Mid-ocean Ridge Axes Research Articles

Reverse-faulting earthquakes and the tectonics of slowly-spreading mid-ocean ridge axes

We examine the distribution and characteristics of 70 reverse-faulting earthquakes at slowly-spreading ridges in the Atlantic and Indian Oceans. All were large enough (Mw≥4.7) for their focal mechanisms to be determined and listed in the gCMT catalogue (www.globalcmt.org/). Of these, 21 were sufficiently large (Mw≥5.3) for detailed body-waveform analysis to determine accurate mechanisms, centroid depths and water depths at their epicentres. All had shallow centroids 1–3 km below the sea bed, and occurred beneath gravity and bathymetric highs on ridge flanks, not in the axial valley, consistent with the locations of reverse-faulting microearthquakes discovered in earlier OBS experiments. We describe a simple quantitative model that explains their occurrence by the bending of an elastic layer about 3 km thick as it is uplifted and exhumed into the dome of a core complex by detachment faulting. The widespread distribution of such events, and their global seismic moment-release rate, suggests that asymmetric spreading in core complexes is the dominant mode of extension at such slowly-spreading ridges. Such an inherently asymmetric process is capable of producing a steady-state axial rift valley, and the overall symmetry of plate creation, but only if its polarity reverses in space and time along the axis of the ridge, for which there is some evidence in the gravity from altimetry.

Geographical structure of endosymbiotic bacteria hosted by Bathymodiolus mussels at eastern Pacific hydrothermal vents

BackgroundChemolithoautotrophic primary production sustains dense invertebrate communities at deep-sea hydrothermal vents and hydrocarbon seeps. Symbiotic bacteria that oxidize dissolved sulfur, methane, and hydrogen gases nourish bathymodiolin mussels that thrive in these environments worldwide. The mussel symbionts are newly acquired in each generation via infection by free-living forms. This study examined geographical subdivision of the thiotrophic endosymbionts hosted by Bathymodiolus mussels living along the eastern Pacific hydrothermal vents. High-throughput sequencing data of 16S ribosomal RNA encoding gene and fragments of six protein-coding genes of symbionts were examined in the samples collected from nine vent localities at the East Pacific Rise, Galápagos Rift, and Pacific-Antarctic Ridge.ResultsBoth of the parapatric sister-species, B. thermophilus and B. antarcticus, hosted the same numerically dominant phylotype of thiotrophic Gammaproteobacteria. However, sequences from six protein-coding genes revealed highly divergent symbiont lineages living north and south of the Easter Microplate and hosted by these two Bathymodiolus mussel species. High heterogeneity of symbiont haplotypes among host individuals sampled from the same location suggested that stochasticity associated with initial infections was amplified as symbionts proliferated within the host individuals. The mussel species presently contact one another and hybridize along the Easter Microplate, but the northern and southern symbionts appear to be completely isolated. Vicariance associated with orogeny of the Easter Microplate region, 2.5–5.3 million years ago, may have initiated isolation of the symbiont and host populations. Estimates of synonymous substitution rates for the protein-coding bacterial genes examined in this study were 0.77–1.62%/nucleotide/million years.ConclusionsOur present study reports the most comprehensive population genetic analyses of the chemosynthetic endosymbiotic bacteria based on high-throughput genetic data and extensive geographical sampling to date, and demonstrates the role of the geographical features, the Easter Microplate and geographical distance, in the intraspecific divergence of this bacterial species along the mid-ocean ridge axes in the eastern Pacific. Altogether, our results provide insights into extrinsic and intrinsic factors affecting the dispersal and evolution of chemosynthetic symbiotic partners in the hydrothermal vents along the eastern Pacific Ocean.

