Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia

The proton microprobe has been used to study the distribution of trace elements in garnet and chromite concentrates from the Udachnaya kimberlite and three smaller, low-grade kimberlites from the Daldyn kimberlite field. Garnet thermobarometry and classical P-T estimates for megacrystalline peridotite xenoliths both suggest a Paleozoic geotherm beneath the Daldyn area that is close to a 35 mW/m² conductive model. Finer-grained xenoliths with T < 1000°C scatter above this geotherm: high-temperature sheared xenoliths lie near or above a 40 mW/m² model geotherm at 1150-1400°C. The higher-T results are interpreted as the result of short-term heating, caused by magmatic intrusion and perturbation of a relatively cool conductive geotherm, especially near the base of the lithosphere. The stratigraphic distribution [inferred from nickel temperature (T_Ni)] of garnets with different major-element chemistry indicates that the lithosphere is strongly layered in terms of rock type; depleted Iherzolites predominate to depths of ca. 150 km, harzburgites comprise up to 60% of the volume between 150 and 180 km, and these are underlain by a mixture of depleted and metasomatically enriched lherzolites. Zinc temperatures (T_Zn) indicate that chromite-bearing peridotites are essentially absent at depths > 190 km. High-T Iherzolite garnets carry a distinctive trace-element fingerprint showing enrichment in Zr, Ti, Y and Ga, interpreted as due to the infiltration of asthenosphere-derived melts. This melt-related metasomatic signature becomes the dominant one at ca. 220-230 km depth, and this is interpreted as the base of the lithosphere. This depth also corresponds approximately to the Lehman Discontinuity at the top of a pronounced low-velocity zone, observed in deep seismic sounding experiments across this part of the Siberian Platform. The techniques used here provide a means of mapping the lithosphere in terms of thermal structure, lithology and fluid-related processes: both lateral (3-D) and temporal (4-D) variations may be mapped using readily available garnet and chromite concentrates from the widespread kimberlite intrusions across the Siberian Platform.