Journal of Conference Abstracts

Kilometer-Scale Chemical Variations Along the Middle Kolbeinsey Ridge Spreading Segment: The Behaviour of the Sub-Axial Magmatic System on a Slow-Spreading Ridge

M. Wieneke Geologisch-Paläontologisches Institut und Museum, Universität Kiel, 24118 Kiel, Germany

mwieneke@email.uni-kiel.de

C. W. Devey Geologisch-Paläontologisches Institut und Museum, Universität Kiel, 24118 Kiel, Germany

D. Ackermand Mineralogisch-Petrographisches Institut und Museum, Universität Kiel, 24118 Kiel, Germany

Global studies of MORB geochemistry have shown variations in major and minor element chemistry related to ridge depth, crustal thickness and tectonic features such as propagating rift tips or fracture zones (Klein and Langmuir, 1987). Small scale correlations of ridge depth and major element chemistry have been made especially for fast- or superfast-spreading ridges in the East Pacific (e.g. Langmuir et al., 1986). Up to the present little is known about the behaviour of the magmatic systems below slow spreading ridges. Seismic tomography data (Detrick et al., 1990) suggest that, in contrast to fast-spreading axes, slow-spreading ridges may not be underlain by significant magma chambers and hence magmas may be able to migrate relatively unhindered from their mantle source to the surface.

The Kolbeinsey Ridge, between Iceland and the Jan Mayen Fracture Zone, represents a very slow spreading ridge with a half spreading rate of about 10 mm/y. It is divided into three main segments (the South, Middle and North Kolbeinsey Ridge a.k.a. SKR, MKR, NKR) by non-transform offsets. The MKR segment is bounded by the Spar Fracture Zone (SFZ, 69°N) in the south and the Eggvin Offset (EOS, 71°N) in the north. Its along-axis morphology is characterised by a central high and deeper tips. At 70°30'N a small discontinuity has been observed with 2 spreading axes separated by 2.5 km and overlapping by 8 km (B. Appelgate, pers. comm.). In contrast to normal slow-spreading ridges, the Kolbeinsey Ridge lacks a pronounced axial valley, and in terms of its morphology is more comparable with fast-spreading ridges. We have collected closely spaced (< 2km) samples from the MKR axis for geochemical analysis. All analyses have been performed on glasses by microprobe.

The MKR samples are all basaltic, with MgO in the range 6.5 to 10%, and so have probably suffered extensive crystal fractionation since leaving their mantle source. They show decreasing CaO and Al₂O₃ contents with falling MgO, in agreement with petrographic evidence showing almost ubiquitous clinopyroxene phenocrysts. Na_8.0 does not vary systematically along the ridge. Although the data show relatively large scatter, it appears that the segment tips have slightly higher MgO than generally found at the segment centre, although, as observed at other regions on the Atlantic ridge axis (Batiza et al., 1988), the mid-segment axial high is also characterised by elevated MgO. The southern tip of the NKR (which topographically corresponds to the shallow Eggvin Bank region) has generally higher K₂O/TiO₂ ratios than the MKR. On a plot of K₂O/TiO₂ vs K₂O (Fig.1), the Eggvin Bank samples lie towards the high K₂O/TiO₂ side of a regression line through the MKR samples, implying relative TiO₂ depletion in the Eggvin magmas. Lavas from the nearby Jan Mayen Island lie to the low K₂O/TiO₂ side of an extension of this line and so it seems unlikely that a Jan Mayen component is contributing to the Eggvin Bank magmatism.

Fig. 1: Plots of K₂O contents against K₂O/TiO₂ ratios. Data from present study except Jan Mayen from Maaløe et al. (1986).

The occurrence of higher MgO values at the tips of the ridge segments may be explained by a reduced thickness of magma chamber/mush zone in which magmas ascending from the mantle can be fractionated. This in turn may be the result of enhanced lithospheric cooling associated with the offsets at the segment tips, leading to rapid solidification of any chamber or mush pile. The occurrence of higher MgO at the segment centre is somewhat more difficult to explain. As this is the region at which crustal thicknesses, and hence magma supply rates, are highest, it is presumably related to the higher magma throughput.

References

Batiza, R., Melson, W.G. & O´Hearn, T., Nature 335, 428-431 (1988).

Detrick, R.S., Mutter, J.C., Buhl, P. & Kim, I.I., Nature 347, 61-64 (1990).

Klein, E.M. & Langmuir, C.H., J. Geophys. Res. 92, 8089-8115 (1987).

Langmuir, C.H., Bender, J.F. & Batiza, R., Nature 322, 422-429 (1986).

Maaløe, S., Sørensen, I. & Hertogen, J., J. Petrol. 27, 439-466 (1986).