Textural and Compositional Zoning of Plagioclase Phenocrysts of the Current Eruption at Arenal Volcano, Costa Rica

Since 1968, Arenal Volcano, Costa Rica has undergone 33 yrs of continuous eruption maintaining a basaltic andesite composition ranging from 54-55 wt.% SiO₂. Lavas are mostly phenocryst-rich (40%) dominated by plagioclase with lesser amounts of orthopyroxene, clinopyroxene, and titanomagnetite. Plagioclase from 20 samples ranging in age from 1968-1996 was studied petrographically using Nomarski Differential Interference Contrast (NDIC) microscopy. Core-rim microprobe traverses collected compositional data every 3-4 μ m for most of the 125 analyzed phenocrysts ranging from 90-3500 μ m in size. Plagioclase of each lava sample display almost the same large variation in compositions (An₉₄-An₅₅) and textural styles. The majority of phenocrysts are cored with mottled (patchy) or flat texture. Mottled cores are irregularly shaped ranging in An from 94-70, whereas flat cores are mainly euhedral with narrower An ranges (94-84 to 95-94). Zoning styles of overgrowth on plag cores include oscillatory zoning (straight, convolute or hieroglyphic), pitted zones (boxy cellular or spongy cellular) or remain unzoned appearing as a texturally homogeneous growth zone. Compositions of most overgrowths (excluding rim) are within An 85-65 and zoning can be normal to reverse. A minor amount of phenocrysts are oscillatory zoned from center to rim and are compositionally alike overgrowths of cored phenocrysts. Regardless of the crystal's interior, most phenocrysts have rims defined by one last growth zone (5-20 μ m). This growth zone is texturally flat but compositionally strongly zoned displaying a drop in An content from 70-57, similar to compositions of microlites in the groundmass. We favor continuous, sporadic, and rapid growth environments to produce homogenous zones, oscillatory zoning and boxy textures, respectively, whereas resorption conditions may create spongy textures. The large range of compositional and textural plag growth stratigraphies in single lava samples can only be the product of magma mixing which has juxtaposed various crystals as late as shortly before the last overgrowth crystallized during final ascent at the time of eruption.