A subduction-modified source for the parental melts to MARID assemblages from the Kaapvaal craton lithosphere and their formation via hybridisation with wall-rock peridotite

Abstract

MARID (mica-amphibole-rutile-ilmenite-diopside) rocks are a suite of mantle xenoliths that provide insight into metasomatic processes in the Earth’s lithospheric mantle. However, many aspects of MARID formation, including the identity of their parental melts, remain uncertain. We present the first set of whole rock Re-Os isotope data from four MARID xenoliths derived from the Kimberley kimberlite cluster, South Africa and combine these results with new in situ zircon U–Pb, Lu–Hf isotope results to provide insight on MARID petrogenesis. MARIDs have highly variable Os isotope signatures (γOs_i = -12 to 116) that can be explained by mixing between a “pure” MARID composition and lithospheric peridotite, clearly indicating MARID formation by melt-rock interaction and “hybridisation”. The highest γOs_i values indicate that their parental magmas are derived from a subduction-related source, stored in cratonic mantle lithosphere. Zircon U–Pb data from MARID sample AJE-2422 yields ²⁰⁶Pb/²³⁸U ages ranging from 86 to 126 Ma (n = 22) with modes at 90.6 Ma, 94.6 Ma, 110.1 Ma, and 124.2 Ma. Zircon Lu–Hf data shows a uniform, non-radiogenic Hf isotope signature (average εHf_i = -17.1 ± 0.9; n = 22) across the span recorded in U–Pb ages. This Hf isotope signature is distinct from typical kimberlites but is similar to South African lamproites and their magmatic source regions. We propose a model where zircons in MARID sample AJE-2422 formed via the passage of a lamproite-like melt coinciding with our oldest observed ages (~ 125 Ma).