Trace and Minor Elements in Magmas at High Pressure: In Situ Structural Insights on Partitioning at Depth

Abstract:

Trace element partitioning between crystals and silicate melts has been successfully modelled using crystal-chemistry rules [1], although these models do not explicitly include melt's properties. However, melt composition has been shown to strongly influence element partitioning [2], implying a control from the 'melt-chemistry' as well. Additionally, element partitioning can be pressure-dependent, in which case it is a useful marker of depth for planetary processes such as mantle melting, and crust or core formation.
To assess the effect of melt structure on partitioning at depth, we have investigated the local structure around trace and minor elements in melts at high pressure using two in situ techniques, high energy x-ray diffraction and EXAFS. X-ray diffraction data, once converted into pair distribution functions, are straightforward to interpret and give access to the full structure although overlaps between different contributions occur. In contrast, EXAFS is a chemically-sensitive probe that describes the local structure around a given element, but is model-dependent. Both methods will be illustrated by results obtained on two types of elements, some volatiles and some lithophiles.

References:

[1] Blundy JD and Wood BJ, Nature 372:452, 1999.

[2] Schmidt MW et al., Science 312:1646, 2006.