Diamonds from the lower mantle?

Abstract

Natural diamonds, because of their great physical resiliency, can preserve information about their formation, storage, and transport conditions for billions of years. Diamond samples therefore provide a unique opportunity to directly study ancient samples of the Earth’s deep interior. To correctly interpret the information diamonds provide, it is essential to accurately constrain the depth of their origin. This depth provenance is usually identified using coexisting minerals that are trapped as inclusions within diamonds during their growth. Comparison of an inclusion’s composition and mineralogy with experimental phase equilibria allows the diamond’s growth conditions to be estimated. While the majority of diamonds likely originate from depths of 140–220 km in cratonic mantle, a small subset appears to have been exhumed from depths extending to >800 km, called “superdeep” or “ultradeep” diamonds (e.g., Walter et al. 2011; Pearson et al. 2014). Inclusions of magnesiowustite are among the most commonly described in sub-lithospheric diamonds and have often been assumed to indicate diamond provenance in the lower mantle because [Mg,Fe]O is not stable at upper mantle conditions in a subsolidus mantle compositions (Tronnes 2009). This is despite the stability field of [Mg,Fe]O extending to ambient pressure conditions and experimental evidence of magnesiowustite stability in equilibrium with diamond throughout the upper mantle (Brey et al. 2004; Thomson et al. 2016). A new study by Uenver-Thiele et al. (2017) in American Mineralogist places important new constraints on the formation and uplift history of inclusions containing magnesioferrite. Studies of magnesiowustite inclusions in diamonds from the Juina region of Brazil often report observation of nanometersized crystals of magnesioferrite ![Formula][1] , which supposedly “confirm” the lower mantle origin of these samples. The magnesioferrite precipitates can occur at the interface between the diamond and [Mg,Fe]O inclusion, or as evenly distributed dislocation “necklaces” within … [1]: /embed/mml-math-1.gif