A Weighty Problem

One of the key assumptions in applying metamorphic petrology to tectonics (among other fields) is the assumption that the pressure of a metamorphic reaction is the same as the weight of the rock above that reaction.  This makes things reasonably straightforward in interpreting the presence of certain minerals: if you see coesite, for instance, in a rock, you should be at a pressure of 28 kbar (2.8 GPa) or higher (a depth of about 100 km if the average density above is 2850 kg/m³). Discovery of very tiny diamonds and other ultra-high pressure minerals have suggested over the past decade that pieces of continental rock make it to great depths in the earth, far below the deepest continental crust.

Lurking in the shadows for several decades has been the specter that the assumption of equivalent pressures is wrong; that is, the existence of differential stresses at a point means that you can grow minerals that “belong” at a different depth (and also that stress concentrations within a polymineralic material could also affect the minerals that are stable). (Differential stress means that the stress in different orientations is different, unlike in a fluid like water, where the pressure in all orientations is the same. Move a balloon up and down in water and you will see it expand or contract; put it in a vice and its shape will change).  A new paper in Geology (accompanied by a nice overview article) brings this problem to a head by explicitly describing how, in a rock with multiple minerals, application of a differential stress can generate new minerals seemingly requiring a different pressure.

While there are limits to this effect, it potentially upsets a lot of apple carts and could lead to wholesale revision of metamorphic petrology textbooks. One, obviously, is the interpretation of the ultra-high pressure minerals; their presence might plausibly reflect high stress and not great depth. The increasing exploitation of extremely tiny crystals embedded in other crystals might also become more challenging. Another, not explored in these two articles, is interpretation of the change in depth with time of a metamorphic rock, the construction of P-T (pressure-temperature) or P-T-t paths (last t  is time).  For instance, in the Basin and Range analysis of P-T conditions in metamorphic rocks has suggested that they were deeply buried and then exhumed in the Eocene, an exhumation that would seem to require a huge amount of extension of the upper crust, but recent work by, among others, Chris Henry, suggests there wasn’t much extension at this time at the surface. Although some interesting ideas are out there to reconcile these without questioning the depth from the metamorphic rocks, another possibility that is now in play is that the apparent change in pressure is a change in the differential stress at depth and not necessarily a large change in pressure.

Does this utterly rewrite what we think we know from metamorphic rocks? It is too early to tell, but the presence of some places where the petrologic depth and the structural depth seem incompatible suggests that this needs to be examined carefully. Much of the story we have on tectonics from older times relies on these metamorphic reactions; knowing how to properly interpret them is more important than ever.