Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

Abstract

Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H 2 O and CO 2 -rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibolerutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…GPa), while age dating demonstrates a close temporal link to the older Group II and subsequent Group I kimberlite magmatism (80-90 Ma) or to the preceding Karroo magmatic event (Konzett et al, 1998(Konzett et al, , 2000(Konzett et al, , 2013Grégoire et al, 2002;Bell et al, 2005;Kobussen et al, 2008;Giuliani et al, 2013Giuliani et al, , 2014Giuliani et al, , 2015b. Phlogopite-bearing polymict breccias from 110-115 km depth have also been linked to group I kimberlite magmatism and were interpreted as failed kimberlite intrusions that paved the way for subsequent kimberlite eruption (Giuliani et al, 2014).…”

Tracing mantle metasomatism using combined stable (S, O, N) and radiogenic (Sr, Nd, Hf, Pb) isotope geochemistry: case studies from mantle xenoliths of the Kimberley kimberlites

“…Phlogopite δ 15 N compositions are between +4.4 and +6.2‰ (five peridotites, one MARID), which is significantly different from the mantle signature of -5 ± 2‰ inferred from studies of diamonds and oceanic basalts (e.g., Cartigny and Marty, 2013). (Konzett et al, 2000); b LIMA U/Pb age (Giuliani et al, 2014b); c maximum zircon U/Pb age (Giuliani et al, 2015); d zircon U/Pb age (Konzett et al, 1998); e metasomatism coeval with kimberlite magmatism in the Kimberley area (Giuliani et al, 2014a) Figure 1. Left panel: Oxygen isotope composition of clinopyroxene grains; the green band represents the mantle range (δ 18 O = 5.57 ± 0.18, 1sd; Mattey et al, 1994).…”

Ultramafic Carbonated Melt‐ and Auto‐Metasomatism in Mantle Eclogites: Compositional Effects and Geophysical Consequences

“…Interaction with a young kimberlite‐like (sensu stricto) agent with OIB‐like isotopic characteristics (Becker & Le Roex, 2006; Tappe et al, 2017) would increase low 87 Sr/ 86 Sr of eclogites with ancient cumulate protoliths, but decrease that of those with ancient evolved protoliths. Proto‐kimberlite and orangeite metasomatism mobilizes aged and isotopically evolved lithospheric metasomes with enriched isotope characteristics (unradiogenic Nd, radiogenic Sr; Becker & Le Roex, 2006; Giuliani et al, 2015; Tappe et al, 2008) and would therefore likely decrease 143 Nd/ 144 Nd and increase 87 Sr/ 86 Sr in affected eclogites. At Lace, although LREE‐enriched samples nearly span the whole range of initial 143 Nd/ 144 Nd, a broad trend toward less radiogenic Nd with increasing Cr abundances is recognized, the lowest initial value, in pyroxenites, being below that of the host orangeite (Figure 6b).…”