A genetic relationship between diamond and subcalcic Cr-pyrope garnet, both being produced by a metasomatic process, can be inferred from the sampling patterns of kimberlites in the Daldyn-Alakit province, Yakutia, Russia. Pressure-temperature estimates for xenoliths and xenocrysts show a strong concentration of highly depleted rocks in a well-defined zone 140–190 km deep; diamond inclusions and diamond-bearing xenoliths show that most diamonds come from harzburgites within this layer. Xenocryst distribution curves indicate that diamondiferous kimberlites have sampled both garnet and chromite from the harzburgitic layer, but low-grade pipes have sampled only chromite. Diamond formation probably is due to the oxidation of methane-rich, silica-bearing fluids: Fe2O3 (in chromite) + CH4 → C + H2O + FeO (in chromite), accompanied by another reaction: chromite ± olivine ± orthopyroxene + Si, Ca (in fluid) → low-Ca, high-Cr garnet. The presence or absence of diamond in kimberlites thus reflects the distribution of metasomatized fluid conduits in a lithospheric mantle that originally consisted of highly refractory harzburgites containing neither garnet nor diamond.