Sunday, September 25, 2022

Scientists revealed the source of enriched mid-ocean ridge basalts

Recycled oceanic lithosphere generates upper mantle heterogeneity.

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity on various scales. However, the origin of these heterogeneities has been highly debated.

Mix of stable Molybdenum (Mo) isotopes and radiogenic isotopes have incredible potential for examining mantle heterogeneities, particularly those subduction-related cycles. Although, the current Mo isotope database for mid-ocean ridge basalts (MORB) is very small. As a result, the possible effects of magma generation and evolution on Mo isotopes remain debated.

In a new study, scientists from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) performed a systematic analysis of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Their research revealed the source of enriched mid-ocean ridge basalt by Mo isotope systematics of lava.

Scientists studied isotopic compositions on samples on and off-axis across morphologically typical ridge segments of EPR and seamounts lavas on the flanks of EPR between 5°N and 12°N. They found significant Mo isotope variations within samples.

According to scientists, this variation may result from MORB melt generation and evolution processes, reflecting mantle isotopic heterogeneity.

Dr. CHEN Shuo from IOCAS said“MORB Mo isotope compositions varied systematically with geochemical parameters indicating mantle enrichment. This is most likely resulted from two-component mixing between an incompatible element depleted endmember with low δ98/95Mo (~-0.21‰) and an incompatible element enriched endmember with high δ98/95Mo (~-0.05‰).”

Considering the association of heavier Mo isotopes composition with the geochemically more enriched MORB, they excluded the previous model of recycled ocean crust with or without sediment. Instead, based on their new data and modeling, they suggested that the enriched endmember was most consistent with localized low degree melt enrichment within the depleted MORB mantle, most likely at the lithosphere-asthenosphere boundary in Earth’s history.

The study reveals that recycled oceanic mantle lithosphere, metasomatized by low degree melt, plays a key role in forming enriched MORB (E-MORB) source lithologies. In addition, it also highlights Mo isotopes can be an effective tool for studying upper mantle processes.

Journal Reference:

  1. Shuo Chen et al. Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt. DOI: 10.1016/j.epsl.2021.117283
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