| The study of ocean basalts provides one window into chemical structure and dynamics of the mantle. MORB are one type, which sample the upper mantle on a global scale as melts of the depleted upper mantle. The majority of MORB are depleted in highly incompatible elements and radiogenic isotopes such as Sr and Pb. In contrast, E-MORB are lower in abundance and are relatively enriched in highly incompatible elements and radiogenic Sr and Pb isotopes. Early studies of ocean ridges located E-MORB near plume-ridge intersections such as Iceland and the Azores. A model evolved whereby E-MORB form as the mixing product of plume and depleted mantle melts. All E-MORB, however, are not found on ridge segments adjacent to plumes.; Two studies, one in the Atlantic and one in the Pacific, led to an increased understanding in the genesis of E-MORB. Both sites located E-MORB on segments far from any known hot spots. The Atlantic site is the MARK area where previous studies failed to recover E-MORB. The high density of sampling in this study led to the discovery of E-MORB in this region. The highly fractionated nature of the trace element pattern of the MARK E-MORB requires low degree melts in its genesis. The E-MORB, however, cannot be a direct low degree melt. Rather its source must have received a low degree some time in the past. The Pacific site is north of the Orozco Fracture Zone where an anomalously inflated segment and abundant off-axis volcanism occurs. A new composition, found at the center of the inflated segment, has low radiogenic Pb isotopes, relatively low La/Sm with high Sm/Yb, and positive Sr and Eu anomalies. Its unique composition is best explained by a recycled gabbro component (recycled lower oceanic crust). Alkaline basalts from seamounts to the west of the ridge axis show evidence for a recycled upper oceanic crust component. Systematic geographic distribution of samples with different chemical compositions calls for a mechanism whereby the recycled oceanic crust components are systematically distributed in the mantle in this region. |
