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Microdiamonds and solidified melt-films preserved in graphite pseudomorphs

26 août 2013 ( dernière mise à jour : 28 août 2013 )

The Beni Bousera massif (northern Morocco) is made of upper-mantle rocks which have been brought to the surface thanks to tectonic activity which led to the formation of the rifean mountain belt during the Alpine orogeny about 25 Ma ago. Upper mantle (UM) is usually sampled through volcanic activity which brings upper-mantle xenoliths up to the surface in lava flows. The good point with tectonic is that huge UM mantle pieces can be exhumed. Upper-mantle structure, deformation and large scale chemical transport (e.g., metasomatism) can be studied in-situ. The bad news is that vertical tectonic motions are relatively slow (mm.yr-1) and rocks are transformed on their way to the surface. Furthermore, exhumation is only possible through intensive rock deformation which induces microtextural changes. Hopefully, deformation is mostly localized along major detachments / faults so that some parts of the mantle units can be preserved. Phase transformations and melting can also occur upon exhumation and, the original mineralogy formed at depth can be altered accordingly. Part of the research carried out by petrologists is, actually, to look for witnesses of the transformation experienced by rocks coming from depth in order to reconstruct their history and, in the best case, their original mineralogy.
In the paper entitled : Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco (European Journal of Mineralogy, 2011, [1]), we have carried out a very fine and detailed characterization of some specific layers composed of pyroxene and garnet (pyroxenite). The co-existence of pyroxene and garnet allows estimating formation pressures (Al barometry). Furthermore, some of the pyroxene layers contain octahedral (and cuboïd) graphite aggregates which are interpreted as graphitized millimetre-sized diamonds. Some of these aggregates were hand-picked and separated into two pieces with a needle (i.e., without the use of diamond-bearing cutting tools). High-resolution electron microscope images showed the presence of sub-micrometric diamond grains. These diamonds could either be residual, in other words, small pieces of diamonds were preserved even though the host rock experienced temperatures higher than 1000°C, in the graphite field. A second alternative would be that the host rock has been recycled under UHP conditions so that newly formed microdiamonds were able to crystallize. In this paper, we favour the first hypothesis since (1) the microdiamonds are recovered in the zone with the larger graphite flakes whereas diamond nucleation would be expected to occur on smaller flakes with a higher amount of crystallographic defects (see Le Guillou et al., 2006) ; (2) there are still no recognized examples of diamond crystallizing directly from graphite in nature. The preservation of diamond remains however a puzzling features in rocks which experienced high temperatures in the graphite field. Another spectacular feature is the presence of silicate films intercalated within the graphitic flakes. We interpret these films as solidified melt films which were preserved in the graphite aggregates. Their basaltic composition supports this view. The preservation of solidified melt is a relatively rare feature. It can occur in the form of “melt inclusions” trapped in magmatic crystals. The originality of our finding relies on the fact that the melt which wetted the graphite flakes, has been chemically isolated from the rock-forming silicates. Therefore, chemical re-equilibration with the host rock might have been limited ; we expect then the observed solid silicate films to display pristine melt compositions. It can be noted that the “melt-trapping” property of graphite, had been utilized, prior to our discovery, for trapping partial melts formed from very low degree of melting in high pressure and high temperature experiments (Laporte et al., 2004).

[1] El Atrassi, F., Brunet, F., Bouybaouène, M.L., Chopin, C., Chazot, G. (2011) Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco. Eur. J. Min, 23, 157-168.

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