Karen Stockstill, Ph.D. Candidate

Karen, working with Mars Exploration Rovers

Department of Earth & Planetary Sci.
Planetary Geosciences Institute
306 EPS Building
University of Tennessee
Knoxville, TN 37996-1410
Phone: (865) 974-2789
E-mail : kstockst@utk.edu

 Ph. D. Research Area

Project #1
"Melt inclusions in augite of the Nakhla Martian meteorite: Evidence for basaltic parental melt"
The Nakhla martian meteorite contains crystallized melt inclusions that were trapped when this rock originally formed on Mars. These inclusions provide a useful tool for delineating the petrogenetic history of this cumulate-rich rock. Our study involved both slow- and fast-heating experiments on multiphase melt inclusions in Nakhla augite. We chose to study melt inclusions trapped within augite because augite reequilibrates with the external melt to a lesser extent than does olivine, which often shows open-system behavior with respect to Fe-Mg exchange. Also, results from augite-hosted melt inclusions allowed for direct comparison with the previous studies of Nakhla melt inclusions. Following homogenization of melt inclusions encapsulated within single mineral grains, the grains were mounted and polished to expose the inclusions and major element chemistry was determined by electron microprobe. The most primitive melt inclusion analyzed in Nakhla, NA03, is basaltic and closely matches some previous estimates of nakhlite parent melt compositions determined by other means.
MELTS equilibrium and fractional crystallization models calculated for NA03 and previous Nakhla parent melt estimates at QFM and QFM-1 produced phase assemblages and compositions that can be compared to petrography of Nakhla. Of these MELTS models, equilibrium crystallization of NA03 at QFM-1 produces the best match to the mineral phases and the mineral compositions observed in Nakhla. In all models, olivine and augite co-crystallize from the estimated parental melt composition, consistent with subsolidus re-equilibration of olivine. In addition, measured melt inclusion compositions are distributed along the MELTS-calculated liquid line of descent and so may represent pockets of melt trapped at various points during the crystallization sequence. Our new experiments are used to resolve discrepancies between estimates of the Nakhla parent melt and to reinterpret the results of a previously published study of melt inclusions. In particular, melt inclusions within Nakhla augite demonstrate that Nakhla is a volcanic rock whose parent melt composition and crystallization history reflect planetary igneous processes.

Project #2
"TES Hyperspectral Analysis of Proposed Paleolake Basins on Mars: No Evidence for In-Place Carbonates"

Several studies have described photogeologic evidence for paleolacustrine basins on Mars, mostly within impact craters. If these basins did contain persistent standing water in the past, they could still contain deposits of evaporite minerals, such as carbonates and sulfates. Many such deposits, if exposed at the surface to a sufficient extent, would be detectable in thermal infrared spectra taken from orbit. Using data from the Mars Global Surveyor Thermal Emission Spectrometer (TES), we have conducted a hyperspectral study of 80 putative paleolake basins to search for the spectral signatures of evaporite minerals exposed at a scale comparable to the spatial resolution of a single TES spectrum (~3 km). Spectral regions of interest (ROIs) within each basin were identified using a principal component analysis on a TES spectral image cube covering the basin and its surroundings. Averaged spectra from the ROIs were evaluated using a dust cover index developed by other workers. Those spectra determined to be "dust-free" were analyzed for composition using linear spectral deconvolution. The same spectra were also analyzed using a set of carbonate indices developed in the present work. This method is estimated to be sensitive to minimum carbonate abundances ranging from 12-53% by area (depending on the specific variety). All TES spectra in this study were well-modeled by deconvolution using only previously-defined TES spectral endmembers. Neither carbonates, nor any of the other common aqueous minerals (e.g., sulfates), were detected. In addition, the carbonate index analysis for these basins indicated that carbonates are not present in abundances greater than the detection limits of the method used. Therefore, this study did not find any spectral evidence for evaporite deposits in the basins studied.


Project #3
"THEMIS Analysis of Proposed Paleolake Basins on Mars"
The Thermal Emission Imaging System (THEMIS) aboard the Odyssey spacecraft is acquiring images at 100 m/pixel resolution, 320 pixels across, in nine spectral bands between 6.6 µm and 15.0 µm with global coverage. Due to its higher spatial resolution, THEMIS is best viewed as a spectral unit mapper whereas, due to its higher spectral resolution, TES is better at mineral identification. Therefore, the high spatial resolution data from THEMIS to produce spectral unit maps will provide a way to search for potential carbonate outcrops of a smaller scale than is discernible using TES data. If potential outcrops are detected, a TES spectrum covering the outcrop region could be analyzed by itself to determine the mineralogy. By pinpointing a potential carbonate outcrop to a single TES observation, the spectral contribution from the carbonate would be increased, perhaps above the detection limits of our carbonate index technique.

Publications

Return to the Top of the Page.

Return to Current Research Page

Return to PGI Homepage