OXY-EXSOLUTION AND SUBSOLIDUS GROWTH TEXTURES IN SYNTHETIC FE-TI OXIDE ASSEMBLAGES

To retrieve experimental data for recalibration of the Fe-Ti oxide thermo-oxybarometer (Buddington & Lindsley, 1964, JPET 5, 310-357; Andersen & Lindsley, 1988, AM 73, 714-726; Ghiorso & Sack, 1991, CMP 108, 485-510), we have synthesized titanomagnetite-ilmenite_ss (Tmt-Ilm_ss) and ilmenite_ss-pseudobrookite_ss (Psb_ss) samples in the Fe-Ti-O system. We started with cylindrical pellets (diameter 5 mm, height >1 mm) of homogeneous Fe₂O₃-TiO₂ mixtures. Experimental conditions were: T: 1000°C-1300°C (subsolidus conditions), fO₂: DFMQ: -4 to +5 (fixed with CO/CO₂ mixtures), P: 1 bar, t: > 24 h (1300°C) up to 240 h (1000°C). Run products were quenched into water.

Except for a thin outer zone, the pellets consist of polycrystalline, roughly equi-granular two-phase aggregates, with ±idiomorphic, homogenous and evenly distributed grains (10-50 µm). This strongly suggests that the inner part of the sample has reached equilibrium.

Nevertheless, the samples are sensitive to oxidation. An outer zone of max. 100 µm may display thin (<5µm) orientated lamellae and rims of the Ti-O-richer phase (Psb_ss in Ilm_ss, resp. Ilm_ss in Tmt). These oxyexsolution features in the sense of Buddington & Lindsley (op. cit.) occur only when the sample was quenched outside the furnace (within few seconds), i.e. when fO₂ increased at high T immediately before quenching. The compositions of the exsolution bodies and their host phases point to higher oxygen fugacity than compositions in the core of the sample.

Pre-quenching textures occur in many samples at the surface of the pellets and along cracks, in form of monomineralic rims (max. 200µm). These rims consist of the more oxidized, Ti-richer phase (Ilm_ss around Tmt+Ilm_ss, Psb_ss around Ilm_ss+Psb_ss), i.e. represent zones of Ti-O-enrichment. In few Ilm_ss-Tmt samples we observed an additional outer concentric Psb_ss rim (30 µm) around the Ilm_ss rim. Phase compositions point to the same fO₂ at the surface as in the core and rather suggest a local shift of the bulk composition during crystal growth.