Abstract
Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates. However, it is unclear whether these deep-carbon reservoirs can be locally remobilized on shorter-term timescales. Here we reveal the fate of carbon released during cold subduction by analyzing an anomalously deep earthquake in December 2020 in the lithospheric mantle beneath Milan (Italy), above a deep-carbon reservoir previously imaged in the mantle wedge by geophysical methods. We show that the earthquake source moment tensor includes a major explosive component that we ascribe to carbon-rich melt/fluid migration along upper-mantle shear zones and rapid release of about 17,000 tons of carbon dioxide when ascending melts exit the carbonate stability field. Our results underline the importance of carbon-rich melts at active continental margins for emission budgets and suggest their potential episodic contributions to atmospheric carbon dioxide.
Highlights
Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates
According to the cellular surface-wave absolute tomography model of the Alpine upper mantle shown in Fig. 13,23, the seismic event originated in a region with shearwave velocities that are slightly lower than those generally observed in the lithospheric mantle of the Adriatic plate (4.25–4.55 km s−1 vs 4.50–4.80 km s−1)
These results are consistent with potential fluid migration within a framework likely controlled by tectonics during AdriaEurope convergence[22,30]
Summary
Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates. The resulting melt network, revealed under favorable conditions by shear-wave low-velocity anomalies in seismic tomography models[12,13], is frozen when the mantle geotherm crosses the carbonated hydrous peridotite solidus[10,11].
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