| It is widely acknowledged that weathering and hydrothermal alteration can change the composition of volcanic rocks. However, the mobility of REE and other trace elements at the earliest stages of these secondary processes is not well understood. Rocks ranging in composition from basalt to dacite were sampled from three geologic units, ranging in age from 0.8 to 2.6 Ma, of the Tecuamburro volcanic complex. Most major elements, particularly Ca, consistently decrease in volumetric concentration from core to rind. Of the trace elements analyzed, only Th is immobile in all samples. Niobium is often depleted in the rind relative to the core, as is Sr. Cerium is generally immobile, while the other REE, Y, and Ba are sometimes enriched in cores and rinds, relative to fresh lavas from the region. This bulk rock REE-enrichment correlates well with the presence of secondary REE-phosphates. Literature suggests that a low pH is necessary for the mobility of the REE, and a high pH, produced by the weathering of primary minerals, provides the conditions for precipitation of the REE-phosphate. Other secondary products observed include clays, barite, hollandite, and gibbsite. This study has identified elements that are mobilized during incipient weathering, and the element enrichment and depletion trends presented should help igneous petrologists identify altered samples before they are used in petrogenetic studies. |
