Evolution of the mantle and crust

The geochemical composition of the mantle and crust have changed over Earths’ history. We reconstruct this compositional evolution using a variety of samples such as mantle xenoliths, diamonds, as well as crustal rocks and sediments. Combining geochronological methods to determine the age of the sample with other geochemical constraints allows us to infer changing mass and element transfer processes between these reservoirs. For example, the of formation and evolution of early Archean continental crust through time is studied through Sr isotope compositions in apatite inclusions in zircons.

Although direct access to mantle material is more difficult, xenoliths (150-250 km) and diamonds from depths of up to 800 km brought up to the surface by kimberlite and lamproite magmas provide rare samples directly from the mantle. Peridotitic mantle xenoliths recovered from kimberlite in cratonic lithospheric mantle and abyssal peridotites from oceanic mantle are used to study the depletion of elements within in the mantle due to crust formation. Inclusions in diamonds provide a unique opportunity to study an unaltered record from the sub-continental lithospheric mantle as well as the sub-lithosphere down to depths of +/- 800 km. Diamonds have been formed over at least the past 3.2 Ga and hence provide the rare chance to answer questions regarding the evolution of the mantle for a large part of Earth’s history. Further, these samples are used to better understand the deep carbon cycle, diamond growth conditions, and redox conditions and other chemical variations over a large depth range, as well as tracing recycling processes in the mantle. 

Mantle xenolith
Mantle xenolith (garnet lherzolite) in kimberlite drilled core.
Eclogitic garnet inclusion
Eclogitic garnet inclusion in diamond from Udachnaya (80x mag). Foto: Suzette Timmerman
Metamorphic Tonality Gneiss
Remelting and melt migration in 3.47Ga Older Metamorphic Tonality Gneiss (OMTG) from Chapua area in Singhbhum Craton. Foto: Sukalpa Chatterjee.