The piston–cylinder apparatus is widely used to investigate rock properties at the conditions of the interior of the Earth, but uncertainty in its calibration of pressure persists, with substantial differences between laboratories. We use a ceramic of magnesium and zinc oxides to measure the conditions experienced by a sample. Routine use of such ceramics provides an archive of experimental conditions, enables interlaboratory comparisons, and resolves long-standing controversies in calibration.

The chemical compositions of the synthetic minerals periclase and zincite in the MgO–ZnO binary system change smoothly and systematically with pressure when they exist together in equilibrium. We have studied these changes experimentally over a wide range of conditions and fitted the results to a thermodynamic model. The model may be used to predict the compositions of the coexisting phases accurately at high pressures and temperatures corresponding to the Earth’s crust and uppermost mantle.

The equilibrium phase of A + HP clinoenstatite = forsterite + water was experimentally investigated at aH₂O = 1 in situ. In cold subducting slabs, it is of relevance to transport water to large depths, initiating the formation of dense hydrous magnesium silicate (DHMS). At normal gradients, the huge water amount from this reaction induces important processes within the overlying mantle wedge. We additionally discuss the relevance of this reaction for intermediate-depth earthquake formation.

The mineralogical and chemical heterogeneity of the mantle is poorly known because it is not able to be directly investigated. Melt–peridotite interaction processes play a fundamental role in controlling the mantle composition. The results of our reaction experiments help us to evaluate the role of temperature and melt composition in the modification of the mantle through the interaction with pyroxenite-derived melts with implications for the evolution of a veined mantle.

Dense hydrous magnesium silicates, like the 3.65 Å phase, are thought to cause deep earthquakes. We investigated the dehydration of the 3.65 Å phase at P and T. In both directions of the investigated simple reaction, additional metastable water-rich phases occur. The observed extreme reduction in grain size in the dehydration experiments might cause mechanical instabilities in the Earth’s mantle and, finally, induce earthquakes.