Showing papers on "Incompatible element published in 1977"

Geochemical evolution of the suboceanic mantle

Abstract: The suboceanic mantle has on the basis of incompatible element and isotope ratios two chemically distinct units; the mantle source for ocean island basalts and the mantle source for ocean ridge basalts. Ocean island alkali basalts have an enriched light rare earth element (REE) source with K/Rb = 400, K/Ba = 28, Zr/Nb = 6.6. Ocean ridge tholeiites have a depleted light REE source, with K/Rb = 1060, K/Ba = no, and Zr/Nb = 37. 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios are generally lower in the ocean ridge basalts than in the basalts from ocean islands. It is suggested that these differences are not a result of simple partial melting, fractional crystallization, zone refining, residual minerals or isotopic disequilibrium between minerals and melt during melting. Instead, the differences between the sources are real, and have existed for some 1500 Ma or more. Nephelinite, alkali basalt, and tholeiites from ocean islands have similar isotopic and incompatible element ratios. The nephelinites and alkali basalts are, however, progressively much more enriched in the light REE than are the tholeiites, yet the light REE in nephelinites and alkali basalts appear to be incompatible. This may be explained by the occurrence of small (average 4 cm thick), closely spaced pyroxenite veins (within tens of cm) in the mantle source for ocean island basalts. The occurrence of a network of veins which melt preferentially may lead to melts which have apparent sources with a much greater enrichment in incompatible elements than the total mantle source and may also give the appearance of a source whose ratio for slightly compatible elements changes as a function of extent of melting.

Open system crystal fractionation and incompatible element variation in basalts

Abstract: O'HARA1 has recently proposed an open-system model for the fractional crystallisation of magmas involving continual replenishment of the magma chamber and only partial extraction of the fractionated liquid at each stage. This could produce a series of chemically uniform lavas (such as basalts) with very little resemblance to the primary mantle melt. It is claimed that the model can also explain large variations in the concentrations of incompatible trace elements and their inter-element ratios which would otherwise be ascribed to variable degrees of partial melting or inhomogeneity in the mantle source region. The purpose of this note is to explore the potential and limitations of O'Hara's model in this respect and to test it against the observed variability of Icelandic basalts.

Strontium isotope evidence for mantle events in the continental lithosphere

Abstract: Following current interpretation of isotope data from oceanic basalts, the case for inheritance of Rb/Sr and 87 Sr/ 86 Sr ratios in continental igneous rocks from long-lived heterogeneities in the mantle is reviewed, both from the point of view of necessary conditions and the evidence of published 9mantle isochrons9. Such inheritance seems unlikely for magmas undergoing plagioclase fractionation and can hardly be claimed for rocks where crustal contamination (especially selective contamination with trace elements or 87 Sr alone) is a feasible alternative. Nevertheless, some unfractionated transitional or undersaturated basic volcanics do show remarkable co-variance of Rb/Sr and initial 87 Sr/ 86 Sr ratios, indicating ages far in excess of extrusion. These and other arguments suggest that heterogeneous mantle, both enriched and depleted in lithophile trace elements, is incorporated into the continental lithosphere during major crust-mantle differentiation events. Re-distribution of isotopes and incompatible elements in fluid phases or small partial melts may be an important mechanism, although others have suggested addition of enriched material from below the asthenosphere.

Instrumental neutron activation analysis and the characterization of granitic rocks

Abstract: It has been shown that instrumental neutron activation analysis with intermediate to long-lived isotopes can be used to determine 25 elements in granites from the Bushveld Igneous Complex, South Africa. By the use of the multivariate technique of discriminant analysis these element concentrations were used to characterize the mineralized granite. The results indicate that because of its ability to determine elements influenced by the process of differentiation, as well as incompatible elements, such as Ta, Th and the heavy rare earths, this method provides a powerful tool for the characterization of granitic rocks.

Lunar differentiation processes as characterized by trace element abundances

Abstract: The pattern of incompatible elements (K, Rb, Ba, r.e.e., H f etc.) is the same for most samples from the lunar highlands. It is suspected that this pattern of incompatible elements is typical for the whole lunar crust. This seems to be a reasonable assumption as one can show from heat flow data that a large part of the Moon’s total U (and consequently other incompatible elements) has to be concentrated in a thin crustal layer, which certainly contributes to the sampled highland rock types. It is supposed that a partial melting process of the major part of the Moon has extracted the trace elements from the interior into the crust. The patterns of incompatible elements of mare basalts are those expected if a second partial melting process were applied to the trace-element-depleted interior. Some consequences of this model are discussed. A relatively constant Sr and Eu distribution through the whole Moon is inferred, implying a positive Eu-anomaly in the lunar interior.