Incipient weathering in silicic rocks indicated by enrichment of ree and trace element concentrations : the High Cascades, Oregon
Date of Award
Union College Only
Bachelor of Science
lava flows; geochemical analysis; geology; samples
Andesite and dacite clasts from unburied Holocene and Pleistocene lava flows in the Deschutes River Basin, Oregon were studied to determine the geochemical and textural signatures of incipient weathering. Through a range of techniques including point counts, photomicrographs, SEM elemental mapping, density measurements, porosity estimates, and geochemical analysis, several weathering signatures were identified. Forty samples were obtained from flows up to .78 million years old in the Oregon High Desert, which receives roughly 20 cm of rainfall throughout the year. Photomicrographs showed no evidence of mineral or glass alteration and major element geochemical analysis showed no significant change from fresh to weathered samples for any of the major mobile elements. However, porosity increases as much as 3 to 20%. Change in density has an inverse relationship to the change in porosity decreasing up to 0.32 g/cm3 from fresh to weathered. Trace element analysis indicates significant enrichment in the weathered clast of trace elements Ba, Sr, Pb, Zr, and Y, and the Rare Earth Elements (REE) compared to the fresh clast. Sr concentrations are enriched as much as 2.5 times and Ba as much as 2.5 in weathered clasts relative to fresh. Enrichment in REE concentration from fresh to weathered clast is more pronounced in the light rare earth elements (LREE), which are enriched to about ~40 to 80 times chondrite values, whereas the heavier rare earth elements plateau at about ~5 to 15 times chondrite values. Possible sources of the enrichment of select trace and REE have been identified as secondary phosphate minerals, clays, and high pH rain water. Since no major mobile element and textural evidence exists for weathering in the samples collected for this study, trace elements could be a useful tool to identify incipient weathering.
Kissane, Matthew T., "Incipient weathering in silicic rocks indicated by enrichment of ree and trace element concentrations : the High Cascades, Oregon" (2010). Honors Theses. 1162.