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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 230Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226Ra excesses at the expense of diminished 230Th 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. [ABSTRACT FROM AUTHOR]

Rhyolite pumices and co-erupted granophyric (granite) xenoliths yield evidence for rapid magma generation and crystallization prior to their eruption at 15·2 ± 2·9 ka at the Alid volcanic center in the Danikil Depression, Eritrea. Whole-rock U and Th isotopic analyses show 230Th excesses up to 50% in basalts <10 000 years old from the surrounding Oss lava fields. The 15 ka rhyolites also have 30–40% 230Th excesses. Similarity in U–Th disequilibrium, and in Sr, Nd, and Pb isotopic values, implies that the rhyolites are mostly differentiated from the local basaltic magma. Given the (230Th/232Th) ratio of the young basalts, and presumably the underlying mantle, the (230Th/232Th) ratio of the rhyolites upon eruption could be generated by in situ decay in about 50 000 years. Limited (∼5%) assimilation of old crust would hasten the lowering of (230Th/232Th) and allow the process to take place in as little as 30 000 years. Final crystallization of the Alid granophyre occurred rapidly and at shallow depths at ∼20–25 ka, as confirmed by analyses of mineral separates and ion microprobe data on individual zircons. Evidently, 30 000–50 000 years were required for extraction of basalt from its mantle source region, subsequent crystallization and melt extraction to form silicic magmas, and final crystallization of the shallow intrusion. The granophyre was then ejected during eruption of the comagmatic rhyolites. [ABSTRACT FROM PUBLISHER]

A suite of peralkaline trachytes from Longonot volcano, Kenya, which erupted during the last 6000 years, has been analysed for major and trace elements, Pb and Nd isotopes, and U–Th–Ra disequilibria. The lavas are divided into three stratigraphic groups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas, designated LMx1 and LMx2, which, respectively, pre-date and post-date the Lt2 lavas. Major and trace elements are consistent, with up to 37% within-group fractional crystallization of predominantly alkali feldspar. The parental magma for the different trachyte groups had a more mafic composition—probably hawaiitic. Nd and Pb isotopes show minimal variation, both within and between magma groups, and indicate that up to 10% comendite magma from the neighbouring Olkaria volcanic field may have intermixed with the Longonot magma. (230Th/238U) disequilibria indicate that limited U/Th fractionation occurred during the past 10 kyr, whereas (226Ra/230Th) disequilibria reflect the effect of alkali feldspar fractionation >8 kyr ago in the Lt2a lavas, between 3 and 7 kyr ago in the Lt2b lavas and in the past 3 kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2b lavas are interpreted using a model that combines the equations of radioactive decay and in-growth with Rayleigh crystallization to give fractionation rates of about 0·2 × 10−4/year for the evolution of hawaiite to trachyte, but more rapid rates of up to 3 × 10−4/year for fractionation within the trachyte sequence. (226Ra/230Th) from two whole-rock–alkali feldspar pairs are interpreted to show the crystals formed at 5800 years BP (Lt2b) and 2800 years BP (Lt3), implying that phenocryst formation continued almost up to the time of eruption. The results strongly indicate that fractionated magmas can be stored for periods on the order of 1000–2500 years prior to eruption, whereas other magmas were erupted as fractionation was proceeding. [ABSTRACT FROM AUTHOR]