Titanium solubility in olivine in the system TiO2–MgO–SiO2: no evidence for an ultra-deep origin of Ti-bearing olivine

Abstract

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO 2 -MgO-SiO 2 system. Experiments at atmospheric pressure in the temperature range 1,200-1,500°C showed that the highest concentration of TiO 2 is obtained when olivine coexists with spinel (Mg 2 TiO 4 ). The amount of TiO 2 in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi 2 O 5 ) and a Ti-Si-rich melt at 1,500°C displays a fourfold lower TiO 2 content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg 2 TiO 4 . The unit cell volume of olivine increases systematically with increasing TiO 2 content demonstrating that the measured TiO 2 contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (A ˚3)=290.12(1) + 23.67(85) N Ti. The partial molar volume of end-member Mg 2 TiO 4 olivine (N Ti =1) is thus 47.24±0.13 cm 3 . The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO 2 contents with pressure and in all the experiments olivine contains $0.2 wt.% TiO 2 . Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

“…The dashed line is a best fit through the deconvoluted data concentration of~60 ppm (Fig. 8), which is close to the maximum solubility of Ti in olivine under anhydrous conditions at the same temperature and pressure, as estimated from the experimental study of Hermann et al (2005). This illustrates how Ti solubility in olivine is dramatically increased at high water activities.…”

“…The R 3+ 4/3 SiO 4 component is the anhydrous substitution of R 3+ into olivine, charge-balanced by octahedral site vacancies (Evans et al 2008), while the Mg 2 TiO 4 component is the anhydrous substitution of Ti for Si in olivine studied by Hermann et al (2005). Therefore, the observation that [Ti] is the dominant mechanism for water incorporation in olivine from San Carlos xenoliths ) may reflect internal re-equilibration during cooling to some extent as yet unknown.…”

Multi-stage emplacement of the Götemar Pluton, SE Sweden: new evidence inferred from field observations and microfabric analysis, including cathodoluminescence microscopy

“…Several authors link the occurrence of blue CL emissions to Ti 4? in tetrahedral sites, for example (albite rich) K-feldspars (Lee et al 2007;Parsons et al 2008), in quartz crystals (Rusk et al 2006), and olivine (Hermann et al 2005;Berry et al 2007). …”

Ti–in–garnet thermometer for ultrahigh–temperature granulites

“…These two are inconsistent each other. The Ti content in olivine was found to increase with pressure (Dobrzhinetskaya et al, 2000) whereas Hermann et al (2005) demonstrated that it decreases with pressure. The Ti-in-quartz thermometer has been experimentally (Wark and Watson, 2006;Ostapenko et al, 2007;Thomas et al, 2010;Huang and Audétat, 2012) and empirically (Kawasaki and Osanai, 2008) calibrated for the temperature estimation of ultrahigh-temperature granulites.…”

“…Sepp and Kunzmann (2001) investigated the substitution of Ti 4+ for Si 4+ on tetrahedral sites in clinopyroxene in the system CaO-MgO-SiO 2 -TiO 2 at pressures from 1 atm to 20 kbar and temperatures from 800 to 1200°C and concluded that the solubility of Ti increases with increasing temperature, whereas it decreases with increasing pressure. The solubility of Ti in olivine coexisting with ilmenite was experimentally calibrated by Dobrzhinetskaya et al (2000) and Hermann et al (2005). These two are inconsistent each other.…”