Public Evening Lecture
Tuesday October 10, 19:00 in lecture room H8, GEO
Prof. Dr. Dan Frost
Bayerisches Geoinstitut Bayreuth, awardee of Leibniz prize
The formation of diamonds
and their journey to the surface
Diamonds are formed in the Earth’s silicate mantle at depths exceeding 140 km. The majority of diamonds appear to have formed within cratonic lithospheric mantle, beneath regions of Archean continental crust where the lithosphere extends to depths of around 200 km. However, some diamonds contain mineral inclusions that attest to formation depths significantly deeper than the lithosphere and were likely formed within the convecting mantle. The occurrence of these so called sub lithospheric diamonds implies the occurrence of a deep carbon cycle that returns carbon from the Earth’s surface within subduction zones to significant depths.
The depth and extent of carbon transport will depend on the forms in which carbon exists under various conditions. In the form of carbon dioxide or carbonate melts carbon can migrate and rise out of the mantle on a relatively short time scale. On the other hand, when carbon forms diamond it remains solid at mantle temperatures and can, therefore, remain fixed in the mantle for billions of years. The redox processes that determine the speciation of carbon have likely had a strong influence on the Earth’s retention of carbon. Sub lithospheric diamonds are only found in regions of thick cratonic lithosphere, implying that solid state convective flow is responsible for their transport from the deeper mantle into the lithosphere.
Diamonds are finally brought to the surface by carbon dioxide-rich volcanic magmas, that are themselves evidence for the deep transport of carbon and its involvement in deep melting processes. Through the analysis of recovered sub lithospheric diamonds in conjunction with interpretation through high pressure experiments we can constrain the role of the Earth’s interior in the carbon cycle, in order to ultimately understand the extent to which the interior may have influenced the surface availability of carbon over Earth’s history.