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 (TNi)] 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 (TZn) 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.
|Number of pages||15|
|Publication status||Published - 30 Sept 1996|