THERMAL ALTERATION OF SEDIMENTS BY BASALTIC FLOWS: GEOTHERMOMETRY BY DIFFUSE REFLECTANCE SPECTROPHOTOMETRY

When lava flows over rock or sediment it thermally alters surface and near-surface materials. On the Taos Plateau, NM, there are numerous exposures of Servilleta Basalt overlying thermally altered Santa Fe Group sediments. This thermal alteration is marked by a red color directly under the flow that gradually changes to orange and yellow as distance beneath the flow increases. In this talk we examine five exposures, four in the Rio Grande Gorge near Arroyo Hondo and one in the canyon of the Rio Pueblo de Taos, that exhibit these thermally induced color changes. The color changes are indicative of the production of iron oxides and oxyhydroxides, mainly hematite and goethite. We analyzed samples from vertical profiles of the exposures with diffuse reflectance spectrophotometry (DRS) because it is extremely sensitive to hematite and goethite and is capable of determining the concentration of these minerals to <0.03% by weight. Results of these analyses demonstrate that those beds immediately below the flow are rich in hematite and the amount of hematite decreases and goethite increases as distance beneath the flow increases. Sediment unaffected by thermal alteration contains no hematite and less than 0.05% goethite.

To determine alteration temperatures under flows we collected samples of unaltered sediment and thermally altered them in the laboratory by heating in a furnace. Temperature profiles under these flows were obtained by comparing the spectra of the naturally thermally altered sediments to laboratory heated samples. Two end points are clearly defined; no alteration occurred at temperatures <300°C and samples melted to a black glassy mass >1175°C.

Comparison of the natural to laboratory thermally altered samples indicates that flow temperatures immediately above the altered sediment varied from 1075°C to 915°C and agrees well with mineral assemblage emplacement temperatures. The thickness of the thermally altered zone beneath the flow varied from ~1.4m to ~0.4m, although the complete alteration sequence was not exposed at every section. Given the temperature profile under the flow and the thickness of the altered zone it may be possible to use thermodynamic relationships to estimate flow duration, a measure not easily determined by other means.