Description: To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67°05′–70°26′N) display (230Th/238U) 〈 1 and (230Th/238U) 〉 1 with (230Th/238U) from 0.95 to 1.30 and have low U (11.3–65.6 ppb) and Th (33.0 ppb–2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. 87Sr/86Sr = 0.70272–0.70301, εNd = 8.4–10.5, εHf = 15.4–19.6 (La/Yb)N = 0.28–0.84) encompass a narrow range of (230Th/232Th) (1.20–1.32) over a large range in (238U/232Th) (0.94–1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96–1.30) are inversely correlated with (234U/238U) (1.001–1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) values (〉=1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment.

Description: Bone morphogenetic protein 9 (BMP9) promotes the acquisition of the cholinergic phenotype in basal forebrain cholinergic neurons (BFCN) during development and protects these neurons from cholinergic dedifferentiation following axotomy when administered in vivo. A decline in BFCN function occurs in patients with Alzheimer’s disease (AD) and contributes to the AD-associated...

Description: The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U–Th–Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9–10°N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large 230 Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226 Ra excesses at the expense of diminished 230 Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9–10°N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9–10°N EPR, the asymmetric, off-axis distribution of E-MORB at 9–10°N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (~0–3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9–10°N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

Description: Abyssal peridotites and mid-oceanic ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts from different mantle depths, their isotopic signature does not directly describe the isotopic composition of the mantle source, but instead describes the local average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on the distribution of isotopic heterogeneities. We analyzed Pb isotopic compositions in sulfide grains from the Southwest Indian Ridge and the Gakkel Ridge (Arctic Ocean) using the high-resolution Cameca 1280 ion microprobe. Sulfide Pb isotope ratios show very large variations, with 16 grains from 1 sample covering ~25% of the entire range observed in the oceanic mantle. Pb isotopes in sulfides preserve a record of mantle compositions not seen in whole-rock MORBs from the same area. Sulfides from the Atlantis II Fracture Zone (Southwest Indian Ridge) confirm the presence of ancient refractory material scatter in the oceanic upper mantle. Gakkel Ridge sulfides define a high degree of isotopic variability, suggesting that oceanic mantle, not subcontinental lithospheric mantle, is the main source of such heterogeneity. Our results confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous and that other mantle end-member components are intimately mixed in. In-situ sulfide analysis is a powerful tool to detect the isotopic diversity of the MORB mantle source.