Multiple episodes of partial melting, depletion, metasomatism and enrichment processes recorded in the heterogeneous upper mantle sequence of the Neotethyan Eldivan ophiolite, Turkey

The Eldivan ophiolite along the Izmir–Ankara–Erzincan suture zone in north-central Anatolia represents a remnant of the Neotethyan oceanic lithosphere. Its upper mantle peridotites include three lithologically and compositionally distinct units: clinopyroxene (cpx)–harzburgite and lherzolite (Group-1), depleted harzburgite (Group-2), and dunite (Group-3). Relics of primary olivine and pyroxene occur in the less refractory harzburgites, and fresh chromian spinel (Cr-spinel) is ubiquitous in all peridotites. The Eldivan peridotites reflect a petrogenetic history evolving from relatively fertile (lherzolite and cpx–harzburgite) toward more depleted (dunite) compositions through time, as indicated by (i) a progressive decrease in the modal cpx distribution, (ii) a progressive increase in the Cr#s [Cr/(Cr+Al)] of Cr-spinel (0.15–0.78), and (iii) an increased depletion in the whole-rock abundances of some magmaphile major oxides (Al2O3, CaO, SiO2 and TiO2) and incompatible trace elements (Zn, Sc, V and Y). The primitive mantle-normalized REE patterns of the Group-1 and some of the Group-2 peridotites display LREE depletions. Higher YbN and lower SmN/YbN ratios of these rocks are compatible with their formation after relatively low degrees (9–25%) of open-system dynamic melting (OSDM) of a Depleted Mid-ocean ridge Mantle (DMM) source, which was then fluxed with small volumes of oceanic mantle-derived melt [fluxing ratio (β): 0.7–1.2%]. Accessory Cr-spinel compositions (Cr#=015–0.53) of these rocks are consistent with their origin as residual peridotites beneath a mid-ocean ridge axis. Part of the Group-2 harzburgites exhibit lower YbN and higher SmN/YbN ratios, LREE-enriched REE patterns, and higher Cr-spinel Cr#s ranging between 0.54 and 0.61. Trace element compositions of these peridotites can be modeled by approximately 15% OSDM of a previously 17% depleted DMM, which was then fluxed (β: 0.4%) with subduction-influenced melt. The Group-3 dunite samples contain Cr-spinel with elevated Cr#s (0.73–0.78) and low-TiO2 contents (<0.13wt.%), implying higher degrees of melting (21–24%) of an already depleted DMM that was triggered by infiltration of low-Ti boninite melt with fluxing rates of 0.4–4.0%. The existence of interstitial, idiomorphic Cr-spinel (high Cr# and low Ti) in the Group-3 dunites is consistent with this interpretation. The occurrence of both MOR- and SSZ-type peridotites in the Eldivan ophiolite suggests that its heterogeneous upper mantle was produced as a result of different partial melting and melt–rock reaction processes in different tectonic settings within the Neotethyan realm.

Numerical model of crustal accretion and cooling rates of fast-spreading mid-ocean ridges

Abstract. We designed a thermo-mechanical numerical model for fast-spreading mid-ocean ridge with variable viscosity, hydrothermal cooling, latent heat release, sheeted dyke layer, and variable melt intrusion possibilities. The model allows for modulating several accretion possibilities such as the "gabbro glacier" (G), the "sheeted sills" (S) or the "mixed shallow and MTZ lenses" (M). These three crustal accretion modes have been explored assuming viscosity contrasts of 2 to 3 orders of magnitude between strong and weak phases and various hydrothermal cooling conditions depending on the cracking temperatures value. Mass conservation (stream-function), momentum (vorticity) and temperature equations are solved in 2-D cartesian geometry using 2-D, alternate direction, implicit and semi-implicit finite-difference scheme. In a first step, an Eulerian approach is used solving iteratively the motion and temperature equations until reaching steady states. With this procedure, the temperature patterns and motions that are obtained for the various crustal intrusion modes and hydrothermal cooling hypotheses display significant differences near the mid-ocean ridge axis. In a second step, a Lagrangian approach is used, recording the thermal histories and cooling rates of tracers travelling from the ridge axis to their final emplacements in the crust far from the mid-ocean ridge axis. The results show that the tracer's thermal histories are depending on the temperature patterns and the crustal accretion modes near the mid-ocean ridge axis. The instantaneous cooling rates obtained from these thermal histories betray these discrepancies and might therefore be used to characterize the crustal accretion mode at the ridge axis. These deciphering effects are even more pronounced if we consider the average cooling rates occurring over a prescribed temperature range. Two situations were tested at 1275–1125 °C and 1050–850 °C. The first temperature range covers mainly the crystallization range that is characteristic of the high temperature areas in the model (i.e. the near-mid-oceanic-ridge axis). The second temperature range corresponds to areas in the model where the motion is mainly laminar and the vertical temperature profiles are closer to conductive. Thus, this situation results in less discriminating efficiency among the crustal accretion modes since the thermal and dynamic properties that are described are common to all the crustal accretion modes far from the ridge axis. The results show that numerical modeling of thermo-mechanical properties of the lower crusts may bring useful information to characterize the ridge accretion structure, hydrothermal cooling and thermal state at the fast-spreading ridges and may open discussions with petrological cooling rate results.

