ABSTRACT

Melt inclusions contained in plagioclase and olivine phenocrysts have been studied in tholeiitic basalts from the Shona ridge-centered hot spot region, South Atlantic Ocean. Two types of primitive melt inclusions exist within a hand sample, Normal-Mid Ocean Ridge Basalt (N-MORB) and Enriched-Mid Ocean Ridge Basalt (E-MORB) as defined by trace elements. The N-MORB melt inclusions have low (La/Sm)_n (0.54-0.72), high Zr/Nb (22-44) and depleted Light Rare Earth Element (LREE) patterns, whereas the E-MORB melt inclusions have high (La/Sm)_n (0.79-1.09), low Zr/Nb (12-21) and flat LREE. The composition of melt inclusions has been modified by post-entrapment crystallization of the host phase, but this effect is considered relatively small and does not affect the incompatible trace element ratios. The matrix glasses have mildly E-MORB compositions that lie between the two different types of melt inclusions, suggesting that they are produced by mixing of primary magmas of E-MORB and N-MORB type.
The N-MORB primitive melt inclusions are geochemically similar to Indian and other South Atlantic N-MORB in having high Ba/Nb ratios (4-10), but for the first time much more extreme values are found (8-18). The reason for this anomalous enrichment can be related to the presence of old pelagic sediments in a regionally depleted asthenosphere.
The melt inclusions of this study display positive anomalies of Sr and Eu, the first time that have been recorded in the South Atlantic. Their characteristics could be explained by partial melting of mantle containing recycled oceanic crust.