Thesis Defense - Spring 2009 - Kean Bliss

Evaluation of XRD and Raman peak broadening in shock-metamorphosed carbonates from selected carbonate-target bolide impact structures

Kean Bliss
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Jared Morrow

Wednesday, May 13th, 2008
CSL 422, 2:30pm



ABSTRACT
Identification of shock metamorphism in minerals is an important line of evidence for confirming the origin of potential impact-generated structures and associated deposits. Unlike common quartz and feldspar occurring in crystalline target rocks, shock-metamorphosed carbonates are not readily identified petrographically. Studies have indicated that X-ray diffraction (XRD) and micro-Raman spectrometry (MRS) peak broadening is a reliable indicator of shock metamorphism in carbonates. Samples from seven confirmed carbonate-target impact structures and other high temperature-pressure regimes (i.e., carbonatite, marble, and fault breccia) were analyzed in order to determine whether peak broadening is unique to impact structures. In addition, the MRS method is further tested for comparability to XRD analyses, and for the effects of different sample preparation (i.e., thin section vs. powder).
XRD analyses reveal that microstructural effects (i.e., crystallite size and lattice strain) contributing to peak broadening in potentially shocked samples follow expected trends of decreased crystallite size and increased lattice strain. However, fault breccias and carbonatites show microstructural effects comparable to those recorded for shocked samples, indicating that peak broadening is not unique to impact-related deposits.
Although XRD and MRS sample trends are similar, machine-derived and sample-derived variability is high in MRS analyses, contributing to uncertainty and less accurate peak width measurements. Large discrepancies exist between thin section and powder MRS analyses of identical samples. Estimated peak shock pressures calculated from previously established calibration lines for dolomite are much higher than expected; indicating that calibration curves should be specific to each impact structure and sample preparation protocol.

Thesis Defense - Spring 2009 - Emily Allen

Physical and Chemical Characteristics of Unweathered Pulverized Rock along the San Andreas Fault, Little Rock, CA

Emily Allen
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Tom Rockwell
Wednesday, May 13th, 2008
CSL 422, 1:00pm



ABSTRACT
We present new observations on pulverized granitic rocks recovered from a shallow core adjacent to the San Andreas fault near Little Rock Creek. The site is characterized by extensive outcrops of granitic rocks with varying degrees of damage, at distances of up to a few hundreds of meters from the fault’s primary active strand. We used an auger drill-rig to recover a continuous core of pulverized rock to a depth of 42 meters. The core is composed mainly of pulverized Si-rich, Si-intermediate, and Si-poor granitoids. It crosses through several high clay content gouge zones, which correspond to secondary fault cores. Detailed results of particle size distribution (PSD) measured using a laser particle analyzer and standard sieving and pipette methods indicate that medium to coarse silt and fine sand (50-600 microns) are the dominant particle sizes for pulverized Si-rich granitoids recovered in the core. Very little clay size particles were observed in both surface and depth samples. Si-rich granitoid samples are comprised only of quartz, feldspar, white mica, and minor biotite with no pedogenic clay. Geochemical data indicate little if any chemical weathering. We conclude that the signature of pulverization results in shattering to silt and sand sized particles with essentially no component of weathering, which is similar to but slightly coarser than prior studies that were conducted on near surface samples at Tejon Pass and along the Garlock fault where a minor weathering component was observed.

Thesis Defense - Spring 2009 - Frank Forcino

Mytilus californianus as an environmental monitor: a case study from central California

Frank Forcino
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Stephen Schellenberg

Wednesday, May 13th, 2008
CSL 422, 10:45am



ABSTRACT
Intraskeletal oxygen stable-isotope ratios (δ18O) and minor element ratios (Mg/Ca) were examined in modern Mytilus californianus specimens to test the hypothesis that these variables record ambient seawater temperature, and therefore can be measured in fossil specimens as a means of reconstructing paleotemperature. Specimens representing a range of ontogenetic stages and two extreme intertidal positions were collected monthly for one year from Santa Cruz, CA. While general linear modeling (GLM) of various monitored environmental parameters revealed temperature as the primary explanatory variable, the best goodness-of-fit GLMs for δ18O and Mg/Ca explained only 37.1% and 28.5% of variation, respectively, with additional significant variance attributable to intertidal positions and specimen size. Thus, M. californianus appears to be an unreliable means to reconstruct paleotemperature given the large amount of unexplained variation in intraskeletal δ18O and Mg/Ca and the role of variables effectively unknowable in the fossil record (e.g., intertidal position). In addition, a proxy species of interest cannot be assumed to be a comparable indicator of environmental conditions to similar species in the same genera or that share ecological characteristics."