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The opportunity to find high-pressure phases in rocks that have equilibrated previously at very high pressures is rare. Only minerals of extremely wide-ranging stability (e.g. zircon) or with extremely strong bonding and very high activation energy of transformation to lower-pressure phases (e.g. diamond) are likely to be preserved. However, neither of these types of phase is very satisfactory for quantifying conditions of maximum depth achieved. The former, because of its wide stability provides no constraints on its own, only in the inclusions it may preserve metastably on the way to the surface. The latter can provide a minimum set of P/T conditions but diamond, for example, can only prove a minimum pressure of ∼4 GPa whereas it might contain other information in inclusions that could demonstrate much higher pressures.
Microstructures, however, especially when combined with experiments, can provide spatial arrangements of minerals stable at low pressures that can suggest and, in some cases, even prove the former presence of very high pressure phases or very-high pressure compositions of otherwise "normal" phases. Here I will summarize examples of such experiment-assisted microstructural analysis that my colleagues and I have published, and the lessons learned about the power and limitations of this approach. Examples of these three basic techniques are:
1. Use of enhanced solubility of trace elements at high pressure to demonstrate peridotite exhumation from > 300 km.
2. Use of compositions and orientations of exsolution lamellae to confirm #1 and to demonstrate in another system the former presence of stishovite in a pelitic gneiss, proving subduction of sediments to >350km and return to the surface.
3. Use of a sequence of experiments to demonstrate that garnet-pyroxene clusters in peridotites reflect decompression of majoritic garnet under conditions of inhibited intracrystalline nucleation or recrystallization during exhumation.
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