A primary goal of the Deep Carbon Observatory is to define how much carbon is present on Earth. In order to do so, it is vital to understand how carbon is cycled between the atmosphere, crust and the deep Earth. In a recent paper published in Nature Geoscience, Galvez et al shed new light on an important facet of deep carbon cycling: The formation of graphite during subduction .
During subduction, oceanic crust and sedimentary rocks are buried to depths that may locally reach the mantle. This process results in a drastic change in the temperature and pressure the rock is exposed to, and in turn induces chemical and physical changes. The elements present in these rocks can be sequestered in the mantle, or returned to Earth’s surface through tectonic activity and volcanism.
By studying high-pressure metamorphic rocks exhumed in mountain belts, scientists can reconstruct the chemical reactions and metamorphic conditions that allowed the observed mineral assemblage to form. Galvez et al analyzed samples from the Malaspina ridge in Alpine Corsica (France), a metamorphic region composed of oceanic units in the French Alps, including metasediments and serpentinites. At a contact between serpentinites and marbles, the authors demonstrated that graphite, a crystalline form of carbon, has been formed by reduction of carbonates during subduction. Similar carbonate reduction to elemental carbon is already known to occur under the very high temperature and pressure conditions of the mantle, but in this case, the studied samples were shown to have never experienced more than 450°C.
If carbonates can be shown to transform to graphite during shallow subduction, this process could significantly change how scientists evaluate the fluxes of carbon in subducted rocks. Indeed, in the form of graphite, a highly refractory phase, carbon is “locked” in the down-going slab. It can then be transferred to the mantle and serve, for example, as a precursor of higher pressure polymorphs of carbon (i.e. diamonds), or, as in this instance, be exhumed and exposed at the surface before entering the erosion cycle. The frequency and mechanism of this low-temperature formation of graphite, however, remains to be fully constrained.
This work was supported by the French Agence Nationale de la Recherche, the CNRS-INSU, and the Ville de Paris (Emergence programme).
Photo credit: A contact between hydrothermally altered mantle rocks and blueschist metasediments in Alpine Corsica, France by Alberto Vitale Brovarone, with cover design by David Shand.