Zincian chromite inclusions in diamonds: Possibility of deep recycling origin

Abstract

It was reported by Meyer and Boyd (1972) that zincian chromite, having 2 wt% to 3 wt% ZnO and high (>0.9) Cr# (= Cr/(Cr + Al) atomic ratio), appears as inclusions in diamonds obtained from kimberlite. Zincian chromite is characterized by a low (<0.03) Mg# [= Mg/(Mg + ferrous Fe + Zn + Mn + Ni + Co) atomic ratio] and an appreciable amount of MnO (0.4 -0.5 wt%). The chemistry of zincian chromite is very different from commonly found chromite inclusions (magnesiochromite) in diamonds, which contain low amounts of ZnO (<0.1 wt%) and MnO (<0.1 wt%). Zincian and manganoan chromites are also commonly found in meteorites and in altered/ metamorphosed peridotites and related rocks. The chromites found in meteorites mostly have an intermediate Mg# (0.2 -0.5) and a high Cr# (mostly >0.8); moreover, the major -element chemistry of these chromites is similar to that of the magnesiochromite inclusions in diamonds. Spinels obtained from altered rocks show a chemical range that comprises the zincian chromite inclusions found in diamonds. The origin of these inclusions possibly lies in deep recycling. They were initially formed at the Earth's surface in altered/metamorphosed peridotites; they then sank deep down into the mantle and were entrained to the surface again by kimberlite magmatism.

Zn-, Mn- and Co-rich chromian spinels from the Bou-Azzer mining district (Morocco): Constraints on their relationship with the mineralizing process

“…1 in Sack and Ghiorso, 1991), it should be expected that complete alteration of olivine would provide the necessary Mn, Zn and Co to reach the contents shown by the Bou-Azzer chromian spinels, as Gahlan and Arai (2007) suggested. Nevertheless, none of the altered chromian spinel studied in completely serpentinized peridotites worldwide exhibits simultaneously such high Mn, Zn and Co contents (Paraskevopoulos and Economou, 1981;Wylie et al, 1987;Bjerg et al, 1993;Challis et al, 1995;Liipo et al, 1995;Melcher et al, 1999;Tesalina et al, 2003;Downes et al, 2004;Pagé and Barnes, 2009;Arai and Ishimaru, 2011;Saumur and Hattori, 2013;Singh and Singh, 2013) suggesting that altering solutions of the Bou-Azzer chromian spinels should be anomalously rich in these metals.…”

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic

“…These features, along with low Ti (<0.21 wt.% TiO 2 ) contents, suggest that the grain cores are not significantly metamorphosed, and probably represent primary compositions. The Zn content in the Cr-spinel cores varies from 0.46 to 0.95 wt.% ZnO, which is unusually high for Crspinels in mantle-wedge peridotites (see compilation by Arai and Ishimaru, 2011).…”

Evolution of the Earth: Plate Tectonics Through Time

“…Sedimentary rocks bearing minerals derived from chromite are also common (Sleep et al, 2012, and references therein). Diamond inclusions derived from subducted weathered chromite even exist (Arai and Ishimaru, 2011). Moores (2002), de Wit (2004, and Friend and Nutman (2010) reviewed candidates for oceanic crust.…”