Petrogenesis and Geodynamic Significance of Xenolithic Eclogites

Kimberlite-borne xenolithic eclogites, typically occurring in or near cratons, have long been recognized as remnants of Precambrian subducted oceanic crust that have undergone partial melting to yield granitoids similar to the Archaean continental crust. While some eclogitized oceanic crust was emplaced into cratonic lithospheres, the majority was deeply subducted to form lithologic and geochemical heterogeneities in the convecting mantle. If we accept that most xenolithic eclogites originally formed at Earth's surface, then their geodynamic significance encompasses four tectonic environments: ( a) spreading ridges, where precursors formed by partial melting of convecting mantle and subsequent melt differentiation; ( b) subduction zones, where oceanic crust was metamorphosed and interacted with other slab lithologies; ( c) the cratonic mantle lithosphere, where the eclogite source was variably modified subsequent to emplacement in Mesoarchaean to Palaeoproterozoic time; and ( d) the convecting mantle, into which the vast majority of subduction-modified oceanic crust not captured in the cratonic lithosphere was recycled. ▪ Xenolithic eclogites are fragments of 3.0–1.8 Ga oceanic crust and signal robust subduction tectonics from the Mesoarchean. ▪ Multiple constraints indicate an origin as variably differentiated oceanic crust, subduction metamorphism, and prolonged mantle residence. ▪ Xenolithic eclogites thus permit investigation of deep geochemical cycles related to recycling of Precambrian oceanic crust. ▪ They help constrain asthenosphere thermal plus redox evolution and contribute to cratonic physical properties and mineral endowments. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.