Have you ever wondered why there are no #Archean UHP eclogites older than c. 0.8 Ga if #plate #tectonics has operated since c. 3 Ga, and isolated #subduction complexes formed even earlier in Earth history? Our new OA paper provides an explanation. (1/X)

https://www.sciencedirect.com/science/article/pii/S167498712100089X

Almost all UHP eclogite in Phanerozoic terranes forms as mafic bodies encased in low-density felsic crust. The felsic crust gives it buoyancy when the terrane is subducted at convergent margins. See Tso Morari, in this picture (2/X) #metamorphism #tectonics #Archean #eclogite

This works well for modern-day crust, which is intermediate ni bulk composition (so buoant at mantle depths); but what about the Archean? Recent geochemical proxy analysis suggests that the "continents" were more mafic than today (3/X) #metamorphism #tectonics #Archean #eclogite

So, Archean "mafic continent" would not be as buoyant when subducted. Does this have implications for the ability to exhume eclogite, which we use as a diagnostic tool to identify subductino/plate tectonics in the past? (4/X) #metamorphism #tectonics #Archean #eclogite

Yes! We modeled the petrophysical implications of having a more mafic crust in the Archean. We show in this new paper that the bulk Archean "continents" would become negatively buoyant in the mantle before reaching UHP conditions (5/X) #metamorphism #tectonics #Archean #eclogite

This precludes buoyancy-driven exhumation of UHP eclogite during this time, but does not preclude its formation! Sensitivity testing of variation in Archean crust petrology shows that few possible 'architectures' should exhume (6/X) #metamorphism #tectonics #Archean #eclogite

There are more implications to having a more mafic crust, though. Calculated lithospheric strength envelopes show that geodynamic models assuming "felsic" crust are probably wrong (oops!). They are using unsuitable flow laws. (7/X) #metamorphism #tectonics #Archean #eclogite

And this has greater implications for isostasy - a more mafic "continent" will have a lower elevation. Likely, the Archean continents were all submerged, with our calculations matching those of independent isotope data. (8/X) #metamorphism #tectonics #Archean #eclogite

No space for tagging people above, but this work was a big collaborative effort. Thanks @CrustalEvo @World_of_Wade @jdpmoore and others! Download the paper and see for yourself! It's open access @OxUniEarthSci