Non-hotspot volcano chains produced by migration of shear-driven upwelling toward the East Pacific Rise

While most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid-ocean ridges (MOR), volcanism occurring off-axis reflects intraplate upper-mantle dynamics and composition, yet is poorly understood. Off the south East Pacific Rise (SEPR), volcanism along the Pukapuka, Hotu-Matua, and Sojourn ridges has been attributed to various mechanisms, but none can reconcile its spatial, temporal, and geochemical characteristics. Our three-dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low-viscosity fingers that are propelled eastward by vigorous sublithospheric flow. This shear-driven upwelling is able to sustain intraplate volcanism that progresses toward the MOR, spreads laterally close to the axis, and weakly continues on the opposite plate. These predictions can explain the anomalously fast eastward progression of volcanism, and its spatial distribution near the SEPR. Moreover, for a heterogeneous mantle source involving a fertile component, the predicted systematics of volcanism can explain the geochemical trend along Pukapuka and the enriched anomaly of SEPR mid-oceanic ridge basalt at 16°–20.5°S. Our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a (deep-rooted) buoyancy source.

Significance of widespread low effusion rate eruptions over the past two million years for delivery of magma to the overlapping spreading centers at 9°N East Pacific Rise

Ever since the discovery that the mid-ocean ridge axis deepens at most axial discontinuities, there has been vigorous debate about whether ridge axis depth and shape reflect magma supply. Here we report evidence, from one of the most thoroughly studied ridge discontinuities on a fast-spreading mid-ocean ridge, that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history. This pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off-axis discordant zone of the East Pacific Rise (EPR) 9°N Overlapping Spreading Centers (OSC) than on typical EPR seafloor. Detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current OSC basin and ridge tips, rather than off-axis. Pillow lava, erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows, primarily builds mounds on the seafloor. Crustal thickness, a proxy for long-term magma supply, does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions. To resolve the problem of achieving the same magma emplacement rate with lower effusion rates, without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations, we propose that eruptions occur across the entire OSC basin. The current configuration of mid-crustal melt sills within the OSC as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the OSC basin via vertically propagating dikes. Instead, we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid-crustal melt sills outside of the overlap zone.

Heat and mass transfer in a thermochemical plume under an oceanic plate far from the mid-ocean ridge axis

Heat and mass transfer processes in the conduit of a thermochemical plume located beneath an oceanic plate far from a mid-ocean ridge (MOR) proceed under conditions of horizontal convective flows penetrating the plume conduit. In the region of a mantle flow approaching the plume conduit (in the frontal part of the conduit), the mantle material heats and melts. The melt moves through the plume conduit at the average velocity of flow v and is crystallized on the opposite side of the conduit. The heat and the chemical dope transferred by the conduit to the mantle flow are carried away by crystallized mantle material at the velocity v. The main equations of heat and mass transfer are obtained for a thermochemical plume interacting with a horizontal convective mantle flow. The joint multiparameter problem of heat and mass transfer is solved for a thermochemical plume located far from an MOR axis. The dope concentration at the base of the plume is found as a function of the Lewis number. The Lewis numbers and, accordingly, the diffusion coefficients of the chemical dope in the plume conduit far from the MOR axis are determined.

Evolutionary ecology during the rise of dioxygen in the Earth's atmosphere.

