Lherzolite xenoliths in kimberlites and basalts: petrogenetic and crystallochemical significance of some minor and trace elements in olivine, pyroxenes, garnet and spinel

Abstract

Electron and ion microprobe analyses for P, Si, Ti, Al, Cr, V, Sc, Fe, Mn, Mg, Ni, Co, Ca, Sr, Na, K and Li in olivine, pyroxenes and garnet in forty-two cold and twelve hot garnet lherzolites from kimberlites, nine spinel lherzolites from kimberlites and eighteen from alkali basalts, and one cold garnet lherzolite from the Malaita alnöite, are compared with published data for minerals occurring in lherzolite, harzburgite and eclogite xenoliths, for silicate megacrysts in kimberlites, and for silicate inclusions in diamonds. Despite wide ranges in the chemistry of minerals from garnet and spinel lherzolites, there are distinct regions in composition space that would enable determination of the parent lithology of disaggregated minerals in kimberlites and alkali basalts. Titanium correlates with Fe3+ in garnets. Chromium, Al, V and Sc are distributed similarly between silicates in lherzolites. Sodium correlates with trivalent ions in olivine, and increases with temperature. The distribution of Na, but not of K and Li, between olivine and clinopyroxene correlates with temperature. The regular partitioning of Ti, Mn and Ni places constraints on crystal-liquid partition coefficients. Above the stability temperature of mica, ‘metasomatising fluids’ may scavenge Cr and other trivalent ions as they increase Ti in silicates.

Melt, crystal, and fluid inclusions in olivine and clinopyroxene phenocrysts from the submarine shield stage hyaloclastites of Gran Canaria, Sites 953 and 956

“…CaO contents (0.21−0.33 wt%) are lower than those in olivine phenocrysts from Miocene subaerial shield basalts on Gran Canaria (Fig. 3A, B; Table 3; Appendix Tables 1-3), but higher than in olivines from mantle xenoliths of the Cr-diopside series xenoliths on Hierro and the spinel-harzburgite suite xenoliths on Lanzarote (Canary Islands; Neumann, 1991;Neumann et al, 1995), as well as in olivines from mantle xenoliths in general (<0.15 wt% CaO; Hervig et al, 1986;Fig. 3B).…”

“…B. CaO contents vs. Fo in olivine phenocrysts and clinopyroxene-hosted inclusions of olivine. All olivine phenocrysts are CaO-rich compared to olivine from mantle xenoliths (e.g., Hervig et al, 1986) implying that the most magnesian olivines Fo >86 from Sample 157-953C-93R-5, 13−27 cm, are phenocrysts of mantle-derived magmas rather than xenocrysts produced by disintegration of mantle peridotite. Compositions of olivine phenocrysts from Miocene shield basalts on Gran Canaria are taken from Gurenko et al (1996) and Gurenko (unpubl.…”

Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa

“…Chromian spinel in wehrlites has lower Cr# (0.67-0.84) and higher Mg# (0.57-0.68; Table 2). The high TiO 2 content of spinel in dunite and wehrlite xenoliths (1.6-3.7 wt%) compared to primary spinel (<2.0 wt%; Carswell et al, 1979;Hervig et al, 1986;Boyd and Mertzman, 1987) reflects its secondary origin as a break down product of rutile, in kelyphite rims of garnet or in glass patches occurring together with phlogopite. It has high Cr 2 O 3 contents (3.9-5.8 wt%).…”

Lithium in carbonatites — consequence of an enriched mantle source?

“…However, contents of Li measured by ion microprobe analysis on olivine, orthopyroxene, clinopyroxene and garnet of typically depleted mantle nodules (Hervig et al, 1986) would tend to suggest somewhat higher average values for the mantle. On the basis of a comparative geochemicaI study of different types of peridotite nodules, Hervig et al (1986) conclude that in the mantle, at temperatures higher than the stability field of mica (another likely Li repository), "metasomatic fluids passing through lherzolite would enrich it in Ti, Na, Li, P, Fe ~+ and Fe 3+ but remove Cr, V and Sc'. Perhaps the most comprehensive account of the geochemistry of metasomatic enrichment of peridotite is that of Hartmann and Wedepohl (1990) who studied xenoliths in basic volcanics of the Hessian Depression, Germany.…”

“…An average Li content for mantle peridotites is cited as 1.9-t-0.2 ppm (Ryan and Langmuir, 1987), and 2.5-1-1.4 ppm for 36 peridotite xenoliths (Hartman and Wedepohl, 1990). However, contents of Li measured by ion microprobe analysis on olivine, orthopyroxene, clinopyroxene and garnet of typically depleted mantle nodules (Hervig et al, 1986) would tend to suggest somewhat higher average values for the mantle. On the basis of a comparative geochemicaI study of different types of peridotite nodules, Hervig et al (1986) conclude that in the mantle, at temperatures higher than the stability field of mica (another likely Li repository), "metasomatic fluids passing through lherzolite would enrich it in Ti, Na, Li, P, Fe ~+ and Fe 3+ but remove Cr, V and Sc'.…”