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

Barva volcano is a dormant stratovolcano in the Cordillera Central region of Costa Rica, rising above the capitol city of San Jose with its 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 will help constrain the recent volcanic stratigraphy of units seen in the quarry and provide information regarding future eruptive behavior.

A complete petrographic and geochemical analysis of nine samples from young lava flows obtained from Tajo La Florida quarry on the flank of Barva has been completed. High resolution backscatter images were obtained on the Phenom Pro SEM and major/minor element analysis were collected on the Cameca SX-100 electron probe, both at University of Tennessee. All lava flow samples are basaltic-andesites; minimal major element variations indicate some level of magma evolution between flows. Observed phase assemblages consist of phenocrysts of plagioclase (11.2 mass%), two pyroxenes (3.6 mass%), and FeTi oxides (<1 mass%) within a groundmass of glass and plagioclase microlites. Groundmass glass compositions range from andesite to rhyolite, with large variations between samples. Glass volume is also variable, with some groundmass nearly glass-free. Mass balance calculations of groundmass composition (glass+microlites) indicate little variation from rock bulk composition, as evidenced by the low phenocryst contents. Several flows have features indicating disequilibrium conditions, including 1) glomerocrysts composed of plagioclase, pyroxene, and small amounts of olivine and 2) clinopyroxene coronas encasing orthopyroxene cores. Trace element data to be obtained Summer 2021 will aid in more precise geochemical differentiation between flows. The data collected will yield new insights into the magma storage system beneath Barva volcano.