Is P really ρgz?

The discovery of widespread ultra-high-pressure phases in many metamorphic terranes–most dramatically the presence of metamorphic diamonds–has fueled speculation on the deep subduction of crustal materials and their subsequent return to the near surface environment. The extent and geologic continuity of some of these terranes has challenged how we think materials move in subduction zones. But all of this depends mightily on the assumption that pressure is, essentially, the weight of overburden. That is, the pressure determined from petrologic changes was simply the weight of the material above (∫ρg dz).

Hovering over this is the recognition that stress can be focused deep within the solid earth.  The assumption has been that such stresses would not materially differ from the weight of the overburden.  It has never been clear that this was a great assumption: Byerlee’s Law, for instance, allows that if pore pressures are near zero, horizontal normal stresses could be three times the overburden weight, meaning that with no special pleading that the pressure (which would be the average of the normal stresses) could be more than twice the weight of the overburden.  This extreme case though caused little concern for deep rocks as many of these are deforming by creep and so will be much weaker.

But what if you could focus all the normal stresses? Such focusing occurs in artificial situations all the time (most extreme case might be hydrogen bombs, which are triggered by focused blasts from smaller atomic bombs). This question was explored in a recent Geology paper by Georg Reuber and colleagues in Germany and Switzerland.

This paper runs some numerical simulations to show that the presence of enclaves of anhydrous crustal blocks in a weak lower crust can result in pressures within the enclave more than twice the weight of overburden. What was more, the duration of such super-lithostatic pressures was in the millions to tens of millions of years.  A block that would see pressures usually associated with depths of 90 km or more could have been only 50 km or so deep. Similarly high pressures can be achieved if the entire lower crust is anhydrous.

This suggests that petrologists now need to consider the broader environment of their samples before inferring great depths (or rapid rise) for samples of ultra-high pressure rocks.  The models in this paper are basically permissive, indicating that it would be the presence of stiff, water-free materials that would be the key in promoting such great overpressures. It will be interesting to see in the coming years whether a reexamination of these rocks confirms the great depths presently ascribed to them or reveals that dynamic pressures from tectonic activity can cloud our ability to interpret depth from petrology.

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  1. Depressurizing Geobarometry | The Grumpy Geophysicist - March 18, 2021

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