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 has yet been rarely mentioned. We report 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 geological and geophysical observations, such as the lesser spreading rate, greater axial depth, and higher mantle Bouguer anomaly of the Seg-1, all indicate magma deficiency and lower melting degree, whereas the segment's lower average Na8 and higher Ca8/Al8 indicate a higher apparent melting degree. These contradictions cannot be reconciled by melting the lithologically homogeneous peridotite mantle. We proposed that fusible pyroxenite components in the Seg-1 source melt preferentially and generate lower Na8 magma. Moreover, it cools the ascending mantle and decreases the supply of melt derived from peridotite, leading to a magma shortage in the segment. The garnet signature revealed by the rare earth elements, higher Zn/Fe and Fe/Mn ratios, and source enrichment evidenced by Sr-Nd-Pb isotopes, support the presence of a trace quantity of garnet-pyroxenite in the mantle source. The mixing model of trace elements supports the origin of Seg-1 MORBs by mixing the depleted peridotite-derived melt and the enriched pyroxenite-derived melt. Based on the significant negative correlation between Fe/Mn and 206Pb/204Pb, the delaminated lower continental crust, which might not be completely homogenized in the convective mantle, may have been transformed into the pyroxene-rich rock. The retained material would then result in lithological heterogeneity, probably in the form of pyroxenite, and cause geochemical anomalies within the source mantle for mid-ocean ridge basalts.