Geochemical stratigraphy and magmatic evolution at Arenal Volcano, Costa Rica

Arenal has been active for at least the past 7000 years. Prior to 3000 years B.P. (before present), Arenal eruptive products consisted of lavas and tephras both with a mafic (basaltic andesitic) composition. At approximately 3000 years B.P. Arenal began producing two discrete tephra compositions, a mafic (basaltic) tephra and a silicic (andesitic to dacitic) tephra as well as the basaltic andesitic lavas whose composition falls into the gap between the two tephra compositions. The amount of phenocrysts in both the mafic tephras and, to a lesser extent, the lavas, has increased steadily over time, demonstrating a gradual increase in the amount of crystal–liquid fractionation. At approximately 3000 years B.P. the fractionation reached a threshold causing the production of silicic tephras in conjunction with the mafic tephras. Modal analyses show that while the mafic tephras become more crystalline over time, the silicic tephras have become glassier. These younger mafic tephras are mineral-rich residues, while the silicic tephras are the incompatible element rich melt. There is also an increase in the crystal–liquid fractionation within the magma forming the lavas and tephras. The concentration of incompatible elements in the lavas has increased over time, while it has decreased in the tephras. In addition to the crystal–liquid redistribution in the magmas at Arenal, there have also been changes in Arenalʹs source region. The amount of flux from the subducting slab and consequently the degree of melting increase up to approximately 3000 years B.P., and then begin decreasing.