Ti substitution mechanisms in hibonite and the determination of Ti3+/Ti4+ by X-ray absorption spectroscopy for use as an oxybarometer with application to early Solar System processes

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Title: Ti substitution mechanisms in hibonite and the determination of Ti3+/Ti4+ by X-ray absorption spectroscopy for use as an oxybarometer with application to early Solar System processes
Authors: Doyle, Patricia Mary
Item Type: Thesis or dissertation
Abstract: Quantitative determination of element species is crucial for understanding geological processes, particularly changing redox conditions. As Ti3+ is stable under very reducing conditions, Ti3+/Ti4+ oxybarometers are suitable for investigating early Solar System processes. The systematics of Ti3+/Ti4+ in Ti-bearing CaO-MgO-Al2O3-SiO2 (Ti-CMAS) glasses and both synthetic and meteoritic hibonite (Ca(Al,Ti,Mg)12O19) were investigated using X-ray absorption spectromicroscopy techniques. For this purpose suites of synthetic samples with 0-100% Ti3+/ΣTi (where ΣTi = Ti3+ + Ti4+) were prepared in a 1 atm gas mixing furnace. The samples were characterised by electron microscopy, and when relevant, by X-ray diffraction and neutron powder diffraction (NPD). The crystal structure of hibonite has five distinct Al sites onto which Ti may substitute. Structural refinements of NPD data using the Rietveld refinement method were used to determine Ti3+ and Ti4+ site occupancy in hibonite. Ti3+ was found to occupy the M4 octahedral site, whereas Ti4+ partitions between the M2 trigonal bipyramidal site and the M4 octahedral site. For hibonite with <1 Ti per formula unit Ti4+ partitions between the M2 and M4 site in a roughly constant ratio of 1:4.3, irrespective of the Ti3+ content. Ti K-edge X-ray absorption near edge structure (XANES) spectra were collected from Ti-CMAS glasses and both synthetic and meteoritic hibonite. Ti3+/Ti4+ oxybarometers were developed using the synthetic samples, and the Ti3+/Ti4+ K-edge oxybarometer was used to quantify Ti3+/ΣTi within hibonite from six different meteorites. Ti3+ hibonite was found in CAIs from the Acfer094 (unique), El Djouf001 (CR2), Murchison (CM2) and NWA1476 (R3) meteorites, whereas hibonite within CAIs from the NWA2446 (R3) and Hughes030 (R3-6) meteorites were Ti3+ poor. Furthermore, the Ti3+/ΣTi varied within hibonite grains in CAIs from Acfer094 and El Djouf001. The presence of Ti3+ within hibonite in CAIs with no alteration minerals, or those that only experienced low temperature alteration, suggests they formed and/or were reprocessed under very reducing conditions (<IW-2.7). Hibonite within CAIs that had undergone extensive secondary alteration were Ti3+-poor. It appears that the Ti3+/ΣTi of the Ti3+-poor CAIs re-equilibrated under oxidising conditions during a high temperature thermal metamorphic process. X-ray photo-emission electron microscopy (XPEEM) images were collected from synthetic and meteoritic hibonites, from which Ti L-edge X-ray absorption spectra were extracted. A preliminary L-edge Ti3+/Ti4+ oxybarometer is presented. Core-to-rim zoning at the 200 nm scale was observed in two meteoritic hibonite samples, which suggests these grains record dynamically changing temperature and fO2 conditions during the CAI forming process. Protocols are described under which the nm scale spatial resolution and spectroscopic capability in XPEEM can be utilised for the non-destructive study of meteorite mineralogy at the sub-micron level.
Issue Date: Sep-2011
Date Awarded: Jan-2012
URI: http://hdl.handle.net/10044/1/9199
Supervisor: Berry, Andrew
Sponsor/Funder: The Engineering and Physical Sciences Research Council (EPSRC) and the Natural History Museum (NHM) provided financial support for this project under a grant obtained by A Berry (Imperial College, IC). P Schofield (NHM) is thanked for preparing successful proposals for peer reviewed experimental time at the ISIS neutron spallation source, Diamond Light Source (i06 and i18) and the Advanced Light Source (on two occasions). The Advanced Light Source (ALS) is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Grants from EnviroSync and the NHM made travel to the ALS possible. The Mineralogical Society and the Geochemical Society are thanked for travel bursaries to present this research at the 2011 Goldschmidt Conference.
Author: Doyle, Patricia Mary
Department: Earth Science and Engineering
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Earth Science and Engineering PhD theses

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