In this chapter, we evaluate how the incorporation of H 2 O as a thermodynamic component influences phase relations in a peridotite composition. This component – present either in the form of hydrous minerals, aqueous fluids and hydrous melts, or as a structurally-bonded trace element at defect sites of nominally anhydrous minerals (NAMs) – may influence upper-mantle rheology in diverse ways. By presenting various natural cases, we identify key incorporation mechanisms and assess their role in the microstructural evolution of ultramafic rocks at different depths in the Earth’s interior. These data suggest that the influence of either aqueous fluids or hydrous melts on rheology out-matches that of NAMs or stable hydrous phases across much of the lithospheric mantle. Consequently, future research is expected to shift towards a better understanding of the transient conditions in the lithosphere that control the availability and transport of aqueous fluids and hydrous melts. These transient conditions are likely to play a more dominant role than the sole ability of hydrous defects in NAMs – a role that is currently less well-constrained experimentally – in controlling the ductile deformation of the upper mantle.

Temperature-dependent equilibrium partitioning of elements between different mineral (or melt/glass) phases forms the basis of geothermometry. In natural rock systems it is necessary to determine whether equilibrium partitioning of a given element was obtained between two phases before calculating temperatures using the tool. With the improvement of spatial resolution of analytical tools and our understanding of solidstate kinetics it has become clear that compositional heterogeneities on different scales exist in mantle rocks because of incomplete equilibration, and a kinetic evaluation is necessary before application of geothermometers. This work summarizes the kinetic situations that may arise and provides some guidelines and criteria for testing whether partitioning equilibrium was obtained. A suite of dunites and harzburgites from an ophiolite suite in the Himalaya (Xigaze, Tibet) is used to illustrate the application of some of these concepts. It is shown that when compositions used for geothermometry are chosen bearing these kinetic considerations in mind, a systematic pattern of freezing temperatures is obtained from the geothermometers. These data provide insights into the cooling histories of these rocks with complex, multistage (e.g. melt percolation) histories. Some potential pitfalls for geospeedometry are also illustrated along the way.