USING XENOLITHS TO INVESTIGATE MAGMA ASCENT: THE MAY 2010 STROMBOLIAN ERUPTION OF PACAYA VOLCANO, GUATEMALA

Pacaya volcano, Guatemala, has been erupting nearly constantly for the last 50 years. Understanding its behavior is very important as it lies at the southern edge of Guatemala City, with a population of ~2.5 million, and a major international airport. The violent Strombolian eruption of Pacaya in May 2010 included both lava flows and ballistic projectiles in the form of volcanic bombs. As basaltic magma traveled from Pacaya’s magma reservoir and through its conduit, rhyolitic xenoliths from the volcano’s basement were incorporated in the magma, and subsequently experienced varying degrees of mixing and mingling with the basalt. We collected six rhyolitic samples from Pacaya. Four were ejected in the form of ballistics and experienced rapid quenching once expelled. The two remaining samples were entrained in effusive lava flows from secondary conduits on the flanks of Pacaya and transported ~4 km in these flows. Backscattered electron images and plane-polarized light photomicrographs reveal a greater amount of dissolution taking place at the contact between rhyolite and basalt in the ballistic samples than at the contact in the effusive samples. Electron micro-probe spot analyses gathered across the contact between xenolith and basalt, illustrate this transition with matrix compositions varying smoothly from rhyolitic to intermediate to basaltic. X-ray mapping of elemental concentrations, reveals uphill diffusion of potassium into the silica-rich molten edge of the xenolith in all samples, with a longer diffusion lengthscale visible in the ballistic samples than the lava flows. Since ballistic samples cooled rapidly after ejection, they must have remained in the conduit for longer than the effusive samples. We infer that magma which had already entrained xenoliths was stored in a shallow reservoir prior to the violent Strombolian eruption, while subsequent effusive flows were fed by magma that ascended rapidly through the conduit without stalling.