Thesis Defense - Spring 2008 - Angela Cavallini

Major element variations of Hawaiian parental magmas:
Mantle source or melting control?

Angela Cavallini
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Friday, May 9th

ABSTRACT
The abundances of the major element oxides (SiO2, TiO2, Fe2O3, MnO, MgO, CaO, NaO2, K2O, P2O5) in Hawaiian lavas are subject to change due to variation in (1) the amount of crystal fractionation or accumulation or (2) the pressure (depth) and degree of partial melting of the mantle, and (3) differences in the composition of the mantle source. Since isotope ratios (e.g. 206Pb/204Pb or 87Sr/ 86Sr) are not subject to the effects of crystal fractionation or partial melting, they are thought to be good indicators of the mantle source composition. Literature data shows that there is a correlation between the isotope ratios and major elements abundances of Hawaiian lavas when the latter are corrected for the effects of crystal fractionation. This suggests a relationship between the major element chemistry and the mantle source composition. However, another possibility is that the variations are related to changes in the depth of melting. Hawaiian volcanoes been categorized into two main geographic trends: the northeastern Kea trend (named after Mauna Kea) and the southwestern Loa trend (named after Mauna Loa). In this study, I summarized major element data for both Kea and Loa lavas from the scientific literature. The Kea trend lavas include Mauna Kea and Kilauea and the Loa trend lavas include Mauna Loa, Koolau, and Kahoolawe. Prior to this study, Loihi lavas were grouped with the Loa trend (based on geography), but I found that their chemistry is actually more similar to the Kea trend. To correct for the effect of crystal fractionation and accumulation, sample data were adjusted to a constant MgO value. This was achieved by running the data through a computer program which added or subtracted small increments of equilibrium olivine to each sample composition until 13 wt. % MgO was reached. My results show that there are significant variations in the major elements chemistry between Kilauea, Kahoolawe, Mauna Kea, Mauna Loa and Loihi lavas. The Kea trend lavas are relatively low in SiO2 and abundant in CaO. The Loa trend lavas have higher SiO2 and lower CaO. Some of the most significant variations are a range greater than 3% in both SiO2 and CaO. A 2% range in SiO2 can possibly be explained by variations in the pressure (depth) of melting. However, these same pressure differences cannot explain the 3% variation in CaO. Thus, these major elements variations are better explained by differences in the mantle source composition of these volcanoes. The lower SiO2 and higher CaO lavas are consistent with melting mantle peridotite, whereas the higher SiO2 and lower CaO lavas are not. These lavas are likely formed by melting of pyroxenite derived from ancient, recycled oceanic crust within the Hawaiian mantle plume.

Thesis Defense - Spring 2008 - Aaron Hebeler

Elemental Transport and Volume Strain of Fault Core and Damage Zone, San Jacinto Fault Zone, California: Assessing The Influence of Weathering

Aaron Hebeler
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
The Clark segment of the San Jacinto Fault Zone is located in southern California. Our study was aimed at evaluating the chemical and physical properties of fault core and damage zone for the affects of chemical weathering. Fault zones are composed of distinct architectural zones that are based on texture, structural fabric, and grain size. Architectural zones commonly include a fault core, damage zone, and wall rocks. These architectural zones are well displayed in an exposure of the Clark segment of the San Jacinto fault zone. At this location measurements of bulk density progressively decrease from high values in the wall rocks to lower values in the fault core. In contrast, porosity trends show an inverse relation with regard to bulk density trends. In A-CN-K space samples from inner damage zone, outer damage zone, and fault core emanate from a clustering of points derived from the wall rocks toward the A-K join in a systematic progressive fashion. This relationship is consistent with petrological observations that suggest that grain size is systematically reduced within each architectural zone culminating in a generally black structureless aphanitic fault core. In A-CNK-FM space samples again spread in a linear fashion from wall rocks to the fault core toward the A-FM join. This spread again correlates well with each architectural zone and grain size. The transport function, τ, suggests that there is progressive loss of Al, Ca, Na, K, and Sr mass toward the fault core. In contrast, the masses of Fe, Mg, Mn are increased in the fault core relative to the adjacent damage zone and wall rock. The above relations are consistent with the idea that fault zones are greatly susceptible to chemical weathering because of their reduced grain size and hence greater surface area. Moreover, our data suggest that at or near the Earth’s surface they are also highly porous. Apparently chemical weathering under such conditions involves degradation of plagioclase, amphibole, and biotite by migrating soil and paralithic fluids. These fluids remove Al, Ca, Na, K, and Sr from degrading plagioclase, and convert amphiboles and biotites to mixed layer expandable clays such as vermiculite and mixed vermiculite and chlorite. The increase in Fe, Mg, and Mn mass within the fault core implies that the fluids were transporting such ions and that they accumulated within the fault core perhaps by some cation exchange process.

Thesis Defense - Spring 2008 - Jennifer Piper

Column Matrix Effects in the Mass Spectrometer and the Effects
on Isotope Analyses

Jennifer Piper
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Friday, May 9th

ABSTRACT
Mass spectrometry can be used to analyze radiogenic and stable isotopes in order to determine the concentrations of different isotopes in a sample, ages of rocks, and sample correlations. Multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has become the preferred method for isotope analysis compared to other methods because it is very precise, the plasma can ionize any element, and there is less time required for each analysis. One of the downfalls of isotope analysis using MC-ICP-MS is that it induces an instrumental mass-dependent fractionation (instrumental mass bias) that needs to be corrected for. This instrumental mass bias is primarily due to the extremely high temperatures of the plasma, which causes a spread in ion energy during ionization and transport within the mass spectrometer.
Instrumental mass bias in the MC-ICP-MS can be corrected using one of two main techniques (1) standard sample bracketing (SSB) or (2) double spiking. SSB is commonly favored, but it is more susceptible to matrix effects that result in inaccurate results. Many different types of matrix effects in the MC-ICP-MS have been identified and corrected for. Spectral matrix effects, or isobaric interferences, result from the occurrence of an element that overlaps in mass with the isotope of interest. Non-spectral matrix effects can create differences in the measured isotope ratios of a sample and the standard.
In a recent study, a new and possibly uncontrollable matrix effect was found that is thought to have come from the separation and purification of molybdenum using an anion exchange resin. It was shown that the collection of a pure Mo-free solution that had been passed through an anion exchange resin and subsequently added to a Mo standard appeared to be isotopically lighter than expected when compared to the same untreated Mo standard. Many tests were performed to try and correct for this “column matrix effect” but all have failed.
The purpose of this experiment is to test for the presence of this column matrix effect in measurements of radiogenic isotopes of U, Th and Pb to see if it affects elements other than molybdenum. In all of the experiments, the standards run with the addition of a column matrix appear to have a higher signal intensity than the untreated standards. However, this effect does not seem as significant when comparing the isotopic ratios of the treated and untreated standards. Based on the data obtained from this experiment, it can be said that there is a column matrix effect for U, Th, and Pb and it is produced from the resin that is used in the column chemistry.