High-temperature inter-mineral magnesium isotope fractionation in eclogite from the Dabie orogen, China

Abstract

To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at high temperatures, we report high-precision analyses of Mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from Bixiling in the Dabie orogen, China. Magnesium isotopic compositions of whole rocks (δ²⁶Mg of − 0.44 to − 0.26‰) are similar to the estimated δ²⁶Mg values of the mantle, suggesting Mg isotopic inheritance from a gabbroic protolith with limited Mg isotope fractionation during eclogite-facies metamorphism. By contrast, mineral separates are highly heterogeneous, with δ²⁶Mg values ranging from + 0.30 to + 0.60‰ in phengite, from + 0.16 to + 0.40‰ in omphacite and from − 0.95 to − 0.74‰ in garnet. Phengite and omphacite are > 1‰ heavier in δ²⁶Mg than coexisting garnet, indicating large high-temperature inter-mineral Mg isotope fractionations. The constant Δ²⁶Mg_{omphacite–garnet} (= δ²⁶Mg_omphacite − δ²⁶Mg_garnet) value (1.14 ± 0.04‰), together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet, suggests an equilibrium Mg isotope fractionation, controlled by the difference in coordination number of Mg between omphacite (six) and garnet (eight). The 1.14‰ fractionation is the largest high-temperature equilibrium inter-mineral Mg isotope fractionation observed so far and makes the omphacite-garnet Mg isotope fractionation a potential geothermometer. By contrast, Mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1.25 to 1.47‰ and from 0.14 to 0.32‰, respectively. This implies Mg isotopic disequilibria between phengite and garnet/omphacite, which might result from the Mg isotopic variation in phengites due to Mg isotope exchange between phengites and retrograde fluids in Bixiling eclogites.

Introduction

Magnesium is a major element in the mantle, crust and hydrosphere with > 8% relative mass difference between ²⁶Mg and ²⁴Mg, making it potentially an excellent isotopic tracer of geological processes. Knowledge on behaviors of Mg isotopes during different geological processes is the prerequisite for applying Mg isotopes as tracers. In particular, studying the magnitude and mechanism of high-temperature inter-mineral Mg isotope fractionation is crucial for applying Mg isotopic systematics in magmatic and metamorphic systems.

Previous studies have found small Mg isotope fractionation between coexisting olivines and pyroxenes in mantle xenoliths (Handler et al., 2009, Yang et al., 2009) and between coexisting hornblendes and biotites in granitoids (Liu et al., 2010). Such limited inter-mineral Mg isotope fractionations are consistent with theoretical studies as all of these major Mg-rich minerals have the same Mg coordination (i.e., six; Deer et al., 1992), which doesn't favor large inter-mineral Mg isotope fractionation at high temperatures (Liu et al., 2010). By contrast, Mg coordination is different among major Mg-rich minerals in eclogites, with coordination number of Mg being eight in garnet and six in omphacite and phengite (Deer et al., 1992). Hence, large Mg isotope fractionation is expected to occur between garnet and coexisting omphacite and phengite in eclogites, with garnet isotopically lighter than omphacite and phengite. However, to our knowledge, no single Mg isotopic datum has been published for eclogites.

In order to investigate the magnitude and mechanism of high-temperature Mg isotope fractionation between coexisting garnet, omphacite and phengite, we carried out high-precision Mg isotopic analyses on whole rocks and mineral separates from Bixiling eclogites in the Dabie orogen, China. Our results demonstrate that Bixiling eclogites have mantle-like Mg isotopic composition, suggesting limited Mg isotope fractionation during eclogite-facies metamorphism of mafic rocks. By contrast, large Mg isotope fractionation occurs between garnet and coexisting omphacite and phengite. Whereas phengite may be in Mg isotopic disequilibrium with garnet and omphacite, the omphacite–garnet is in equilibrium, with garnet 1.14‰ lighter than omphacite. The largest so far recognized high-temperature equilibrium Mg isotope fractionation between omphacite and garnet makes it a potential geothermometer.