INITIAL MODELING OF THE EFFECT OF CO2 ON CHEMICAL WEATHERING OF SILICATE MINERALS, A FIELD APPROACH

Global warming trend models use the chemical weathering of silicate minerals as a sink for atmospheric carbon dioxide (CO2). The relationship between weathering rates and dissolved CO2 concentration is not well established for field conditions. Indian Wells Valley, CA is an area that is known to be geothermally active and water samples from the area display high concentrations of CO2. Satellite imagery and visual inspection of the geomorphology have indicated enhanced weathering of granite and granodiorite terrains. Analyses of stream chemistries in the area indicate a high variability of TDS that appears to be directly related to the spatial variability of CO2. Initial geochemical modeling with PHREEQC shows that the stream chemistry can be reproduced by varying the amount of CO2 as snow melt waters are reacted with bedrock minerals previously described such as albite, microcline, quartz, and biotite.

In order to develop site-specific models, detailed mineralogical characterization was performed using optical petrology and X-ray diffraction techniques. The results show that the most volumetrically primary important minerals are plagioclase (An30), hornblende, olivine and microcline. The weathering products from these minerals are kaolinite, amorphous silica and clays in the smectite group. It has also been shown that halite and gypsum are introduced into the system by dust deposition, these minerals are included in the models.