The phase relations of mafic rocks within the Earths mantle and geobarometers for eclogite and pyroxenite rocks to conditions of the Earths transition zone.

• Applicant: Professor Dr. Daniel J. Frost
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 264531645

While the bulk of the Earths upper mantle is considered to be comprised of olivine dominated ultramafic rocks, there is evidence from mantle xenoliths and inclusions in diamonds for the presence of mafic rock compositions such as eclogites and pyroxenites. Eclogites are likely present in the mantle as a result of subduciton of oceanic crust, however, many pyroxenite rocks may also be present as a result of melting or metasomatic processes occurring in the mantle. As they provide evidence for mantle processes, studying these rocks and interpreting their depth of origin and also their influence on mantle seismic properties is of great importance. Some studies on seismic properties have even proposed that the convecting mantle may comprise a mechanical mixture of ultramafic and mafic rocks. In this study the phase relations of such eclogitic/pyroxenitic rocks will be studied up to conditions of the Earths transition zone, and mainly in the range where these rocks are composed principally of clinopyroxene and garnet. Using these relations, two geobarometers will be calibrated; one that employs the proportion of Al hosted in the clinopyroxene Si-site coexisting with garnet and the other that employs the majorite-garnet transition within such mafic assemblages. Due to the high chemical variance and small number of phases, the mineral compositions within such assemblages are strongly dependent on bulk compositions. In this study phase relations over a wide range of plausible eclogite and pyroxenite compositions will be studied and to maintain internal consistency multiple sample chambers will be employed so many bulk compositions will be equilibrated at the same pressure and temperature simultaneously. In addition further experiments will be performed to asses the effects of oxygen fugacity and H2O on these barometer equilibria. These barometers are essential for determining the depth of formation of mineral inclusions in diamonds. It is increasingly recognised that many such inclusions are formed at sublithospheric conditions in the Earths convecting mantle. This growing field of diamond study presents a remarkable opportunity to investigate processes in the deep mantle. Using the results the conditions of origin of a range of diamond inclusions and xenoliths will be investigated. In addition by combining a thermodynamic model for mafic rock phase relations with elasticity data, the seismic velocity of subducted oceanic crust compositions will be calculated and compared with seismic observations.

• DFG Programme: Research Grants