Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Eclogitic and websteritic diamond sources beneath the Limpopo Belt - is slab-melting the link?

Abstract: Trace-element concentrations in eclogitic and websteritic inclusions in diamonds from Venetia (South Africa) were analysed using an ion microprobe (SIMS). Garnets of both parageneses show similar, positive LREEN/HREEN slopes, but eclogitic garnets have higher MREE and almost flat MREEN–HREEN, and are also different in having significantly higher Sr and Zr. The occurrence of negative and positive Eu anomalies in garnets and clinopyroxenes of both parageneses points towards feldspar fractionation and accumulation in a magmatic precursor, suggesting subducted oceanic crust as a common protolith. Assuming equilibrium between clinopyroxene and garnet included in the same diamond, a bulk eclogite was reconstructed from these inclusions plus (expected) accessory rutile. The whole rock has a trace-element pattern lying between oceanic gabbro and EMORB, but is depleted in highly incompatible elements relative to these possible precursors. Quantitative modelling shows that relative and absolute trace-element abundances of the reconstructed eclogite and the hypothetical oceanic precursor agree if the latter is subjected to a loss of partial melts after subduction into the eclogite stability field. Major- and trace-element characteristics of websteritic inclusions could imply a more mafic precursor, which may have been part of a heterogeneous oceanic crust. However, new experimental data show that major- and trace-element compositions of websteritic inclusions in diamond are consistent with a mixing model in which they result from the reaction of slab-derived melts with surrounding mantle peridotite. This reaction generates major element contents that are intermediate between those of eclogitic and peridotitic sources whereas trace-element characteristics, such as Eu anomalies, are inherited from the melt source.

Geochemistry of lamprophyres from the Western Alps, Italy: implications for the origin of an enriched isotopic component in the Italian mantle

Abstract: Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.