Wieneke, Martin (1998) Geochemical variations along the Kolbeinsey Ridge : the behaviour of the sub-axial magmatic system on a slow-spreading ridge. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 99 pp.

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Abstract

The Kolbeinsey Ridge is situated directly north of Iceland and is part of the global midocean ridge system with a total length of over 50 OOO km of presently active sea-floor spreading. It is the world's shallowest ridge segment and represents a very slow-spreading ridge with a half-spreading rate of 1 cm/a. The Kolbeinsey Ridge extends about 500 km from the Tjörnes Fracture Zone in the south to the Jan Mayen Fracture Zone (JMFZ) in the north. It is subdivided into three major segments called the South, Middle and North Kolbeinsey Ridge (SKR, MKR and NKR), which are separated by non-transform overlapping spreading centres, namely the Spar offset (~69 N)and the Eggvin offset (~70 45'N). During two cruises of the "R.V. POSEIDON" closely spaced (< 2 km) samples were recovered by dredging the neovolcanic zones of the SKR, MKR and NKR. Major element, trace element, and isotope compositions of selected samples have been analysed in order to investigate magmatic processes beneath the Kolbeinsey Ridge and the nature and composition of mantle sources. This study focusses on mantle melting processes and the magma supply system feeding the presently active spreading axes and their respective implications for along-axis geochemical variations. Moreover, the context of the Kolbeinsey Ridge in the North Atlantic ridge framework and the relationship to the Iceland plume are examined in terms of differing mantle sources and domains. The samples are generally both basaltic pillows and sheet flow fragments, mostly with well preserved glass rims. Geochemical compositions show clear changes occuring at the ridge offsets. The lavas from the MKR are highly depleted N-MORB, those from the SKR show enrichments in some elements relative to the MKR. The largest compositional change occurs at the Eggvin offset where incompatible element ratios jump from very low values on the MKR to higher and generally much more variable values in the highly enriched NKR lavas. Fractionational crystallization of olivine, clinopyroxene, and plagioclase is the dominant process controlling the major element variations, whereas the variation in (incompatible) trace elements reflects the effects of both partial melting and magma mixing. Modelling of the behaviour of rare earth elements during the melting process shows that the effects of partial melting can account for most of the trace element variations seen in MKR and SKR lavas, whereas the variation in NKR lavas is mainly controlled by mixing processes between an enriched NKR and a depleted MKR endmember. Furthermore, different attempts to quantify the depth and degree of melting based on trace and major element information are discussed. Calculations of the melting parameters show that Kolbeinsey lavas have undergone relatively high degrees of melting with an onset of melting in the garnet stability field. The melting parameters show no significant changes along the investigated ridge. Nevertheless, systematic along-axis within-segment geochemical variations occur and suggest that no segment-scale magma-homogenisation or -distribution system, such as a continuous large crustal magma chamber or extensive lateral magma transport in dykes, is present. The overlappers appear, therefore, directly to coincide with the boundaries of mantle domains. Each segment erupts geochemically distinct lavas, although there is evidence for
lateral mixing between the sources. The highly depleted MKR lavas are probably not influenced by an Iceland plume component but reflect a widely distributed North Atlantic N-MORB source. This source appears to act as a primitive mixing endmember incorporated in lavas from several other North Atlantic Ridges. In contrast, lavas from the SKR generally show similarities to picrites from Theistareykir (North Iceland) and to lavas from West Greenland related to the initial stage of the Iceland plume. The NKR magmas show some similarities to enriched Icelandic magmas in terms of both trace elements and isotope ratios, whereas no relation to nearby Jan Mayen exists. This implies that the geochemical influence of the Iceland plume may extend at least up to the JMFZ. This is supported by the fact that the whole Kolbeinsey Ridge is anomalously shallow, which has long been interpreted as an effect of a thermal influence of the Iceland plume. The differering mantle domains may have been present in the region at least since the early Tertiary, whereby the North Atlantic N-MORB source has not undergone significant changes and the Icelandic plume component have changed composition with time. In conclusion, solving the North Atlantic "ridge and plume puzzle" will require a model which incorporates both the temporal and spatial evolutions of the Iceland plume and its influence on the North Atlantic Ridges. The present study provides new information and constraints for this model.

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