Canadian Mineralogist Vol. 25, pp. 515-525 (1987) ABSTRACT The mantle-derived xenolith population found in the Ham kimberlite (Somerset Island, Northwest Territories) consists predominantlyof coarse granular-, porphyroclastic- and mosaic-textured garnet lherzolite and subordinate, weakly deformed spinel lherzolite, Compositional data are presented for the constituentmineralsof the lherzolitesand used to calculate temperatures and pressures of equilibra- tion. The variously textured garnet lherzolites have equilibrated over similar relatively limited rangesof tem- perature and pressure (coarse granular texture: 1022-1094" C, 35.5-38.4 kbar; porphyroclastic texture: 1038-l 15loC, 35,7 -4.2 kbar: mosaic texrure: 927 -1140" C, 32.5-39.5kbar) that do not coincidewith thoseof the theo- retical steady-state shield geothenn and which lie outside of the diamond stabilily field. The wide rangeof equilibra- tion temperatures determined for spinel therzolites Q62-1142'C) suggest that they represent disequilibrium assemblages. Garnet lherzolites are believed to be derived from a thermal aureolesurroundinga mantle diapir. Defor- mation associated with crack formation and propagation above this diapir is considered to give rise to zones of mosaic- and porphyroclastic-textured lherzolite within an otherwise coarse granular-textured mantle. Samplingof this heterogeneous mantle by an ascending kimberlitic magma accounts for the observed suite of xenoliths. Keywords: kimberlite, garne[ lherzolile, spinellherzolite, geothermobarometry, Somerset Island, Northwest Ter- ritories. Souualnr La population de nodulesddriv6s du manteaudans la kimberlite de Ham, sur l'ile de Somerset, aux Territoires du Nordouest, contient surtout des lherzolites i grenatgra- nulaires d granulomdtrie grossibre,porphyroclastiques ou d texturemosaique, ainsi que des lherzolites d spinelle fai- blementd6formds. L'information sur la compositionchi- mique des mindraux des lherzolitesseft e calculerleurs tem- p6ratures et pressions d'6quilibrage. Les lherzolitesd grenat ont atteht l'6quilibre sur un intervalle relativement restreint de temperature et de pression (texture granulaire d gros grains:1022-1091oC, 35.5-38.4 kbar; texrure porphyroclas- tique: 1038-ll5loc, 35.7-40.2 kbar; texture mosaique: 9n-114C,32.5-39.5 kbar), valeurs qui ne coihcident pas avec cellespr6dites pour le g6otherme stable de la r6gion et qui sont en dehors du champ de stabilitd du diamant. La grande variabilitd danslestunp6raturesde rd-6quilibrage des lherzolitesd spinelle Q62-ll42.C) font penserqu,il s'agit d'assemblages en dds€quilibre. Les lherzolites d gre- nat seraient d€rivees d'une aurdolede contact entourant un 515 ULTRABASIC XENOLITHS FROM THE HAM KIMBERLITE, SOMERSET ISLAND, NORTHWEST TERRITORIES BRUCE C. JAGO AND ROGER H. MITCHELL Department of Geologlt, Lakehead University, Thunder Bay, Ontario pZB sEl diapir dans le manteau. La d6formation associ6e i la for- mation ete la propagation defissures au-dessus de ce dia- pir aurait cause des zones delherzolites a textures mosai- queet porphyroclastique dans un domaine autrement e granulomdtrie grossibre. Un 6chantillonage dece manteau hdtdrogdne par le magma kimberlitique ascendant rend compte de la suite de nodules observds. (Traduit par Ia Redaction) Mots-cl4si kimberlite, lherzolite d grenat, lherzolite d spi- nelle, geothermobarom€trie, ile deSomerset, Territoi- resdu Nord-Ouest. INTRoDUCTIoN The Ham kimberliteis the northernmost manifes- tation of the Somerset Island kimberlite province (Mitchell 1976),It consists of a main bell-shaped intrusive body (Ham diatreme) and a single dyke (Ham dyke) located 1.5 km to the east.A detailed descriptionof the mineralogyand petrology of the Ham kimberlite is given by Jago (1982) and Jago & Mitchell (1985). Ovoid xenoliths of garnet and spinel lherzolite, up to l0 cm in diameter,were found within the rego- lith overlying the Ham diatremeand in an area of outwash occruring downslope from the intrusive body to the west. Although no lherzolite xenoliths werefound in the Ham dyke, macrocrysts of chrome- diopside and Cr-rich, Ti-poor pyrope garnet were found in heavy-mineral concentrates from the kim- berlite (Jago 1982),indicating that such xenoliths were fragmented during kimberlite emplacement. Xenoliths of mantle-derived garnet and spinellher- zolite havebeenfound at five other localities in the Somerset Island kimberlite province. Rare garnet lherzolitesare present in the Ameyersuk,Tunraq and Inugpasugsuk kimberlites, but are more plentiful, with garnet lherzolite predominatingover spinellher- zolite, in the Elwin Bay and Nanorluk kimberlites (Mitchell 1978,1987). At the Ham kimberlite, gar- net lherzolites are the most abundanttype of xeno- Iith found. PETRoGRAPHY oF THE XENoLITHS The Ham lherzolites may be classified texturally using the scheme of Harte (1977). A complete gra-