Pre-photosynthetic niches were meagre with a productivity of much less than 10(-4) of modern photosynthesis. Serpentinization, arc volcanism and ridge-axis volcanism reliably provided H(2). Methanogens and acetogens reacted CO(2) with H(2) to obtain energy and make organic matter. These skills pre-adapted a bacterium for anoxygenic photosynthesis, probably starting with H(2) in lieu of an oxygen 'acceptor'. Use of ferrous iron and sulphide followed as abundant oxygen acceptors, allowing productivity to approach modern levels. The 'photobacterium' proliferated rooting much of the bacterial tree. Land photosynthetic microbes faced a dearth of oxygen acceptors and nutrients. A consortium of photosynthetic and soil bacteria aided weathering and access to ferrous iron. Biologically enhanced weathering led to the formation of shales and, ultimately, to granitic rocks. Already oxidized iron-poor sedimentary rocks and low-iron granites provided scant oxygen acceptors, as did freshwater in their drainages. Cyanobacteria evolved dioxygen production that relieved them of these vicissitudes. They did not immediately dominate the planet. Eventually, anoxygenic and oxygenic photosynthesis oxidized much of the Earth's crust and supplied sulphate to the ocean. Anoxygenic photosynthesis remained important until there was enough O(2) in downwelling seawater to quantitatively oxidize massive sulphides at mid-ocean ridge axes.

Hydrothermal cooling of midocean ridge axes: Do measured and modeled heat fluxes agree?

Accurate estimates of near-axis hydrothermal cooling are keys to understanding many geological, chemical, and biological processes at midocean ridges. At present, however, field observations ( H, heat flux/vent field) are few and imprecise, and model results ( H m, heat flux/km of axis) vary widely depending on assumptions about crustal formation, crustal composition, and the cooling efficiency and spatial and temporal extent of hydrothermal circulation. In this paper I compare all published vent-field-scale measurements of H to a range of published H m for slow (≤ 40 mm/yr) and fast (> 40 mm/yr) ridges to assess the present state of knowledge about the magnitude of near-axis hydrothermal cooling. H has been evaluated 28 times, either by point measurements using inventories of individual sources (362 ± 552 MW, n = 12 measurements), or by integrated measurements using water column observations (876 ± 1011 MW, n = 16). Comparable estimates of mean vent field “expected H” at any spreading rate can be derived by merging H m from various models with the predicted global inventory of active vent sites. These estimates range from ∼ 1–1.5 GW/field and are approximately constant for all spreading rates. This range agrees with the best-constrained results of H from slow-ridge vent fields (1669 ± 1354 MW) but is considerably higher than those from fast ridges (245 ± 170 MW). Inferences about the spatial and temporal variability of H can be gained by defining the parameter L = H / H m, equivalent to the axis length needed to continuously power a hydrothermal field. The resultant large L values (13–333 km for slow ridges, 1–32 km for fast) demand punctuated cooling, where for short periods H > H m. The percentage of time that a given vent field might be active can be speculatively estimated by dividing L by the length of axis the field cools. Examples of this percentage range from a low of < ∼ 5% for a slow-ridge field to > ∼ 50% for some fast-ridge fields.

Hydrothermal fluid fluxes calculated from the isotopic mass balance of thallium in the ocean crust

Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Thallium is a unique tracer of ocean floor hydrothermal exchange because of its contrasting behavior during seafloor alteration at low and high temperatures and the distinctive isotopic signatures of fresh and altered MORB and seawater. The calculated high temperature hydrothermal water flux is (0.17–2.93) × 10 13 kg/yr with a best estimate of 0.72 × 10 13 kg/yr. This result suggests that only about 5 to 80% of the heat available at mid-ocean ridge axes from the crystallization and cooling of the freshly formed ocean crust, is released by high temperature black smoker fluids. The residual thermal energy is most likely lost via conduction and/or through the circulation of intermediate temperature hydrothermal fluids that do not alter the chemical budgets of Tl in the ocean crust. The Tl-based calculations indicate that the low temperature hydrothermal water flux at ridge flanks is (0.2–5.4) × 10 17 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1 to 3.6 °C relative to ambient seawater. If these low temperatures are correct then both Sr and Mg are expected to be relatively unreactive in ridge-flank hydrothermal systems and this may explain why the extent of basalt alteration that is observed for altered ocean crust appears insufficient to balance the oceanic budgets of 87Sr/ 86Sr and Mg.

