Correlated Sr Isotope and Geochemical Variations in Basalts and Basaltic Andesites from Guatemala

Abstract

The incompatible elements in the high alumina basalts and basaltic andesites of six composite volcanoes in Guatemala are most abundant in the volcanoes of San Pedro, Atitlán and Tolimán which lie in the 84 000 year old cauldron of Lake Atitlán. Modelling of the major and trace elements, using the high alumina basalts of Fuego as the parental magma, indicates that the five other volcanoes contain excess incompatible elements but are deficient in Na. We infer the excesses to mean that the magmas have evolved through cycles of magma replenishment and fractional crystallization prior to their delivery to the individual volcanoes, probably in magma ponds at the base of the crust (Carr, 1983). The Na deficiency may reflect either the crystallization of sodic plagioclase as would be expected at high pressures, or the diffusional loss of Na as the magmas rose through continental crust.

The average ⁸⁷Sr/⁸⁶Sr ratios, which range from 0.70385 to 0.70405, correlate positively with average incompatible element abundances and Rb/Sr and incompatible element ratios, and negatively with Sr abundances for each volcano. Because the isotope ratios also correlate with the incompatible element excesses, it would appear the process adding ⁸⁷Sr to the magmas was linked to the process leading to incompatible element enrichments. We conclude that the explanation lies in the size and longevity of the magma ponds at the base of the crust, where longevity controls the length of time through which the assimilative addition of ⁸⁷Sr, and incompatible element enrichment through magma replenishment-fractionation cycles, have taken place. The data suggest that the chemical composition of the lower crust lies within the compositional spectrum of the erupted lavas and consists of gabbroic cumulates.

The chemical evolution of the magma ponds in or at the base of the crust may change abruptly after the volcanoes supplied by the ponds become dormant, because the magmas (previously buffered to uniform compositions by magma replenishment triggered by the removal of eruptive magma batches) would be free to evolve through fractionation and assimilation to silicic compositions. Silicic magmas may also have been produced by the partial melting of the lower crust, where the magmas ponded at the base have contributed the thermal energies for melting.