Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Cratons represent the oldest preserved lithospheric domains. Their lithosphere (lithospheric mantle welded to overlying Precambrian crystalline basement) is considered to be particularly robust and long-lived due to the protecting presence of buoyant and rigid “keels” made up of residual harzburgites. Although the cratons are mostly assumed to form in the Archaean, the timing of their formation remains poorly constrained. In particular, there are very few datasets describing concurrently the age of both the crustal and mantle portions of the lithosphere. In this study, we report new U–Pb ages and Hf isotope compositions for zircons in crustal xenoliths from the Udachnaya kimberlite in the central Siberian craton; this dataset includes samples from both the upper and lower portions of the crust. The zircon ages agree well with model melt-extraction Re–Os ages on refractory peridotite xenoliths from the same pipe; taken together they allow an integrated view of lithosphere formation. Our data reveal that the present day upper crust is Archaean, whereas both the lower crust and the lithospheric mantle yield Paleoproterozoic ages. We infer that the deep lithosphere beneath the Siberian craton was not formed in a single Archaean event, but grew in at least two distinct events, one in the late Archaean and the other in the Paleoproterozoic. Importantly, a complete or large-scale delamination and rejuvenation of the Archaean lower lithosphere (lower crust and lithospheric mantle) took place in the Paleoproterozoic. This further demonstrates that craton formation can be a protracted, multi-stage process, and that the present day crust and mantle may not represent complementary reservoirs formed through the same tectono-magmatic event. Further, deep cratonic lithosphere may be less robust and long living than often assumed, with rejuvenation and replacement events throughout its history.
ISSN: 0012-821X

hal-01467648
https://hal.archives-ouvertes.fr/hal-01467648
DOI : 10.1016/j.epsl.2016.09.046

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A subject of continuing debate is how the Earth's lithospheric portion of the upper mantle has remained the thickest (> 200 km) and oldest (> 3 Gy) beneath cratons and is yet surrounded by a vigorously convecting asthenosphere. It is generally admitted that water is a key parameter in the strength and longevity of cratonic roots, because olivine, the main phase of the lithospheric mantle, becomes stronger if its water content decreases. Expanding upon the work presented in Novella et al. (2015) and Taylor et al. (2016), we report new water contents for additional olivine inclusions in diamonds together with the trace-element composition for all olivine inclusions, as well as for mantle xenoliths from various kimberlite pipes located on the Siberian craton. The olivine diamond inclusions from this study have systematically low-water contents (< 50 ppmw H2O), moderate to high forsterite (e.g., Fo91–94) contents and low Ni, Co, and Zn ppm contents (e.g., < 2848, < 108, and < 47 ppm, respectively). In contrast, olivines from Siberian craton mantle xenoliths have a wide range of water contents (6–323 ppmw H2O) and extend to lower-Fo (91–92), Ni, Co, and Zn-rich compositions, compared to the diamond inclusions. Depleted incompatible trace-element concentrations in olivine (0.1–0.001 × Primitive Mantle) advance our hypothesis for the protogenetic origins for the majority of Siberian diamond inclusions. These observations are consistent with the peridotite xenoliths as representing a part of the cratonic lithosphere that has experienced melt re-fertilization, which has also transported water. The olivine diamond inclusions, on the other hand, preserve “micro-samples” of an initial, dry cratonic lithosphere, mostly resulting from melting events. These inclusions are likely sourced from the initial cratonic mantle lithosphere, which thereby, resisted delamination over time, due to its buoyancy and strength, imparted from melt and water depletion, respectively. And thus, our data provides a major argument that the kimberlite-hosted mantle xenoliths may be more metasomatized than common rocks at the base of the Siberian and other cratonic roots away from kimberlite fields.
ISSN: 0024-4937

hal-01467731
https://hal.archives-ouvertes.fr/hal-01467731
DOI : 10.1016/j.lithos.2016.07.023

