Prospective Pyroxenite–Peridotite Mixed Mantle Source for the Northern Carlsberg Ridge

Abstract

The geochemical heterogeneity of the mantle source beneath the mid-ocean ridge has been well studied, while the lithologic variability within the source yet rarely mentioned. We reported new major, trace elements and Sr-Nd-Pb isotopic data of fresh mid-ocean ridge basalts from the northern part of Carlsberg Ridge (i.e., CR Seg-1). The data favors the presence of a small amount of garnet-pyroxenite in the peridotitic mantle source. The smaller spreading rate, larger axial depth, and higher mantle Bouguer anomaly of the Seg-1, all indicate magma deficiency and smaller melting degree, while the lower average Na 8 and higher Ca 8 /Al 8 show a higher calculated melting degree for this segment. The contradictions may require additional fertile material in the mantle source apart from the peridotite. The garnet signature revealed by the rare earth elements, source enrichment evidenced by Sr-Nd-Pb isotopes, and low CaO content and high Zn/Fe and Fe/Mn ratios, all supports the presence of a small amount of garnet-pyroxenite. The lower 206 Pb/ 204 Pb indicates that the enriched components may have originated from the lower continental crust materials or subducted materials involved during the breakup of the Gondwana supercontinent. The significant positive correlation between Fe/Mn, and 206 Pb/ 204 Pb, indicates that the pyroxene-rich rock may have been transformed from the delaminated continental crust. Thus, we proposed that the recycled materials may not be completely homogenized in the convective mantle. The retained material would then lead to lithological heterogeneity, probably in the form of pyroxenite, and cause geochemical anomalies within the source mantle for mid-ocean ridge basalt.

Keywords: Carlsberg Ridge, mantle source, lithological heterogeneity, pyroxenite, Gondwana Supercontinent