CHEMICAL WEATHERING RATES OF OLIVINE AND SERPENTINE IN NATURAL ENVIRONMENTS

Chemical weathering is an important process that is a critical component in soil formation, global carbon cycles, and controlling the chemistry of natural waters. With growing concerns due to global climate change, it is important to understand the dissolution of mafic minerals because they consume a significant amount of carbon dioxide during the weathering process. Therefore, quantifying the field dissolution rates of the mafic minerals olivine and serpentine may help in our understanding of natural atmospheric carbon dioxide uptake.

Olivine xenoliths were collected from the Black Rock Flow basalt flow in central Nevada. Scanning electron microscopy (SEM) was used to identify and measure weathering pits on olivine xenoliths in the basalt. The rate of olivine dissolution will then be calculated using the volume of the etch pits, as measured by SEM, and the age of the basalt exposure previously measured by cosmogenic dating.

Serpentine soils and serpentinite rock were collected in depth profiles from the Trinity ultramafic body of the Klamath Mountains in northern California. Chemical and mineralogical analyses of the serpentine soils and parent rock include X-ray fluorescence (XRF), X-ray diffraction (XRD), SEM, energy dispersive spectrometry (EDS), and biological activity reaction tests (BARTs^TM). Results from XRD indicate lizardite in the parent material, and lizardite, smectites, and iron oxides in the weathered material. The bulk chemistry of the soils, as measured by XRF, was normalized to an assumed immobile element and to concentrations in the parent rock. Normalized concentrations of Si, Fe, and Mg throughout the soil profile are depleted relative to the parent rock, indicating the dissolution of parent minerals. Depletion profiles were used to calculate a field weathering rate for serpentine.