Assessing the presence of garnet‐pyroxenite in the mantle sources of basalts through combined hafnium‐neodymium‐thorium isotope systematics

Abstract

[1] The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinelgarnet transition are different if garnet-pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230 Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet-peridotite will have significant 230 Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts.

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

“…Pertermann and Hirschmann (2003) showed that the melt productivity of synthetic pyroxenite G2 in their experiment increased up to 70%/ GPa before the onset of peridotite melting, partly due to heating of the pyroxenite by the enclosing peridotite. These characteristics potentially result in disproportionate contribution of the pyroxenite component to the final erupted melt (Hirschmann and Stolper, 1996;Stracke et al, 1999;Phipps Morgan, 2001;Pertermann and Hirschmann, 2003).…”

“…Pyroxenite, for example, has a larger melt productivity than peridotite, resulting in a higher melting rate (e.g., Hirschmann and Stolper, 1996;Stracke et al, 1999;Pertermann and Hirschmann, 2003;Prytulak and Elliott, 2009) (Section 4.3). The presence of small-scale source heterogeneity, inferred to represent pyroxenitic recycled oceanic crust (Chauvel and Hémond, 2000;Hanan et al, 2000;Skovgaard et al, 2001;Stracke et al, 2003b;Kokfelt et al, 2006;Stracke and Bourdon, 2009) could therefore significantly affect the U-series disequilibria in Icelandic basalts.…”

“…Russo et al (2009) suggested that melting of fertile pyroxenite veins could explain the relationships between trace element ratios and 230 Th and 226 Ra excesses observed in mid ocean ridge basalts (MORB) from the South-East Indian Ridge. The quantitative effect of lithological source heterogeneity on the U-series disequilibria, however, remains difficult to predict due to the uncertainty attached to the relevant melting and partitioning behaviour of lithologically different sources (e.g., Stracke et al, 1999;Bourdon and Sims, 2003).…”

“…Generally, the above-mentioned models make the simplifying assumption that the mantle source has a homogeneous mineralogical composition. The presence of lithological heterogeneity in the mantle, however, may change its melting behaviour, because different lithologies have different modal composition, trace element partitioning characteristics, and melt productivity (Lundstrom et al, 1995;Bourdon et al, 1996;Hirschmann and Stolper, 1996;Stracke et al, 1999Stracke et al, , 2003aHirschmann, 2003, 2004;Elliott, 2007, 2009).…”

“…Specifically, they reveal information about mantle upwelling velocity, melt extraction rate, and residual porosity, but also about intrinsic properties of the mantle source such as its modal mineralogy and melt productivity (e.g., Condomines et al, 1981;Williams and Gill, 1989;McKenzie, 1985;Cohen and O'Nions, 1993;Iwamori, 1993;Spiegelman and Elliott, 1993;Turner et al, 1997;Bourdon et al, 1998Bourdon et al, , 2005Bourdon et al, , 2006Sims et al, 1999;Stracke et al, 1999Stracke et al, , 2006Stracke et al, , 2003aPeate et al, 2001;Kokfelt et al, 2003;Lundstrom et al, 2003;Pietruszka et al, 2009;Prytulak and Elliott, 2009).…”