Re-Os Isotope Systematics of Peridotite Xenoliths from Dreiser Weiher and Gees, Eifel, Germany

A. S. Cohen Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

O. M. Burnham Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

N. W. Rogers Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

K. Martyn Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK

One of the key elements in gaining a clearer understanding of the evolution of the continental crust is a knowledge of the timing of stabilisation of the underlying mantle lithosphere from which the crust was derived. Lithosphere becomes isolated from the underlying asthenosphere as a result of melt extraction, and thus dating the depletion places an important constraint on lithosphere stabilisation and its relation to the overlying crust. Suitable samples of lithospheric mantle are available from orogenic peridotite massifs, such as the Ronda and Pyrenean ultramafic complexes, or in the form of peridotite xenoliths which are brought to the surface in volcanic fields. However, attempts to characterise and unequivocally date depletion events in such material using conventional isotope systems (Rb-Sr, Sm-Nd and U-Pb) have had limited degrees of success, principally because these elements are all incompatible and their abundances in depleted peridotites are very low. For this reason, the Nd, Sr and Pb isotopic signatures of peridotite samples are very susceptible to the later effects of metasomatism and alteration within both the mantle and crust.

The geochemical behaviour of Os contrasts markedly with that of Nd, Sr and Pb because Os is highly compatible during mantle melting and is thus retained by the residue. At the same time, Re is lost preferentially to the melt, hence samples of depleted peridotite possess high Os abundances and low Re/Os ratios and may be used to estimate minimum ages for mantle depletion and melt extraction. Furthermore, the high Os abundances of depleted peridotites make them relatively insensitive to the effects of subsequent metasomatism and alteration. This approach has been used with considerable success in studies of garnet lherzolite xenoliths from kimberlites (Walker et al., 1989; Pearson, 1995; Pearson et al., 1995), of ultramafic xenoliths from basaltic volcanics (Carlson and Irving, 1994), and of massif peridotites (Reisberg and Lorand, 1995; Burnham, 1995).

The present study is centered on a suite of spinel peridotite xenoliths from Dreiser Weiher and Gees in the Eifel region of Germany. The samples consist of both hydrous and anhydrous harzburgites, dunites and wehrlites which have been variably enriched metasomatically after initial extraction of some 13-15% melt; all are enriched in LREE. Three samples from Gees have Os abundances between 1.55 and 2.80 ppb, with 187Os/188Os ratios in the range 0.1141-0.1184. In comparison, the six samples from Dreiser Weiher show a much greater range in both their 187Os/188Os ratios and Os abundances. There is a good positive correlation between 187Os/188Os ratio and Al2O3 content for most samples, with Al2O3 acting as a proxy for Re/Os ratio (Reisberg and Lorand, 1995; Burnham, 1995), although the samples with low Os abundances fall off this trend. The correlation indicates a 187Os/188Os ratio of ~0.112 at an Al2O3 content of 0.6%, which may be used to provide an estimate of the initial ratio of the xenolith suite at the time of melt extraction (Burnham, in prep.), corresponding to a depletion age of ~2 Ga. This result is identical to the most reliable Nd and Sr model age of ~2 Ga reported previously for the Eifel lithospheric mantle, obtained from an anhydrous lherzolite from Dreiser Weiher (Stosch and Lugmair, 1986), and is somewhat older than a 1.5 Ga Nd model age from mafic granulite xenoliths from the region (Stosch et al., 1986). Os model ages of ~2-2.5 Ga have also been obtained from Pyrenean lherzolites (Reisberg and Lorand, 1995; Burnham, 1995). Taken together, these results suggest that the stabilisation of an appreciable part of the lithospheric mantle beneath Europe took place some ~2 Ga ago.


Burnham, O.M., Unpublished Ph.D thesis, Open University (1995).

Carlson, R. W. & Irving, A. J., Earth Planet. Sc. Lett. 126, 457-472 (1994).

Pearson, D.G. et al.., Geochim. Cosmochim. Acta 59, 959-977 (1995).

Pearson, D.G., Carlson, R.W., Shirey, S.B., Boyd, F.R. & Nixon, P.H., Earth Planet. Sc. Lett. 134, 341-357 (1995).

Reisberg, L. & Lorand, J.-P., Nature 376, 159-162 (1995).

Stosch, H.G. & Lugmair, G.W., Earth Planet. Sc. Lett. 80, 281-298 (1986).

Stosch, H.G., Lugmair, G.W. & Seck, H.A., In The Nature of the Lower Continental Crust (Dawson, J.B., Carswell, D.A., Hall, J. & Wedepohl, K.H., eds.), 309-317 (Geological Society, London, 1986).

Walker, R.J., Carlson, R.W., Shirey, S.B. & Boyd, F.R., Geochim. Cosmochim. Acta 53, 1583-1595 (1989).