Chemical Variation Among French Ultramafic Xenoliths—Evidence for a Heterogeneous Upper Mantle

R. Hutchison1, A. L. Chambers, D. K. Paul and P. G. Harris
Department of Earth Sciences, The University, Leeds LS2 9JT
1Now at: Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD.

Summary: Some 200 ultramafic xenoliths and their basaltic hosts from five French localities were studied. New analyses are presented, which show the five host-rocks to be nepheline- and olivinenormative. Seven bulk analyses of xenoliths from four localities, together with analyses of their constituent diopsides and, for six, of their orthopyroxenes, are also presented. Xenoliths from four occurrences appear to have equilibrated at pressures between about 8 to 18 kb at sub-basaltic solidus temperatures. Suites of xenoliths are chemically different. Histograms were used to determine compositions of depleted and ‘undepleted’ upper mantle. A suite of peridotite xenoliths from the Bultfontein kimberlite pipe is no less depleted in fusible oxides than xenoliths from two French localities. ‘Undepleted’ upper mantle is very similar to ‘pyrolite’ in composition, except that the latter has much higher TiO2, Na2O, and K2O contents. No xenolith encountered in this work has a bulk composition that could yield more than 12% oceanic tholeiite on partial melting.

Clay Minerals; June 1975 v. 40; no. 310; p. 153-170; DOI: 10.1180/minmag.1975.040.310.05
© 1975, The Mineralogical Society
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