Megacryst suites in kimberlite

Large, single crystals (> 1cm) are a familiar component of mantle xenolith suites of many kimberlites.  Confusion between different suites exists in the literature, however, which affects petrogenetic models, and some clarification is warranted.  Megacrysts of the Cr-poor suite[1] are most common.  Cr-poor silicates (garnet, clinopyroxene, orthopyroxene, olivine) are characterized by lower Mg/(Mg+Fe) and Cr₂O₃ and higher TiO₂ values than typical mantle peridotite minerals.  Strong geochemical trends in most occurrences of Cr-poor megacryst suites (e.g., concomitant decrease in Mg/(Mg+Fe) and Cr₂O₃) are interpreted by most authors as the result of fractional crystallization of a kimberlite, or kimberlite-like, magma.   

The Cr-rich megacryst suite, comprising garnet, clinopyroxene, orthopyroxene and olivine, but not ilmenite, was described from the Sloan/Nix kimberlites in northern Colorado[1].  Constituent minerals, all four of which are essential to the definition of the suite, are characterized, in part, by high and restricted values of Mg/(Mg+Fe) and wt% Cr₂O₃ (e.g., 0.791 to 0.837 and 6.1 to 13.0, respectively, in garnet [2]).  Elsewhere, large crystals with Mg/(Mg+Fe) and Cr₂O₃ values higher than Cr-poor suite minerals do occur, but none correspond to the Sloan-Nix Cr-rich suite in paragenesis, size and/or composition[2].  For example, almost no garnet megacrysts described as “Cr-rich” or “high-Cr” from other localities (e.g., refs 3-6) contain >6 wt% Cr₂O₃ and even garnets with <2 wt% Cr₂O₃ are termed “Cr-rich” or “high-Cr”.  Most, or all, of these so-called “Cr-rich garnet megacrysts” are simply xenocrysts from coarse-grained peridotite. 

The “Granny Smith” suite, first described from Kimberley and Jagersfontein [7], is dominated by Cr-clinopyroxene associated with phlogopite (and ilmenite at Kimberley), with uncommon olivine or rutile.  Garnet and orthopyroxene do not occur in this suite, which is neither equivalent to nor a subset of the Cr-rich megacryst suite.  Other suites dominated by Cr-clinopyroxene, also not shown to coexist with garnet and orthopyroxene, have been described from Orapa and Bobbejaan [6] and Grib [8], though analogies have been drawn with the Cr-rich megacryst suite despite compositional and paragenetic differences.  A similar megacrystalline assemblage (Cr-cpx, ilmenite, phlogopite, olivine) has been described from Attawapiskat [9] and at Balmoral megacrysts of Cr-cpx occur with ilmenite, Nb-Cr rutile and zircon [10].

All of these suites of Cr-cpx +/- ilmenite, rutile, phlogopite, olivine, zircon (lacking garnet/opx), though varied, have more in common with each other than with the Cr-rich megacryst suite.  All might be best termed “Granny Smith”, and may have common origins.  The only feature they share with the Sloan-Nix Cr-rich megacryst suite is the presence of large chromian clinopyroxene.  Use of such populations as equivalents of the Sloan-Nix Cr-rich megacryst suite in mantle petrogenetic schemes can lead to faulty conclusions. 

References:  1) Eggler et al. (1979) The Mantle Sample, 2) Schulze (2022) Goldschmidt Conf. Abstr., 3) Hunter and Taylor (1984) Am. Min., 4) Kopylova et al. (2009) Lithos, 5) Bussweiller et al. (2018) Min. Pet., 6) Nkere et al. (2021) Lithos, 7) Boyd et al. (1984) GCA, 8) Kargin et al. (2017) Lithos, 9) Hetman (1996) MSc., 10) Schulze, unpub. data.