Petrology and geochemistry of basalts from the American-Antarctic Ridge,Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from 4° W to 18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have enriched geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and ¹⁴³Nd/¹⁴⁴Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and ⁸⁷Sr/⁸⁶Sr (0.70263–0.70295) ratios and have (La/Sm)_N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and ¹⁴³Nd/¹⁴⁴Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), ⁸⁷Sr/⁸⁶Sr (0.70307–0.70334) and (La/Sm)_N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and ¹⁴³Nd/¹⁴⁴Nd (0.51287) ratios and high Zr/Y (7.1), ⁸⁷Sr/⁸⁶Sr (0.70351) and (La/Sm)_N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically enriched MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains 4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of enriched lavas and by implication enriched mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.     

Tectonics of ridge-transform intersections at the Kane fracture zone

The Kane Transform offsets spreading-center segments of the Mid-Atlantic Ridge by about 150 km at 24° N latitude. In terms of its first-order morphological, geological, and geophysical characteristics it appears to be typical of long-offset (>100 km), slow-slipping (2 cm yr^-1) ridge-ridge transform faults. High-resolution geological observations were made from deep-towed ANGUS photographs and the manned submersible ALVIN at the ridge-transform intersections and indicate similar relationships in these two regions. These data indicate that over a distance of about 20 km as the spreading axes approach the fracture zone, the two flanks of each ridge axis behave in very different ways. Along the flanks that intersect the active transform zone the rift valley floor deepens and the surface expression of volcanism becomes increasingly narrow and eventually absent at the intersection where only a sediment-covered ‘nodal basin’ exists. The adjacent median valley walls have structural trends that are oblique to both the ridge and the transform and have as much as 4 km of relief. These are tectonically active regions that have only a thin (<200 m), highly fractured, and discontinuous carapace of volcanic rocks overlying a variably deformed and metamorphosed assemblage of gabbroic rocks. Overprinting relationships reveal a complex history of crustal extension and rapid vertical uplift. In contrast, the opposing flanks of the ridge axes, that intersect the non-transform zones appear to be similar in many respects to those examined elsewhere along slow-spreading ridges. In general, a near-axial horst and graben terrain floored by relatively young volcanics passes laterally into median valley walls with a simple block-faulted character where only volcanic rocks have been found. Along strike toward the fracture zone, the youngest volcanics form linear constructional volcanic ridges that transect the entire width of the fracture zone valley. These volcanics are continuous with the older-looking, slightly faulted volcanic terrain that floors the non-transform fracture zone valleys. These observations document the asymmetric nature of seafloor spreading near ridge-transform intersections. An important implication is that the crust and lithosphere across different portions of the fracture zone will have different geological characteristics. Across the active transform zone two lithosphere plate edges formed at ridge-transform corners are faulted against one another. In the non-transform zones a relatively younger section of lithosphere that formed at a ridge-non-transform corner is welded to an older, deformed section that initially formed at a ridge-transform corner.     

Seasonal variation of hepatic EROD activity in flounder (Platichthys flesus) in the Dutch Wadden Sea

The use of hepatic 7-ethoxyresorufin O-deethylase (EROD) activity in flounder (Platichthys flesus) as a potential biomarker of marine pollution by organic chemicals (as indicated by hepatic PCB-153 concentration) was investigated in the Dutch Wadden Sea in 1989 and 1990. Particular attention was paid to the seasonal variation. The results indicate that EROD activity was reasonably stable during the year, except for a period in January–February when a marked peak occurred. This peak coincided with the absence of mature flounder from the sampling area probably due to the spawning migration. Generally a good relationship existed between hepatic EROD activity and PCB-153 concentration, with one exception. During the period January–April 1990 a peak in PCB-153 concentration occurred immediately after the observed peak in EROD activity. It is concluded that the best period for monitoring EROD activity in flounder is the summer, corresponding to the non-migratory period of the species.     

Farming as a livelihood source for the urban poor of Nakuru, Kenya

Farming among urban dwellers in Sub-Saharan Africa is a common phenomenon. The present study, carried out in a medium-sized town in Kenya, not only confirmed this but also showed that farming by urban dwellers in the rural areas was even more important for these households’ livelihoods than farming in town. However, those who could benefit most from farming, i.e. the urban poor, appeared to be underrepresented among urban farming households and those urban poor who did farm, either in urban or in rural areas, performed worse than the farming non-poor. The importance of farming as a livelihood source is illustrated by the fact that in years with very little or no harvest due to drought, many of the poor farming households faced food shortages.     

Magmatic and metamorphic evolution of the oceanic crust in the western flank of the MAR crest zone at 15°44′N: Investigation of cores from sites 1275B and 1275D,JOIDES resolution Leg 209

This paper reports the results of an investigation of a representative collection of samples recovered by deep-sea drilling from the oceanic basement 10 miles west of the rift valley axis in the crest zone of the Mid-Atlantic Ridge at 15°44′N (sites 1275B and 1275D). The drilling operations were carried out during Leg 209 of the drilling vessel JOIDES Resolution within the framework of the Ocean Drilling Program (ODP). The oceanic crust was penetrated to a depth of 108.7 m at Site 1275B and 209 m at Site 1275D. We reconstructed the following sequence of magmatic and metamorphic events resulting in the formation of a typical oceanic core complex of slow-spreading ridges: (1) formation of a strongly fractionated (enriched in iron and titanium) tholeiitic magmatic melt parental to the gabbroids under investigation in a large magma chamber located in a shallow mantle and operating for a long time under steady-state conditions; (2) transfer of the parental magmatic melt of the gabbroids to the base of the oceanic crust, its interaction with the host mantle peridotites, and formation of troctolites and plagioclase peridotites; (3) intrusion of enriched trondhjemite melts as veins and dikes in the early formed plutonic complex, contact recrystallization of the gabbro, and development in the peridotite-gabbro complex of enriched geochemical signatures owing to the influence of the trondhjemite injections; (4) emplacement of dolerite dikes (transformed to diabases); (5) metamorphism of upper epidoteamphibolite facies with the participation of marine fluids; and (6) rapid exhumation of the plutonic complex to the seafloor accompanied by greenschist-facies metamorphism. The distribution patterns of Sr and Nd isotopes and strongly incompatible elements in the rocks suggest contributions from two melt sources to the magmatic evolution of the MAR crest at 15°44′N: a depleted reservoir responsible for the formation of the gabbros and diabases and an enriched reservoir from which the trondhjemites (granophyres) were derived.