First natural CaSiO3 perovskite inclusion within a diamond from Cullinan kimberlite

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Marco E. Ciriotti
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First natural CaSiO3 perovskite inclusion within a diamond from Cullinan kimberlite

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 11:31

▪ Nestola, F., Korolev, N., Kopylova, M., Rotiroti, N., Pearson, G., Pamato M., Alvaro, M., Peruzzo, L. (2018): First natural CaSiO3 perovskite inclusion within a diamond from Cullinan kimberlite. IMA218 Abstract book. Unseen but integral to the Earth’s interior: How minerals determine properties and processes, 379.

Laboratory experiments and indirect observations from seismology have provided a relatively clear scenario of the most abundant minerals of our
Planet down to its deepest regions. Natural super-deep diamonds (those rare diamonds considered to have crystallized at depths greater than at least 250 km) appear to be the best tool to preserve and bring to us real fragments of deep Earth. Recently, a super-deep diamond from Brazil revealed the first terrestrial ringwoodite, which in turn proved to be extremely hydrous, opening a new scenario on the location of Earth’s deep water (Pearson et al. 2014). In terms of crystallization depth, the presence of ringwoodite within the diamond would indicate that it formed at least between 525 and 660 km, in the transition zone. However, many studies indicate that diamonds could be derived from even deeper, within the lower mantle (i.e. below 660 km depth). Previously, the idea that diamonds could form in the lower mantle was founded on the assumption that phases included within diamonds, like CaSiO3 walstromite-type (a high-pressure polymorph of wollastonite showing the structure of walstromite, BaCa2Si3O9, found only within diamonds) and/or enstatite are the back-transformation products of CaSiO3 perovskite and bridgmanite, respectively (Harte et al., 1999). The most abundant phase in super-deep diamonds appears to be ferropericlase, (Mg,Fe)O, but its very wide pressure-temperature stability field (from the lower mantle to the shallowest regions of the upper mantle) makes it an insensitive indicator of a lower mantle origin for the host diamond. In addition, so far, no un-inverted bridgmanite and/or CaSiO3-perovksite were reported as inclusions in diamond, leaving some remaining doubt about the origin of diamonds from the Earth’s lower mantle. Here, we report - for the first time - the identification of a CaSiO3-perovskite phase within a diamond from Cullinan kimberlite (Nestola et al., 2018) (in figure a 232-carats diamond from Cullinan mine is shown; courtesy image Petra Diamonds). The inclusion is intergrown with about six per cent CaTiO3 perovskite. The two phases show the same crystal structure type and the bulk composition calculation not only reveals a composition consistent with derivation from basaltic oceanic crust but a depth of formation of about 780 km (Kubo et al., 1997), well within the lower mantle. The relatively ‘heavy’ carbon isotopic composition of the surrounding diamond provides evidence of the recycling of oceanic crust and surficial carbon to lower-mantle depths.
Marco E. Ciriotti

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