The effect of Ti on Ca-pv and Mg-pv phase stability

Perovskite-bearing crystal-controlled oxide-silicate mantle xenoliths: Resolution to controversial origins?

“…For Ca, there is a limited solid solubility of davemaoite (CaSiO 3 ) in Brg ( 33 ), but increased miscibility to a single phase can be achieved in the presence of high iron (Fe 2+ Fe 3+ ) concentrations ( 63 , 64 ) and extreme P (>90 GPa) as determined experimentally by ( 33 ) and thermodynamically by ( 62 , 64 ). A similar effect is demonstrated for Ti in which Ilm-Cpx intergrowths are stable experimentally at ~55 GPa and Opx-Ilm at 35 GPa ( 33 ), and thermodynamically for the binary Pvr-Brg with the corresponding values of 80 and 45 GPa, respectively ( 34 ). In addition, proof-of-element (Fe-Ti-Al)–enriched Brg is reported as diamond inclusions from Brazil ( 48 ).…”

“…8E), the xenolith ratios are 0.88 and 0.96, well within the uncertainties for pyrolite (0.94) and chondritic (0.99) models [Ca is not considered in (80)]; the xenoliths, however, are depleted in SiO 2 (~25 versus ~48 wt %) and enriched in TiO 2 (~15 versus ~0.2 wt %), possibly indicative of LLSVP heterogeneity. Titanium is essential to the xenoliths discussed and in a pyrolytic lower mantle, where thermodynamic mixing with Ca and Fe is greatly enhanced to the point that solid state Opx-Ilm and Cpx-Ilm are predicted at ~50 GPa, and ~85 GPa along a 2000 K geotherm (34) in accord with the experimental data shown in Fig. 8.…”

“…Perovskite is stable to at least 60 GPa, ~1800 km depth ( 26 , 27 ). Phase equilibria at high P-T demonstrate extensive to complete solid solution for CaTiO 3 -CaSiO 3 ( 28 ), CaTiO 3 -FeTiO 3 ( 29 , 30 ), and FeTiO 3 -MgTiO 3 ( 31 ), is limited for CaSiO 3 -MgSiO 3 along an upper mantle adiabat ( 32 ), but complete at P > 85 GPa in the presence of Fe and Ti ( 33 , 34 ). Single-phase En-Gk is stable at P > 25 GPa, 1600°C ( 35 ).…”