Peridotite xenoliths from the Olmani cinder cone, northern Tanzania, possess distinctive mineralogical and chemical features interpreted to result from interaction of ultra-refractory peridotite residues with carbonatite melts. Chief among these are as follows: (1) The presence of unusually low Al₂O₃ clinopyroxene and the lack enstatite in refractory dunites (with olivines up to Fo₉₄). (2) The presence of monazite and F-rich apatite in refractory harzburgite and wehrlite xenoliths, respectively. (3) LREE enrichment and strong Ti depletion relative to Eu in all but one peridotite. Ca/Al ratios of the clinopyroxene-bearing dunites are some of the highest ever measured for peridotites (up to 10.8, relative to chondritic Ca/Al of 1.1). In addition, Zr/Hf and Ca/Scratios correlate positively, increasing with greater influence of carbonatite on the whole rock. (4) Clinopyroxene-bearing samples have a restricted range of isotopic …