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Two types of olivine occur in kimberlites from Greenland, Canada and southern Africa. The first, xenocrystic olivine, displays several different forms. Most distinctive are 'nodules', a term we use to describe the large (1-5 mm), rounded, single crystals or polycrystalline aggregates that are a common constituent of many kimberlites. Olivine compositions are uniform within single nodules but vary widely from nodule to nodule, from Fo81 to 93. Within many nodules, sub- to euhedral asymmetric tablets have grown within larger anhedral olivine grains. Dislocation structures, particularly in the anhedral grains, demonstrate that this olivine was deformed before being incorporated into the kimberlite magma. Olivine grains in the kimberlite matrix between the nodules have morphologies similar to those of the tablets, suggesting that most matrix olivine grains are parts of disaggregated nodules. We propose that a sub- to euhedral form is not sufficient to identify phenocrysts in kimberlites and provide some criteria, based on morphology, internal deformation and composition, that distinguish phenocrysts from xenocrysts. The second type of olivine is restricted to rims surrounding xenocrystic grains. Only this olivine crystallized from the kimberlite magma. Major and trace element data for the rim olivine are used to calculate the composition of the parental kimberlite liquid, which is found to contain between about 20 and 30% MgO. The bulk compositions of many kimberlites contain higher MgO contents as a result of the presence of xenocrystic olivine. The monomineralic, dunitic, character of the nodules, and the wide range from Fo-rich to Fo-poor olivine compositions, provide major constraints on the origin of the nodules. Dunite is a relatively rare rock in the mantle and where present its olivine is persistently Fo-rich. The dunitic source of the nodules in kimberlites lacked minerals such as pyroxene and an aluminous phase, which make up about half of most mantle-derived rocks. It appears that these minerals were removed from the mantle peridotite that was to become the source of the nodules, and the Fo content of the retained olivine was modified during interaction with CO2-rich fluids whose arrival at the base of the lithosphere immediately preceded the passage of the kimberlite magmas. Fragments of the resultant dunite were entrained into the kimberlite, where they were retained both as intact nodules and as disaggregated grains in the matrix.
ISSN: 0022-3530

insu-00543321
https://hal-insu.archives-ouvertes.fr/insu-00543321
BIBCODE : 10.1093/petrology/egp080
DOI : 10.1093/petrology/egp080

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report new data on the geochemical evolution, metasomatic and deformation processes in the lower layers of cratonic lithospheric mantle based on a detailed study of extraordinarily fresh and large deformed peridotite xenoliths from the Udachnaya kimberlite pipe, Siberia. Based on our T-P estimates, the deformed peridotites are localized in a depth range of 170 to 220 km near the base of cratonic mantle. The degree of deformation is not correlated with the depth and equilibration temperatures of the xenoliths. The deformed peridotites are depleted in magmaphile major elements indicating their residual nature, but are enriched in incompatible trace elements, Fe and K. The deformed peridotites experienced a complex evolution, i.e., they were formed initially as high-degree melt extraction residues and later were subjected to three main stages of metasomatic modification. (1) An old, mostly cryptic metasomatism by melt/fluid of carbonatitic composition, which formed garnets with sinusoidal REE patterns (Smn/Ern > 1). (2) Silicate metasomatism, which led to the most significant changes in mineralogical and chemical composition of the deformed peridotites. (3) Fe and Ti metasomatism just before the entrainment of the deformed rocks into kimberlite magma. Enrichment of the peridotites in large-ion lithophile elements (K, Rb, Ba) is related to the formation of kelyphitic rims around garnet. The nature of the metasomatic agent of silicate metasomatism was evaluated from mass-balance of measured and calculated whole-rock compositions, ratios of highly incompatible elements (D < 0.1) and fractional crystallization modeling. All this evidence indicates that the agent of silicate metasomatism had a composition intermediate between that of kimberlites and HIMU OIB. The metasomatic processes responsible for the formation of deformed peridotites and precipitation of megacryst suites cannot be widespread at the base of the cratonic mantle. Rather, they are local features only existing in the cratonic mantle below kimberlite fields and localized along the metasomatic vein system.
ISSN: 0024-4937

hal-00941010
https://hal.archives-ouvertes.fr/hal-00941010
DOI : 10.1016/j.lithos.2012.11.014