Oxygen isotope constraints on proto-kimberlite melt modification through assimilation of low δ18O recycled crust in the deep lithosphere

Abstract

Megacrysts in kimberlite are large (>1 cm) mineral grains that crystallized from sub-lithospheric proto-kimberlite melts in the lower sub-continental lithospheric mantle (SCLM), often after complex melt-SCLM interactions. Understanding the development of the proto-kimberlite melt system is important for two primary reasons: 1) a control on the melt diversification and range in Mg# observed in kimberlites that erupt at the surface and 2) it is associated with diamond-destruction in the deep lithosphere. We present δ¹⁸O values for a well-characterized suite of megacrysts from the Monastery kimberlite, South Africa, to evaluate variations in melt δ¹⁸O, the effect of melt-SCLM interactions on the primary δ¹⁸O of mantle-derived magmas, and further constrain the development of the proto-kimberlite melt system in the deep lithosphere.

The δ¹⁸O values of the megacrysts from the early-crystallising assemblages of Fe-poor (Mg# 83–88) olivine and relatively primitive (Group 1) ilmenite are: δ¹⁸O_{Fe-poor ol} = 5.43 to 5.84 ‰ ($\overline{x}$ = 5.23 ‰, σ = 0.10, n = 10); δ¹⁸O_{Grp 1 ilm} = 3.88 to 4.35 ‰ ($\overline{x}$ = 4.10 ‰, σ = 0.15, n = 8). The calculated δ¹⁸O value of melts in equilibrium with these megacrysts (5.4–5.8 ‰), indicates that they are typical of mantle-derived melts (i.e., 5.7 ± 0.2 ‰; Eiler, 2001). Garnet, clinopyroxene, and orthopyroxene have δ¹⁸O of: δ¹⁸O_gt = 5.12 and 5.25 ‰ (n = 2); δ¹⁸O_cpx = 4.72 and 5.02 ‰ (n = 2); δ¹⁸O_opx = 5.20 and 5.55 ‰ (n = 2). The calculated melts in equilibrium with these phases range from 4.9 to 5.4 ‰, which overlaps those calculated for Fe-poor olivine and Group 1 ilmenite but extend to slightly lower δ¹⁸O. The δ¹⁸O values of the megacrysts from later crystallising assemblages of Fe-rich olivine (Mg# 78–83), moderately evolved (Group 2) ilmenite, phlogopite, and zircon have δ¹⁸O values are: δ¹⁸O_{Fe-rich ol} = 4.53–4.94 ‰ ($\overline{x}$ = 4.75 ‰, σ = 0.15, n = 5); δ¹⁸O_{Grp 2 ilm} = 2.74–4.46 ‰ ($\overline{x}$ = 3.56 ‰, σ = 0.45, n = 11); δ¹⁸O_phlog = 4.25–5.73 ‰ ($\overline{x}$ = 5.08 ‰, σ = 0.48, n = 8); and δ¹⁸O_zir = 4.87–5.09 ‰ ($\overline{x}$ = 4.98 ‰, σ = 0.08, n = 6). These phases have calculated equilibrium melts (3.8–5.6 ‰) predominantly below the typical mantle range. The δ¹⁸O values of the Group 3 ilmenites, representing the last stage of crystallisation, have δ¹⁸O_{Grp 3 ilm} = 2.93–4.05 ‰ ($\overline{x}$ = 3.59 ‰, σ = 0.36, n = 7).

Based on the most primitive megacrysts (Fe-poor olivine and Group 1 ilmenite), we show that the Monastery proto-kimberlite melt had a mantle-like δ¹⁸O value of 5.61 ± 0.15 ‰ upon entering the SCLM. The magma then experienced several stages of evolution in the deep lithosphere. The lower-than-normal mantle δ¹⁸O values of melts in equilibrium with, the more evolved megacrysts, Fe-rich olivines, Group 2 ilmenites, phlogopites, and zircons can be explained by the assimilation of low-δ¹⁸O eclogitic material. An increased Cr# during evolution, marked by the Group 2 ilmenites, suggests additional assimilation of a Cr-rich peridotitic component. We present a three-step model for the Monastery proto-kimberlite involving fractional crystallisation along with complex open-system processes involving melt-SCLM interactions and show that the δ¹⁸O value of primary mantle-derived magmas can be significantly modified during ascent through the lithospheric mantle. Furthermore, assimilation of an eclogitic component in the deep lithosphere may contribute to the Fe-enrichment observed in megacryst-rich kimberlites globally.