Most Hawaiian basaltic shield volcanoes are capped by moderately to strongly evolved alkalic lavas (MgO<4.5 wt.%).="" on="" mauna="" kea="" volcano="" the="" cap="" is="" dominantly="" composed="" of="" hawaiite="" with="" minor="" mugearite.="" although="" these="" lavas="" contain="" dunite="" and="" gabbroic="" xenoliths,="" they="" are="" nearly="" aphyric="" with="" rare="" olivine="" and="" plagioclase="" phenocrysts="" and="" xenocrysts.="" the="" hawaiites="" are="" nearly="" homogeneous="" in="" radiogenic="" isotope="" ratios="" (sr,="" nd,="" pb)="" and="" they="" define="" coherent="" major="" and="" trace="" element="" abundance="" trends.="" these="" compositional="" trends="" are="" consistent="" with="" segregation="" of="" a="" plagioclase-rich="" cumulate="" containing="" significant="" clinopyroxene="" and="" fe-ti="" oxides="" plus="" minor="" olivine.="" elements="" which="" are="" usually="" highly="" incompatible,="" e.g.,="" rb,="" ba,="" nb,="" are="" only="" moderately="" incompatible="" within="" the="" hawaiite="" suite="" because="" these="" elements="" are="" incorporated="" into="" feldspar="" (rb,="" ba)="" and="" oxides="" (nb).="" however,="" in="" the="" most="" evolved="" lavas="" abundances="" of="" the="" most="" incompatible="" elements="" (p,="" la,="" ce,="" th)="" exceed="" (by="">5–10%) the maximum enrichments expected from models based on major elements. Apparently, the crystal fractionation process was more complex than simple, closed system fractionation. The large amounts of clinopyroxene in the fractionating assemblage and the presence of dense dunite xenoliths with CO₂ inclusions formed at minimum pressures of 2 kb are consistent with fractionation occurring at moderate depths. Crystal segregation along conduit or magma chamber walls is a possible mechanism for explaining compositional variations within these alkalic cap lavas.