ELSEVIER Chemical Geology 134 (1996) 159-179 ,NCLoDTNc zyxwvutsrqponml ISOTOPE GEOSCIENCE Silicate glasses in spine1 lherzolites from Yemen: origin and chemical composition Gilles Chazot a’ *, Martin Menzies a, Ben Harte b a De,uartment of Geology, Royal Holloway University of London, E&am, Surrey TW 20 OEX, UK b Department of Geology and Geophysics, University of Edinburgh, Kings Buildings, W est Mains Road, Edinburgh EH9 3JW , UK Received 1 November 1995; accepted 1 February 1996 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Anhydrous and amphibole-bearing spine1 lherzolites found in alkali basalts from Southern Yemen contain melt-pockets with silicate glasses from which have crystallized euhedral clinopyroxenes, olivines and spinels. These glasses have variable major element compositions related to variations in the composition of the melting phases. In the hydrous lherzolites, the variations in the major element composition of the glasses are directly correlated to the variations in the composition of residual amphiboles in the melt-pockets. The same observations can be made for the composition of clinopyroxene and glass in the anhydrous lherzolites. The trace element compositions of the residual phases and the glasses are consistent in the different samples and all these observations indicate that the glasses originated by in-situ melting of amphibole or clinopyroxene in the lherzolites. If infiltration of a metasomatic melt or fluid caused the melting event, these data are indicative of very low fluid/rock ratios and attainment of conditions close to equilibrium between the fluid and the solid phases. Surprisingly, Cl-rich apatite, when present, was not melted in these samples, which indicates that apatite can be a residual phase during partial melting at mantle conditions. Comparison of the glasses in the Yemen lherzolites with published chemical compositions for mantle glasses shows a dichotomy between glasses produced by (a) in-situ melting of amphibole or clinopyroxene ( f phlogopite), either in closed-system or during reaction with a metasomatic fluid at low fluid/rock ratios, and (b) glasses resulting from interaction between the peridotite and a metasomatic melt or fluid at higher fluid/rock ratios. Keywords: Geochemistry Mantle; Lherzolites; Metasomatism; Trace elements; Glasses; Melting 1. Introduction Mantle xenoliths brought to the surface of the Earth by lavas prov:ide an insight into the composi- tion of the mantle and the processes which affect its evolution. They are also important to our understand- ing of the genesis of silicate melts at mantle pres- sures and temperatures. Some xenoliths contain small amounts of glass representing quenched silicate melt. * Corresponding author. Address: Laboratoire de Gkologie, Universid Blake Pascal, 5, rue Kesseler, 63038 Clermont-Ferrand cedex, France. E-mail: g.chazot@opgc.univ-bpclermorkfr. FAX: (33) 73 34 67 44. It is important to know if these glasses represent in-situ partial melting of the mantle (thus providing direct information upon magma genesis) or if they represent the result of interactions between peri- 0009-2.541/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SOOOS-2541(96)00086-l