In: Exploration Overview 1996, E. Igboji (compiler); NWT Geology Division, Department of Indian and Northern Affairs, Yellowknife, p. 3-24.
M.KOPYLOVA, H.COOKENBOO, J.K.RUSSELL
The Jericho kimberlite located 400 km northeast of Yellowknife contains abundant, well-preserved mantle xenoliths, which provide an excellent opportunity to investigate mantle lithologies and thermal state. A sample suite of 156 thin sections of xenoliths included eclogite (41%), peridotite (51%), and pyroxenite (8%).
Peridotitic xenoliths include both low-Ti and rare (3%) high-Ti peridotite. High-Ti peridotite is characterized by the presence of abundant orange-coloured Ti-pyrope and ilmenite and by a mosaic-porphyroclastic texture comprising mainly (more than 90%) small olivine neoblasts.
The low-Ti peridotite shows diverse textural habits including coarse, granular (61%), porphyroclastic non-fluidal (35%) and, less commonly, porphyroclastic fluidal (4%). The group of coarse peridotite consists of mainly harzburgite, lherzolite, and wehrlite and contain subhedral, coarse-grained forsterite and enstatite, anhedral finer-grained Cr-diopside and Cr-pyrope and intergranular vermicular chromite. In porphyroclastic non-fluidal peridotites up to 50% of olivine is present as neoblasts, whereas pyroxene is less deformed. Fluidal porphyroclastic peridotite is characterized by highly attenuated bands or lenses of pyroxene neoblasts which stretch out from larger pyroxene porphyroclasts. These lenses are most commonly surrounded by a matrix that is dominantly mosaic-textured olivine neoblasts. These peridotite samples are mineralogically-banded and show a strong preferred orientation. The most deformed samples show a disrupted texture characterized by disaggregated Cr-pyrope.
Eclogitic xenoliths are massive or textured, with a range of transitional varieties, and comprise mainly pyrope and omphacite. Accessory phases include rutile, zircon and olivine in massive samples, and kyanite in foliated samples showing mineral banding or/and preferred mineral orientation. The latter group of eclogite samples are metasomatized, as evidenced by the presence of abundant volatile-rich secondary phases, chemically-zoned garnet, and high-Nb (3% Nb2O5), high-Fe rutiles.
Coarse-grained pyroxenite exhibits allotriomorphic textures with both anhedral, extremely large grains of pyroxenes and unequilibrated magmatic structures, such as lamellae and symplectite intergrowths.
Electron microprobe analyses of coexisting mineral compositions from peridotitic and pyroxenitic xenoliths have been used to estimate mantle P-T conditions. The xenoliths derive from depths consistent with pressures of 40-65 kb according to different P - T solutions. However, irrespective of the solution employed, most of them lie within the diamond stability field. Fig.1 (omitted here) shows equilibrium conditions for Jericho ultramafic rocks calculated by methods advanced by MacGregor (1974) and Finnerty and Boyd (1987).
Compositions of mineral parageneses in samples of eclogite provide an alternate estimate of temperature (Ellis-Green, 1979), but do not constrain pressure. Using an average equilibration pressure of 50 kb, eclogite samples from the Jericho pipe show a unimodal, symmetric distribution of temperatures with a mode between 1000 and 1100oC and a total range of 600 to 1300oC. Similar to peridotites, eclogites are equilibrated within the diamond stability field. Assuming that the thermal regime reconstructed for Jericho peridotites also controls mineral chemistry of eclogites, the majority of the eclogites originated at 45-60 kb.
The calculated temperature of the mantle sampled by the Jericho kimberlite at depths from 130-200 km, is slightly cooler than those for mantle underlying other reported North American occurences of kimberlite (Meyer et al., 1994). Deformed porphyroclastic peridotites are commonly taken as representative of the petrological asthenosphere; the samples of such peridotites from the Jericho kimberlite appear to derive from depths of 180 km. This is a minimum depth to the petrological asthenosphere, and, in conjunction with the thermobarometry results, defines a "diamond window" of 850-1000oC and indicates favourable conditions for storage of diamonds.
