Fe-rich ferropericlase and magnesiowüstite inclusions reflecting diamond formation rather than ambient mantle

Abstract

Diamond BZ270 derives from within a 30 km radius of the small Rio São Luiz, near the town of Juina. The sampling site for diamond JUc4 can be more precisely constrained to 261000 m E and 8708000 m N in WGS84 UTM-projected Zone 21S (Agrosi et al. 2017; Figure DR1). METHODS Micro-Computed X-ray Tomography Micro-Computed X-ray Tomography (Cnudde and Boone, 2013) was carried out using a Scyscan 1172 microtomograph, located at the University of Bari (Italy). A 45-kV X-ray source was used with a current of 218 µA. A total of 1200 absorption radiographs were acquired over a 360 ° rotation with an angular step of 0.3 °. Random movement of the vertical axis and multiple-frame averaging were used to minimize the Poisson noise in the projection images. Beam hardening was reduced by the presence of a 0.5 mm Al-filter between the source and the detector. The nominal spatial resolution for the resulting model was 4.75 µm. The raw data were reconstructed into two-dimensional slice images using the software "NRecon, Skyscan, Belgium". Corrections for the beam-hardening effect and ring artifacts were also applied during the reconstruction process. Micro-CT data were analyzed using the software "CT-analyser, Skyscan, Belgium". Cathodoluminescence Cathodoluminescence images were collected on a Centaurus detector attached to a Hitachi S-3500N scanning electron microscope (SEM; School of Earth Sciences, University of Bristol). Samples were carbon coated prior to imaging. Accelerating voltages were varied between 10 and 20 kV to obtain the best quality images.

Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us?

“…Wheeler et al 2001). These arguments were used, for instance, by Nimis et al (2018) to propose epitaxial growth of Fe-rich ferropericlasemagnesiowüstite inclusions in sublithospheric diamonds; in this case, the high-stress environment in which the diamonds were formed made the static recrystallization hypothesis highly unlikely. This may not hold true, however, for the relatively undeformed lithospheric diamonds, in which mchr inclusions are found.…”

“…Crystallographic orientation relationships (CORs) in crystalline inclusion-host systems have often been used to derive inferences about the origin of the inclusions and the relative timing of growth of inclusions and their hosts (e.g., Hwang et al 2007Hwang et al , 2011Hwang et al , 2015Zhang et al 2011;Nestola et al 2014Nestola et al , 2017Nimis et al 2018). A frequent underlying assumption is that a COR that differs from random reflects minimization of interfacial energies and indicates a control by the host's crystal lattice during the formation of the inclusion.…”

“…Despite the limited number of supporting data, the existence of specific CORs has long been considered as one piece of evidence for syngenesis of inclusions and diamonds (e.g., Futergendler and Frank-Kamenetsky 1961;Sobolev 1977;Orlov 1977;Harris and Gurney 1979;Meyer 1985Meyer , 1987Pearson and Shirey 1999). More recently, specific CORs were reported for five mchr inclusions in a single diamond (Wiggers de Vries et al 2011) and for nine Fe-rich ferropericlase-magnesiowüstite inclusions in two diamonds (Nimis et al 2018), whilst an extensive study of olivine inclusions by Nestola et al (2014) did not reveal any preferred orientation. Neuser et al (2015) suggested non-trivial crystallographic orientations in eight olivine inclusions in four diamonds, but these orientations were re-interpreted, against a larger dataset, by Milani et al (2016), and shown to represent random distributions.…”

Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond

“…We combine textural and chemical analyses on ferropericlase inclusions contained in a diamond sample and demonstrate that the observed association of magnetite + FeNi metal exsolved from the matrix results from postentrapment decomposition at the expense of a natural Fe 4 O 5 phase recently discovered and characterized in high-pressure experiments. exsolved from ferropericlase, then confirmed in a number of studies (20,(22)(23)(24)(25), and estimated it to be 6-7 vol % of the original mineral. A similar estimate has been made on the same sample examined in the present study (26).…”

“…Ferropericlase inclusions have been linked to the presence of Fe metal in the deep mantle. However, their wide range in Fe# suggests either local chemical heterogeneities (17,18) or kinetically controlled chemical (redox) reactions that promote the encapsulation of ferropericlase during diamond formation (6,(19)(20)(21). Ref.…”