Thallium isotope constraints on the water fluxes of ridge flank hydrothermal systems

Hydrothermal activity in the oceans is not restricted to the high-temperature (T) vents that are found at mid-ocean ridge axes. Rather, ridge flanks have a total hydrothermal power output which exceeds the axial output by at least a factor of 3. As the temperatures are comparatively low at ridge flanks, the respective water flux must be several orders of magnitude larger than on-axis, to remove the inferred power output. Even small changes in the composition of the circulating fluids can therefore produce very significant chemical fluxes. Our understanding of the importance of these chemical fluxes with respect to global geochemical budgets is limited, however, because the partitioning of heat between warmer, more reactive fluids with T 40 °C and cooler fluids, which have T 20 °C, is only poorly constrained. In this study, we have used Tl isotope and concentration data for altered ocean crust and hydrothermal fluids to constrain the water fluxes and average fluid exit temperatures of ridge flank hydrothermal systems. The calculations are based on the observation that the upper ocean crust, which has been altered at low temperature, has fractionated Tl isotope compositions and elevated Tl concentrations. The observed systematics can be exploited to calculate the flux of low-T hydrothermal fluids (FLT) by mass balance: FLT×[Tl]sw×fupt=Fuoc×?[Tl]uoc Here [Tl]sw is the Tl concentration of seawater, fupt is the fraction of Tl removed from seawater by alteration, Fuoc is the mass flux of upper ocean crust affected by low-T hydrothermal alteration, and ?[Tl]uoc is the average change of Tl concentration observed for low-T altered ocean crust. The calculations indicate that the hydrothermal water flux of ridge flanks is (0.2–5.4) × 1017 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1–3.6° relative to ambient seawater. Such low temperatures should severely restrict the effect of ridge flank hydrothermal systems on the marine budgets of 87Sr/86Sr and Mg. This conclusion is in accord with the results of previous rock alteration studies, which concluded that the fluxes of low-T hydrothermal systems are insufficient to balance the oceanic budgets of these elements.

Hydrothermal Vent Geology and Biology at Earth’s Fastest Spreading Rates

Earth’s fastest present seafloor spreading occurs along the East Pacific Rise near 31°–32° S. Two of the major hydrothermal plume areas discovered during a 1998 multidisciplinary geophysical/hydrothermal investigation of these mid-ocean ridge axes were explored during a 1999 Alvin expedition. Both occur in recently eruptive areas where shallow collapse structures mark the neovolcanic axis. The 31° S vent area occurs in a broad linear zone of collapses and fractures coalescing into an axial summit trough. The 32° S vent area has been volcanically repaved by a more recent eruption, with non-linear collapses that have not yet coalesced. Both sites occur in highly inflated areas, near local inflation peaks, which is the best segment-scale predictor of hydrothermal activity at these superfast spreading rates (150 mm/yr).

A new isostatic model of the lithosphere and gravity field

Based on the analysis of various factors controlling isostatic gravity anomalies and geoid undulations, it is concluded that it is essential to model the lithospheric density structure as accurately as possible. Otherwise, if computed in the ‘classical’ way (i.e. based on the surface topography and the simple Airy compensation scheme), isostatic anomalies mostly reflect differences of the real lithosphere structure from the simplified compensation model, and not necessarily the deviations from isostatic equilibrium. Starting with global gravity, topography and crustal density models, isostatic gravity anomalies and geoid undulations have been determined. The initial crust and upper-mantle density structure has been corrected in a least squares adjustment using gravity. To model the long-wavelength (>2000 km) features in the gravity field, the isostatic condition (i.e. equal mass for all columns above the compensation level) is applied in the adjustment to uncover the signals from the deep-Earth interior, including dynamic deformations of the Earth’s surface. The isostatic gravity anomalies and geoid undulations, rather than the observed fields, then represent the signals from mantle convection and deep density inhomogeneities including remnants of subducted slabs. The long-wavelength non-isostatic (i.e. the dynamic) topography was estimated to range from –0.4 to 0.5 km. For shorter wavelengths (<2000 km), the isostatic condition is not applied in the adjustment in order to obtain the non-isostatic topography due to regional deviations from classical Airy isostasy. The maximum deviations from Airy isostasy (−1.5 to 1 km) occur at currently active plate boundaries. As another result, a new global model of the lithosphere density distribution is generated. The most pronounced negative density anomalies in the upper mantle are found near large plume provinces, such as Iceland and East Africa, and in the vicinity of the mid-ocean ridge axes. Positive density anomalies in the upper mantle under the continents are not correlated with the cold and thick lithosphere of cratons, indicating a compensation mechanism due to thermal and compositional density.

