Modeling alkali-exchange alteration in volcanic glass to obtain original rhyolitic melt compositions and magma storage pressures for the Permian Ora Ignimbrite (Italy)

Volcanic glass contains a wealth of information about the magma reservoirs that feed eruptions; however, glass is highly susceptible to secondary processes that modify original compositions. Here, we investigate well-preserved, glass-bearing fiamme from two vitrophyre horizons in the ~ 275 Ma Ora Ignimbrite, a crystal-rich (~ 40%), supereruption-sized (~ 1,300 km 3 ) rhyolite in northern Italy. Glass compositions from these fiamme present a paradox: average K 2 O concentrations are too low to crystallize sanidine (2.3—3.6 wt. % K 2 O); yet, all fiamme contain abundant euhedral sanidine and have glass compositions strongly depleted in Ba, consistent with sanidine crystallization. Glass data also show a strong negative correlation between K 2 O and Na 2 O, suggestive of alkali-exchange alteration. We test if 1:1 alkali exchange is a viable mechanism for reduction of K 2 O in the Ora glasses. Our results suggest that Ora glasses experienced “reverse” alkali-exchange alteration, in which K is replaced by Na on a 1:1 atomic basis. We generated a series of ten synthetic compositions for each fiamma by starting with the measured composition and iteratively adding 0.5 wt. % K 2 O and subtracting 0.33 wt. % Na 2 O at each step, following a 1:1 exchange on a molar basis of the oxides. We employ rhyolite-MELTS pressure calculations to find compositions consistent with equilibration between melt, quartz, plagioclase, sanidine, and a fluid phase. By progressively replacing K with Na in the glass compositions, the model returns compositions saturated in quartz + sanidine + plagioclase (Q2F), in agreement with the mineralogy and trace-element geochemistry observed in Ora fiamme. We thus present a new application of rhyolite-MELTS geobarometry that provides estimates of original glass compositions and magma storage pressures from Q2F-bearing rhyolitic fiamme for glass affected by Na–K exchange. These results demonstrate that subtly altered volcanic glass can still provide meaningful information about their magmatic origins.