Experimental investigations into the Earth’s deep chemical and physical recycling processes

Large-scale tectonic processes introduce a range of crustal rocks into the Earth’s deep mantle. The detectability of large- and small-scale heterogeneities caused by remnants of former recycled crustal material by means of seismic velocities remains notoriously challenging [e.g. 1]. Yet, consensus suggests that many mantle-derived magmas form during partial melting of a complex mix of rocks; including (i) peridotite – the main rock of the mantle – and (ii) oceanic crust (eclogite) recycled into the mantle at subduction zones [e.g. 2].

However, there is little knowledge of interactions between subducting slabs and the mantle along subduction and adiabatic pressure-temperature paths. It is uncertain which rocks are produced by this process, how such material melts, the types of liquids it produces and how these are extracted from the mantle. These are critical questions, which must take into account the abundant evidence that melts have inherited isotopic signatures from recycled crustal components [e.g. 3].

These hundred-km-scale processes leave fingerprints that can be investigated by various sophisticated micro-analytical and micro-imaging techniques such as diffraction (density, phase assemblages, structure, rheology), absorption spectroscopy (redox state and partitioning), scanning electron microscopy (texture), electron microprobe (quantitative major and minor element analyses), secondary ion mass spectrometry (quantitative trace and volatile element analyses), Moessbauer spectroscopy (quantitative Fe³⁺ analyses), and by other advanced micro- and nano-beam electron, ion and x-ray and mass spectroscopies (for defect incorporation, trace element and isotopic signatures). Most are adapted only to recovered material, but, where possible, data have been obtained at mantle relevant pressures and temperatures.

In this presentation, I will highlight some recent results from these techniques on samples produced at extreme conditions to track the recycling of oceanic crust into the Earth’s mantle, its movement between reservoirs, how it affects phase, redox and density changes in the deep mantle, forms diamonds and melts and, eventually, how these then return to the atmosphere.

[1] Tkalčic et al. (2009). Geophys J Internat 179, 425-443. [2] Sobolev et al. (2007). Science 316, 412-417. [3] Hofmann, A. W. (2003). Treatise Geochem 2, 2.03, 61-101.