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 petrology of dolerite sills in the Huab River Valley, Damaraland, north-western Namibia

Abstract: 2and individual sills are up to 130 m thick. They intrude both the Damara basement and overlying Karoo Sequence sediments and are frequently transgressive. The sills and associated dykes are separated into four distinct dolerite magma types which are distinguished from each other by different ratios of incompatible element abundances. All the dolerite types are tholeiitic, containing both normative hypersthene and olivine. They are relatively primitive in character with rather high concentrations of Mg, Ni and Cr and low concentrations of incompatible trace elements (e.g. Ba, Zr, and Nb). The Huab sills are compositionally different from the basaltic lavas of the overlying Etendeka Formation and from most of the dolerite types which intrude the lavas. They are closest in composition to the Horingbaai dolerites, a suite of late-stage intrusives into the Etendeka Formation with MORB affinities. The compositional variations within individual sills are consistent with crystal fractionation processes which appear to have operated both by gravitational separation and accumulation of early-formed phenocrysts, and by flow differentiation.

Thermal Aspects of the Origin of Hebridean Tertiary Acid Magmas. II. Experimental Melting Behaviour of the Granites at 1 kbar PH20

Abstract: Experimental melting equilibria at 1 kbar P H2O are reported for three early Tertiary granites from Skye and one from Rhum, together with a quartz monzonite from Ardnamurchan. The Hebridean Province leucogranites show cotectic melting behaviour, whilst associated ‘primitive’ (mostly adamellitic) acid plutons have silicate liquidi ∼ 925 °C and plagioclase liquidi ∼ 875 °C. At such high temperatures the morefusible crustal rock types of the region would be almost completely molten. If the Tertiary ‘primitive’ acid magmas were simple crustal melts, they would inherit the incompatible element ratios of their sialic progenitors. This is demonstrably not so. Minor element ratios in these plutons are consistent with extensive fractional crystallization during the evolution of their magmas. But it is, in most individual instances, extremely difficult to specify, either by thermal or geochemical arguments, whether (1) fractional crystallization followed sialic contamination of basic magmas or (2) mixing occurred between fractionation residua and sialic melts. Both the minor element ratios and cotectic melting behaviour of the Hebridean Province leucogranites are consistent with the view that they are products of extreme fractional crystallization of (sial contaminated) more-basic magmas. Nevertheless, it is clear—when the sequence and timing of emplacement of acid magmas within individual intrusive centres is considered—that very complex polycyclic combinations of fusion, fractionation, and re-fusion were involved in the evolution of these granites. The solidi of the samples studied experimentally, together with published data, show a progressive rise in temperature as the compositions of the rocks diverge from the minimum melting composition in NaAlSi3O8-KAlSi3O8-SiO2 at 1 kbarH2O. This relationship is important when considering the likely nature of sialic contamination of basic to intermediate continental magmatic suites fed through reservoir systems which are swarms of dykes and, or, sills. Because salic low-solidus crustal rock types also show near-cotectic melting behaviour, the composition of the contaminant may be strongly biased towards these (usually minor) crustal units, rather than the ‘bulk’ or average crustal composition.

Incompatible element ratios in French Polynesia basalts: describing mantle component fingerprints

Abstract: The chemical composition and origin of the isotopically identified end-member mantle components have been difficult to describe because their melted samples—oceanic basalts—are affected by many processes such as variable degrees of partial melting Exploratory data analysis [multidimensional scaling (MDS)] applied to ∼200 basalts from French Polynesia reveals over 100 ratios of similarly incompatible elements (SIER) that are minimally affected by these processes Ratios from elements with dissimilar incompatibility are identified as affected by melting percentages When basalt samples are compared simultaneously using ∼100 SIER and MDS, they organise in the same way that they would with isotopes, according to mantle component type Applying discriminant analysis to the most extreme French Polynesia samples representing each mantle component yields preliminary discrimination diagrams that improve the description of chemical variation in the mantle As a test of their utility and reliability, they are used

Geochemical and Petrological Significance of the Archaean-Proterozoic Boundary

Abstract: S U M M A R Y : A significant change in the nature of igneous activity responsible for continental growth apparently occurred at the Archaean-Proterozoic boundary. The sedimentary record is consistent with an Archaean upper crust dominated by the bimodal basic-felsic suite, with both units derived by melting at mantle depths. In contrast, the Proterozoic (and younger) upper continental crust is dominated by K-rich granites, derived by intracrustal melting. This major episodic event produced an upper crust enriched in incompatible elements (e.g. Rb, U) as attested inter alia by the increase in 87Sr/S6Sr in marine carbonates in the early Proterozoic, and the abundance of early Proterozoic uranium deposits. This event seems to have been connected with the widespread initiation of subduction related calcalkaline volcanism, which, since that time, has been the principal contributor to continental growth. Although the change at the Archaean-Proterozoic boundary was the major episodic event in continental growth, minor episodes of cratonization occurred in the early Archaean, and some greenstone belt development lingered into the early Proterozoic. More recent episodes of continental growth (e.g. 1800 Ma event) appear to have been on a smaller scale. The sedimentary record is transparent to them, and they do not represent any fundamental change in the igneous processes responsible for continental growth.

Geochemistry of metasomatised spinel peridotite xenoliths from the Dariganga Plateau, South-eastern Mongolia

Abstract: One fresh (green), one altered (black) and one composite (green/black) peridotite xenolith from the Neogene-Quaternary basalts of the Dariganga Plateau, SE Mongolia, were studied by electron microprobe, X-ray fluorescence, wet chemical and instrumental neutron activation analysis. The history of the upper mantle underneath the Dariganga Plateau has been complex and is characterised by elemental depletions and enrichments processes. The rocks investigated appear to have been processed in several steps, have been moderately depleted (relative to the primitive upper mantle composition) in incompatible elements and subsequently metasomatically enriched in alkalis, Fe, Ca, LREE, Th and U. As a result, most peridotites are moderately depleted in Si, Cr, Ti, HREE and Hf, are slightly enriched in LREE and have elevated Th and U abundances. The minerals in all rocks are out of chemical equilibrium. In the green peridotites disequilibrium is modest but it is severe in the blackened lherzolites. The latter have experienced strong Fe metasomatism accompanied by strong oxidation. As a result, Mg-rich olivines formed by oxidation and precipitation of Fe oxides in the primary olivines (blackening) and Fe-rich olivines formed in the Fe metasomatic event. The latter could only have taken place after the oxidising event, otherwise the Fe-rich olivines would also have been affected by it. Three of the four rocks show negative anomalies (relative to the Ce abundance) of Hf and Ti, one is enriched in these elements, which is considered an indication of the action of carbonatitic melts/fluids in the upper mantle. Enrichment of U over Th in some of our samples seems also to indicate the presence of water in the fluid phase, however, the lack of (OH)-bearing minerals in the Dariganga xenoliths suggests a low activity of water in these fluids. The latest of the metasomatic events probably took place shortly before entrapment of the rocks by the basaltic lava that carried them to the earth’s surface. The composite sample consisting of a green harzburgite and a black lherzolite suggests that blackening took place at the original location of the rock rather than in the basaltic tuff because the latter should have altered the whole xenolith. It also demonstrates that metasomatic processes in the upper mantle can be confined to rather restricted locations with sharp boundaries towards the wall rocks. Blackening as well as the metasomatic events apparently took place because of a better permeability in one part of the rock as compared to the other, probably the result of tectonisation.