Ti solubility in diopsidic pyroxene from a suite of New South Wales leucitites (Australia)

Abstract

The pyroxene chemistry from a suite of leucitites and related differentiates from New South Wales has been investigated with the aim of understanding the physico-chemical factors which may influence the Ti solubility in the pyroxene, relative to the chemistry of the host rocks. These are characterized by high and variable Ti (TiO 2=3.9−8.0 wt.%) and a high agpaitic index, ( Na+K) Al =0.85−1.0 , combined with Mg ( Mg+Fe 2+) =0.79−0.63 . The pyroxenes from this suite are notable in that they contain insufficient Al to complete the occupancy of the tetrahedron to 2.000 atoms per formula unit and Ti occurs both as Ti IV and Ti VI. The Ti solubility in the investigated pyroxenes depends on the partitioning of Ti to Ti IV and Ti VI, Ti IV being favoured in the early-crystallized (core) and Ti VI in the late-crystallized (rim) compositions, relative to Al. Therefore, the well-known and common substitution couple Ti VI-Al 2 IV is associated with the Ti IV-Si substitution, predicted by T.F.W. Barth and later by E.R. Segnit from synthetic pyroxenes. Further, ti solubility in the New South Wales pyroxenes depends on the Ti/Al ratio of the liquid from which the pyroxene crystallized and not necessarily that corresponding to the bulk rock. Ti partitioning between pyroxene and coexisting magnetite varies significantly with temperature and f O 2 , increasing dramatically in favour of the coprecipitated pyroxene at relatively low temperature and higher f O 2 . However, the pyroxene Ti content is independent of tetrahedral Si, which remains high and relatively insensitive to variations in silica and silica saturation, respectively, in the host rock.