Provenance analysis of porphyritic volcanic materials in San Diego using portable X-ray Fluorescence

Abstract

Geochemical research using aphanitic volcanic rocks such as basalt and obsidian has long contributed to archeological understanding. Porphyritic materials have proven less amenable to methods of compositional analysis largely due to their complex structure. Under some circumstances, similarities in structure can mask geochemical signatures indicative of localized formations. Fine-grained volcanic materials comprise the majority of lithic assemblages in San Diego County, California, yet include a wide variety of geologic formations that each contains rocks with similar structural features and quality for producing lithic tools. This combination of diversity and overlapping structure have led to a dominant assumption that materials were either locally procured from the nearest available source of tool stone, or attributed to the Santiago Peak Volcanic formation as a known source of high-quality fine-grained volcanic materials. This study investigates the potential for using pXRF for provenance research on fine-grained volcanic materials in southern California. Results indicate that volcanic materials can be suitably discriminated using pXRF that sourcing porphyritic volcanic materials is possible and can be applied to archeological assemblages.