Correlated Sr isotope and geochemical variations in basalts and basaltic andesites from Guatemala.

Abstract

Two features of the geochemical and isotopic variations between six composite volcanoes in Guatemala volcanoes are: 1) the magmas evolved in open-system magma chambers subjected to repeated cycles of magma replenishment and crystal fractionation before they were delivered to each volcano; 2) the 87Sr/86Sr ratios show positive correlations with the incompatible element abundances and ratios which imply that mixing or assimilation processes (not simple two-component mixing) have played a part in the high-P evolution of the magmas. Both features may be related to the longevity of the magma ponds in which these magmas evolved prior to their delivery to the individual volcanoes. This explanation is consistent with: 1) absence of correlation between 87Sr/86Sr ratios and major elements, 2) incompatible element excesses (e.g. excess Rb) are focused on the three volcanoes with the greatest volume (San Pedro, Atitlan, Toliman), 3) successful solutions to the least squares mixing calculations for most major elements imply that magmatic chemical evolution did not include addition of components with chemical compositions distinct from the high Al basalt- basaltic andesite spectrum or the linear extrapolation of that spectrum. -J.M.H.