Mineralogy and geochemistry of sands from Playa las Golondrinas, Puerto Rico: an approach to establishing a geogenic background

Abstract

Sands from the dune, berm, and shore environments at Playa las Golondrinas (18° 30′ 51″ N, 67° 3′ 26″) were investigated to explore how beach sands could be applied as a potential environmental (geogenic) background for the local region. Grain size is dominantly unimodal classifying as fine to medium sand. Hydraulic conductivity values range from 1.07 cm/s (berm) to 1.49 cm/s (shoreface). Sample mineralogy as constrained by X-ray diffraction (XRD) reveals a dominance of quartz and feldspar with minor Mg-calcite, pyroxene, and olivine. Light microscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM–EDS) support XRD data and indicate the presence of oxide-bearing lithic fragments in addition to biologic materials (e.g., corals. forams). Reflective spectra are consistent with XRD and microscopy. Bulk element concentrations determined using inductively coupled plasma—mass spectrometry (ICP-MS) are consistent with derivation from the arc-related rocks of Puerto Rico’s interior exhibiting LILE enrichment, Pb-enrichment, and associated Nb–Ta depletion. The majority of the bulk elemental concentrations are below those of average upper continental crust (UCC) values and element co-variation trends (e.g., wt. % Fe2O3 vs. As) are interpreted as geogenic in origin. Berm sands are enriched in Fe, Mn, Ni, Cr, V, and As compared to dune and shore samples and this signature is interpreted as being from a wind-driven winnowing effect. The exact form of As (As3+ or As5+) remains unconstrained and thus it is unknown if As is mobile in this environment. Reflective spectra, supported by grain size, mineralogy, and bulk chemistry, enables future remote sensing investigations by providing detailed constraints on sand in environmentally sensitive areas. This study therefore provides local context for metal pollution studies across the region.