Heavy 𝛿57Fe in ocean island basalts: A non-unique signature of processes and source lithologies in the mantle

Soderman, Matthews, Shorttle, Jackson, Ruttor, Nebel, Turner, Beier, Millet, Widom, Humayun, Williams, GCA 292, 309-332 (2021). doi: 10.1016/j.gca.2020.09.033

Despite being many tens of kilometres beneath our feet, Earth’s mantle plays an important role in the development of our planet. It acts as a vast chemical reservoir, exchanging with the surface through volcanism and subduction of tectonic plates. The mantle may also act as an archive of the chemical and tectonic changes that have occurred during our planet’s life.

An important tracer of past tectonics is the presence of ancient recycled crust, returned to the mantle by subduction. The presence of recycled crust has been inferred beneath volcanic islands such as Hawaii and Iceland, thought to have been transported in hot upwellings from the base of the mantle. However, most of the geochemical tools available to us only imply the presence of recycled crust indirectly. A more direct observation must relate to the mineralogical makeup of the mantle component, also known as its lithology.

Fe makes up a significant proportion of mantle rocks, and (in-part) determines their lithology. Subtle fractionations in its isotopes exist between different minerals due to variations in the way Fe atoms are bonded in their crystal structures. ‘Heavy’ Fe-isotope signatures have been linked to melting of recycled crust, but lithology is not a unique control on Fe-isotope fractionations. Here we combined new data with some novel models to assess the role of recycled crust in generating the Fe-isotope variability we observe in erupted lavas.

The image is Figure 6 from the manuscript. This shows the results of THERMOCALC phase-equilibra calculations for two mantle lithologies: KLB-1 (more ‘normal’ mantle) and G2 (‘recycled’ mantle material). Software we developed allowed us to calculate the degree of Fe-isotope fractionation that would be generated as these mantle components produce magma.

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