Recent Hypothesis
A correlation has been proposed between the episodic formation of UHT granulites and the episodic assembly and disruption of supercontinents or the plume activity during various periods in Earth history. UHT granulites are generally characterized by dry mineral assemblages, the stability of which require low water activities. The direct evidence for the involvement of CO2-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO2 fluid inclusions in buffering the water activity and stabilizing the anhydrous mineralogy of UHT rocks have come from the finding of abundant pure CO2 fluid inclusions in these rocks. UHT metamorphism has been recently evaluated in the plate tectonic context using modern analogues and it has been suggested that both post-collisional extension and rifting play a crucial role. The abundant CO2 liberated by subsolidus decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect thereby triggering the deglaciation of snowball Earth. Based on the distributions of carbonated subcontinental mantle and crustal domains that have undergone CO2-aided dry metamorphism at extreme conditions, Santosh and Omori (2008b) speculated that the UHT rocks might represent windows for the transfer of CO2 from the mantle into the mid crust and ultimately to the atmosphere.
Read more about this topic: Ultra-high-temperature Metamorphism
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