OXYGEN ISOTOPE ZONING IN HIGH TEMPERATURE/HIGH PRESSURE GARNETS

Garnet is a recorder of metamorphic history and can preserve evidence, through chemical zoning, of chemical changes and fluid interactions even under high temperature and high pressure conditions. δ¹⁸O is a valuable monitor of metamorphic history, especially fluid interactions and temperature. Measurement of oxygen isotope ratios at 10 mm scale using secondary ion mass spectrometry (SIMS) is possible with a precision of ±0.3‰ (2sd) and accuracy of ≈0.5‰, if multiple garnet standards are used and chemical composition is analyzed by EPMA (Page et al., 2010). The coupling of major element zoning and δ¹⁸O zoning from core to rim in garnets provides information on relative timing of chemical changes and fluid interactions during prograde metamorphism.

Garnets were selected from the Spacice eclogite boudin and a nearby (≈200m) cpx-granulite boudin in the Béstvina felsic granulite, Moldanubian Zone, Bohemian Massif, Czech Republic. Peak metamorphic conditions are 893°C and 18.2 kbar and 827°C and 11.8 kbar, respectively. Juxtaposition of boudins with surrounding felsic granulite occurred during Variscan high pressure metamorphism (Medaris et al. 1998).

Cation and oxygen isotope zoning in prograde garnet from the granulite is muted. A slight variation in δ¹⁸O (VSMOW) correlates with changes in grossular content; however, most of the δ¹⁸O values are within 3.7±0.4‰ with analytical uncertainty of 0.3‰ (2sd).

The analyzed prograde garnet grain from the Spacice eclogite does not show the full zoning described by Medaris et al. (2006) for other garnets, but still records a strong correlation between d¹⁸O and XPrp. From the center of the analyzed grain, where δ¹⁸O = 2.7 to 2.0‰ and pyrope ≈ 24 mol %, there is an increase in pyrope content and δ¹⁸O to rim values of approximately 27% pyrope and δ¹⁸O = 3.7‰. This trend is opposite of that expected for a single post-magmatic event. We interpret the inverted oxygen isotope pattern to reflect metamorphism of a hydrothermally altered basaltic protolith. Variability in d¹⁸O is not a result of changes in T or P. Instead, the core of the garnet has inherited a low d¹⁸O signature from hydrothermal alteration of lower oceanic crust. Subsequent exchange with higher d¹⁸O minerals and fluids during subduction, metasomatism, and HP/HT metamorphism has caused d¹⁸O to increase during garnet growth.