PETROGRAPHY AND PETROLOGY OF A BASALTIC ANDESITE SAMPLE FROM ARENAL VOLCANO, COSTA RICA

Arenal Volcano, Costa Rica, has continuously erupted basaltic andesites of bulk compositional uniformity over the last 35 years of its current eruption phase. These phenocryst-rich andesites contain plagioclase, clinopyroxene, orthopyroxene, spinel, titanomagnetite and may also contain olivine. Individual crystals usually have compositional zoning and mineral inclusions that represents different crystallization environments as ascending mafic magma becomes more andesitic through assimilation of country rock, fractionation and mixing. Plagioclase phenocrysts are the most common (68% abundance) and are texturally and compositionally complicated, ranging from An₉₄ to An₅₅. They have mottled to “flat” cores with overgrowths of oscillatory zoning that may exhibit reverse zonation. Clinopyroxene phenocrysts (8% abundance) are also compositionally complicated and show complex oscillatory zonation and inclusions. Most have Fe-rich outer rims. Orthopyroxene phenocrysts (12% abundance) show little compositional variation, but most have inclusions and some degree of oscillatory zoning. Most show Fe-rich outer rims. Olivine is rare (<1%) and occurs as orthopyroxene rimmed phenocrysts or as inclusions in clinopyroxenes. Spinel usually occurs as inclusions in clinopyroxene phenocrysts, but may occur as individual phenocrysts. Groundmass (10%) consists of residual glass, with microlites and microphenocrysts of mainly plagioclase, with lesser amounts of clinopyroxene, orthopyroxene, and titanomagnetite. Thus the individual phenocrysts reveal a very complex nature to the generation of magmas in individual eruption events that the whole-rock composition does not.