O ZONING IN ECLOGITE GARNET - A RECORD OF EVOLVING HIGH PRESSURE FLUIDS

Eclogites contain garnet, a mineral that records metamorphic history and preserves evidence of P-T changes and fluid interactions through chemical zoning. Understanding the characteristics of fluids present in the burial of supracrustal rocks to great depth is vital to studies of metamorphic reactions, earthquakes, and melting. O isotope ratios in minerals are valuable as monitors of these fluid interactions. Secondary Ion Mass Spectrometry (SIMS) allows the measurement of δ¹⁸O on a 10 mm scale in garnet with a precision of ±0.3‰ (2SD).

12 prograde garnets from eclogites in country-rock gneisses were examined for δ¹⁸O zoning by SIMS, selected from 65 eclogite garnet samples analyzed by laser fluorination. These 12 eclogites are from the Variscan Bohemian Massif; Trescolmen, Alps; the Chatanika eclogite, Yukon-Tanana Terrane, Alaska; and the Scandian Nordfjord region, Norway. Bulk garnet δ¹⁸O of these eclogites are outside the typical mantle range (5-6‰), and include both low (-0.4 to 3.7‰) and high (8.2 to 13.8‰) δ¹⁸O values.

This first detailed study of δ¹⁸O zoning in eclogite garnet reveals a common pattern for garnet (independent of bulk garnet δ¹⁸O): lower δ¹⁸O cores with higher δ¹⁸O rims, differing by ~1.5-2.5‰. This pattern, present in 9 out of 12 garnets, correlates with an increase in the mole fraction of pyrope from core to rim, and suggests fluid infiltration during garnet growth. Intragranular diffusion distances are small (<3-20μm) in all but one of these nine garnets, and fractionations between garnet and other minerals at high temperature are minimal, indicating that δ¹⁸O zoning is not caused by diffusion or varying fractionation. Possible fluid sources include dehydration reactions in surrounding rocks where evolved δ¹⁸O(fluid) can be low, for rocks that exchanged with meteoric water, or high for carbonate-bearing sedimentary rocks. The mantle wedge may also supply interacting fluids or melts if eclogites represent subducted oceanic crust.

A new tool for understanding metamorphism, δ¹⁸O zoning preserved in eclogite garnet records fluid evolution as eclogite precursors are metamorphosed during burial to high pressures. In addition, interpretations of eclogite precursors based solely on laser fluorination data of bulk garnet separates should be viewed with caution, as garnets may be zoned in δ¹⁸O.