CALCITE-CORUNDUM OXYGEN ISOTOPE FRACTIONATIONS IN NAXOS META-BAUXITES: SELECTING SAMPLES FOR THERMOMETRY

Discontinuous horizons of corundum-rich lenses, interpreted as meta-karst bauxites, are enclosed within greenschist (~400°C) to amphibolite facies (~700°C) marble on Naxos, Cyclades, Greece. The large range of metamorphic temperature, mineralogy, grain size, and modal proportions permit tests of different strategies to improve accuracy of oxygen isotope thermometry. Diffusion modeling predicts that Refractory Accessory Minerals (RAM) will provide accurate thermometers when applied to rocks modally dominated by a mineral with faster diffusion and below the closure temperature (Tc, for diffusion of oxygen) of the RAM (Valley, RIMG 2001). Corundum (Crn) has Tc > 700°C even for slow cooling and fine grain size. Calcite (Cc) has significantly faster diffusion. Appropriate samples for RAM thermometry should have: (1) high modal Cc/Crn ratio; (2) absence of thick Ca-Al silicates reaction rims (0.1-1 m) and low abundance (<10%) of margarite and anorthite within the sample; (3) lack of secondary or recrystallized calcite; and (4) no growth zoning in corundum. Corundum and calcite coexist abundantly in cm-scale lenses and boudins from the Ky-St pelitic zone. Of the 14 rocks sampled from different localities within this zone 4 have observable calcite-veins, 4 are Ca-Al-silicate-rich and have low Cc/Crn and 6 satisfy all the criteria for RAM thermometry. Corundum was separated either by hand picking and/or as acid-residue. All samples show highly reproducible d¹⁸O_Crn ( ± 0.1 ‰ in a single sample) regardless of grain-size, color or separation method. Crn-Cc is thus an accurate RAM thermometer in the bauxite-marble system on Naxos. Cm-scale variability of d¹⁸O_Cc is ± 1.5 ‰ in the veined samples, ± 0.5 ‰ in the Cc-poor samples and ± 0.12 in the ones defined as suitable for thermometry. The latter give average d¹⁸O_(Cc-Crn)=3.80 ± 0.15 ‰. Calcite-magnetite oxygen isotope thermometry in these samples gave 607° ± 15°C, in agreement with petrologic estimates. Meta-bauxites characterized as suitable for RAM thermometry yielded reproducible and geologically reasonable results, whereas other samples yielded d¹⁸O_(Cc-Crn)=3.74 ± 0.80 ‰. This lays the groundwork for new studies of the thermal gradient during orogenic extension on Naxos that have not been previously possible.