Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P,T

Abstract

Experiments on hydrous mafic, intermediate and felsic compositions at 7.5–30 kbar and 900–1100°C allow broad delineation of TiO 2 contents for a range of silicate liquids coexisting with a Ti-rich accessory phase. Different starting compositions, including unadulterated fused natural rocks, and rocks enriched with various Ti-rich phases, give mutually consistent results. The TiO 2 content of liquids saturated in a Ti-rich accessory phase is shown to decrease with decreasing temperature, increasing pressure, increasing SiO 2, alkali and rare-earth element content (at % level) of the liquid. Water content has little observable effect, while increasing oxygen fugacity lowers the TiO 2 content of liquid coexisting with a Ti-rich phase. A separate Ti-rich accessory phase may be expected to crystallize from liquids at 1000°C in the deep crust (pressures corresponding to 7.5–12 kbar) if the TiO 2 content exceeds ∼ 3.0 wt.% for mafic, ∼ 1.8 wt.% for intermediate, and ∼ 1.2 wt.% for felsic compositions. At the same depth but at 950°C, these TiO 2 values change to ∼ 1.7, 1.2 and 0.9 wt.%, respectively. At upper-mantle depths (20–30 kbar) the corresponding TiO 2 saturation levels at 1000°C are ∼ 2.0, 1 and 0.7 wt.%, respectively. In natural igneous rock series the contrasting TiO 2SiO 2 systematics in alkaline and tholeiitic series compared with calc-alkaline series are well established, as are the characteristic, though non-unique, low TiO 2 values for rock series in convergent plate regions. The present results allow experimental confirmation of observed and/or predicted crystallization of Ti-rich phases in these rock series. They also provide reasonable constraints on conditions under which TiO 2 contents of convergent plate magmas may be buffered by a residual Ti-rich phase in the source region, and indicate that TiO 2 contents of mafic parent magmas in these areas are unlikely to be controlled in such a manner. In contrast, it is almost inevitable that felsic magmas, generated by partial melting of the lower crust, are saturated with respect to a Ti-rich phase in their source regions.