Geochemists’ toolkit for tracing the global deep carbon cycle has recently been augmented by the analysis of magnesium isotopes. Light Mg-isotopic compositions normally occur uniquely in carbonates precipitated from seawater, either in the form of sedimentary limestones/dolomites or as sea-water-precipitated alteration products impregnating the oceanic crust. But Chinese geochemists have found that large numbers of volcanic rocks from the eastern part of China also contain the tell-tale signature of marine carbonates, indicating that these have been recycled through the upper mantle and into the sources of the volcanic rocks. Li and co-authors have reviewed the available data, adding many new analyses, in order to map the regional distribution of these anomalous Mg-isotopic compositions . What emerges is a remarkable isotopic map of young volcanic rocks from eastern China, showing light Mg-isotopic compositions in a swath of land extending from the east coast approximately 500 km inland and from Hainan Island in the south to the Russian border in the north. Even more remarkably, this largely matches the extent of a slab of flat-lying but deeply subducted lithosphere, which is located at a depth of 400–600 km in the so-called mantle transition zone, near the base of the upper mantle beneath eastern China . This so-called ‘stagnant slab’ has been identified and imaged by the techniques of seismic tomography, which works in ways analogous to medical X-ray tomography but uses seismic (earthquake) waves, which traverse the Earth’s interior.
The authors argue that the two phenomena are related. Subduction of altered oceanic crust introduced (originally) marine carbonate into the deep upper mantle. The carbonates that survive subduction to this depth are predominantly Mg carbonates, and their melting point is several hundred degrees lower than the melting point of silicates. They will therefore form carbonate melt, which infiltrates the overlying more ordinary mantle rocks, the so-called asthenosphere. During this process, the carbonate melt is likely to be reduced to diamond, and consequently the infiltrated rock assemblage solidifies. Ultimately, though, this material is transported to shallower levels by solid-state mantle convection until it undergoes a second stage of melting at depths between 300 and 100 km. These melts eventually reach the surface to generate the volcanoes that still remember the subducted magnesium isotopes.
A potentially important ramification is that, if it is possible to identify large mantle regions containing subducted carbonates, this will give us new information about the down-going part of the global carbon cycle. Previous estimates of carbon flux entering the convecting mantle via subduction range from essentially zero to 52 Mt carbon per year , and it has therefore not been clear whether the carbon exchange between mantle and exosphere is actually a cycle or a one-way process of cumulative CO2 outgassing. Future investigations will show whether or not the present story of extensive carbon subduction will hold up elsewhere. Thus, ‘stagnant slabs’ have also been found in other parts of the world . If similar signatures with subducted carbonate can be identified in those regions as well, we may finally get a better grip on the down-going limb of the deep-mantle carbon cycle.
Albrecht W. Hofmann
Max Planck Institute for Chemistry,
Hahn-Meitnerweg 1, Mainz, Germany
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See the article: Li, S.-G.*, Yang, W., Ke, S., Meng, X., Tian, H., Xu, L., et al. (2017). Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China. National Science Review, 4(1), 111–120. PDF