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Lu–Hf and geochemical systematics of recycled ancient oceanic crust: evidence from Roberts Victor eclogites

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Abstract

Eclogites from the Roberts Victor mine, Kaapvaal craton are classic examples of subducted Achaean oceanic crust brought up as xenoliths by kimberlite. New in situ trace element and oxygen isotope data (δ18O=3.09–6.99‰ SMOW) presented here reemphasise their origin from seawater-altered plagioclase-rich precursors. Their Hf–Nd isotopic compositions are not in agreement with compositions predicted by geochemical modelling of the isotopic composition of aged subducted oceanic crust. Instead, Hf isotopic compositions are very heterogeneous, varying between 0.281625 and 0.355077 (−37.8 and +2561 ɛHf) at the time of kimberlite emplacement (128 Ma) in keeping with equally variable Nd isotopic compositions (0.511124–0.545092; −26.3 to +636 ɛNd). However, most samples plot on the terrestrial array. The isotopic compositions of some samples are too extreme to play a major role in mixed peridotite-eclogite melting in basalt source regions, whereas the isotopic composition of other samples is reconcilable with a contribution of up to ca. 15% of eclogite partial melt to the MORB source. Most importantly, our results show that ancient subducted oceanic crust is not isotopically homogeneous and should not be treated as a “component” or “reservoir” during geochemical modelling. The heterogeneity reflects radiogenic in-growth starting from small compositional heterogeneities in gabbroic protoliths, followed by modification during sea-floor alteration, subduction and emplacement into the subcratonic lithosphere.

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Acknowledgements

J.B. Dawson, E. Jagoutz and G. Brey are thanked for access to their Roberts Victor eclogite samples. We are also grateful to J.J. Gurney and the Kimberlite Research Group at the University of Cape Town for providing a piece of HRV247 for this study. D.J. gratefully acknowledges financial support by the DFG (Forschungsstipendium Ja 781/2). M.B. acknowledges financial support from FIU and NSF Grant OCE-0241681 to G. Sen. Reviews by H. Brueckner and S. Weyer helped to clarify some of the points stressed in this article.

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  1. Institut für Geowissenschaften, Universität Mainz, Becherweg 21, D-55099 Mainz, Germany

    D. E. Jacob

  2. NHMFL and Department of Geology, Florida State University, 1800 E Paul Dirac Drive, Tallahassee, FL, 32306, USA

    M. Bizimis & V. J. M. Salters

  3. Department of Earth Sciences, Florida International University, Miami, FL, 33199, USA

    M. Bizimis

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Appendix

Appendix

Secondary effects on whole rock Hf and Nd isotopic compositions

To constrain the effects of secondary processes on the whole rock eclogite Hf and Nd isotopic data, Hf and Nd isotopic as well as solution ICP–MS analyses of trace elements were carried out on both leached and unleached eclogite powders. Whole rock powders were leached twice with warm 2.5 N HCl for 30 min, subsequently dissolved in HF/HNO3 and split into two aliquots. One aliquot was analysed for trace element concentrations with a Finnigan Element ICP–MS at NHMFL/FSU (see Bizimis et al. 2003; Stracke et al. 2003 for detailed analytical techniques); the other was submitted to the chemical separation scheme outlined above and measured for Hf and Nd isotopic compositions. Leachates were evaporated, re-dissolved in 2.5 N HCl and measured for their trace element concentrations by solution ICP-MS. The amount of leached material was found to vary between 6.7 and 8.6 wt% and contained 8.8–32.4% of the rock’s Nd and Sm budget (Table 9). Only 3.8–7.4% of the Hf and 4.7–10.2% of the Lu budgets were in the leachates. Consistent with earlier findings (Zindler and Jagoutz 1988; Barth et al. 2001) the leachate is enriched in LILE and LREE, whereas Ti, Zr and Hf are immobile, especially in samples that contain rutile (e.g. sample DEJ5). Unleached bulk rocks have very variable isotopic compositions (grey diamonds, Fig. 6), whereas leached residues (open diamonds) converge towards the field defined by Group II kimberlites (to which the Roberts Victor kimberlite belongs) in both Nd and Hf isotopic composition, but do not agree with the reconstructed bulk compositions (black triangles). This is in agreement with the presence of both leachable and non-leachable foreign components in the eclogite xenoliths, introduced by metasomatizing fluids and deuteric alteration that have also been described from peridotitic xenolith from kimberlites (Richardson et al. 1985). These findings once again highlight the necessity to reconstruct “clean” bulk eclogite compositions based on mineral analyses and modes and that bulk trace element and isotopic analyses have to viewed with great caution.

Table 9 Isotopic composition of unleached (“bulk”) and leached samples (“residue”) compared to recalculated bulk compositions (from Table 6 and Jacob and Jagoutz 1995) and selected trace element compositions (in ppm) of unleached whole rocks (with subscript WR) and leachates (subscript L)
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Fig. 6
figure 6

Comparison of isotopic compositions of bulk eclogite samples (grey diamonds) with leached bulks (white diamonds) and reconstructed bulk compositions (black triangles). Acid leaching moves the isotopic compositions towards the composition of the kimberlite (grey bar, Smith et al. 1985). This illustrates that secondary phases are introduced into the xenoliths by infiltration and that especially interaction with the kimberlite magma leads to formation of not leachable minerals (silicates, oxides). This is consistent with findings in the literature that describe secondary phlogopite, amphibole, spinel and pyroxenes in eclogite xenoliths from kimberlites (e.g. McCormick 1994) and with comparable studies on peridotitic xenoliths from kimberlites (Richardson et al. 1985). Reconstructing the bulk via mineral analyses and modal estimations, in contrast, gives a more reliable estimate of the primary composition.

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Jacob, D.E., Bizimis, M. & Salters, V.J.M. Lu–Hf and geochemical systematics of recycled ancient oceanic crust: evidence from Roberts Victor eclogites. Contrib Mineral Petrol 148, 707–720 (2005). https://doi.org/10.1007/s00410-004-0631-x

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