically, this volcanic center is the least complex of all the Pleistocene to Holocene volcanoes in the Roman Comagmatic Province, and the 87Sr/86Sr uniformity reflects this (0.71024–0.71091). Whole-rock δ18O = +5.6 to +9.8, but many samples were enriched in18O by post-eruption hydration, evidenced by the good correlation between H2O content (up to 5 wt. %) and δ18O. Correcting for these effects, we obtain a δ18O range of +5.6 to +7.8 for the original magmas. No other volcanic center in the Roman Province displays such uniform strontium and oxygen isotopic compositions; thus, this volcano provides special insight into the origin of the High-K Series magma end-member in Central Italy. Three groups of lavas are recognized on 87Sr/86Sr-1/Sr, δ18O-87Sr/86Sr, and K2O-SiO2 graphs; all of these groups, as well as the major caldera-forming eruption (Villa Senni Tuff), are derived from a very uniform, LIL-enriched, metasomatized source in the upper mantle. This source had δ18O = +6.5 ± 1.0 and 87Sr/86Sr= 0.71030 ± 0.00010. The δ18O values correlate positively with 87Sr/86Sr, indicating minor interaction with the continental crust. Essentially all chemical and isotopic variations in the primitive (low-SiO2, high-Ca, high-Sr) potassic lavas in Italy can be explained by mixing in the upper mantle between this Alban Hills end-member and a Low-K Series Roccamonfina-type end-member.

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