Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
We report new textural and chemical data for 10 garnet peridotite xenoliths from the Udachnaya kimberlite and examine them together with recent data on another 21 xenoliths from the 80-220 km depth range. The samples are very fresh (LOI near zero), modally homogeneous and large (>100 g). Some coarse-grained peridotites show incipient stages of deformation with <10 % neoblasts at grain boundaries of coarse olivine. Such microstructures can only be recognized in very fresh rocks, because fine-grained interstitial olivine is strongly affected by alteration, and may have been overlooked in previous studies of altered peridotite xenoliths in the Siberian and other cratons. Some of the garnet peridotites are similar in composition to low-opx Udachnaya spinel harzburgites (previously interpreted as pristine melt extraction residues), but the majority show post-melting enrichments in Fe and Ti. The least metasomatized coarse peridotites were formed by 30-38 % of polybaric fractional melting between 7 and 4 GPa and ≤1-3 GPa. Our data together with experimental results suggest that garnet in these rocks, as well as in some other cratonic peridotites elsewhere, may be a residual mineral, which has survived partial melting together with olivine and opx. Many coarse and all deformed garnet peridotites from Udachnaya underwent modal metasomatism through interaction of the melting residues with Fe-, Al-, Si-, Ti-, REE-rich melts, which precipitated cpx, less commonly additional garnet. The xenoliths define a complex geotherm probably affected by thermal perturbations shortly before the intrusion of the host kimberlite magmas. The deformation in the lower lithosphere may be linked to metasomatism.
ISSN: 0010-7999

hal-00941252
https://hal.archives-ouvertes.fr/hal-00941252
DOI : 10.1007/s00410-013-0855-8

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27 peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively). The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (<= 1 anhydrous wt.%), high Mg# (>91.5), and bulk rock trace elements compositions mostly below 0.1 x primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites 1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ. The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally supposed below slow spreading centers.
ISSN: 0012-821X

hal-00412001
https://hal.archives-ouvertes.fr/hal-00412001
DOI : 10.1016/j.epsl.2007.11.058