Granites from the Earth’s Mantle

Fragments of the deep interior of the Earth, the mantle, are extremely rare at the surface. What should never be found in these regions are areas of granitic rock for there is no simple way in which the two should ever be linked. Thus when we discovered granitic intrusions in the mantle section of the Oman ophiolite we knew we had stumbled upon something remarkable. The research was funded by a grant from National Geographic and we commissioned detail geochemical analysis of the samples in Canada. Our results showed that the granites had a geochemical fingerprint of melted sediments allowing us to build a geochemical model of how this strange geological association has come into being. In order to explain the unusual field relationship and the geochemical signals we infer that sediment from the ocean floor was subducted deep into the mantle where it melted and then rose up through the overlying mantle to become frozen in place where we find it today.

Collaborative research with colleagues at Curtin University in Western Australia has shown that tiny zircon crystals from the granites about 1 mm in length also contain a remarkable chemical signal. Christopher Spencer and colleagues have shown that the oxygen isotope signal of these zircon crystals is higher than any known terrestrial samples, further confirming that this material, now found deep in the Earth, originated at the Earth’s surface. These results demonstrate that our granitic material was originally marine mud transported many tens of kilometres into the deep Earth where it melted and rose to solidify as granite in the Earth’s mantle.‌

Contact: Professor Hugh Rollinson