A MONAZITE OXYGEN ISOTOPE THERMOMETER

We have made a preliminary calibration of a quartz-monazite oxygen isotope thermometer based on d¹⁸O measurements of mineral separates from monazite-bearing rocks and on pilot piston cylinder calcite-monazite oxygen exchange experiments.

The production of POF_x compounds during fluorination of phosphate minerals complicates the accurate determination of d¹⁸O_mnz. We evaluated this potential problem by extracting oxygen from monazite using three techniques: CO₂ laser fluorination, conventional fluorination, and in situ UV laser extraction. 100 mbar of BrF₅ and 200 mbar of F₂ gas were used as reagents for laser fluorination. Analyses with F₂ result in d¹⁸O_mnz ~1‰ higher than analyses with BrF₅, but ~1‰ lower than conventional fluorination analyses. The 1s precision of all bulk fluorination techniques is ≤±0.2‰. Measurements of oxygen extracted in situ from ~200 mm domains are similar to values from laser fluorination with F₂ and are reproducible within ±0.3‰ (1s).

Mineral separates were analyzed via laser fluorination with F₂. Monazite-calcite high-pressure exchange experiments (95 wt% mnz, 5 wt% cc) were conducted at 1000°C for varying lengths of time. Measured d¹⁸O_cc and calculated d¹⁸O_mnz (assuming mass balance) reached equilibrium in ~1 day. A linear regression through empirical and experimental data plotted as 1000lna_qtz-mnz vs. 10⁶/T² has a slope of 2.2 and a y-intercept of 1.2‰. If laser fluorination with F₂ results in d¹⁸O_mnz ~1‰ too low, the theoretical constraint that the regression line pass through the origin is satisfied. We therefore suggest that conventional fluorination results in accurate determinations of d¹⁸O_mnz and that a_qtz-mnz=2.2. This value is large enough that an increase in the precision of monazite analyses to ±0.1‰ will allow temperatures of sub-granulite facies metamorphism to be estimated with an uncertainty of < ±50°C.

Measurements of D¹⁸O_qtz-mnz in a granulite facies orthogneiss from the Wind River Range are consistent with the conclusion that monazite escaped the retrograde reequilibration suffered by magnetite and orthopyroxene and preserves peak metamorphic temperatures of 815 ±50°C estimated by Sharp and Essene (1991). This evidence implies that monazite can be used to constrain peak metamorphic conditions in amphibolite and granulite grade rocks.