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The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation

Jingao Liu1, James M. Scott2, Candace E. Martin2, D. Graham Pearson1

1Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, Alberta, Canada T6G 2E3; ;

2Department of Geology, University of Otago, Leith Street, Dunedin 9054, New Zealand; ;


This webpage is a summary of: Liu J. et al., 2015. The longevity of Archean mantle residues in the convecting upper mantle and their role in young continent formation. Earth. Planet. Sci. Lett. 424, 109-118.


Earth’s continents are underlain by melt-depleted lithospheric mantle that protects them from the disruptive effects of asthenospheric mantle convection. The role that buoyant, viscous, highly depleted mantle has played in preserving Archean cratonic crust through to the present day has become clear through Os isotope studies of peridotite xenoliths erupted by kimberlites (Walker et al., 1989; and more recent studies reviewed by Pearson & Wittig, 2014). Less clear is the relationship between post-Archean crust and its underlying lithospheric mantle. For example, could ancient lithospheric mantle act as a “life-raft” that promotes attachment of younger crust (McBride et al., 1996; Handler et al., 1997, 2003; Peslier et al., 2000a,b; McCoy-West et al., 2013; Mundl et al., 2015)? Alternatively, are the relicts of ancient melting events entrained in the convecting mantle flow and later sampled by volcanic or orogenic events, as suggested for oceanic settings (Parkinson et al., 1998; Brandon et al., 2000; Meibom and Frei, 2002; Harvey et al., 2006; Bizimis et al., 2007; Pearson et al., 2007; Liu et al., 2008; Simon et al., 2008; Dijkstra et al., 2010; Stracke et al., 2011; Rampone & Hofmann, 2012; Lassiter et al., 2014)? To answer these questions, we studied peridotite xenoliths from Earth’s youngest continent–Zealandia–and report the largest decoupling of young crust and underlying old lithospheric mantle age (~2.4 Ga) so far observed on Earth.  We examined the potential cause for such extreme age decoupling and this sheds light on the questions posed above.

Within Zealandia, intraplate basaltic-to-lamprophyric lavas have transported many mantle peridotite xenoliths to the surface over the last ~85 Ma. These peridotite xenoliths (Figure 1) permit investigation of the lithospheric mantle beneath Zealandia, where no crust older than 520 Ma has so far been found. In central Zealandia, the xenoliths are primarily in the East Otago and West Otago areas (Figure 1; Reay & Sipiera, 1987; Scott et al., 2014a). Osmium and hafnium isotope data from a small selection of fairly fertile-to-moderately-depleted peridotite xenoliths on the eastern side of New Zealand (East Otago; Figure 1) show that parts of the sampled lithospheric mantle experienced Paleoproterozoic melt depletion (McCoy-West et al., 2013; Scott et al., 2014a). This has led to the suggestion that this young continent is underlain by an ancient lithospheric mantle that has provided a stable platform, since the Paleoproterozoic, onto which various fragments of continental materials were accreted (McCoy-West et al., 2013). 


Figure 1: A satellite gravity map of central-southern Zealandia showing the distribution of peridotite xenolith locations for which there are Re-Os isotope data (modified from Scott et al. 2014a). White symbols are data from McCoy-West et al. (2013) where the circles define their “Waitaha domain”; gray symbols are from our study. The number inside each symbol indicates the number of samples analysed from each location. Dark fields show the extent of xenolith-bearing magmatism in West Otago and East Otago. The Australia-Pacific plate boundary is shown in red. The thick black line shows the important Zealandia geological subdivision between the Eastern Province (Carboniferous and younger terranes <300 Ma) and the Western Province (Cambrian-Early Paleozoic terranes < 520 Ma; Scott, 2013).


A suite of newly discovered peridotite xenoliths from the western side of New Zealand (Lake Wanaka and Mt. Alta, West Otago; Figure 1) has strong compositional affinities to cratonic mantle (Figure 2) supporting the possibility of an Archean mantle root beneath much younger (< 300 Ma) crust. However, this inference lacks geochronological evidence. The 187Re–187Os isotopic system in mantle peridotites has been extensively used because it provides a reliable means of dating formation of lithospheric mantle, while platinum group element (PGE) systematics can be used to investigate the extent of melt depletion and potential subsequent alteration in the peridotites (Pearson et al., 2004; Liu et al., 2011). Using whole-rock Re-Os isotopes and PGE we examined the timing and extent of melt depletion processes in the lithospheric mantle beneath this region to try to establish the temporal relationships between crust and lithospheric mantle and to evaluate the role of ancient depleted lithospheric mantle in the formation of a much younger continent.


Figure 2: A). Olivine Mg# (100 x Mg/(Mg+Fe2+)) versus olivine modal abundance for mantle peridotites from West Otago, New Zealand. The schematic melting curve is from Boyd (1989). The field labelled “Greenland cratonic peridotites” contains data from the Ubekendt (Bernstein et al., 2006), Wiedemann (Bernstein et al., 1998), and Sarfartoq (Garrit, 2002) suites. The large symbols labeled with upper-case letters indicate the average for each suite, with 1σ error bars. The arrow field marks possible silica addition to account for the high orthopyroxene modes. B). Olivine Mg# versus spinel Cr# (100 x Cr/(Cr + Al)). ‘OSMA’: olivine-spinel mantle array from Arai (1994). Given the large errors in the spinel Cr# of the cratonic peridotites, the error bar marks the total range, whereas it indicates 1σ for the West Otago peridotites.


The Lake Wanaka and Mt Alta peridotites from West Otago have rhenium-depletion Os model ages that vary from 0.5 to 2.7 Ga (Figure 3), firmly establishing an Archean depletion event. However, the vast range in depletion ages does not correlate with melt depletion or metasomatic tracer indices, providing little support for the presence of a significant volume of ancient mantle root beneath this region. Instead, the chemical and isotopic data are best explained by mixing of relict components of Archean depleted peridotitic mantle residues that have cycled through the asthenosphere on timescales of Ga, along with more fertile convecting mantle (Figure 3). Extensive melt depletion associated with the “docking” of these melt residues beneath the young continental crust of the Zealandia continent explains the decoupled age relationship that we observe today. Hence, the newly formed lithospheric root incorporates a mixture of ancient and modern mantle derived from the convecting mantle, cooled and accreted in recent times. We argue that in this case, the ancient components played no earlier role in continent stabilization, but their highly depleted nature along with that of their younger counterparts now represents a highly viscous, stable continental keel. This model could account for the large spectrum of ages observed in fertile-to-moderately-depleted peridotites sampled from lithospheric mantle beneath SE Australia, W Antarctica and other locations in Zealandia, as well as the oceanic mantle (Figure 3). Our data confirm the longevity and dispersal of ancient depleted mantle domains in the convecting mantle and their re-appearance beneath young continents.


Figure 3: (A) 187Os/188Os vs. Al2O3 content, (B) whole-rock or olivine Mg#, (C) Pt/Ir and (D) Pd/Ir. The mixing scenarios are calculated, with increments of 5%, between relict Archean component (represented by sample LWA-1 but having variable siderophile-element concentrations, e.g., Os concentration ranges from a 0.25 to 1.0 and half of that of PUM) and ambient fertile mantle (PUM, modern or 0.5 Ga). PUM: 187Os/188Os = 0.1296 from Meisel et al. (2001), and calculated 187Os/188Os = 0.1260 at 0.5 Ga. Arrow fields mark the possible additional processes to account for the data. Data sources are given in Liu et al. (2015). Click here or on Figure for enlargement.



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last updated 2nd July, 2015