Has Metamorphism Always been the Same?

Has Metamorphism Always been the Same?

THIS VERY INTERESTING ARTICLE follows the debate whether metamorphism has always been the same. The minerals in a metamorphic rock can tell you its temperature and pressure history. 

It has long been suspected that the average T/P conditions revealed by metamorphic rocks have varied over geological time, this article discusses A RECENT PAPER which tries to put some numbers to this speculation. And also A DISCUSSION of the paper.

At the heart of the discussion is thermobarometric data - derived from mineral assemblages which indicate the temperature and pressure at which they were formed. The 564 data points are, as one would expect, not evenly scattered through time. And clusters of data points show a wide scatter of T/P. 

But the original authors manage to extract a smooth curve showing that T/P lowered with time and suggest this is linked to the development of modern plate tectonics. 

The author of the discussion thinks the lowering of P/T is due to a fall in mantle temperature. 

All this is far above my competence - I leave you to read the articles and decide for yourselves!

The latest division in pressure-temperature space of different styles of metamorphism (colours) and the main mineral equilibria (dashed lines) that define them. Note the three T/P lines. Did early rocks follow the 30℃/km line and later ones the 10℃/km line? - very roughly speaking and using averages and with all sorts of caveats!

Peter Cawood’s ‘take’ on the relationship between tectonic development and other important variables in the Earth-system with the estimate by Brown et al. of the mean metamorphic T/P (‘thermobaric’) variation through Earth history