Tracking continental-scale modification of the Earth’s mantle using zircon megacrysts

Abstract

doi: 10.7185/geochemlet.1727 Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth's mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu-Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km 2 . These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.

Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir

“…Furthermore, the linear arrays evolving with time back towards the mantle array would require a highly systematic controlwhereas crustal contamination is usually seen as a more random process, and zircon megacrysts, which have not experienced any upper level crustal contamination, show very similar time-composition relationships 44 . Finally, any kimberlite experiencing such extreme levels of crustal contamination would no longer retain its primary magmatic characteristics.…”

Mesozoic denudation history of the lower Orange River and eastward migration of erosion across the southern African Plateau

“…The Gibeon cluster has generally been considered to be 65-75 Ma ( Fig. 1; Allsopp et al, 1989), but a younger U-Pb date of 57.6 ± 0.6 Ma was recently reported for a zircon megacryst from Mukurob, Namibia, one of the Gibeon pipes (Woodhead et al, 2017). We therefore adopt 58-75 Ma as the age of the Gibeon cluster.…”

Preservation of systematic Ni and Cr heterogeneity in otherwise homogeneous mantle olivine: Implications for timescales of post-metasomatism re-equilibration

“…Bultfontein are interpreted to represent 'failed' kimberlite melts that did not make it to the surface (Giuliani et al, 2013a) and recently Jollands et al (2018) identified two stages of metasomatism preserved in garnet, the second of which was interpreted as reactive infiltration of a silico-carbonate-rich melt, assumed to be a proto-kimberlite melt. (Woodhead et al, 2017) have identified isotopic homogeneity amongst mantle-derived zircon megacrysts from kimberlites in the Kaapvaal craton, they attribute this to a continent-wide metasomatic event that occurred between Ma and several hundred million years ago. Our work provides evidence for a more localised metasomatic event, associated with the Bultfontein kimberlite, that occurred within half a million years of emplacement.…”

“…The sub-continental lithospheric mantle represents one of Earth's largest and most longlived chemical reservoirs. Direct evidence from mantle xenoliths shows that beneath the ancient cores of continents a once refractory melt residue has undergone billions of years of refertilization (Gibson et al, 2008;Menzies and Hawkesworth, 1986;Pearson, 1995;Shu and Brey, 2015;Simon et al, 2007;Woodhead et al, 2017). This long-term metasomatic enrichment results from reactive percolation of low-viscosity melts, such as kimberlites and carbonatites (e.g.…”