MAGMATIC ENVIRONMENT PRODUCING TEXTURAL AND COMPOSITIONAL ZONING IN PLAGIOCLASE PHENOCRYSTS OF THE 1968-1996 ERUPTION AT ARENAL VOLCANO, COSTA RICA

Since 1968, Arenal Volcano, Costa Rica has undergone steady eruption maintaining a basaltic andesite composition ranging from 54 to 55 wt.% SiO₂. Lavas are mostly phenocryst-rich (40%) dominated by plagioclase (plag).

Plagioclase from 15 samples covering the period 1968-1996 was studied by Nomarski Differential Interference Contrast microscopy and core-rim microprobe traverses collected compositional data every 3-4 µm for most of 125 analyzed phenocrysts. Analyzed plag phenocrysts are between 90-3500 µm across, indicate normal or reverse An zoning, and each lava sample displays ~ same large range in textures and compositions (An₉₄-An₅₅).

Most phenocrysts contain either irregularly shaped mottled cores with average An from 89 to 81, or flat, euhedral cores with a slightly higher An (An_90-85). Zoning styles of overgrowth on cores include 1) oscillatory zoning: straight (An_77-68), convolute (An _75-68) or hieroglyphic (An_76-68), 2) pitted zones: boxy cellular (An _81-72), spongy cellular (An _86-75) or pitted texture (An_82-72) or 3) remain unzoned appearing as a texturally homogeneous growth zone (An_70-62). We favor continuous, sporadic, and rapid growth environments to produce homogenous zones, oscillatory zoning and boxy textures, respectively, whereas resorption conditions may create spongy and pitted textures.

Phenocrysts share the last crystallization episode resulting in a homogenous growth zone (5-20 µm) at the rim that is texturally flat but strongly zoned from An₇₀ to An₆₂. Evidently, phenocryst rim crystallization occurred concurrently to growth of compositional similar microlites. Conversely, vastly different and complicated growth histories of phenocrysts in individual lava samples suggest recurring and multiple growth environments juxtaposing phenocrysts during mixing as late as shortly before rim crystallization in final ascent during eruption. The chief control on plagioclase compositional variation appears to be Ca/Na of the melt and/or decompression during ascent. Strong variation in H₂O saturation levels appears unlikely because it would be inconsistent with overall strong decline from An₉₀ in core to An₅₅ in rim (also observed in a 90 µm micro-phenocryst), and preliminary data on glass in matrix and inclusions suggest H₂O saturated conditions for much of the compositional range.