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OXIDATION STATE of Ti in SYNTHETIC PYROXENE, OLIVINE, Fe-Ti

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OXIDATION STATE of Ti in SYNTHETIC PYROXENE, OLIVINE, Fe-Ti XANES Ti K-EDGE STUDY ON THE COORDINATION CHEMISTRY AND OXIDATION STATE OF Ti IN SYNTHETIC PYROXENE, OLIVINE, Fe-Ti OXIDES AND BASALTIC GLASSES UNDER LUNAR fO2. F.P. Leitzke1, R.O.C. Fonseca1, J. Göttlicher2, R. Steininger2, M. Lagos1 1Steinmann Institute for Geosciences, University of Bonn, Germany 2Institute for Photon Science and Synchrotron Radiation, ANKA, Karlsruhe Institute of Technology, Germany. Introduction which occur at high Ti concentrations in fluorescence mode, the sample shift in energy for absorption edge features from oxidizing to reducing Discussion: Lunar petrogenesis has occurred under oxygen fugacity(fO2) surface was aligned 85 ° towards the beam (close to perpendicular conditions [3,8], for example, about 3 eV from TiO2 to Ti2O3 at the main- Studies of direct measurement in pyroxene and olivine conditions that are below the Fe-FeO (Iron-Wüstite – IW) position) and the sample surface – detector angle resulted then in 5 ° edge feature (Figure 3 and 4). Pre-edge Main-edge from natural lunar samples originated under reducing conditions have 0.0 redox buffer [e.g., 1], which is ca. three orders of magnitude more (close to grazing exit conditions). Sample chamber was kept under 3+ Augite (IW-1.5) reported Ti /∑Ti average values of 0.1-0.2 [6] or 0.3-0.4 [12] for ) reduced than the average value for the Earth’s upper mantle [2]. vacuum in order to avoid absorption of the low energy X-rays under air. 0 -10.0 FMQ -E primitive lunar magmas, which would become more oxidized as WM E Augite (IW-1.8) Even though the TiO2- Ti2O3 redox buffer is more reduced than IW, Energy shifts were monitored by measuring Ti metal foil between every ( 3+ 2 IW 3+ µ fractional crystallization continues, and Ti is retained at the source due -2 (this study) d IW-1.5 to IW Diopside (IW-1.5) the idea that Ti cannot coexist with high FeO contents have been five points and aligned to 4966 eV (first maximum of the first derivative). e z i -20.0 l log fO refuted by direct measurements in refractory inclusions in Because crystal orientation can be a source of uncertainty [3], we have a to its higher crystal/silicate melt compatibility. Some of these values are m 3+ iO - TiO2 r T 1.5 o lower than the average reported here, which can be explained by the fact meteorites, which yielded a Ti /∑Ti ratio from 0.37 to 0.79 [3], as measured several points randomly allocated among the crystals in n -30.0 3+ well as by the discovery of Tistarite (Ti2O3) in the Allende meteorite different experiments at the same fO2. To determine Ti coordination and that in our experiments the Ti content of the crystals is solely a function 1000 1100 1200 1300 4960 4980 5000 5020 5040 [4]. Regarding lunar magmatism, the dataset of studies dealing with oxidation of state in the experiments, the 1s-3d energy shifts were used, of the fO2 imposed by the experimental atmosphere. When compared to Temperature (°C) Energy (eV) X-ray absorption near edge structure (XANES) spectroscopy to following [3,8,9], and the lever rule between standards representative Figure 2 – Comparison between oxygen Figure 3 – Normalized XANES experimental results on equivalent compositions, our data for pyroxene determine redox transition of Ti is scarce, with recent works in lunar of each one of the end-members. XANES data were processed with the fugacity values vs. temperature for common spectra showing difference in pre- mineral redox buffers. edge and main-edge for pyroxene. are in good agreement to [5], who reported Ti valence of ca. 3.5 for pyroxene and olivine from experiments and natural samples Athena programme of the IFEFFIT software package [10]. pyroxenes equilibrated at ΔIW -2. Previous microprobe detections of Ti3+ 3+ showing either significant [5, 6] or negligible [7] amounts of Ti , A Armalcolite B Armalcolite C Preliminary XANES data yield an average valence for Ti of 3.6, i.e., a 3+ 3+ for lunar Fe-Ti oxides [e.g.,13,14] pointed to a Ti /∑Ti ratio of 0.1-0.2, depending on petrologic history, mineral and mantle source composition. Glass Ti /∑Ti value of 0.4 for crystalline phases with a lower limit of 0.1 and which is within error of the ones obtained by our X-ray absorption Therefore, we present new Ti K-edge XANES spectroscopy an upper limit of 0.7 (Figure 5). Silicate glasses show that Ti is mainly spectroscopic measurements. When recalculated to TiO2(t), our estimates measurements performed on a wide range of minerals (pyroxene, olivine, Glass tetravalent, which indicates that a redox exchange occurred in the glass Pyroxene Fe-Ti oxides) and basaltic glasses (Fig. 1) under lunar conditions of fO , during the experiments, quenching or after samples were mounted of ca. 30-40 % Ti2O3 can be used to correct the excess of TiO2 measured by 2 Pyroxene Glass 3+ (Figure 5). Pre-edge features also show that the coordination number of EMP-WDS. Future work on the substitution mechanisms of Ti3+ in the which can be used to better constrain the Ti /∑Ti of the lunar mantle. 