Incompatible element

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

Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic

Abstract: Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present.

Whole-Mantle Convection and the Transition-Zone Water Filter

Abstract: Because of their distinct chemical signatures, ocean-island and mid-ocean-ridge basalts are traditionally inferred to arise from separate, isolated reservoirs in the Earth's mantle. Such mantle reservoir models, however, typically satisfy geochemical constraints, but not geophysical observations. Here we propose an alternative hypothesis that, rather than being divided into isolated reservoirs, the mantle is filtered at the 410-km-deep discontinuity. We propose that, as the ascending ambient mantle (forced up by the downward flux of subducting slabs) rises out of the high-water-solubility transition zone (between the 660 km and 410 km discontinuities) into the low-solubility upper mantle above 410 km, it undergoes dehydration-induced partial melting that filters out incompatible elements. The filtered, dry and depleted solid phase continues to rise to become the source material for mid-ocean-ridge basalts. The wet, enriched melt residue may be denser than the surrounding solid and accordingly trapped at the 410 km boundary until slab entrainment returns it to the deeper mantle. The filter could be suppressed for both mantle plumes (which therefore generate wetter and more enriched ocean-island basalts) as well as the hotter Archaean mantle (thereby allowing for early production of enriched continental crust). We propose that the transition-zone water-filter model can explain many geochemical observations while avoiding the major pitfalls of invoking isolated mantle reservoirs.