The Lherz spinel lherzolite: Refertilized rather than pristine mantle

TLDR: In this article, structural and geochemical data from the Lherz massif, the type-locality of lherzolite, were used to show that most of these massifs were formed through a refertilization process involving interaction of refractory, lithospheric mantle with upwelling asthenospheric partial melts.

About: This article is published in Earth and Planetary Science Letters.The article was published on 2007-07-30 and is currently open access. It has received 336 citations till now. The article focuses on the topics: Mantle wedge & Transition zone.

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Table 2 Compositional ranges in major and minor elements of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx) and spinel (Spl) for harzburgites and lherzolites, and ranges of Mg# in olivine (=Mg/Mg+Fe cationic ratio) and Cr# in spinel (=Cr/Cr+Al cationic ratio)

Table 3 REE compositions of clinopyroxene in four representative peridotites: a harzburgite and a lherzolite distal from harzburgite–lherzolite contacts, and the same rock types adjacent to a contact (Section 2 in Table 1)

Fig. 1. (a) Geological map of the Lherz peridotite massif showing anastomosed fertile lherzolites enclosing irregularly-shaped refractory harzburgite bodies. Foliation in harzburgites shows a constant N40–N60 orientation with a gentle southward dip, while lherzolite foliation and websterite layering show variable directions and a steep southward dip. Right insert: localization of the massif. (b) 3-D bloc diagram of the upper part of the massif made after the dips of websterite layering at the contact.

Fig. 4. MgO, TiO2 and Cr vs. Al2O3 concentrations in whole-rock harzburgites and lherzolites from Lherz (Table 1) compared with literature data for mantle xenoliths and orogenic peridotites (Canil, 2004). Empty squares = harzburgites; full circles = lherzolites. The variations observed within a few meters across harzburgite–lherzolite contacts in Lherz encompass almost the whole range of mantle peridotites worldwide.

Table 1 Whole-rock major- and trace-element contents, and modal compositions, of Lherz peridotites collected along two sections across harzburgite– lherzolite contacts and in distal peridotites

Fig. 7. TiO2 concentrations in clinopyroxene vs. whole rocks for the harzburgites and lherzolites (same symbols as in Fig. 4). Error bars indicate the variation in Cpx composition at the sample scale. Grey symbols represent Cpx compositions recalculated for near-solidus temperature (1250 °C) using inter-mineral partition coefficients derived from Wood and Blundy (2003). Numerical models of TiO2 variations during partial melting and magmatic refertilization are shown for comparison. For the melting model, we used the modal composition of the most fertile lherzolite, and its TiO2 content in Cpx and whole rock, as the source composition. The melting reaction was taken from Walter et al. (1995) and the mineral/melt partition coefficients from Wood and Blundy (2003). The results reported here were calculated with the fractional melting model. However, changing the melting model (e.g. for batch or incremental melting) does not significantly modify the variation trend of TiO2 in Cpx vs. whole rock. Magmatic refertilization was simulated with the Plate Model of Vernières et al. (1997) applied to the following melt-rock reaction: fi.melti + xol.Ol→ fr.meltr + xopx.Opx+ xcpx.Cpx+ xspl.Spl, where melti and meltr stand for the infiltrated and residual melts, respectively, fi and fr for the corresponding melt mass fractions, xol for the mass fraction of dissolved olivine, and xopx, xcpx and xspl for the mass fractions of crystallized Opx, Cpx, and Spl. Reaction parameters fi, fr, xol, xopx, xcpx and xspl were adjusted to reproduce the modal variations and the Ti contents in Cpx and whole rocks. The protolith is a highly refractory harzburgite (83% Ol, 15% Opx, 1.5% Cpx, 0.5% Spl, 0.02wt.% TiO2 in whole rock and 0.05wt.% TiO2 in Cpx). The Ti content of the infiltrated melt (TiO2=2wt.%) was derived from the composition of clinopyroxene in the most fertile lherzolite and Cpx/ melt partition coefficients (Wood and Blundy, 2003).