Mineralogical anatomy and implications of a Ti–Sc-rich ultrarefractory inclusion from Sayh al Uhaymir 290 CH3 chondrite

Titanium-rich minerals are common in Ca–Al-rich inclusions from primitive chondrites. They are important not only for testing the condensation models for a gas with a solar composition, but also for constraining the redox conditions of the early solar nebula. In this study, we report the detailed mineralogical features and its oxygen isotope compositions of a Ti–Sc-rich ultrarefractory inclusion A0031 from a CH3 chondrite Sayh al Uhaymir 290. The A0031 inclusion has a compact and layered texture with the interior consisting of panguite, Sc-rich anosovite, Ti-rich davisite, and anorthite. A few hexaferrum, perovskite, and spinel crystals are present as inclusions in these minerals. Outside of Ti-rich davisite are a layer of Al–Ti-rich diopside and two grains of enstatite. This texture strongly suggests that A0031 has a condensation origin. Panguite is its third occurrence in nature and similar in composition to the type panguite from the Allende meteorite. Sc-rich anosovite in A0031 has a chemical formula of (Ti^(4+),Ti^(3+),Mg,Sc,Al)_3O_5 with the pseudobrookite structure. This is the second report of Ti_3O_5 in nature, but is the first description of anosovite formed in the solar nebula as an ultrarefractory phase. The discovery of Sc-rich anosovite in A0031 reveals the stability of Ti_3O_5 in the early solar nebula and supports the prediction of previous equilibrium condensation calculations. The panguite, Sc-rich anosovite, and Ti-rich davisite in A0031 show a large variation in Ti^(3+)/Ti^(tot). The primitive nature of A0031 implies that the variations in Ti^(3+)/Ti^(tot) among different Ti-rich minerals are primary features. We propose that the distribution of Ti^(3+) and Ti^(4+) could be controlled mainly by their various competition abilities of incorporating into these Ti–Sc–Al-rich minerals. Similarity of Ti^(3+)/Ti^(tot) value between Ti-rich davisite from A0031 and those in other carbonaceous chondrites indicates that most refractory inclusions might have formed in highly reducing nebular settings. The ^(16)O-depleted isotope compositions of A0031 confirm the existence of diverse oxygen reservoirs for CH Ca–Al-rich inclusions in the solar nebula