Type of Document Dissertation Author Gaboardi, Mabry Author's Email Address firstname.lastname@example.org URN etd-05082009-164852 Title Geochemical Analyses of Paleoenvironments and Processes: Peking Man to Stardust Degree Doctor of Philosophy Department Geological Sciences, Department Advisory Committee
Advisor Name Title Munir Humayun Committee Co-Chair Yang Wang Committee Co-Chair Vincent Salters Committee Member William Parker Committee Member Philip Froelich Outside Committee Member Keywords
- Peking Man
- Laser Ablation Inductively Coupled Plasma-Mass Sp
- NASA Stardust
Date of Defense 2009-04-13 Availability unrestricted AbstractThis dissertation employs chemical analysis as a tool to explore past environments and processes, and to examine the viability of matrix-independent standardization of laser ablation inductively coupled plasma mass spectrometry.
The Pleistocene deposits at Zhoukoudian, often referred to as the “Peking Man” Site, contain dental remains from a diverse group of herbivores, including Equus sanmeniensis, Cervus elaphus, Cervus nippon, Megaloceros pachyosteus, Sus lydekkeri, and Dicerorhinus choukoutienensis. The carbon and oxygen isotopic compositions of structural carbonate within the enamel of these teeth are used to reconstruct the paleodiet and paleoenvironment of the mammals. The δ13C values of enamel from Zhoukoudian range from −2.3‰ to −13.0‰, indicating that these mammals consumed between ~25% to 100% C3 plants. The presence of significant amounts of C4 plants in the diets of some herbivore species indicate that at the beginning of the Middle Pleistocene local habitats included mixed C3/C4 vegetation. By approximately 470,000 yr ago, C3 plants dominated the diets of herbivores studied, suggesting that the abundance of C4 flora had decreased in the area. For all deer analyzed in this study, the values of δ13C and δ18O decrease substantially from about 720,000 to 470,000 yr ago. This trend may be due to a strengthening of the winter monsoon during the Middle Pleistocene.
In preparation for the analysis of NASA Stardust material, matrix-independent LA-ICP-MS standardization is explored and applied to the measurement of elemental abundances in aerogel, the silica foam matrix in which Stardust materials are embedded. To determine the degree to which matrix affects elemental abundance measurements in LA-ICP-MS analysis, and describe these effects in term of elemental properties, elemental sensitivity ratios (ESRs) from a multi-element solution and from the laser ablation of a range of silicate standards are measured. For all analyses, ESRs depend strongly on element mass and first ionization potential (FIP), as expected. For laser ablation of the nearly transparent SRM 612, ESRs show an additional dependence on elemental temperature of condensation (Tc). To explore the relationship between this volatility dependence and matrix transparency, the range of transparencies available in the SRM 61X series is exploited. With increasing transparency of glasses, LA-ICP-MS analyses show progressive volatility dependence, with measured concentrations of refractory elements being lower, and measured concentrations of volatile elements being higher, than reported concentrations. This effect is minimized, and possibly negated, by increasing laser energy output. All non-transparent glasses analyzed (MPI-DING glasses and USGS BHVO-2G and BCR-2G) show no resolvable matrix-dependent elemental fractionation and may be used interchangeably with an accuracy of better than 10%.
Based on the measured concentration offsets in the above silicate standards, the approximate offsets for NASA Stardust samples, given their Fe to Si ratios, are < 20%. As comet particles are composed of metals and sulfides, in addition to silicates, standardization investigations are extended to include these materials by calibrating metal and sulfide standards against ESRs measured from a basalt glass. Independent of standard matrix, measured metal and sulfide concentrations are accurate to within ~20%. Concentration offsets show no observable relationship to element volatility.
To assess the magnitude of the aerogel background in LA-ICP-MS analysis of Stardust material, the elemental abundances and distributions of 48 elements in pre-flight aerogel are analyzed. Measurements in this study agree well with previous bulk analysis (Tsou et al., 2003), and obtained detection limits are about 100-fold lower than those reported for bulk analysis. The amount of material of chondritic composition necessary to overcome the aerogel blank is calculated: for a 20 μm diameter particle most elements would be recovered; for a 100 μm particle all elements except for Sn would be detected. Finally, two Stardust analogues are evaluated, demonstrating our ability to recover elemental abundances and confirming the lack of a substantial volatility-dependent fractionation during LA-ICP-MS analysis of materials embedded in aerogel. Unfortunately, during the creation of the embedded analogue, contamination was introduced by the firing of the gas gun. The accuracy of the LA-ICP-MS technique, therefore, cannot be quantified with analogues produced in this manner.
We conclude that 1- matrix-independent standardization of LA-ICP-MS analysis of non-transparent silicates and ceramics is an accurate and viable tool that can best be employed by avoiding calibration with transparent standards, 2- the LA-ICP-MS analysis of materials embedded in aerogel should not be hindered by volatility-dependent elemental fractionation, and 3- matrix-independent standardization between silicates, metals, and sulfides is appropriate for the LA-ICP-MS analysis of Stardust materials.
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