Study reveals low-degree melt metasomatic origin of heavy Fe isotope enrichment in mid-ocean ridge basalt mantle

Studies of mid-ocean ridge basalts (MORBs) show a variable Fe isotope composition of the oceanic upper mantle, beyond explanation by processes of mantle melting from a uniform source and magma evolution, indicating Fe isotope heterogeneity in the oceanic upper mantle. However, the origin of upper mantle Fe isotope heterogeneity remains enigmatic.

In order to verify the hypothesis that MORB mantle heavy Fe isotope (56, ⁵⁷Fe vs. light ⁵⁴Fe) enrichment results from the same process of incompatible element enrichment associated with low-degree melt mantle metasomatism in Earth's history, a research team led by Dr. Guo Pengyuan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) conducted an Fe isotope study of well-characterized MORB samples from a magmatically robust segment (OH-1) of the Mid-Atlantic Ridge (MAR) at ~ 35°N.

The study was published in Earth and Planetary Science Letters.

The studied 18 samples showed large ^δ56Fe variation, reflecting a rather large MORB Fe isotope variability on such a ridge segment scale. "What is striking is the statistically significant correlations of ^δ56Fe_corr with the abundances and ratios of more-to-less incompatible elements, and even with Sr-Nd-Hf isotopes," said Dr. Guo.

The large ^δ56Fe variation and the significant correlations with abundances and ratios of incompatible elements and with radiogenic isotopes of the OH-1 lavas were not caused by seawater alteration, partial melting, magma evolution and mixing in the magma chamber; they reflected a clear mantle source inheritance beneath this part of the MAR.

Low-degree melting can effectively fractionate elements of varying incompatibilities, especially between elements with small or subtle incompatibility difference. Thus, the correlated Fe isotope variation (^δ56Fe = +0.03 to +0.18 ‰) with the abundances and ratios of incompatible elements indicates a common process that leads to enrichment of heavy Fe and incompatible elements.

"We explain this process as low-F melt mantle metasomatism taking place at the oceanic lithosphere-asthenosphere boundary in earth's history," said Dr. Guo. Melting of such recycled composite lithologies beneath the present-day sub-ridge produces MORB melts with geochemical correlations as observed at ridge segment OH-1 of the MAR. The positive correlation of ^δ56Fe with ⁸⁷Sr/⁸⁶Sr and negative correlations with ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf further indicate an ancient formation of the low-F melt metasomatic mantle.

"We suggest that low-F melt mantle metasomatism causing upper mantle heavy Fe isotope enrichment is a globally common process and widespread, because the available global MORB Fe isotopes show systematic correlations with the abundances and ratios of their incompatible elements," said Dr. Guo.