Local and regional heterogeneity in MORB from the Mid-Atlantic Ridge between 54.5°S and 51°S: Evidence for geochemical enrichment

Abstract

Basaltic lavas from the southern Mid-Atlantic Ridge between the Bouvet triple junction at 54.5°S 1°W and 51°S, show a range in texture from aphyric to highly olivine or plagioclase porphyritic. Olivine ± Cr-spinel ± plagioclase are the dominant phenocryst phases, and clinopyroxene occurs as a phenocryst or microphenocryst phase in many lavas. The range in differentiation can be attributed to crystal fractionation accumulation ; basalts from fracture zones show a greater range than basalts from ridge segments. The fracture zone basalts also show a minor transform fault effect in that their compositions are consistent with lower degrees of partial melting than the ridge axis basalts. This difference is, however, not manifested in incompatible trace element ratios. The incompatible trace element ratios range from normal depleted (N-type) MORB through transitional varieties to moderately enriched (E-type) MORB, yet have higher Sr and lower Nd isotopic ratios than is generally accepted for depleted MORB. Characteristic trace element and isotopic ratios of these lavas are: Zr Nb = 8–38 , Y Nb = 2.1–15.7 , Zr Y = 2.0–4.0 , 143Nd 144Nd = 0.51303–0.51282 , 87Sr 86Sr = 0.70290–0.70364 . The geochemical signature of enriched MORB from this region is distinct from that associated with the Bouvet mantle plume and enriched MORB from the Southwest Indian and the American-Antarctic Ridges. This is best illustrated in terms of the covariation of Zr Nb ratio and 143Nd 144Nd and 87Sr 86Sr ratios. The source of enrichment in these lavas may be the recently proposed Shona hotspot. The enriched signature results from source region mixing achieved by veining of normal depleted mantle by low volume partial melts associated with the rising Shona plume. Subsequent partial melting, during upwelling below the ridge axis, has led to the geochemical enrichment observed in MORB from this region.