200 μm Ti varies from average V-fold in the silicate glass, which is in agreement crystals and combining XANES data on experiments and natural samples Experimental and Analytical Techniques: Figure 1 - Back-scattered electron images of typical experimental run products: a) Large euhedral to subhedral armalcolite crystals in equilibrium with silicate glass; b) to the expected for basaltic glasses [11], to VI-fold in the Fe-Ti oxides will provide new insights into lunar magmatism and the fO2 evolution of Experiments were conducted in vertical tube gas-mixing furnaces Euhedral orthopyroxene (enstatite) crystals in equilibrium with armalcolite and and a mixture between VI and IV-fold coordination in the pyroxenes. the solar system. through the loop technique, using either Pt or Re wire, for Fe-poor and silicate glass; c) Euhedral clinopyroxene (diopside) crystals in chemical equilibrium with silicate glass. The maximum amount of Ti in tetrahedral coordination in the References: Fe-rich compositions respectively. Temperature ranged between 1100 Results: pyroxenes in our study is ca. 40 %. [1] Papike J. J. et al. (2005) Am Mineral., 90, 277-290. and 1300 °C and fO2 between IW-1.5 and IW-1.8 (Fig. 2), imposed via 1.4 [2] Ballhaus C. (1993) CMP, 114, 331-348. Run products had large (> 100 µm) euhedral to subhedral A B 1.2 [3] Simon S. B. et al. (2007) GCA, 71, 3098-3118. a CO-CO atmosphere. Run products were analyzed for major elements ) 2 1.0 3+ E NaTi Si O ( crystals (diopside, augite, enstatite, forsterite, armalcolite and ilmenite) 2 6 [4] Ma C. and Rossmann G.R. (2009) Am Mineral, 94, 841-844. CaTiSiO µ 0.8 5 d (Titanite) e (CaO, MgO, Al O , SiO , Cr O , TiO , and FeO) using a JEOL JXA 8900 z 2 3 2 2 3 2 i l 0.6 [5] Krawczynski M. J. et al. (2010) LPSC XLI, #1825. in chemical equilibrium with quenched silicate glass (Figure 1). FeTiO3 a m (Ilmenite) r 0.4 o Ti2O3 ) n electron microprobe in WDS (Wavelength Dispersive) mode, with 0 [6] Simon S. B. et al. (2016) LPSC XLVII, #1251. 1s-3d TiO TiO2 (Brookite) 2 Among the silicates, cpx crystals had the largest TiO contents -E 0.2 Average of XANES spectra for 2(t) E ( armalcolite crystals at lunar fO2 [7] Simon S. B. et al. (2014) LPSC XLV, #1063. µ 0 accelerating voltage at 15 kV, 15 nA beam current and an electron beam d Ti O (Powder) e 3 5 z i ranging from 0.49 to 2.89 wt. % and 0.01 to 0.09 atoms per formula unit l C [8] Waychunas G. (1987) Am Mineral., 72, 89-101. a 0.12 E) m Ti2O3 (Powder) ( 3+ r o defocused to 5 µm. Ti K-edge XANES spectra were acquired using a Si µ 0.10 n d [9] Farges F. et al. (1995) GCA, 60, 3023-3038. (apfu), which is not high enough for determining Ti /∑Ti using e 0.08 z i l IV Ti (Metal) a 0.06 [10] Ravel B. and Newville M. (2005) JSR, 12, 537-541. (111) double crystal monochromator at the SUL-X beamline of the m r the electron microprobe [3]. The Al of cpx did not exceed 0.10 apfu. o 0.04 [11] Farges F. and Brown Jr. G.E. (1997) GCA, 61, 1863-1870. ANKA synchrotron facility (Karlsruhe, Germany). The X-ray beam was 0.02 Enstatite and Forsterite had a maximum Ti content of 1.43 wt. % (0.04 0 [12] Sung, C. et al. (1974) LPSC V, 717-726. -0.02 derivative n focused to 50 µm at sample position with a KB mirror system on each -0.04 [13] Pavicevic et al. (1972) LPSC III, 295-303. apfu) and 0.13 wt.%, respectively. Armalcolite crystals ranged from Fe- 4960 4980 5000 5020 5040 4960 4980 5000 5020 phase. Measurements had to be performed in fluorescence mode due to 3+ Energy (eV) Energy (eV) [14] Stanin F.T. and Taylor L. (1980) LPSC XI, 117-124. Mg-rich and Ti /∑Ti calculated using microprobe data is ca. 0.1-0.2, the the thickness of the polished sections. To reduce self-absorption effects Figure 4 – Normalized XANES spectra Figure 5 – Ti valence state for crystals and Acknowledgements: F.P.L. was supported by a PhD scholarship from CNPq (grant same as observed for ilmenite.
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