THE PRESSURE AND TEMPERATURE DEPENDENCE OF ZR AND TI SUBSTITUTION IN ALMANDINE

The P-T dependence of Zr and Ti incorporation in almandine garnet has been assessed in high pressure and temperature experiments employing a piston-cylinder apparatus with NaCl-graphite furnaces. Starting materials were well-characterized, synthetic pure and Zr- or Ti-doped almandine (1000 and 5000 ppm, respectively), and natural quartz, zircon, and rutile. Experiments were unbuffered with respect to H₂ fugacity. Trace to minor Zr substitution (50 –1000 ppm) was studied by equilibrating stoichiometric almandine, zircon and quartz in nominally dry Ag₈₀Pd₂₀ capsules at 800–1100 ºC, 5-20 kbar. This substitution appears to follow a kimzeyite-type coupled exchange (Zr^VIAl^IV Al^VI_-1Si^IV_-1) as buffered by the assemblage almandine-zircon-quartz (GZQ) via the reaction Fe₃Al₂Si₃O₁₂ + ZrSiO₄=Fe₃(AlZr)(Si₂Al)O₁₂ + 2SiO₂. EMPA and SIMS analyses on run-product almandine show a strong decrease in Zr with pressure and a strong increase with temperature. Compositions were reversed by equilibrating the Zr-doped almandine with zircon and quartz over the same P-T range. Rims of lower Zr-almandine are evident in these run products, particularly in those of the highest temperature runs. The newly grown rims are extremely thin and difficult to analyze but do appear to be approaching the values of the forward reaction. A multiple linear regression of our data yields the following equation for Zr content in almandine: Zr (ppm)=–453.9140 – 19.2721P + 0.8644T,where P is in kbars, T is in ºC and the R²=0.977. P-T estimates made using the GZQ thermobarometer on natural samples agree well with conventional K_D thermobarometers and P-T net predictions. Minor substitution of Ti (1000-5000 ppm) in garnet was investigated by equilibrating the assemblage almandine-quartz-rutile (GRQ) at 800-1000° C, 13-20 kbar. This substitution appears to be controlled by the coupled exchange Ti^VIAl^IV Al^VI_-1Si^IV_-1 in the GRQ system via the reaction Fe₃Al₂Si₃O₁₂ + TiO₂=Fe₃(AlTi)(Si₂Al)O₁₂ + SiO₂. Preliminary analyses of the product almandine indicate isobaric increase in Ti with T and a strong isothermal decrease with P. This is consistent with observations in natural assemblages and will have application to eclogites and other high and ultra-high pressure rocks.