Eclogite formation beneath the northern Slave craton constrained by diamond inclusions: Oceanic lithosphere origin without a crustal signature

We report the geochemical and oxygen isotope compositions for eclogitic mineral inclusions in diamonds hosted by high-MgO eclogite xenoliths from the Jericho kimberlite, Canada. These data are used to constrain the nature and evolution of the eclogite protolith. The garnet and clinopyroxene diamond inclusions (DIs) are compositionally different than their host eclogite counterparts. In particular, garnet DIs have much lower Mg-numbers (54 vs. 82) and Cr2O3 contents (0.1 vs. 0.6 wt.%) and higher CaO contents (7.6 vs. 4.3 wt.%) than host eclogite garnet. DI and host eclogite clinopyroxenes are more similar but differences include lower Mg-numbers (78-81 vs. 93) and higher Na2O contents (23 vs. 1.8 wt.%) in the DIs. The DIs lack typical shallow oceanic crust signatures such as strong positive Eu and Sr anomalies, and oxygen isotope compositions that deviate significantly from the pristine mantle average. On the contrary, both the Jericho Dls and host eclogite garnets have small negative Eu and Sr anomalies, fractionated HREE patterns ((Lu-N/Gd-N)similar to 3-5) and pristine mantle-like delta O-18 values of 5.2-6.0%. indicating that shallow, plagioclase-rich oceanic crust protoliths are unlikely.

The eclogitic DI trace-element characteristics require that both garnet and plagioclase were present in the protolith, which likely crystallized in the shallow upper mantle. DI-based reconstructed whole-rock eclogite compositions have higher Mg-numbers and lower Al2O3 contents than found in typical basaltic or gabbroic oceanic crust, and are similar to pyroxenitic veins found in orogenic peridotite massifs. Due to the lack of clear oceanic crust signatures and the mantle-like delta O-18 values of the studied Dls, we propose that the Jericho diamond eclogites originally crystallized as pyroxenite cumulates that formed veins within the oceanic mantle lithosphere. Following partial melt extraction, the eclogite protoliths were subducted into the diamond stability field beneath the evolving Slave craton. Hence, the Jericho Dls and host high-Mg eclogites may represent an example of eclogite formation in an oceanic setting without the diagnostic 'crustal signatures' that are typically observed in cratonic eclogite xenolith suites worldwide. (C) 2011 Elsevier B.V. All rights reserved.