Incompatible element

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

Melt Geometry, Movement and Crystallization, in Relation to Mantle Dykes, Veins and Metasomatism

Abstract: Consideration of theoretical, experimental and natural rock data show that basic-ultrabasic melt will disperse along mineral grain edges in olivine-rich mantle rock and thereby form a connected three-dimensional network throughout the rock even when present in only small (less than 1%) volumes. The viscosity of such melts will also allow small (less than 1-5%) volumes to move on appropriate geological timescales as a result of gravity-driven compaction. These features mean that small volume basic-ultrabasic melts are capable of infiltrating and metasomatizing mantle peridotites. Modally metasomatized mantle xenoliths are commonly closely associated with an array of dyke-like and vein injection phenomena. Textural, structural and modal characteristics of a wide array of mantle dykes, veins and metasomatic rocks suggest that such rocks have certain features in common with cumulates, and might usefully be distinguished as dyke cumulates and metasomatic infill cumulates . They represent partial crystal precipitates from melt flowing along channelways or pervasively through peridotite, and their bulk rock compositions provide poor guides to actual mantle melt compositions. The crystallization of the minerals in dykes/veins/ metasomites causes differentiation of the melt by crystal fractionation processes, but at the same time the melt may maintain equilibrium with host rock phases (e.g. olivine) and chromatographic column or percolation effects will control the range of transport of different chemical components by the melt. These combined processes are referred to as percolative fractional crystallization . Data on the actual trace element compositions of melt in equilibrium with the minerals of mantle dykes/veins/metasomites are calculated from trace element analyses of the minerals by using partition coefficients. For a wide variety of metasomatic suites, the calculated melt compositions show a progression of trace element abundances from ones similar to primitive asthenospheric OIB-like compositions towards more incompatible element enriched compositions. Thus they support the hypothesis that fractional crystallization and percolative fractional crystallization processes operating upon initial primitive asthenospheric melts may yield melt compositions matching those necessary for wide varieties of mantle metasomatism. The differentiation of the melts and evolution of the metasomatic rocks proceed together. No evidence for the involvement of volatile-rich fluids distinct from melts has been found. The trace element compositions of many kimberlitic and lamproitic melts may also arise by processes of percolative fractional crystallization of initially primitive melts with oIB-like trace element compositions, as a result of flow through mantle peridotite.

On the Causes of Electrical Conductivity Anomalies in Tectonically Stable Lithosphere

Abstract: Magnetotelluric (MT) data can image the electrical resistivity of the entire lithospheric column and are therefore one of the most important data sources for understanding the structure, composition and evolution of the lithosphere. However, interpretations of MT data from stable lithosphere are often ambiguous. Recent results from mineral physics studies show that, from the mid-crust to the base of the lithosphere, temperature and the hydrogen content of nominally anhydrous minerals are the two most important controls on electrical conductivity. Graphite films on mineral grain boundaries also enhance conductivity but are stable only to the uppermost mantle. The thermal profile of most stable lithosphere can be well constrained, so the two important unknowns that can affect the conductivity of a lithospheric section are hydrogen content and graphite films. The presence of both of these factors is controlled by the geological history of the lithosphere. Hydrogen in nominally anhydrous minerals behaves as an incompatible element and is preferentially removed during melting or high-temperature tectonothermal events. Grain-boundary graphite films are only stable to ~900 °C so they are also destroyed by high-temperature events. Conversely, tectonic events that enrich the lithosphere in incompatible elements, such as interaction with fluids from a subducting slab or a plume, can introduce both hydrogen and carbon into the lithosphere and therefore increase its electrical conductivity. Case studies of MT results from central Australia and the Slave Craton in Canada suggest that electrical conductivity can act as a proxy for the level of enrichment in incompatible elements of the lithosphere.

Elemental fingerprints of isotopic contamination of hebridean Palaeocene mantle-derived magmas by archaean sial

Abstract: One of the major puzzles presented by the geochemistry of the Palaeocene plateau lavas of Skye and Mull (N.W. Scotland) is that, although a very strong case can be made that the magmas are variably isotopically contaminated by Archaean Lewisian continental crust, little evidence has been gleaned to date from their major- and trace-element compositions to illuminate this hypothetical process. The combined results of published Sr-, Nd- and Pb-isotope studies of these lavas allow the basalts and hawaiites to be divided into three broad groups: essentially uncontaminated; contaminated with granulite-facies Archaean crust; contaminated with amphibolite-facies Archaean crust. Members of each group show distinctive chondrite-normalised incompatible-element patterns. The processes which gave rise to isotopic contamination of these lavas also affected the abundances and ratios of Ba, Rb, Th, K, Sr and light REE in the magmas, whilst having negligible effects on their abundances and ratios of Nb, Ta, P, Zr, Hf, Ti, Y and middle-heavy REE. Because such a wide range of elements were affected by the contamination process, it is postulated that the contaminant was a silicate melt of one or more distinctive crustal rock types, rather than an aqueous or similar fluid causing selective elemental movements from wall rocks into the magmas. As previous experimental and isotopic studies have shown that the Skye and Mull basic magmas were not constrained by cotectic equilibria at the time when they interacted with sial, the compositions of the contaminated lavas have been modelled in terms of simple magma-crust mixtures. Very close approximations to both the abundances and ratios of incompatible elements in the two groups of contaminated basalts may be obtained by adding 15% to 20% of Lewisian leucogneisses to uncontaminated Palaeocene basalt. Nevertheless, major-element constraints suggest that the maximum amount of granitic contaminant which has been added to these magmas lies between 5% and 10%. These estimates may be reconciled by postulating that the contaminants were large-fraction cotectic partial melts of Lewisian leucogneisses, leaving plagioclase residua. A corollary of this hypothesis is that it is necessary to postulate that the “magma chambers” where the sialic contamination occurred were, in fact, dykes or (more probably) sills. The very large surface-to-volume ratios of such magmas bodies would permit the systematic stripping, by partial melting, of the most-easily-fusible leucogneisses and pegmatites from the Lewisian crust, whilst failing to melt its major rock types. A present-day analogue to this situation may be the extensive sill-like magma bodies detected by geophysical methods within the continental crust beneath the Rio Grande Rift, southwestern U.S.A.