Metamorphic CO2-production in Himalaya: Where? How? How much? When? And then?

Metamorphic degassing from active collisional orogens supplies a significant fraction of CO2 to the atmosphere, playing a key role in the long-term global carbon cycle1-4. Primary geologic settings for the production of significant amounts of metamorphic CO2 include “large-hot” collisional orogens, where decarbonation reactions occur at a relatively high temperature within carbonate-bearing metasediments (e.g. calc-silicate rocks), in which metamorphic reactions between carbonates and silicates trigger CO2 production. Being the most prominent recent and still active “large-hot” orogen on Earth, the Himalayan belt is the best candidate for the generation of a significant amount of metamorphic CO2 during the Cenozoic. Moreover, the widespread occurrence of high CO2-bearing hot springs and gaseous CO2 discharges from the ground located along the major tectonic discontinuities5-9 are indicators for a contemporary metamorphic CO2 production in Himalaya. The nature and magnitude of the metamorphic CO2 cycle in the Himalaya, however, is still poorly known. Basing on the results of our recent studies10-13, I will try to answer the following key questions: • Where? - Which are the CO2-source rocks, their abundance and distribution in the Himalayan orogen? Which are their bulk compositions and mineral assemblages? From which protoliths do they derive? • How? - Which are the CO2-producing metamorphic reactions? At which pressure and temperature did these reactions occur? Which was the composition of the produced fluid? • How much? - How much CO2 was produced through these reactions? • When? - At which time did the metamorphic CO2 production occur? Was the CO2 production a continuous process or a sum of multiple discrete events? Which was the metamorphic CO2 flux from the Himalayan orogen? • And then? - Which was the fate of the CO2 produced? Was the CO2 able to reach the surface or it was sequestered through graphite and/or carbonate precipitation? Our estimated flux of CO2 extrapolated to the scale of the whole orogen is similar to that actually measured from spring waters, thus suggesting that CO2-producing processes similar to those occurred in the past are still occurring along the active Himalayan orogen.