Basaltic island sand provenance

The Hawaiian Islands are an ideal location to study basaltic sand provenance in that they are predominantly composed of basaltic flows and pyroclastic deposits that are exposed in arid to humid climates on progressively older islands. The major components of Hawaiian beach sands are calcareous bioclasts, volcanic lithic fragments, and monomineralic grains of dense minerals and plagioclase. A small to moderate percentage of volcanic lithic fragments are altered to iron oxides and/or clay minerals. The relative proportions of bioclastic, unaltered, and altered volcanic grains in these samples are highly variable, but are generally a function of island evolution, reflecting the degree of reef development and the intensity and duration of weathering and erosion. Hawaiian volcanic lithic fractions consist of brown and black glassy volcanic fragments exhibiting vitric, microlitic, and lathwork textures and "other" holocrystalline volcanic lithic fragments. The proportions of these volcanic lithic types do not appear to vary systematically for sands with tholeiitic versus alkalic basalt provenance. Although these basaltic sands compositionally overlap sands from undissected magmatic arcs on traditional ternary plots (e.g., QFL, QmPK, LmLvLs), they can be compositionally and texturally distinguished from sands derived from island arc volcanic sources. In general, Hawaiian basaltic sands contain more olivine/pyroxene grains, fragments of tachylitic black glass, holocrystalline volcanic lithic fragments, and volcanic grains exhibiting lathwork textures.