Wyoming craton mantle lithosphere: reconstructions based on xenocrysts from Sloan and Kelsey Lake kimberlites

Book synopsis: The structure of the lithospheric mantle of the Wyoming craton beneath two Paleozoic kimberlite pipes, Sloan and Kelsey Lake 1 in Colorado, was reconstructed using single-grain thermobarometry for a large data set (>2,600 EPMA analyses of xenocrysts and mineral intergrowths). Pyrope compositions from both pipes relate to the lherzolitic field (up to 14 wt% Cr2O3) with a few deviations in CaO to harzburgitic field for KL-1 garnets. Clinopyroxene variations (Cr-diopsides and omphacites) from the Sloan pipe show similarities with those from Daldyn kimberlites, Yakutia, and from kimberlites in the central part of the Slave craton, while KL-1 Cpx resemble those from the Alakit kimberlites in Yakutia that sample metasomatized peridotites. LAM ICP analyses recalculated to parental melts for clinopyroxenes from Sloan resemble contaminated protokimberlite melts, while clinopyroxenes from KL-1 show metasomatism by subduction fluids. Melts calculated from garnets from both pipes show peaks for Ba, Sr and U, and HFSE troughs, typical of subduction-related melts. Parental melts calculated for ilmenites from Sloan suggest derivation from highly differentiated melts, or melting of Ilm-bearing metasomatites, while those from Kelsey Lake do not display extreme HFSE enrichment. Three P-Fe# (where Fe# = Fe/(Fe + Mg) in atomic units) trends within the mantle lithosphere beneath Sloan have been obtained using monomineral thermobarometry. At the base, the trends reveal melt metasomatized (possibly sheared) peridotites (Fe# = 13–15 %), refertilized peridotites (Fe# = 10–11 %) and primary mantle peridotites (Fe# = 7–9 %). Anomalous heating was found at depths equivalent to 4.0 and 3.0–2.0 GPa. The mantle section beneath KL-1 is widely metasomatized with several stages of refertilization with dispersed Ilm–Cpx trends. The step-like subadibatic heating in the mantle column beneath the Sloan pipe is strong in the base and the middle part and weaker within 2–3 GPa. Heating beneath the KL-1 pipe is more evident in the base and middle part from 7.0 to 3.0 GPa

The origin of coarse garnet peridotites in cratonic lithosphere: New data on xenoliths from the Udachnaya kimberlite, central Siberia

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. © 2013 Springer-Verlag Berlin Heidelberg

Petrology, bulk-rock geochemistry, indicator mineral composition and zircon U–Pb geochronology of the end-cretaceous diamondiferous mainpur orangeites, Bastar Craton, Central India

The end-Cretaceous diamondiferous Mainpur orangeite field comprises six pipes (Behradih, Kodomali, Payalikhand, Jangara, Kosambura and Bajaghati) located at the NE margin of the Bastar craton, central India. The preservation of both diatreme (Behradih) and hypabyssal facies (Kodomali) in this domain implies differential erosion. The Behradih samples are pelletal and tuffisitic in their textural habit, whereas those of the Kodomali pipe have inequigranular texture and comprise aggregates of two generations of relatively fresh olivines. The Kosambura pipe displays high degrees of alteration and contamination with silicified macrocrysts and carbonated groundmass. Olivine, spinel and clinopyroxene in the Behradih and the Kodomali pipes share overlapping compositions, whereas the groundmass phlogopite and perovskite show conspicuous compositional differences. The bulk-rock geochemistry of both the Behradih and Kodomali pipes has a more fractionated nature compared to southern African orangeites. Incompatible trace elements and their ratios readily distinguish them from the Mesoproterozoic Wajrakarur (WKF) and the Narayanpet kimberlites (NKF) from the eastern Dharwar craton, southern India, and bring out their similarity in petrogenesis to southern African orangeites. The pyrope population in the Mainpur orangeites is dominated by the calcic-lherzolitic variety, with sub-calcic harzburgitic and eclogitic garnets in far lesser proportion. Garnet REE distribution patterns from the Behradih and Payalikhand pipes display “smooth” as well as “sinusoidal” chondrite-normalised patterns. They provide evidence for the presence of a compositionally layered end-Cretaceous sub-Bastar craton mantle, similar to that reported from many other cratons worldwide. The high logfO₂ of the Mainpur orangeite magma (ΔNNO (nickel-nickel oxide) of +0.48 to +4.46 indicates that the redox state of the lithospheric mantle cannot be of first-order control for diamond potential and highlights the dominant role of other factors such as rapid magma transport. The highly diamondiferous nature, the abundance of calcic-lherzolitic garnets and highly oxidising conditions prevailing at the time of eruption make the Mainpur orangeites clearly “anomalous” compared to several other kimberlite pipes worldwide. U–Pb dating of zircon xenocrysts from the Behradih pipe yielded distinct Palaeoproterozoic ages with a predominant age around 2,450 Ma. The lack of Archean-aged zircons, in spite of the fact that the Bastar craton is the oldest continental nuclei in the Indian shield with an Eoarchaean crust of 3.5–3.6 Ga, could either be a reflection of the sampling process or of the modification of the sub-Bastar lithosphere by the invading Deccan plume-derived melts during the Late Cretaceous