Researchers validate the origin of mantle eclogites

Kimberlite is a carrier of diamonds, as well as mantle eclogite xenoliths. Mantle eclogite, dubbed as Christmas rocks, is a recorder for the formation and evolution of craton, growth of continental crust and other geological processes. However, its origin has long been debated with two hypotheses: subduction model suggests that eclogites represent the ancient oceanic crust by subducted metasomatism; magmatic model argues that mantle eclogites were melts crystalized in deep mantles.

A research team led by Prof. HUANG Jian from the University of Science and Technology of China (USTC) and his collaborators utilized Zn-, Mg- and O- isotope as footprint, and validated that eclogoites from the Roberts Victor (RV) kimberlites from the Kaapvaal craton, South Africa, were crystalized from melts in deep, high temperature mantles. This work was published in Geology. 

RV eclogite can be divided as Type I and Type II. Type I eclogites are derived from intense metasomatism of Type II ones which contain the properties of protoliths. The carbonate-rich, altered oceanic crust is enriched in light Mg- and heavy O isotope, which is inconsistent with Type II ecogites characterized by light Mg- and O isotope. Hence, the subducted oceanic crust recycling is insufficient to interpret the origins of eclogites. The deep mantles in Kaapvaal craton area contain compositions of light O isotope, and the negative correlation between δ66Zn and δ26Mg of Type II ecogites, reflecting kinetic isotopic fractionation of Zn- Mg- isotope produced by deep melts-peridotites. 

The light O isotope-rich magma intruded upward and the kinetic isotopic fractionation of Zn- Mg- isotope triggered by the melt-peridotites reaction, leading to the heavy Zn- and light Mg isotope enrichment in the migrating melts. The melts further invaded the lithosphere-asthenosphere boundary (LAB), cooled and crystalized to Type II eclogites. 

This study verifies the hypotheses of the magmatic model, and sheds new light on the origins of mantle eclogites and the kinetic isotopic fractionation at mantle depths.