COMPARATIVE GEOCHEMICAL ANALYSIS OF RECENT VOLCANIC FLOWS ON BARVA VOLCANO, COSTA RICA

ABSTRACT

Barva Volcano is a dormant stratovolcano in the Cordillera Central region of Costa Rica, rising above the capital city of San Jose with a population of 347,000. Barva’s recent inactivity and extensive forestation have resulted in fewer investigations of its composition, petrogenesis, and eruptive history. The objective of this research is to analyze the geochemical data from several of the most recent lava flows, evaluate variability between eruptive units, and ascertain physical conditions within the Barva magma storage region. The data collected in this study helps constrain the geochemistry of units seen in the quarry and provides information regarding past and future eruptive behavior.

A complete petrographic and geochemical analysis of nine samples from young lava flows obtained from the Tajo La Florida quarry on the flank of Barva has been completed. SEM images and electron microprobe (EMPA) data were collected for major/minor element analysis at the University of Tennessee. All lava flow samples are basaltic-andesites, with observed phenocryst phase assemblages (averaging ~15 %) of plagioclase, orthopyroxene, clinopyroxene, and iron titanium oxides within a groundmass of plagioclase and pyroxene microlites, with some glass. Groundmass glass compositions range from andesite to rhyolite, with large variations between samples. While the percent of crystallization and the matrix glass composition in the samples vary, mass balance calculations of groundmass composition (glass plus microlites) indicate little variation from rock bulk composition. The collected trace element data also reinforces the homogenous nature of the described units. Two-pyroxene temperatures were calculated providing crystallization temperatures for both core and rims of phenocrysts. Transects from core to rim of a limited number of zoned pyroxenes provide relative cooling rates and indicate rapid cooling. The collected data all provide evidence of a stable magma system that exhibits evidence of a minor reheating event, which resulted in a cleansing of the magma chamber in multiple pulses over an unknown period of time.