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Langmuir, Charles

Basalts from the Mid-Atlantic Ridge change progressively in composition with increasing distance from the Azores platform. Study of the Lucky Strike and Menez Gwen segments reveals much complexity in the gradient. Both segments contain only basalts enriched relative to normal mid-oceanic ridge basalt, but in two distinct groups. Moderately enriched basalts occur throughout the segments, with proximal Menez Gwen enriched relative to Lucky Strike. Highly enriched basalts occur at segment centers. Incompatible element ratios of the highly enriched basalts exceed those of the Azores platform, while isotopic compositions are less enriched. These observations can be explained by a low-degree melt of garnet-bearing Azores mantle added to mantle depleted by previous melt extraction. Melting this “metasomatized” mantle produces lavas that match the enriched samples. The Azores gradient cannot be explained by simple two-component mixing; rather, it reflects recent melt extraction and addition processes related to southward flow of the Azores plume. The Azores gradient also permits tests of segmentation models. Central supply models predict step functions in chemical compositions between segments. Within-segment gradients require vertical supply. Central supply is supported by robust central volcanoes, thicker crust at segment centers, and a step function in isotopes between the segments. The lava diversity at segment centers, however, requires batches of distinct magma that are preserved through melting and melt delivery. Within-segment gradients in moderately incompatible element ratios support a component of multiple supply. The data suggest partial homogenization of magma within a segment and preferential melt focusing to segment centers with some vertical transport.

High-pressure, low-temperature (HP–LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500 °C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low-T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P–T–x paths. Li concentrations ((\leq 90 ppm)) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP–LT rocks are significantly higher than MORB (4–6 ppm), but similar to HP–LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP–LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP–LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5–10% of the Li in these fluids is derived from sediments. Model results predict that the dehydrated bulk ocean crust is isotopically lighter ( (\sigma)(^{7})Li (\leq) +1 ± 3‰) than the depleted mantle (∼+3.5 ± 0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC–GLOSS composition over the full range of model temperatures (50–600 °C) gives an average fluid (\sigma)(^{7}) Li (∼+7 ± 5‰ 1(\sigma)) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50–260 °C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid ( (\sigma)(^{7})Li = +4 ± 2.3‰ 1 (\sigma)), is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (∼+5 ± 3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using (\sigma)(^{7})Li alone.

The Gakkel ridge, which extends under the Arctic ice cap for similar to 1,800 km, is the slowest spreading ocean ridge on Earth. Its spreading created the Eurasian basin, which is isolated from the rest of the oceanic mantle by North America, Eurasia and the Lomonosov ridge. The Gakkel ridge thus provides unique opportunities to investigate the composition of the sub- Arctic mantle and mantle heterogeneity and melting at the lower limits of seafloor spreading. The first results of the 2001 Arctic Mid- Ocean Ridge Expedition ( ref. 1) divided the Gakkel ridge into three tectonic segments, composed of robust western and eastern volcanic zones separated by a `sparsely magmatic zone'. On the basis of Sr - Nd - Pb isotope ratios and trace elements in basalts from the spreading axis, we show that the sparsely magmatic zone contains an abrupt mantle compositional boundary. Basalts to the west of the boundary display affinities to the Southern Hemisphere 'Dupal' isotopic province(2), whereas those to the east - closest to the Eurasian continent and where the spreading rate is slowest display affinities to 'Northern Hemisphere' ridges. The western zone is the only known spreading ridge outside the Southern Hemisphere that samples a significant upper- mantle region with Dupal- like characteristics. Although the cause of Dupal mantle has been long debated, we show that the source of this signature beneath the western Gakkel ridge was subcontinental lithospheric mantle that delaminated and became integrated into the convecting Arctic asthenosphere. This occurred as North Atlantic mantle propagated north into the Arctic during the separation of Svalbard and Greenland.

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry ( or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

The Mid-Atlantic Ridge (MAR) south of the Kane Fracture Zone at similar to23degreesN (the MARK area) is distant from hot spots and a type area for "normal" mid-ocean ridge basalt (N-MORB) depleted in highly incompatible elements. High-density sampling reveals that a small proportion of basalt are enriched in incompatible elements (enriched mid-ocean ridge basalts, E-MORB) from the MARK area. It is apparent that enriched magma sources, not associated with hot spots, are widespread in the upper mantle and are a common occurrence on both fast- and slow-spreading ridges. Evaluation of the trace-element systematics shows that E-MORB generation requires two stages. Low-degree melts metasomatise the upper mantle to create an enriched source, which later undergoes large extents of melting. A significant time lapse between the two events is required by differences in radiogenic isotope ratios. Atlantic, Pacific, and Indian ocean ridges that are far from hot spots show "mantle isochron" ages of similar to300 Ma for the Sm-Nd, Rb-Sr, and U-238-Pb-206 systems after corrections for melting, but these ages need not be indicative of a specific event. Instead, they can result from continuous processes of formation and destruction of enriched mantle sources by melting and convective mixing. A two-box model describing these processes illuminates relationships between mantle isochron ages and upper mantle dynamics. If formation destruction of enriched mantle is at steady state, constant "mantle isochron" ages are maintained and depend on the residence time of enriched mantle sources, the half-life of the radioactive system, and the daughter element behavior during mantle melting. The common ages of the Sr, Nd, and Pb systems reflects their long half-lives and similar melting behavior. In contrast, Pb-207/Pb-204-Pb-206/Pb-204 ages are approximately twice as old due to the short half-life U relative to the age of the Earth. For the long-lived systems, the mantle isochron ages approximate the residence time of the enriched reservoir, if its mass is a few percent of the system.

