Work

Sulfur isotope evidence for shale recycling in pyroxenites from the Beni Bousera orogenic massif (Morocco); insights on the HIMU mantle endmember

Year: 2026

Type: article

Abstract: The recycling of surface materials into the mantle shapes the chemical diversity of oceanic basalts. The Beni Bousera orogenic massif (Morocco) provides a unique window into these processes, exposing a great diversity of mantle pyroxenites, including samples formed from the recycling of lower oceanic crust. This massif hosts especially rare graphite-bearing pyroxenites, with evidence that graphite was once diamond. Here we present sulfur isotope compositions (δ 34 S, Δ 33 S, Δ 36 S) of graphite-free and graphite-bearing pyroxenites from Beni Bousera. Graphite-free pyroxenites preserve near-zero sulfur isotope signatures with low sulfur contents (~300 ppm), similar to MORB. In contrast, graphite-bearing pyroxenites exhibit fractionated isotopic signatures, with δ 34 S ~ −3.5‰, Δ 33 S ~ +0.065‰ and Δ 36 S ~ −0.45‰. Those are associated with extreme sulfur enrichment, up to 5600 ppm. Mass balance calculations indicate that these signatures reflect the addition of ~5% black shale to an igneous oceanic crust protolith. The Δ 36 S/Δ 33 S systematics and the magnitude of Δ 33 S anomalies suggest the recycled shale is of post-Archean age. The sulfur isotope signatures documented in Beni Bousera are similar to those observed in basalts from the Mangaia HIMU plume. We propose that partial melting of shale-hybridized pyroxenites, analogous to those at Beni Bousera, generates metasomatic melts that transfer carbon and sulfur with fractionated isotope signatures to plume sources. The radiogenic lead isotope composition of Mangaia, however, remains consistent with the classical model of slab devolatilization and U/Pb fractionation during subduction. Our mechanism reconciles the classical HIMU model with the growing evidence for carbon and sulfur enrichment in the Mangaia HIMU mantle source.

Source: Chemical Geology

DOI: 10.1016/j.chemgeo.2026.123735

Authors: Jabrane Labidi, Jean-Pierre Lorand, Pierre Cartigny, Fatima El Atrassi