Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions.

A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

33S solid state NMR of sulphur speciation in silicate glasses.

33S solid-state NMR is reported from some model crystalline sulphides, sulphates, sulphites and thiosulphates. This is the first report of (33)S NMR signals of the latter two species from a solid. Good quality spectra, which have distinct, well separated shift ranges can be identified for all these groupings, except for sulphites and hydrogen sulphites whose resonances are very broad. Nonetheless the presence of sulphites and hydrogen sulphites can be confirmed from their characteristic sharp, intense time domain echoes. (33)S MAS NMR is also applied to a range of dry and hydrous silicate glasses with approximately 1wt% 99at% (33)S-enriched sulphur. It is clear that in all these glasses sulphate is present, and in mixed cation systems there is some evidence of preferential association of sulphate with a specific cation. In a dry potassium silicate glass sample two resolved (33)S resonances are observed, a sulphate, and the second from thiosulphate. Hence solid state (33)S NMR is shown to be a feasible probe that can be applied to this problem that can readily distinguish different sulphur species.

Observation of hydroxyl groups by 17O solid-state multiple quantum MAS NMR in sol-gel-produced silica.

17O NMR parameters (CQ, eta, delta(iso) and T1) are reported for both Si-O-Si and Si-OH fragments within a silica gel. The Si-OH units have a wide spread of parameters but are typically characterised by a very short T1 (approximately 0.1 ms) and CQ < 200 kHz. These observations have extremely important implications for the quantification of such units in these gels and related glassy materials by 17O NMR. In light of these observations, the 17O NMR experiments have been optimised and a distinct resonance from the OH group is observed in 1D static and magic angle spinning (MAS) NMR measurements as well in the multiple quantum (MQ) experiment.