The effects of magmatic processes and crustal recycling on the molybdenum stable isotopic composition of Mid-Ocean Ridge Basalts

Bezard R., Fischer-Gödde M., Hamelin C., Brennecka G., Kleine T.

Abstract

Molybdenum (Mo) stable isotopes hold great potential to investigate the processes involved in planetary formation and differentiation. However their use is currently hampered by the lack of understanding of the dominant controls driving mass-dependent fractionations at high temperature. Here we investigate the role of magmatic processes and mantle source heterogeneities on the Mo isotope composition of Mid-Ocean Ridges Basalts (MORBs) using samples from two contrasting ridge segments: (1) the extremely fast spreading Pacific–Antarctic (66–41°S) section devoid of plume influence and; (2) the slow spreading Mohns–Knipovich segment (77–71°N) intercepted by the Jan Mayen Plume (71°N). We show that significant variations in Mo stable isotope composition exist in MORBs with δ98/95Mo ranging from −0.24‰ to +0.15‰ (relative to NIST SRM3134). The absence of correlation between δ98/95Mo and indices of magma differentiation or partial melting suggests a negligible impact of these processes on the isotopic variations observed. On the other hand, the δ98/95Mo variations seem to be associated with changes in radiogenic isotope signatures and rare earth element ratios (e.g., (La/Sm)N), suggesting mantle source heterogeneities as a dominant factor for the δ98/95Mo variations amongst MORBs. The heaviest Mo isotope compositions correspond to the most enriched signatures, suggesting that recycled crustal components are isotopically heavy compared to the uncontaminated depleted mantle. The uncontaminated depleted mantle shows slightly sub-chondritic δ98/95Mo, which cannot be produced by core formation and, therefore, more likely result from extensive anterior partial melting of the mantle. Consequently, the primitive δ98/95Mo composition of the depleted mantle appears overprinted by the effects of both partial melting and crustal recycling.