S. Escrig

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80u2009kmu2009Myr–1 (refs 1–4). These faults can accommodate extension for up to 1–3u2009Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of ∼2,500u2009km of the Mid-Atlantic Ridge between 12.5 and 35°u2009N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults in oceanic lithospheric accretion.

Chemical Systematics and Hydrous Melting of the Mantle in Back‐Arc Basins

This paper explores the chemical systematics of back-arc basins and the physical processes that give rise to them, making use of published data from the Scotia, Mariana, Lau, and Manus Basins. A new low-pressure fractionation model is used to back-correct data with greater than 5.5 wt.% MgO. Even after hydrous correction, back-arc basin basalts (BABB) have low TiO 2 and FeO contents relative to basalts from other ridges. The low TiO 2 both absolutely and relative to Na 2 O requires a source depletion followed by a Na enrichment. This signature is critical to evaluate the range of mantle temperature at back-arc basins, which is about 100°C. In addition to a subduction component and wedge depletion, BABB reflect a prevalent enriched component akin to enriched ocean ridge basalts worldwide, despite the absence of mantle plumes. Data from the Mariana Basin suggest this component arises from very recent addition of low-degree (low-F) melts, which may be an important general agent of mantle heterogeneity. Important aspects of the back-arc data are the linear relationships among all major element parameters with each other and with water. Previous models involving isothermal, isobaric melting with increasing water contents do not account for these relationships. A constraint on permissible physical models is that both trace element and major element data show no inherited effects from garnet in back-arc basins, which constrains generation and transport of melts from great depth to the base of the melting regime. Quantitative modeling of the effects of water on mantle melting shows that previous conclusions based on the MELTS thermodynamic approach are not consistent with experimental data for the mantle. Our new models can account for the back-arc systematics by mixing between dry, pooled fractional melts, formed similarly to open ocean ridges, with hydrous melts generated from sources enriched in H 2 O, Na 2 O, and K 2 O that have equilibrated at low pressure. Thus, successful physical models must be able to produce melts by these two different mechanisms. The effects of H 2 O in back-arc basin ridges and open ocean ridges contrast markedly. In the open ocean, increased water is associated with lower mean extents of melting, increased TiO 2 contents, and an increased garnet effect. In back-arc basins, increased water is associated with increased extents of melting, lower TiO 2 , and no garnet influence. These differences can be accounted for by contrasts in the melting regimes and tectonic setting of the two environments. In the open ocean, deep, low-degree, hydrous melts are produced in the wings of the melting regime and combine with higher degree drier melts produced at a range of shallower pressures. At back-arcs, geometrically and thermally, there is no room for wings on the arc side of back-arc spreading centers. On the arc side of the spreading center, where water is added, shallow hydrous melting is important, and melt must get to the surface in the context of descending mantle flow. On the back side, dry melting under relatively anhydrous conditions occurs, similar to open ocean ridges. Mixing between melts from the dry side and the wet side should then lead to the characteristic spectra of parental BABB compositions. Both the geometry of melting and the fact of continual rifting of young lithosphere may contribute to the very different water signatures in the open ocean and back-arc settings.

Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin-Tonga arc system

[1]xa0New high-density sampling of the Eastern Lau Spreading Center provides constraints on the processes that affect the mantle wedge beneath a back-arc environment, including the effect of the subduction input on basalt petrogenesis and the change in subduction input with distance from the Tonga arc. We obtained trace element and Pb-Sr-Nd isotopic compositions of 64 samples distributed between 20.2°S and 22.3°S with an average spacing of ∼3.6 km. The trace element and isotope variations do not vary simply with distance from the arc and reflect variations in the mantle wedge composition and the presence of multiple components in the subduction input. The mantle wedge composition varies form north to south, owing to the southward migration of Indian-like mantle, progressively replacing the initially Pacific-like mantle wedge. The mantle wedge compositions also require an enriched mid-ocean ridge basalt–like trace element enrichment that has little effect on isotope ratios, suggesting recent low-degree melt enrichment events. The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas. The combination of the back-arc and arc data allows identification of several components contributing to the subduction input. These are a fluid derived from the altered oceanic crust with a possible sedimentary contribution, a pelagic sediment partial melt, and, in the southern Lau basin, a volcaniclastic sediment partial melt. While on a regional scale, there is a rough decrease in subduction influence with the distance from the arc, on smaller scales, the distribution of the subduction input reflects different mechanisms of the addition of the subduction input to a variable mantle wedge.

Origins of chemical diversity of back‐arc basin basalts: A segment‐scale study of the Eastern Lau Spreading Center

We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and well-developed spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a “wet component” with strong affinities to arc volcanism and a northern province influenced by a “damp component” intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High–field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H_(2)O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.

Enriched basalts at segment centers: The Lucky Strike (37°17′N) and Menez Gwen (37°50′N) segments of the Mid-Atlantic Ridge

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.

Characterizing the effect of mantle source, subduction input and melting in the Fonualei Spreading Center, Lau Basin: Constraints on the origin of the boninitic signature of the back-arc lavas

[1]xa0New major element, trace element and Pb-Sr-Nd isotope data for glasses from the Fonualei Spreading Center (FSC) constrain the genesis of back-arc basin basalts and the origins of boninites. The FSC is an end-member for global back-arc lavas in terms of low Ti8.0 and Na8.0, and contains lavas with a boninitic signature. Latitudinal variations reveal a correspondence in location between back-arc and adjacent arc volcanism. The locations of spikes in subduction input and positive bathymetric anomalies along the FSC correspond to the projected location of the arc volcanoes, likely reflecting 3-D convective structure of the mantle wedge. Non-mobile trace elements in arc and back-arc lavas reveal an increasing proportion northward of a re-enriched refractory mantle source, which is supported by isotope data. Quantitative modeling constrains the extents of melting, fraction of enriched mantle and subduction input. For the FSC, extents of melting are exceptionally large. We show a general relationship between extent of melting, subduction input and distance from the arc that applies to both the Eastern Lau Spreading Center and the FSC segment closer to the arc. In the center of the FSC where the arc volcanism is captured by the back-arc, exceptionally high subduction input and even greater extents of melting are observed, producing melts with boninitic signature. Boninitic samples require the juxtaposition of high subduction input and refractory mantle, leading to large integrated extents of melting, an occurrence that can be produced by multiple causes.