Volatile contents in subduction-related basaltic magmas from Central Mexico: considerations on mantle enrichment processes and low pressure magma degassing during basaltic eruptions

Abstract

Melt inclusions in olivine phenocrysts from primitive lavas from five different volcanoes in the Sierra Chichinautzin (Central Mexico) were analyzed by infrared spectroscopy and electron microprobe to determine their major element chemistry and their H₂O and CO₂ concentrations. The compositions of the inclusions range from basalt to andesite and include both alkaline and calc-alkaline varieties. Most inclusions have low H₂O concentrations (average values 0.4[]0.2 wt.% H₂O for Xitle, 0.6[]0.5 wt.% for Tuxtepec, 0.6[]0.3 wt.% for Ixcatepec, 0.8[]0.5 wt.% for Las Tetillas, 1.0[]0.3 wt.% for Jumiltepec) and no measurable CO₂. The lack of CO₂, which has strongly pressure-dependent solubility in silicate melts, and the low and variable H₂O contents indicate that low pressure degassing is a widespread phenomenon during basaltic cinder cone eruptions. One model is presented to explain the extensive low pressure degassing in a well studied volcano (Xitle). The model explains the formation of an initial tephra cone and its temporal growth by explosive events followed by a switch in eruptive style to effusive. The model is based on the fact that cinder cones are highly porous, which allows efficient degassing of reticulating, low viscosity basaltic magma. The presence of alkaline and calc-alkaline primitive lavas throughout the volcanic field indicates heterogeneities in source region compositions. Rare melt inclusions in the primitive lavas contain measurable CO₂ and were trapped at relatively high pressures (~1 to 3 kb). These inclusions constrain the primary H₂O contents of the magmas before low pressure degassing. The results clearly show that the calc-alkaline magmas contain higher H₂O (~2 to 4 wt%) than the alkaline (~1 wt.%). Calculations of a hypothetical mantle source and comparison with EMORB and NMORB mantle sources shows that these volcanics were formed by two different processes: (1) partial melting caused by fluxing of the mantle wedge with a water-rich component from the subjected slab (calc-alkaline magmas) and (2) partial melting of the mantle wedge by upwelling-induced decompression (alkaline magmas). These two processes can be considered as end-members in arc magma genesis.