VOLATILE CONTENTS AND DEGASSING OF PRIMITIVE MAGMAS FROM THE WESTERN MEXICAN VOLCANIC BELT

Olivine-hosted glass inclusions from scoria cones in the Colima and Mascota volcanic fields of western Mexico were analyzed for volatiles. With compositions ranging from basanite to lamprophyre, these rocks are distinct from, yet intimately associated with the andesitic volcanism that is prominent in the area. We interpret the most primitive magmas as representing nearly pure partial melts of phlogopite-rich veins in the mantle, that with dilution by peridotite melting result in the calc-alkaline basalts that differentiate to form andesites. High concentrations of volatiles in some glass inclusions and estimates of magmatic fO₂ from bulk-rock Fe³⁺/Fe²⁺ and glass-inclusion %S⁶⁺ indicate a volatile-rich and highly oxidized (several log units above the Ni-NiO buffer) environment. The highest volatile contents (7% H₂O, 1460 ppm CO₂, ~2% SO₃^Total, 2400 ppm Cl, and ~1% F) were recorded for an inclusion from a Colima minette (phlogopite-bearing lamprophyre). A calculated gas saturation pressure of 6660 bars for this sample corresponds to a depth of 24.1 km. This magma can be modeled as a parent to most other glass inclusions under closed-system degassing with a high wt.% exsolved vapor. This model is consistent with curious inclusions with multiple bubbles, from a Mascota minette, that appear to represent entrapment of a foaming melt. It is notable that many of the inclusions from minettes contain 2% water or less. Given the wide variations in water content and relatively poor correlation with K, P, S, and Cl, diffusion of H through the olivine host may be partially responsible. However, it is reasonable to assume that glass inclusions get trapped at different stages along the ascent path. A value of 2% water corresponds to water saturation at a pressure of 415 bars (1600 m. depth). Published phase-equilibrium experiments on an olivine-bearing minette imply that phlogopite is stable down to pressures of 200-400 bars, allowing the possibility that these low-water inclusions were trapped at shallow depth. The range in volatile contents, the degassing model, and the presence of vesicular glass inclusions are consistent with olivine crystallization at depth (24 km), followed by rapid ascent and degassing, and then stagnation in the upper crust, where olivine continued to crystallize and trap inclusions of melt.