Alkali olivine basaltf rom Tahiti contains zoned titanaugite microphenocrysts, phenocrysts, and overgrowths on lherzolite xenoliths, plus olivine (Fo88 – 49), plagioclase (An61 – 27), alkali feldspar (Or72), leucite, titanomagnetite (Mag30 Usp70), and ilmenite (Ilm94 Hem6). Augite rims contain up to 8.8 weight percent TiO2, the most Ti-rich yet documented from earth, accompanied by very low SiO2 (37.3 wt%) and high Al2O3, (13.6 wt%). Pyroxene stoichiometry suggests a modest amount of Fe is Fe3+, and leads to the following formulas for low- and high-Ti augites respectively:
(Na0.04Ca0.84Mn0.01Fe0.112+)(Fe0.052+Mg0.83Fe0.043+Cr0.03Ti0.034+Al0.03)(Al0.10Si1.90)O6
(Na0.07Ca0.89Mn0.01Fe0.032+)(Fe0.172+Mg0.43Fe0.103+Cr0.00Ti0.254+Al0.05)(Al0.57Si1.43)O6

The Tahiti composition trend suggests substitution of Fe2++Ti4++2Al for 2Mg+2Si, leading to the idealized end-member

CaFe0.5Ti0.54+AlSiO6
⁠. By contrast, stoichiometrya nd spectrald ata of high-Ti augitesf rom the Moon (up to 9.3 wt% TiO2) and Allende meteorite (up to 17.7wt% TiO2) indicate that half or more of the Ti is Ti3+. Thus, although the Allende augites are much richer in Ti (with much as CaTi3+AlSiO6), the Tahiti specimens are the most titanian yet reported. Magmatic conditions at Tahiti which favored crystallization of titanian augite included low P, high TiO2, low SiO2, high Ca/Al and Mg/Fe ratios, and low fO2. Coexisting magnetite-ilmenite indicate log fO2 = –11 atm at 1020°C, slightly more reducing than FMe buffer, but much more oxidizing than lunar and Allende meteorite conditions.

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First page of Zoned titanian augite in alkali olivine basalt from Tahiti and the nature of titanium substitutions in augite
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