In this study the partitioning of Ti between gt and cpx both in high pressure experiments and in high pressure rocks is investigated to test its potential as geobarothermometer.
Dry and H2O-bearing (3 wt.%) experiments were carried out on Fe-rich (MG 0.32) and Mg-rich (MG 0.74 ) natural eclogites in a piston cylinder and a belt apparatus from 2.0 to 5.5 GPa and from 1100 to 1440 °C. The starting materials were enriched in TiO2 (to 7.5 wt.%). Run products - garnet, clinopyroxene, rutile and melt - were analysed by EMP. The experimental results are compared with those of natural eclogites and garnet peridotites (Fett, 1995).
Experimental results show an increase of Ti solubility both in gt and cpx with temperature. Ti in cpx decreases with pressure whereas Ti in gt is almost independent of pressure. Ti solubility in gt and cpx of the Fe-rich starting material is distinctly higher than in gt and cpx of the Mg-rich starting material. Further, Na solubility in gt increases both with pressure and temperature. Na in cpx increases with pressure but decreases with temperature due to preferred partitioning into coexisting melt.
Different coupled substitution mechanisms are proposed to incorporate fourvalent Ti into gt like
(I) R2+[8]-Al[6] * Na[8]-Ti[6] and
(II) 2Al[6] * R2+[6] + Ti[6] (Akella and Boyd, 1972; Bishop et al., 1978; Huggins et al., 1977). For cpx proposed coupled substitutions for Ti incorporation are
(III) CaTiAl2O6 (TiTs) * R2+[6]-2Si[4] and
(IV) Na[8]-Ti[6] or
(V) Ti[6]-Al[4] with an endmember like the NaTiAlSiO6 component (Akella and Boyd, 1972; Papike, 1982; Carswell, 1991).
(I) is the dominating coupled substitution mechanism for Ti incorporation in gt of natural eclogites and of the Fe-rich starting material. For Ti incorporation in gt and cpx of garnet peridotites and of the Mg-rich starting material besides of (I) a further substitution mechanism like (II) must be influential. In cpx for Ti incorporation (III) and a combination of (IV) and (V) seem to dominate. As a consequence, Ti incorporation in gt and cpx is also influenced by bulk rock composition since realisation of reactions (I), (IV) and (V) depends on the Na offer of the system.
The values of Ti partition coefficients (based on wt.%) both of the high pressure experiments and of natural eclogites and of garnet peridotites are independent of the composition of the bulk rock and in the range of 0.2 to 1.6. Partitioning of Ti between gt and cpx in the Fe-rich starting material is mainly pressure dependent and nearly independent of temperature. In contrast, Ti partitioning in the Mg-rich starting material is influenced stronger through temperature than pressure. Application of the experimental results on natural eclogites and on garnet peridotites indicates both a slight pressure and temperature dependency of the Ti partitioning. Therefore, Ti partitioning between gt and cpx is a function of pressure and temperature which control the dominating Ti substitution mechanisms. Besides of pressure and temperature, metasomatic overprinting of the host rock also can significantly influence the Ti partitioning.
Akella, J. & Boyd, F.R., Carneg. Inst. Year Book 71, 378-384 (1972).
Bishop, F.C., Smith, J.V. & Dawson, J.B., Lithos 11, 155-173 (1978).
Carswell, D.A., Min. Mag. 55, 19-31 (1991).
Fett, A., Ph.D.-thesis, Joh. Gutenberg-Universität Mainz, Germany (1995).
Huggins, F.E., Virgo, D. & Huckenholz, G. , Amer. Mineral. 62, 475-490 (1977).
Papike, J.J., Rev. Mineral. 7, 495-525 (1982).