Mantle-Melt Interaction Recorded in Spinel Lherzolite Xenoliths from the Alligator Lake Volcanic Complex, Yukon, Canada

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Abstract
Magmas which have equilibrated with the Earth's upper mantle are generally assumed to be compositionally buffered by spinel lherzolite as represented by Cr-diopside series xenoliths found in alkaline lavas. The fact that the mineral equilibria preserved in such xenoliths typically reflect re-equilibration at sub-solidus mantle conditions, however, has discouraged attempts to use the compositional variation observed in spinel lherzolite xenoliths to constrain the compositions of melts extracted from the upper mantle. A suite of mantle-derived xenoliths from the Alligator Lake volcanic center in the southern Yukon, Canada, exhibits a bimodal xenolith population consisting of lherzolites, the most fertile of which approach pyrolite in composition, and relatively depleted harzburgites. If a source-residue relationship is assumed between the two, then the extracted melt was a picritic magma (∼17 wt. per cent MgO, 23 Mg cation units) with low Fe but relatively high Si contents, similar to picritic lavas associated with subduction margins. The compositional variation within the lherzolite xenoliths, however, is not towards the majority of the harzburgite xenoliths, but towards relatively rare, Fe-rich harzburgites. Reactions observed between the xenoliths and their alkaline host lavas may provide an analogue for the upper mantle process which produced this trend. The observed reactions result in the loss of an Al and Si-rich melt associated with the preferential destruction of pyroxene and spinel and a concomitant rise in the Fe content of residual olivine. The result of such an interaction in the upper mantle would be the development of a Fe and oli vine-rich residue similar to the observed Fe-rich harzburgites. In turn, the magma responsible would be forced to evolve towards more Si-rich, but Fe-poor compositions than would otherwise be possible by closed system, crystal fractionation. A comparison with other mantle xenolith suites indicates that the compositional spectra of many of those associated with continental alkaline basalts can be interpreted in terms of the extraction of picritic magmas similar to that calculated for Alligator Lake. Xenolith suites from oceanic islands such as Hawaii, in contrast, contain fertile lherzolites which are considerably more Fe-rich than pyrolite. The associated refractory xenoliths, however, are similar to those at Alligator Lake and their derivation from such fertile lherzolites would require the extraction of a picritic melt which was both Fe and Si-rich, similar to the observed tholeiitic picritcs of the shield-building stage of Hawaiian volcanism. Alternately, the Fe-rich lherzolites may represent samples of upper mantle which have reacted extensively with the relatively Fe-rich Hawaii magmas. Xenolith suites from kimberlites, on the otheT hand, are dominated by refractory harzburgites which are richer in Si but poorer in Fe than the Alligator Lake harzburgites. They suggest that the lower continental lithosphere is both more orthopyroxene-rich and more depleted than the upper mantle sampled by alkaline basalts. In general, the derivation of depleted harzburgite xenoliths by the partial melting of a pyrolite mantle source seems to require the extraction of picritic magmas. If the majority of terrestrial basaltic magmas are not derived from picritic parental magmas, they require the existence of mantle source regions more Fe-rich than standard pyrolite models.

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