Spinel-free dunites as a proxy to komatiitic melt activity in the mantle

abstract Article history:Received 11 July 2014Accepted 31 December 2014Available online 16 January 2015Keywords:Oman ophioliteSpinel-free dunitesKomatiitic meltCumulate dunitesHigh-Ca boninites High-Mg dunites, which are spinel-free or spinel-poor, occur as veins that cut across a pod of chromitite withinthe mantle section deep below the Moho transition zone of the northern Oman ophiolite (Wadi Rajmi). Onlythick (N50 cm) veins were considered in order to avoid the effects of subsolidus Mg–Fe 2+ exchanges with thechromitite wall rock on the compositions of olivine. The spinel-free dunites contain especially high-Mg (Fo N93 )olivines,whereasolivinesinthespinel-poorduniteshavelowerFocontents(Fo 91–93 ).Inaddition,chromianspi-nels inthe spinel-poor dunites display highvalues of Cr# up to 0.8 [Cr# = Cr/(Cr + Al) atomic ratio].We foundthatthesekomatiiticdunitesformedviaanolivinefractionalcrystallizationprocess,andnotbyreplacement,andthatthechromititesservedasisolatingcapsuleswheremeltswerepreservedfrommelt–rockreactionswithpe-ridotite minerals. The olivines show a continuous trend in their chemistry from the spinel-free dunites, throughthe spinel-bearing dunites, to the high-Ca boninites described from the northern Oman ophiolite (Fizh Block).This strongly suggests a genetic linkage between the high-Mg spinel-free or spinel-poor dunites and the high-Caboninites,whichrespectivelyrepresentcumulatesandtheresultantfractionatedmeltsofaprimarykomatiiticmelt.Inthispaper,wechallengetheprevalentmodelsthatruleoutthegenesisofkomatiitesinsupra-subductionzones and shed light on the conditions of the mantle at an early stage of arc evolution.© 2015 Elsevier B.V. All rights reserved.