Sr-Nd-Pb isotopes of fluids in diamond record two-stage modification of the continental lithosphere

High-density fluid (HDF) microinclusions in diamonds allow direct investigation of deep carbon- and water-rich fluids that influence the properties of Earth’s mantle. Identifying the sources and evolution of such fluids in the context of different potential mantle reservoirs is difficult due to the limited radiogenic isotope data. Here, we report Sr-Nd-Pb isotope compositions of silicic to low-Mg carbonatitic HDFs in a suite of diamonds from a single source in Canada. Relationships between isotopes and trace element compositions indicate the contribution of two distinct sources within the continental lithosphere: one with relatively primitive isotopic compositions characterised by εNd of −0.2, ⁸⁷Sr/⁸⁶Sr of 0.7044 and ²⁰⁶Pb/²⁰⁴Pb of 17.52, and another with more unradiogenic εNd < −16 and radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb > 0.713 and 18.3, respectively. We suggest that the latter reflects an old metasomatic event in the continental lithosphere involving fluid addition from a subducting slab, most probably in the Paleoproterozoic. HDFs formed and their host diamonds crystallised in a more recent metasomatic event, indicated by the unaggregated nitrogen of the diamonds, where fluids from both sources mixed. HDFs from Canada, Botswana, and Congo have comparable isotope-trace element relationships, suggesting contributions of similar sources in distinct lithospheric provinces worldwide.