The nature of the lower crust at ultra-slow ridges

• Applicant: Dr. Astri J. S. Kvassnes
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2005 to 2012
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5449772

Theories of how and where melt is produced in the mantle beneath mid-ocean ridges are based almost exclusively on experimental attempts to reproduce the compositions of mid-ocean ridge basalts (MORB) although it is by no means clear to what extent the compositions of the eruptives reflect the composition of the crust as a whole. Phenocrysts and glass-compositions of MORB show, however, that very few, if any, magmas that erupt along the global spreading system are in equilibrium with the mantle. Extensive modification of the magma compositions must have occured, via such processes as fractional crystallization and melt-rock reaction during magma ascent. A clear picture of what type of magmas are being supplied to the crust from the mantle and what modification processes they experience in the crust can only be gained by studying the rock they produce, namely cumulates in lower crust and mantle. Up to present, the low trace element contents of these cumulates, the need to do mineral rather than bulk-rock analyses in most cases, and the global paucity (relative to basalts) of relevant samples has meant that these studies have not been attempted. The high magma productionrates and relatively thick crust produced at fast-spreading ridges yield cumulates that have a complicated history of repeated melt percolation not conducive to approaching this problem. It is at the other end of the spreading-rate scale, the ultraslow ridges, where magma production rates should be low enough that the magmatic systems are likely to be characterized by single magma batches crystallizing and ascending to the surface. The present project aims to use studies of the trace element- and isotope chemistry of minerals in cumulate rocks recovered from ultraslow-spreading ridges in the Arctic and Southern Ocean to approach the following fundamental questions:What is the composition of melt derived directly from the mantle at ultraslow spreading rates?How is this composition modified by crystallization and melt/rock reaction?What is the average trace element composition of the oceanic crust?For this study, we will have access to a unique suite of cumulate samples from the slowest spreading ridges on Earth. This study will be carried out in close collaboration with leading researchers in the field of ultraslow spreading using the world-class analytical facilities of the partner institutes with the aim of developing comprehensive models of magmatic processes at ultra-slow ridges.

• DFG Programme: Research Grants
• Participating Person: Professor Dr. Colin Devey