Geochemistry, Mineralogy and Petrogenesis of Lavas Erupted along the Southwest Indian Ridge Between the Bouvet Triple Junction and 11 Degrees East

Abstract

Basaltic lavas from fracture zones and ridge segments have been systematically sampled along the Southwest Indian Ridge from the Bouvet triple junction to 11 ° E. Detailed studies of the major and trace element and isotopic variations in these lavas serve to distinguish three compositional groupings which occur throughout this region. The one type comprises basalt with typical ‘depleted’ (N-type MORB) characteristics with high Zr/Nb (>17), Y/Nb (>8) and 143Nd/144Nd (0·51312 ± 2–0·51302 ± 2) ratios, LREE depletion (La/YbN = 0·35–1·1) and with relatively low 87Sr/86Sr (0·70267 ± 4–0·70297 ± 4) ratios. The second, least abundant, type has typical ‘plume’ characteristics (P-type MORB) with low Zr/Nb (5·8–6·8), Y/Nb (0·9–1·2) and 143Nd/144Nd (0·51295 ± 2–0·51286 ± 2) ratios, strong LREE enrichment (La/YbN = 4·8–6·9) and relatively high 87Sr/86Sr (0·70356 ± 4–0·70364 ± 4) ratios. These compositional features are similar to those found in the nearby Bouvet Island lavas. The third type has a transitional chemistry (T-type MORB) with relatively low Zr/Nb (7·7–11·8), Y/Nb. (1·3–3·0) and l43Nd/144Nd (0·51301 ± 2–0·51284 ± 2) ratios, slight LREE enrichment (La/YbN = 1·7–4·3) and with relatively high 87Sr/86Sr (0·70291 ± 4–0·70370 ± 4) ratios. All three lava types show similar variations in terms of major element composition (e.g. Mg #). N- and T-type lavas are volumetrically the most abundant and lavas from both groups occur at most localities that have been sampled. P-type lavas have been found at approximately half the localities sampled and crossing REE patterns are a common phenomenon of these data sets. There is no evidence for a ‘geochemical gradient’ away from the present location of the Bouvet mantle ‘plume’; rather a juxtapositioning of all three lava types occurs along this entire length of the Southwest Indian Ridge system. A comparison of lavas dredged from fracture zones and ridge segments shows no inherent geochemical differences which can be related to the source regions giving rise to these lavas. The fracture zone basalts do however tend to more evolved compositions, a feature attributed to the cooling effect of the adjacent, cold passive lithospheric plate. Consideration of trace element and isotopic systematics supports the concept that the lavas have been derived from a mantle that is chemically heterogeneous on the scale of tens of kms. The available data are generally consistent with a model in which the lavas with P-type characteristics have been derived from an ‘enriched’ source region related to the upwelling associated with the Bouvet mantle ‘plume’ and similar to that giving rise to the Bouvet Island lavas, while the N-type lavas have been derived from normal ‘depleted’ sub-oceanic mantle. It is suggested that the T-type lavas represent mixing products derived from an N-type source region that has experienced less enrichment (by veining by small degree partial melts and/or metasomatic fluids) than that giving rise to the P-type magmas.