Slab-derived fluids and partial melting in subduction zones: insights from two contrasting Mexican volcanoes (Colima and Ceboruco)

Abstract

Colima and Ceboruco are the only two historically active volcanoes in the western portion of the Mexican Volcanic Belt. Contrasts in eruptive style and mineralogy indicate that magmas from Colima are significantly more hydrous. Eruptions at Colima are commonly explosive, and the volcano is blanketed by a thick sequence of pyroclastic deposits. In contrast, only one major pyroclastic eruption is known for Ceboruco. Hornblende is a common mineral at Colima, but is extremely rare at Ceboruco. These volcanoes also display strong contrasts in whole-rock, mineral, and groundmass compositions. This study compares pre-eruptive temperatures, oxygen fugacities, and water contents calculated for andesites-dacites from Colima and Ceboruco, along with published whole-rock chemical compositions. Temperatures were estimated from pyroxene thermometry, oxygen fugacities were inferred from whole-rock Fe2O3/FeO values, and water contents were calculated from plagioclase-groundmass compositional relationships. At a given whole-rock SiO2 content, magmas from Ceboruco have significantly higher pre-eruptive temperatures and significantly lower water contents. The Ceboruco magmas are also less oxidized, with calculated oxygen fugacities about 1 log unit ƒO2 below those for Colima. At a given SiO2 content, whole-rock samples from Ceboruco are strongly enriched in Ti, K, P, Rb, Zr, Ba, La, Yb, Hf, Th. U. and other incompatible trace elements compared to samples from Colima, but the latter have higher values of many ratios that are diagnostic of subduction-zone magmas: Ba/La, Sr/La, K/La, Rb/La, K/Th, and K/Ti. Thus, water does not behave as an incompatible “element” in these systems. The source regions for magmas from both Colima and Ceboruco are presumed to lie in the mantle wedge above the subducting slab. The differences between these two volcanoes indicate that the mantle wedge beneath Colima is more strongly affected by fluids rising from the slab; perhaps because Colima lies closer to the trench than any other Mexican volcano. The mantle wedge beneath Colima is apparently strongly “polluted” by K, Ba, Sr, and Rb relative to other elements, and strongly hydrated, which leads to higher percentages of partial melting compared to the Ceboruco source. Consequently, the parental melts to the Colima suite have relatively lower absolute abundances of incompatible elements, but higher ratios of K, Ba, Sr, and Rb to La, Th, Ti and other elements. The higher melt fractions in the Colima source region are also evident in the considerably higher magmatic eruption rate compared to Ceboruco, and the hydrous nature of the Colima magmas is manifested in the abundance of hornblende and frequency of explosive eruptions. The higher temperatures of Ceboruco magmas probably reflect relatively greater depths to the slab and the overlying mantle source region.