Study of changes in oxidative/antioxidative status of low density lipoproteins in middle aged, elderly and long-living men in Novosibirsk

The Eldivan ophiolite along the Izmir–Ankara–Erzincan suture zone in north-central Anatolia represents a remnant of the Neotethyan oceanic lithosphere. Its upper mantle peridotites include three lithologically and compositionally distinct units: clinopyroxene (cpx)–harzburgite and lherzolite (Group-1), depleted harzburgite (Group-2), and dunite (Group-3). Relics of primary olivine and pyroxene occur in the less refractory harzburgites, and fresh chromian spinel (Cr-spinel) is ubiquitous in all peridotites. The Eldivan peridotites reflect a petrogenetic history evolving from relatively fertile (lherzolite and cpx–harzburgite) toward more depleted (dunite) compositions through time, as indicated by (i) a progressive decrease in the modal cpx distribution, (ii) a progressive increase in the Cr#s [Cr / (Cr + Al)] of Cr-spinel (0.15–0.78), and (iii) an increased depletion in the whole-rock abundances of some magmaphile major oxides (Al2O3, CaO, SiO2 and TiO2) and incompatible trace elements (Zn, Sc, V and Y). The primitive mantle-normalized REE patterns of the Group-1 and some of the Group-2 peridotites display LREE depletions. Higher YbN and lower SmN/YbN ratios of these rocks are compatible with their formation after relatively low degrees (9–25%) of open-system dynamic melting (OSDM) of a Depleted Mid-ocean ridge Mantle (DMM) source, which was then fluxed with small volumes of oceanic mantle-derived melt [fluxing ratio (β): 0.7–1.2%]. Accessory Cr-spinel compositions (Cr# = 015–0.53) of these rocks are consistent with their origin as residual peridotites beneath a mid-ocean ridge axis. Part of the Group-2 harzburgites exhibit lower YbN and higher SmN/YbN ratios, LREE-enriched REE patterns, and higher Cr-spinel Cr#s ranging between 0.54 and 0.61. Trace element compositions of these peridotites can be modeled by approximately 15% OSDM of a previously 17% depleted DMM, which was then fluxed (β: 0.4%) with subduction-influenced melt. The Group-3 dunite samples contain Cr-spinel with elevated Cr#s (0.73–0.78) and low-TiO2 contents (< 0.13 wt.%), implying higher degrees of melting (21–24%) of an already depleted DMM that was triggered by infiltration of low-Ti boninite melt with fluxing rates of 0.4–4.0%. The existence of interstitial, idiomorphic Cr-spinel (high Cr# and low Ti) in the Group-3 dunites is consistent with this interpretation. The occurrence of both MOR- and SSZ-type peridotites in the Eldivan ophiolite suggests that its heterogeneous upper mantle was produced as a result of different partial melting and melt–rock reaction processes in different tectonic settings within the Neotethyan realm.

Segmentation and melt supply at the Southwest Indian Ridge

The segmentation of mid-ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust. We present seismic refraction results from three segments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variations in maximum crustal thickness between segments, we infer that melt generation and/or delivery is focused at segment midpoints and varies between segments. Anomalously thin crust—2.0–2.5 km beneath the nontransform segment boundaries and 3.5–6.0 km beneath the segment mid-points—is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center.

Cross-sectional areas of mid-ocean ridge axes bounding the Easter and Juan Fernandez microplates

