The Growing Inventory of Ti 3+ -bearing Objects from the Solar Nebula Steven B . Simon 1 , Stephen R. Sutton 1 , 2 and Lawrence Grossman 1 , 3 1 Dept. of the Geophysical Sciences, 2 Center for Advanced Radiation Sources (CARS), 3 The Enrico Fermi Institute
Steven B. Simon1, Stephen R. Sutton1, 2and Lawrence Grossman1, 3
1Dept. of the Geophysical Sciences, 2Center for Advanced Radiation Sources (CARS), 3The Enrico Fermi Institute
The University of Chicago, Chicago, IL 60637
It has been known for some time that the Ti-rich phases fassaite, hibonite and rhönite, found in coarse-grained refractory inclusions, contain trivalent Ti, an indicator of formation under very reducing conditions. The development of Ti-XANES (X-ray absorption near-edge structure) spectroscopy has allowed measurement of Ti valence to be performed on relatively Ti-poor phases such as spinel and olivine. These are important phases among early solar system materials, and the valence of Ti in them can tell us whether they formed in a reducing, nebular gas or under more oxidizing conditions. We have investigated a wide variety of materials for the occurrence of Ti3+. This report is a summary of those findings. We have also recently begun an investigation of the valence of Ti as a function of metamorphic grade in L and LL chondrites, and some preliminary results are reported.
Samples were selected for analysis after characterization by scanning electron microscope and electron microprobe. Titanium K XANES spectra were collected using the GSECARS X-ray microprobe in fluorescence mode, with a 3 µm X-ray beam. Results were determined following the methods of [1, 2] and are reported as values from 3 to 4, representing the average Ti valences of the analytical volume.
That RF might contain Ti3+ was suggested by , who calculated refractory lithophile element contents of a melt that would be in equilibrium with RF. They predicted enrichments of ~10-40xC for most elements, but Ti enrichments of 50-65xCI would have to be inferred if it were only present as Ti4+. Our measurements of Ti3+/Ti4+ in RF combined with an olivine/liquid D for Ti3+ of 0.355 interpolated from Ds for V3+ and Sc3+  indicate a liquid with 28xCI, well within the range predicted by  for other refractory lithophiles, supporting their model for the formation of RF.
The lack of Ti3+ in Fo-bearing refractory inclusions (FOBs) is not understood. One of the inclusions we studied is an isotopically fractionated (FUN) inclusion, a type thought to have undergone evaporation, so it may have been exposed to oxidizing conditions, but the other FOB we studied is not a FUN inclusion.
The occurrence of Ti3+ in Type I and Type II chondrules suggests that both types had reduced precursors. The valence of Ti in chondrules does not reflect the near-IW conditions under which FeO-bearing olivine forms, and the Fa contents of chondrule olivine do not reflect the reducing conditions (ƒO2≤IW-3) that stabilize Ti3+ in chondrule-composition melts . Either Fe attains redox equilibrium more rapidly than Ti, or the Fe/Mg ratio of olivine and pyroxene can increase by inward diffusion of Fe without disturbance of the Ti3+/Ti4+ ratio.
Measurements on L chondrites show that variation in Ti valence in olivine and pyroxene persists to metamorphic grades at which Mg/Fe ratios are equilibrated. The high proportions of tetrahedral Ti4+ indicate formation under anhydrous conditions . These results and the chondrule data show that the Ti valence of olivine and pyroxene is not easily disturbed.
Fig. 2. (a, b)Backscattered electron images of a Type IAB (a) and a Type IIA chondrule from the Semarkona LL3 chondrite, showing locations of XANES analysis spots. Brightness of a mineral is proportional to the average atomic number of the elements in it. (c, d) Plots of mole % fayalite (Fa) and Ti valence as a function of distance across single olivine grains in Semarkonachondrules. No correlation is observed between Fa content and Ti valence within either grain. (e) Plot of Ti valence in phenocryst cores as a function of grain size.
Ti valence in AOA olivine
Fig. 1.Plot of normalized intensity of pre-edge peak vs. energy; these parameters vary systematically as a function of valence and coordination number. Mixing lines are calculated from endmember values shown and are used to calculate valences in samples. Fig. 1 shows that almost all of the Ti in the isolated RF grains (open squares) is trivalent, Ti in the chondrule RF (open circles) is 80-90% trivalent, and that in forsterite-bearing inclusions is tetravalent.
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