IDENTIFICATION AND CORRELATION OF VOLCANIC DEPOSITS BY MINERALOGY, GEOCHEMISTRY, AND LUMINESCENCE AGE ANALYSIS, WITH IMPLICATIONS FOR ASSESSING VOLCANIC ACTIVITY AND RISK: THE CASE OF TWO DORMANT VOLCANOES IN COSTA RICA

Understanding risk in volcanic landscapes requires dating and correlation of volcaniclastic deposits to facilitate determination of recurrence intervals or probabilities, but the lack of dateable organic matter often limits age analysis. Recent advances in luminescence dating of plagioclase and the need to understand risk provide opportunity and impetus to date pyroclastic flows deposits, lahars, lava flows and ash fall deposits on the flanks of two volcanoes in Costa Rica, Barva and Miravalles. Barva looms over the Valle Central, home to 2.5 million people, and field relations associated with the Monte de la Cruz cinder cone suggest that the most recent activity likely was in the early Holocene. Miravalles is in the rural north of Costa Rica, and ages of a few lahars are known, ranging from 300 to 2200 years ago based on ¹⁴C dates from logs and buried soils; many lahars, however, remain undated due to lack of organic matter. In total, 58 samples from the two volcanoes were obtained for XRD, XRF, VSWIR and ICPMS analysis of mineralogical and geochemical signatures for correlation, and a subset of 16 samples are being dated by luminescence. Many of the samples are minimally weathered paleosols that occur below lahars, lava flows or ash deposits -- these paleosols generally contain A horizons and minimally developed B-C horizons, indicating sufficient exposure at the surface to reset the luminescence signal prior to deposition of overlying lahars, lava flows or ash. The approach to dating in this study is to determine the age of burial of a given paleosol or pre-existing land surface to date the overlying deposit. Unweathered plagioclase is required for this approach, so powder XRD was used to identify and quantify plagioclase; in two cases, plagioclase from buried paleosols was too weathered (to halloysite) to qualify for dating, but the majority of the paleosols contain plagioclase sufficiently robust for luminescence analysis. XRD, XRF and VSWIR preliminary analysis indicate that compositional variations (mainly due to varied degrees of chemical weathering) facilitate recognition and correlation of deposits across the landscape which will augment luminescence age analysis in understanding timing and extent of erosional and depositional events on the flanks of Barva and Miravalles volcanoes.