Eclogites and garnet pyroxenites: Similarities and differences

Eclogites and garnet pyroxenites are genetically linked to basaltic precursors. Traditionally garnet pyroxenites are linked to fractionation of basaltic magmas over a range of mantle pressures and more recently they have been implicated in the genesis of ocean islands. In contrast eclogites are linked to the subduction of slab basalt and gabbro precursors which may be hydrothermally altered. Recently this subduction paradigm has been questioned. We present mineralogical, trace element and O isotopic data for eclogites and garnet pyroxenites that reveal some similarities but also important chemical and isotopic differences that support a distinct provenance (i.e., age and process). Continental and oceanic garnet pyroxenites have relatively homogeneous O isotope compositions identical to peridotitic mantle and P-type solid inclusions in diamonds. In contrast on- and off-craton continental eclogites have heterogeneous O isotopes with similarities to E-type solid inclusions in diamonds. This is consistent with garnet pyroxenites being high temperature derivatives of melts in equilibrium with mantle peridotite but requiring eclogites to have a more multi-stage history. This invalidates arguments that eclogites and garnet pyroxenites are identical, and also raises questions about petrogenetic models for OIB production that have an implicit link between eclogite and garnet pyroxenite.