Depth of diamond formation obtained from single periclase inclusions

Super-deep diamonds (SDDs) are those that form at depths between ~300 and ~1000 km in Earth’s mantle. They compose only 1% of the entire diamond population but play a pivotal role in geology, as they represent the deepest direct samples from the interior of our planet. Ferropericlase, (Mg,Fe)O, is the most abundant mineral found as inclusions in SDDs and, when associated with low-Ni enstatite, which is interpreted as retrogressed bridgmanite, is considered proof of a lowermantle origin. As this mineral association in diamond is very rare, the depth of formation of most ferropericlase inclusions remains uncertain. Here we report geobarometric estimates based on both elasticity and elastoplasticity theories for two ferropericlase inclusions, not associated with enstatite, from a single Brazilian diamond. We obtained a minimum depth of entrapment of 15.7 (±2.5) GPa at 1830 (±45) K (~450 [±70] km depth), placing the origin of the diamond-inclusion pairs at least near the upper mantle–transition zone boundary and confirming their super-deep origin. Our analytical approach can be applied to any type of mineral inclusion in diamond and is expected to allow better insights into the depth distribution and origin of SDDs. INTRODUCTION Diamonds, and the mineral inclusions they trap during their growth, are pristine samples from Earth’s mantle and provide information on processes operating in inaccessible regions of our planet. This information is particularly valuable if it can be combined with depth estimates. Based on the mineral inclusions, the majority of diamonds (99%) originate within the lithosphere (Stachel and Harris, 2008). The other 1% are sublithospheric and formed at depths between ~300 and ~1000 km, and hence are called super-deep diamonds (SDDs) (Walter et al., 2011; Pearson et al., 2014; Smith et al., 2016; Nestola et al., 2018). Based on experimental evidence, bridgmanite and ferropericlase are the most abundant minerals in the lower mantle, composing approximately ~75 and ~17 wt%, respectively (Stixrude and Lithgow-Bertelloni, 2012, and references therein). On decompression, bridgmanite inverts to Al-rich, low-Ni enstatite (Stachel et al., 2000), while ferropericlase can remain stable to room pressure. Numerous lines of evidence, particularly association with low-Ni enstatite, have indicated that ferropericlase has a lower-mantle origin (Harte et al., 1999; Stachel et al., 2000). However, the association of ferropericlase with olivine and jeffbenite (previously known as TAPP; Nestola et al., 2016) (Hutchison et al., 2001) casts doubt on that conclusion. Indeed, ringwoodite is in equilibrium with ferropericlase at 24 GPa (Brey et al., 2004) and could have later reverted to olivine, whereas jeffbenite is stable only up to 13 GPa (Armstrong and Walter, 2012), even as its origin is still controversial. In addition, the observation of droplets of Fe-Ni alloys in some Fe-enriched ferropericlases induced Hayman et al. (2005) to outline a model that ascribes the Fe-rich character to equilibration with silicates in the deeper part of the lower mantle (1700–2900 km). On the other hand, synthesis of ferropericlase and diamond by carbonate melt–peridotite reactions (Thomson et al., 2016) suggested that ferropericlase inclusions with variable Fe contents can form at lower upper-mantle to transition-zone depths. The presence of nanometric exsolutions of magnesioferrite in some ferropericlase inclusions (Harte et al., 1999; Wirth et al., 2014; Kaminsky et al., 2015) led Palot et al. (2016) to propose an origin in the uppermost part of the lower mantle, but Uenver-Thiele et al. (2017a, 2017b) showed that magnesioferrite cannot exsolve directly from ferropericlase in the lower mantle. In aiming to identify a method for determining the depth of origin of ferropericlase inclusions completely independent of mineral paragenesis, Hutchison (1997) combined sophisticated thermoelastic modeling with measurements of periclase cell parameters before and after release from diamonds from the São Luiz River (Juina, Brazil) and Guinea. Hutchison and Harris (1998) reported an absolute minimum depth of formation of 320 km (equivalent to an entrapment pressure, Ptrap, of 11 GPa), uncorrected for the brittle deformation evident in the diamond host. This study provided strong evidence for super-deep origins for the samples analyzed, however limitations were imposed by uncertainties in the Gandolfi camera measurement technique available at the time and full quantification of plastic and brittle diamond deformation. In this study, we have been able to extend the original work with improved certainty and propose an updated method for determining minimum Ptrap applied to two ferropericlase inclusions in a further diamond from the São Luiz River (sample AZ1; Fig. 1). The reverse calculation of Ptrap was performed by applying the elastic geobarometry *Current address: Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada; E-mail: chiara.anzolini@phd.unipd. it. CITATION: Anzolini, C. , et al., 2019, Depth of diamond formation obtained from single periclase inclusions: Geology, v. 47, p. 1–4, https://doi.org/10.1130/G45605.1. Manuscript received 16 September 2018 Revised manuscript received 19 December 2018 Manuscript accepted 3 January 2019 https://doi.org/10.1130/G45605.1 © 2019 Geological Society of America. For permission to copy, contact editing@geosociety.org.