Evolution Of Mantle Xenolith-bearing Alkali Basalts During Ascent

Studies have shown that mantle xenolith-bearing magmas must ascend rapidly to be able to carry mantle xenoliths to the surface.It has thus been inferred inadvertently that such rapid ascending melt must have undergone little crystallization or evolution.However,this inference is inconsistent with the widespread observation that the xenolith-bearing alkali basalts are variably evolved with Mg#?72,the minimum value required for the melt to be in equilibrium with mantle olivine in both asthenospheric source region and lithospheric mantle magma conduit(e.g.,the mantle xenoliths).In this paper,we review this fundamental,yet overlooked,petrological problem and offer new perspectives with evidence.We analyzed the Cenozoic mantle xenolith-bearing alkali basalts from several locations in Southeast China that have experienced varying degrees of fractional crystallization(Mg#=?48.6-66.1).We show that the variably highly evolved composition of the host alkali basalts is actually not in contradiction with rapid ascent,but rather reflects inevitability of crystallization during ascent.Calculations for equilibrium temperatures and pressures of clinopyroxene(Cpx)megacrysts and phenocrysts show a near liquidus ascent path of the host melt with dT/dP?10.4?/kbar.This path is steeper than the expected adiabat of molten mantle(4-6 ?/kbar),which is consistent with the effect of conductive cooling and melt crystallization during the ascent because TMelt>TLithosphere.Furthermore,decompression-induced volatile exsolution can result in liquidus temperature rise,and can thus further facilitate crystallization.The equilibrium pressure of 10.0-29.3 kbars for Cpx megacrysts and phenocrysts suggests that the crystallization takes place under lithospheric mantle conditions.This can occur in magma conduits,but lithospheric mantle magma chambers are required to provide a stable environment for crystallizing megacrysts(1.5-3.0 cm)of uniform compositions.The emplacement of mantle xenoliths is controlled by the volatile content in the host melt,which is ultimately controlled by the mantle source compositions as well as the extent of melting.Melts produced by lower extent of melting of a more enriched mantle source will have high contents of alkalis and volatiles.Exsolution of volatiles during ascent due to decompression results in bulk volume expansion,increasing buoyancy,rapid ascent,and enhanced mechanical power to break off the lithosphere conduit wall materials entrained as mantle xenoliths directly carried to the surface.However,tholeiitic melts produced by higher extent of melting of a less enriched mantle source will have low contents of volatiles and alkalis,ascend slowly and lack the energy to fragment the lithosphere as xenoliths.Such melt could also be ponded at the level of neutral buoyancy in deep crustal magma chambers,where higher extent of fractional crystallization and crustal assimilation take place.