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

Magmas which have equilibrated with the Earth's upper mantleare generally assumed to be compositionally buffered by spinellherzolite as represented by Cr-diopside series xenoliths foundin alkaline lavas. The fact that the mineral equilibria preservedin such xenoliths typically reflect re-equilibration at sub-solidusmantle conditions, however, has discouraged attempts to usethe compositional variation observed in spinel lherzolite xenolithsto constrain the compositions of melts extracted from the uppermantle. A suite of mantle-derived xenoliths from the AlligatorLake volcanic center in the southern Yukon, Canada, exhibitsa bimodal xenolith population consisting of lherzolites, themost fertile of which approach pyrolite in composition, andrelatively depleted harzburgites. If a source-residue relationshipis assumed between the two, then the extracted melt was a picriticmagma (17 wt. per cent MgO, 23 Mg cation units) with low Febut relatively high Si contents, similar to picritic lavas associatedwith 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. Reactionsobserved between the xenoliths and their alkaline host lavasmay provide an analogue for the upper mantle process which producedthis trend. The observed reactions result in the loss of anAl and Si-rich melt associated with the preferential destructionof pyroxene and spinel and a concomitant rise in the Fe contentof residual olivine. The result of such an interaction in theupper mantle would be the development of a Fe and oli vine-richresidue similar to the observed Fe-rich harzburgites. In turn,the magma responsible would be forced to evolve towards moreSi-rich, but Fe-poor compositions than would otherwise be possibleby closed system, crystal fractionation. A comparison with other mantle xenolith suites indicates thatthe compositional spectra of many of those associated with continentalalkaline basalts can be interpreted in terms of the extractionof picritic magmas similar to that calculated for AlligatorLake. Xenolith suites from oceanic islands such as Hawaii, incontrast, contain fertile lherzolites which are considerablymore Fe-rich than pyrolite. The associated refractory xenoliths,however, are similar to those at Alligator Lake and their derivationfrom such fertile lherzolites would require the extraction ofa picritic melt which was both Fe and Si-rich, similar to theobserved tholeiitic picritcs of the shield-building stage ofHawaiian volcanism. Alternately, the Fe-rich lherzolites mayrepresent samples of upper mantle which have reacted extensivelywith the relatively Fe-rich Hawaii magmas. Xenolith suites fromkimberlites, on the otheT hand, are dominated by refractoryharzburgites which are richer in Si but poorer in Fe than theAlligator Lake harzburgites. They suggest that the lower continentallithosphere is both more orthopyroxene-rich and more depletedthan the upper mantle sampled by alkaline basalts. In general,the derivation of depleted harzburgite xenoliths by the partialmelting of a pyrolite mantle source seems to require the extractionof picritic magmas. If the majority of terrestrial basalticmagmas are not derived from picritic parental magmas, they requirethe existence of mantle source regions more Fe-rich than standardpyrolite models.