Natural experimental charges: an ion-microprobe study of trace element distribution coefficients in glass-rich hornblendite and clinopyroxenite xenoliths

Igneous glasses found in hornblende- and clinopyroxene-rich xenoliths represent former melt pools and contain important information about magmatic processes in mafic alkaline magmas. Rare glass-bearing hornblendite and clinopyroxenite xenoliths entrained in intraplate alkali basalts from Germany and Israel contain up to 11% interstitial glass surrounded by euhedral crystals of mafic minerals (hornblende, clinopyroxene, phlogopite). These xenoliths represent natural experimental charges of alkaline magma quenched by rapid eruption whilst crystallising mafic mineral phases. Bulk rock compositions of the xenoliths are basanitic, considered as representing the starting compositions of the natural experiment, whereas the interstitial glasses are chemically more evolved (highly alkaline foidites) as a result of closed system crystallisation of mafic minerals. Ion-probe trace element data for glass and mineral phases (hornblende, clinopyroxene, phlogopite and apatite) yield in situ distribution coefficients (K(D)s) between magma and minerals. The results generally confirm distribution coefficients derived from experimental data and from melt-bearing peridotite xenoliths, but tend to be slightly higher, perhaps because of the evolved nature of the melt. Disruption of these xenoliths, which must have been semi-consolidated when entrained, may help to explain the origin of clinopyroxene, amphibole and phlogopite megacrysts that are commonly observed in alkali basalts. (C) 2004 Elsevier B.V. All rights reserved.