Carbon … deep downLife on Earth is based on carbon. Plants and animal, oceans and atmosph

Carbon … deep downLife on Earth is based on carbon. Plants and animal, oceans and atmosphere, all form part the the cycle of this vital element. Less obvious is the fact that the deep Earth itself forms a crucial part of the global carbon cycle.Carbon in fossil fuels like oil, gas and coal are clearly part of the geological reservoir of subterranean carbon. But more recently, focus has turned towards the role of carbon in controlling and reflecting the chemistry of our planet from crust to core.Diamonds, erupted from hundreds of kilometres deep within rocks called kimberlites, are one vestige of this deep carbon. Mike Walter and colleagues previously showed that minerals trapped within the diamonds while they grow demonstrate that the can form as deep as 700 km. But the isotope ratio of carbon-13 to carbon-12 for the same diamonds matches better the signature of Earth’s shallow crust: the carbon used to make these deep diamonds first came from the surface and has been drawn into the depths of Earth’s mantle as part of the tectonic cycle of subduction.A paper in the journal Science discusses further evidence for the importance of carbon in shaping our planet. Cottrell and Kelly have measured the oxidation states of iron in volcanic rocks erupted at mid-ocean ridges. Geochemists have long postulated that variations in chemistry of these rocks indicate at least two different deep sources (or reservoirs) for the lavas from which they form: one enriched in elements like barium, thorium or niobium, and one that is depleted in these elements.Cottrell and Kelly’s findings indicate that the enriched reservoir contains iron that is less oxidised than iron in the depleted reservoir. Furthermore, enrichment is postulated to be due to recycling of Earth’s crust into the depths of the mantle, while the depleted source may be a more pristine (possibly deeper) and uncontaminated part of the Earth. Enriched subducted crust is also high in carbon: carbon is a great reducing agent, sucking oxygen from metal oxides like coke in a blast furnace, and this is thought to be the reason that the iron in the minerals of the enriched reservoir is slightly reduced compared to the primitive depleted mantle.So, variations in the chemistry of rocks and melts deep in the Earth appear to be controlled by carbon drawn down into the planet during plate tectonic recycling. As well as decreasing the oxidation of the enriched mantle material, carbon also acts as a flux to lower its melting point. Thus the same variations in carbon may be key in generating the melts that form at mid-ocean ridge spreading centres. Earth’s mantle is heterogeneous in chemical composition and in oxidation state, and carbon may be the key.~SATRImage: Magma erupted onto the seafloor freezes to glass that contains clues to its origin in Earth’s deep interior and ancient past (field of view ~1 cm). Credit: Glenn Macpherson and Tim Goodinghttp://www.sciencemag.org/content/340/6138/1314.abstracthttp://www.sciencemag.org/content/334/6052/54.abstract -- source link

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