Active carbon sequestration in the Alpine mantle wedge and implications for long-term climate trends

Abstract

The long-term carbon budget has major implications for Earth’s climate and biosphere, but the balance between carbon sequestration during subduction, and outgassing by volcanism is still poorly known. Although carbon-rich fluid inclusions and minerals are described in exhumed mantle rocks and xenoliths, compelling geophysical evidence of large-scale carbon storage in the upper mantle is still lacking. Here, we use a geophysical surface-wave seismic tomography model of the mantle wedge above the subducted European slab to document a prominent shear-wave low-velocity anomaly at depths greater than 180 km. We propose that this anomaly is generated by extraction of carbonate-rich melts from the asthenosphere, favoured by the breakdown of slab carbonates and hydrous minerals after cold subduction. The resulting transient network of carbon-rich melts is frozen in the mantle wedge without producing volcanism. Our results provide the first in-situ observational evidence of ongoing carbon sequestration in the upper mantle at a plate-tectonic scale. We infer that carbon sequestered during cold subduction may partly counterbalance carbon outgassed from ridges and oceanic islands. However, subducted carbon may be rapidly released during continental rifting, with global effects on long-term climate trends and the habitability of planet Earth.

Synchronous Periadriatic magmatism in the Western and Central Alps in the absence of slab breakoff

“…We interpret the ε Hf (t) systematic decrease from the Central to the Western Alps as the evidence of an increased input of continental material into the mantle source by subduction. This scenario is in line with palinspastic reconstructions showing that the inception of continental subduction migrated progressively from the Western to the Central Alps (Ford, Duchêne, Gasquet, & Vanderhaeghe, ; Malusà et al., , ) (Figure a–c).…”

Evidence for a serpentinized plate interface favouring continental subduction

“…1), which includes different slivers of serpentinites with eclogites 6 . During subduction, meta-ophiolites experienced breakdown of amphibole and zoisite, and partial dehydration of antigorite that was likely completed at~180-km depth along the cold Alpine geothermal gradient (<7°C km −1 ) 6,43,44 . Continental rocks experienced progressive dehydration breakdown of zoisite, chlorite, biotite, talc, and lawsonite, but the low geothermal gradient of the Alpine subduction zone prevented the breakdown of phengite in UHP continental rocks.…”

“…Continental rocks experienced progressive dehydration breakdown of zoisite, chlorite, biotite, talc, and lawsonite, but the low geothermal gradient of the Alpine subduction zone prevented the breakdown of phengite in UHP continental rocks. Dehydration of metasediments and mafic rocks was likely completed at 200-250 km 44 . Substantial amounts of aqueous fluids were thus released at subarc depths (80-180 km), promoting further serpentinization both in the subduction wedge and along the plate interface above the subduction channel.…”

“…Substantial amounts of aqueous fluids were thus released at subarc depths (80-180 km), promoting further serpentinization both in the subduction wedge and along the plate interface above the subduction channel. Minor amounts of fluids are possibly released from the slab even today, despite the fact that subduction is no longer active, thus promoting further serpentinization and lowering seismic velocities 44 .…”

Rapid migration of CO2-rich micro-fluids in calcite matrices

“…In the case of lubrication of faults, calcite is an important, and in some cases dominant mineral in seismically active regions worldwide, leading an extra anisotropy in P- or S-wave velocities. Fast fluid movement at the microscale and nanoscale have significant consequences in interpreting anisotropy in fluid-bearing rock systems during carbon crust cycling 41–43 and geological CO 2 sequestration 17,44 . High mobility of fluids in crystals may also have important implications for fluid-mediated metal transport.…”