Composition of the Lithospheric Mantle in the Siberian Craton: New Constraints from Fresh Peridotites in the Udachnaya-East Kimberlite
Résumé
We present petrographic, major element and trace element data for bulk-rocks and minerals for a suite of 34 remarkably fresh peridotite xenoliths in the Udachnaya kimberlite recovered from deep horizons of the open pit mine. The xenoliths are spinel peridotites, granular and deformed garnet peridotites (as well as a megacrystalline dunite) in proportions similar to those reported by previous work on altered xenoliths from less deep levels in the mine. Equilibration temperatures (T) range from 760–965°C for the spinel peridotites to 1200–1320°C for sheared garnet peridotites; the latter yield pressures (P) of 5·4–6·6 GPa. The majority of the granular garnet peridotites equilibrated at 860–1000°C and 2·6–5·5 GPa, but two samples yield much higher T and P values (1176°C, 6·1 GPa; 1340°C, 6·8 GPa) indicating the presence of undeformed rocks near the base of the lithosphere. Strong enrichments in silica and opx relative to dry melting residues are not common in our granular peridotites (e.g. 11 samples out of 18 analyzed plot on Boyd’s ‘oceanic trend’); they have distinctly lower modal opx than low-T peridotites from the Kaapvaal craton and could be formed by ∼30–45% of melt extraction at 1–5 GPa. Minor to moderate enrichments in silica relative to experimental melting trends seen in some xenoliths may indicate that subduction-related settings were involved in their origin. Garnets are usually unzoned but some are enriched in Ti at the rims. Their contents of CaO (4–7 wt %) and Cr2O3 (2–12 wt %) show inverse power-law correlations with whole-rock CaO, Al2O3 and Al/Cr ratios; the latter can be estimated from garnet analyses. Garnets from the dominant cpx-free and cpx-bearing harzburgites and less common low-cpx (5–6%) lherzolites fall into the G9 ‘lherzolite’ field; garnets from three cpx-rich (9–15%) lherzolites are in the G5 ‘pyroxenite’ field. Rare earth element (REE) patterns in cpx and garnet range from sinusoidal to bell-shaped and light REE (LREE)-depleted; the latter are common in sheared rocks and may be close to equilibrium with metasomatic liquids. Garnets from some granular rocks show low heavy REE (HREE) and continuous depletion in LREE, as in melting residues. Trace element patterns and ratios in many bulk peridotites are distinct from those in the host kimberlites and may not be much affected by contamination. Bulk-rock abundances of moderately incompatible elements are lowest in spinel harzburgites (20–50 times lower than in primitive mantle for HREE) and highest in sheared rocks. Some bulk-rocks show depletion from Lu to Er attributed to 30–40% of melt extraction first in the presence of garnet, then in the spinel field. Overall, the observations on fresh Udachnaya peridotites show less evidence for silica and opx enrichments and smaller compositional effects of syn- and post-eruption alteration than earlier work. The abundances of cpx and garnet and bulk-rock Al2O3 do not vary systematically with depth except that rare sheared xenoliths anomalously enriched in cpx and garnet occur near the base of the lithosphere. The remarkable preservation of olivine and opx shows in detail the transformation of granular microstructures into different types of porphyroclastic fabrics and their relations to chemical compositions.