We propose that the first stage of melting occurs at depth in subduction zones where the mantle wedge is enriched by the addition of low-degree melts of subducted crust. The second stage of greater extents of melting occurs beneath ocean ridges. The model results suggest that the mantle is currently in quasi-steady state and that the size of the system (N-MORB plus EMORB sources) is similar to the upper mantle. The time scale of similar to300 Ma for survival of E-MORB sources indicates rapid convective stirring and efficient reprocessing of the upper mantle by plate tectonics.

Geochemical studies of Plio-Quaternary volcanic rocks from the Valle de Bravo-Zitacuaro volcanic field (VBZ) in central Mexico indicate that slab melting plays a key role in the petrogenesis of the Trans-Mexican Volcanic Belt. Rocks from the VBZ are typical arc-related high-Mg andesites, but two different rock suites with distinct trace element patterns and isotopic compositions erupted concurrently in the area, with a trace element character that is also distinct from that of other Mexican volcanoes. The geochemical differences between the VBZ suites cannot be explained by simple crystal fractionation and/or crustal assimilation of a common primitive magma, but can be reconciled by the participation of different proportions of melts derived from the subducted basalt and sediments interacting with the mantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlate inversely with Pb and Sr isotopic compositions, indicating that the Sr and Pb budgets are strongly controlled by melt additions from the subducted slab. In contrast, an inverse correlation between Pb(Th)/Nd and Nd-143/Nd-144 ratios, which extend to lower isotopic values than those for Pacific mid-ocean ridge basalts, indicates the participation of an enriched mantle wedge that is similar to the source of Mexican intraplate basalts. In addition, a systematic decrease in middle and heavy rare earth concentrations and Nb/Ta ratios with increasing SiO2 contents in the VBZ rocks is best explained if these elements are mobilized to some extent in the subduction flux, and suggests that slab partial fusion occurred under garnet amphibolite-facies conditions.

The magmatic record of the easternmost part of the Trans-Mexican Volcanic Belt elucidates how temporal changes in subduction parameters influence convergent margin volcanism. In the Palma Sola massif, three phases of magmatic rocks with distinct chemical characteristics were emplaced in a relatively short time span (similar to17 Ma): Miocene calc-alkaline plutons, latest Miocene-Pleistocene alkaline plateau basalts, and Quaternary calc-alkaline cinder cones. Plutons have arc-like trace element patterns (Ba/Nb=16-101), and their Sr, Nd, and Pb isotopic compositions become more "depleted'' with increasing SiO2 contents. Their Pb isotopes are bracketed by the subducted sediments and Pacific mid-ocean ridge basalts (MORB), requiring the participation of an unradiogenic component that mixes with a sediment contribution. High Sr/Y and Gd/Yb ratios in the least radiogenic pluton might indicate a melt coming from the subducted MORB. Trace element patterns of the plateau basalts show moderate or negligible subduction contributions (Ba/Nb=6-31). Rocks without subduction signatures are similar to ocean island basalts, indicating melting of an enriched mantle wedge. The plateau basalts form an array in Pb-206/Pb-204-Pb-207/Pb-204 space that trends toward the composition of the subducted sediment. The sediment component is also indicated by the inverse correlations between Pb isotopes and subduction signals. This component has high Th/Nd coupled with low Nd-143/Nd-144, but lower Pb/Nd and Sr/Nd ratios than the bulk sediment. These suggest melting of a sediment that has lost fluid mobile elements prior to melting. The Quaternary cinder cones have moderate subduction signals (Ba/Nb=16-41), and their isotopic compositions correlate with differentiation indices. Contamination with the local Paleozoic basement can explain the petrogenesis of the youngest rock suite. The geochemical differences among the suites indicate temporal modifications in the chemical characteristics of the slab input. These variations can be associated with modifications in the Pacific subduction regime. We suggest the Miocene magmatic phase was formed by an essentially flat subduction angle that favored melting of the subducted oceanic crust. Slab rollback in the Pliocene allowed melting of deeper portions of the wedge by the injection of dehydrated sediment melts. In the Quaternary, an even steeper subduction angle provided negligible slab contributions to the Palma Sola region, and upper crustal contamination largely controls the petrogenesis.