Basalt Genesis Across the Solar System: Observational and Experimental Studies of Shergottite and Eucrite Meteorites

• Author / Creator

  Castle, Nicholas

• This thesis consists of three projects based on exploring igneous processes on two parent bodies: the HED (howardite-eucrite-diogenite) parent body and Mars. The HED parent body is an asteroidal parent body (possibly Vesta) that differentiated rapidly in the early solar system; Mars is a planet with an extensive geologic history and active igneous processes from the beginning of the solar system through the present. Similar experimental, analytical, and conceptual techniques were used between the projects, creating a unified theme despite the differences in petrogenesis on the two parent bodies. This first project examines the petrology of the Tissint meteorite, an olivine phyric shergottite fall belonging to the depleted geochemical trend of Martian basalts. Tissint is one of very few shergottites with a near-liquid composition, allowing for the calculation of a robust parental liquid composition. Mineral assemblages in Tissint are consistent with two different oxygen fugacities (fO2s), QFM-3.5 for the early-formed olivine-spinel-pyroxene mineral assemblage, and QFM-1.4 for the late-formed titanomagnetite-ilmenite mineral assemblage. Geothermometer/oxybarometer (GTOB) estimates and MELTS calculations were compared to experimental results; GTOB estimates tend to overestimate temperatures by 100-150°C, but fO2 estimates were close to experimental settings, and the crystallization curve produced by MELTS was close to the experimentally derived crystallization curve. MELTS was used to estimate the maximum extent of auto-oxidation of the Tissint magma to ~0.8 log units, far short of the 2.1 derived from GTOB estimates, leading to the conclusion that Tissint had undergone an oxidation event during petrogenesis. This is the first time the phenomenon has been observed in a depleted shergottite, providing insights into volatiles in the Martian interior. The liquid line of descent demonstrates that QUE 94201, a basaltic shergottite, could be derived from a Tissint-like magma; however, both Shergotty and Zagami, also basaltic shergottites, could not. Further, the parental composition for EETA79001, another olivine-phyric shergottite, could not be derived from a Tissint-like magma, suggesting that there is additional magmatic variety within each of the textural types. The second project aims to determine the mechanism by which Cr in olivine was overprinted in experiments on the Tissint parental composition that were subjected to oxidation during crystallization (hybrid fO2 experiments). Chromium profiles in the Tissint shergottite are similar to the experimentally-derived overprinted olivine, and not to the olivine produced by cooling rate experiments without an oxidation event. Several variants of the original hybrid experiment were performed, each allowing evaluation of a different proposed mechanism. The diffusion length scale was found to be too short to cause the change in Cr observed in the experiments. Although oxidation-induced peritectic resorption of olivine, forming pyroxene, is expected during oxidation of the Tissint liquid during cooling, the large degree of resorption required to account for the overprinting was not observed. Oxidation-induced exsolution of spinel from olivine appears consistent with the observed overprinting, and occurs on a length scale closer to the diffusion length scale. An experiment doped with V demonstrated that V behaves similarly to Cr during the experiment; Co and Ni are also suspected to behave similarly to Cr, partitioning into spinel during exsolution, but additional experimentation is required to confirm. The third project involved partial melting of a zoned eucrite, NWA 7035, to provide an experimental test of the hypothesis that the incompatible trace-element (ITE) enrichment of the Stannern trend (ST) can be formed from the main group trend (MGT) by mixing with such a partial melt. A zoned eucrite was selected for experimentation because it represents an unmetamorphosed igneous sample, similar to what is expected in a primitive eucritic crust on the HED parent body. Partial melts of NWA 7035 have an ITE enrichment capable of creating a ST-like rare earth element composition from a MGT composition, but would have an accompanying enrichment in both Sc and Fe that is incompatible with derivation of the ST from a MGT magma. A larger plagioclase component in melting, either from a longer duration equilibration time or a higher plagioclase fraction in the melting crust, may resolve the Sc enrichment. The drop in Mg# might be countered by starting with a more magnesian composition of eucrite, but additional experiments on such a composition are required to evaluate.

• Subjects / Keywords

  • Tissint
  • Experimental Petrology
  • NWA 7035
  • Oxygen Fugacity
  • Mars

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VQ2SP22

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.