CONTRASTS IN TRACE ELEMENT AND REE BEHAVIOR DURING INCIPIENT WEATHERING OF BASALTIC-DACITIC LAVAS IN THE DESCHUTES WATERSHED OF CENTRAL OREGON

The trace element and REE signatures of eleven basalt, andesite, and dacite clasts (48-65 wt% SiO₂) from unburied Holocene and Pleistocene lava flows in the Deschutes River Basin of central Oregon suggest a systematic difference in incipient physical and chemical weathering, correlated to silica content. The eastern side of the High Cascades of central Oregon is considered a high desert due to the overall elevation (>3000 m) of the region and the location within the rain shadow (40 cm rainfall/yr) of the N-S trending mountains. Due to the young age of the lavas and relatively arid climate, the degree of physical and mineralogical weathering is not significant. Basaltic clasts (<52 wt% SiO₂) have weathering rinds up to 5 cm, characterized by a color change and breakdown of olivine and plagioclase phenocrysts. The interiors of these basaltic clasts have not experienced mineralogical weathering. More silicic clasts (>54 wt% SiO₂) do not exhibit weathering rinds or visible mineralogical changes in either fresh or weathered lavas. With respect to major element chemistry, none of the lavas show depletions in the elements generally considered mobile during weathering (i.e. Mg, Ca, Na). However, more interesting trends are apparent in the trace and REE chemistry. The weathered basaltic samples show depletion in most mobile trace elements (Sr, Ba, Rb) and REE compared to unaltered cores/clasts, whereas weathered basaltic-andesites, andesites, and dacite clasts are enriched in trace and REE compared to more pristine, fresh samples. The enrichment in REE concentration from silicic fresh to weathered clasts is more pronounced in the light rare earth elements (LREE), which are enriched ~40-80 times chondrite values, whereas the heavier rare earth elements plateau ~5-15 times chondrite values. The transition in both trace and REE behavior appears to occur between 51 and 53 wt% SiO₂, with weathered basaltic samples showing depletions and more silicic weathered samples showing enrichment relative to unweathered clasts. Possible sources of the enrichment of select trace and REE are secondary phosphate minerals, clay minerals from feldspar breakdown, and glass alteration, which may be more significant in more silicic lavas.