MICROCT IMAGING OF TRAVERTINE SEEP DEPOSITS ASSOCIATED WITH SERPENTINIZING GROUNDWATERS

Serpentinization, the aqueous alteration of ultramafic rock units, is an exothermic, volume-increasing process. In subsurface environments, the alteration of ultramafic rocks is known to generate hydrogen and complex organic compounds, making this geologic process of interest regarding the sustenance of Earth’s deep, chemosynthetic biosphere. When serpentinization-associated groundwater reaches the ground surface in springs/seeps, carbonate mineralization takes place, as the cations in the groundwater react with atmospheric CO₂. We obtained mini-cores of such travertine mineral deposits in August 2012 near Lower Lake, CA, within the McLaughlin Natural Reserve (administered by UC Davis) and upslope of the Coast Range Ophiolite Microbial Observatory groundwater monitoring well array, which plumbs serpentinizing groundwater in the vicinity. Mini-cores were obtained with a hand-held ASC Scientific Electric Core Drill Model DE-T3, based on a Makita high-powered 18-volt Lithium cordless drill. Mini-cores were then presented to a SCANCO Medical MicroCT 40 scanning unit at the McCulloch μCT imaging facility at Rhode Island Hospital (partially funded by Award NIH P20-GM104937 for the COBRE Imaging CORE Project). Mini-core scans were conducted at 15-μm vertical steps, with the top of each core as the uppermost CT scan in the image stack. Raw image DICOM files were downloaded to OsiriX Imaging Software (32-bit, free version; www.osirix-viewer.com) as a “patient” files. By right clicking “linear attenuation” in the patient column, DICOM files were exported to “TIFF” and “Movie” files. Results presented here indicate changing density fields with depth in the mini-cores, and two different sections of a single mini-core from the White Seep site #3 (WS-3) are analyzed. Corresponding thin section images are also shown, in plane polarized and cross polarized light. We integrate µCT data with thin section petrography to infer changing depositional characteristics of this travertine.