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1 Institute of Geology and Mineralogy SD RAS, Koptyug ave 3, Novosibirsk, Russia (e-mail: Igor.Ashchepkov{at}uiggm.nsc.ru)
2 Institute of Geochemistry SD RAS, Favorsky Str. 1a, Irkutsk, 664033, Russia
3 Central Science and Research Geology and Prospecting Institute of the Stock Company ‘ALROSA’, Chernyshevsky Str, 7, Mirny, 678170 Russia
4 Aykhal Mining Enterprise of Stock Company ‘ALROSA’, Almazny Str., 3-26, Aykhal, 678170 Russia
Four original monomineral methods for mantle peridotite associations are used to reconstruct P–T conditions beneath the kimberlite pipes of Yakutia. The clinopyroxene Jd–Di method gives the closest coincidence with Opx barometry in accord with all physico-chemical boundaries. Garnet thermometers calibrated using Opx, Gar–Cpx and Ni-garnet thermometers and two variants of barometers were developed separately for pyroxenites and peridotites. A Cr–Sp thermobarometer uses the monomineralic version of the Ol–Sp thermometer and a newly calibrated Cr–Sp barometer. A picroilmenite method uses the Ol–Sp thermometer and a pressure-calibration of the geikielite component. Each mantle column is divided into two (upper and lower) sections by a pyroxenite layer located near 40 kbar. Below the pyroxenite layer, the lower section comprises 3–4 lithologically distinct horizons, with a thermally perturbed layer at the base. Above the pyroxenite layer are 3–5 lithologically distinct horizons, which are more fertile than the lower sections. Splitting of the geotherms characterizes most P–T diagrams and is ascribed to multistage melt percolation processes typical for the mantle beneath kimberlite pipes. The largest pipes are diamond-bearing and have a highly depleted peridotite lens above the asthenospheric layer.