We have calculated cross-sectional areas for the ridges bounding the Easter and Juan Fernandez microplates, 22°–28°S and 31°–35°S, obtaining accurate results where complete bathymetric data exist and estimates in other regions with partial bathymetric coverage and predicted bathymetry. We consider the reliability and usefulness of global predicted bathymetry in these calculations and the possible application of this dataset in other localities. The spreading rates on ridges bounding these microplates span the range from slow to superfast, allowing an investigation of ridge axis inflation over most of the rates active on Earth today. The across-axis areas of the Easter microplate ridge axes range from −29 km2 to 7 km2, while the Juan Fernandez ridge axis areas range from −27 km2 to 8 km2. Positive values correlate with regions usually interpreted as magmatically robust. Negative values arise from calculations in areas of propagating rift tips and deep grabens, such as Pito and Endeavor Deeps. Geochemical trends of Easter microplate axial basalts show decreasing MgO toward propagating rift tips and slight positive correlations between variables such as MgO vs. cross-sectional area, Na8.0 vs. axial depth, and Na8.0 vs. cross-sectional area. We document the decrease in the axial area approaching segment ends and propagating rift tips along both the West and East ridges of the microplates. On the Easter microplate both East and West ridge systems undergo large variations in spreading rate from >130 km Myr−1 to <50 km Myr−1. Inflation on these ridge segments is highly variable and only weakly correlated with spreading rate. On the Juan Fernandez microplate, West ridge spreading rates vary only between ∼115–140 km Myr−1 and are systematically faster than on the East ridge, where rates vary between ∼10–35 km Myr−1. Cross axis areas are systematically greater and significantly less variable on the faster spreading West ridge. Overall, compared to oceanic spreading centers bounding major plates with similar spreading rates, the axial areas are smaller on the microplate ridge systems, possibly because their rapidly changing configurations create a lag in the mantle response to the rigid plate boundary.

The dynamics of off-axis plume-ridge interaction in the uppermost mantle

Two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant, off-axis mantle plumes feed and interact with diverging plates at mid-ocean ridge axes which act as sinks of upper mantle material. Here we present results from two-dimensional (2-D) numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume-ridge-upper mantle system in order to test the feasibility of this plume-ridge interaction model. Specifically, experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume-ridge dynamics. Results indicate that both transient and steady-state connections may be established between off-axis plumes and ridges for a range of realistic mantle conditions. The presence of a strongly sloping rheological boundary layer (RBL) is a necessary condition for long-term communication between an off-axis buoyant upwelling and a spreading ridge. The flux of buoyant material to the ridge is also shown to increase with increasing plume-to-mantle density contrast, decreasing plume viscosity and smaller plume-ridge separation distances. Plume-ridge interaction regimes are defined based on the competing effects of plate-driven and buoyancy-driven flow. Thermal erosion of the viscous lithosphere strongly inhibits long-term plume-ridge interaction by enhancing the plates ability to deflect the plume, both head and conduit, away from the ridge axis.

Dynamics of melt generation beneath mid-ocean ridge axes: Theoretical analysis based on 238- 230Th- 226Ra and 235U- 231Pa disequilibria

Although slow melting favors the generation of basaltic melt from a mantle matrix with large radioactive disequilibrium between two actinide nuclides ( Mckenzie, 1985a), it results in long residence time in a magma chamber, during which the disequilibrium may be removed. An equilibrium melting model modified after Mckenzie (1985a) is presented here which suggests that, for a given actinide parent-daughter pair, there exists a specific melting rate at which disequilibrium between these two nuclides reaches its maximum. This melting rate depends on the decay constant of the daughter nuclide concerned and the magma chamber volume scaled to that of its source. For a given scaled chamber size, large radioactive disequilibrium between two actinide nuclides in basalts will be observed if the melting rate is such that the residence time of the magma in the chamber is comparable to the mean life of the daughter nuclide. With a chamber size 1% in volume of the melting source, the melting rates at which maximum disequilibrium in basalts is obtained are 10 −7, 2 × 10 −7 and 3 × 10 −6 y −1, respectively, for 238U- 230Th, 235U- 231Pa and 230Th- 226Ra. This implies that, while large disequilibrium between 238U- 230Th and between 235U- 231Pa may occur together, large 230Th- 226Ra disequilibrium will not coexist with large 238U- 230Th disequilibrium, consistent with some observations. The active mantle melting zone which supplies melt to a ridge axis is inferred to be only about 10 km thick and 50 km wide. The fraction of melt present in such a mantle source at any time is about 0.01 and 0.04, respectively, if melting rate is 10 −7 and 10 −6 y −1. The corresponding residence time of the residual melt in the matrix is 10 5 and 4 × 10 4 y.

Crustal control of ridge segmentation inferred from observations of the Reykjanes Ridge

LARGE-AMPLITUDE variations in topography and inferred crustal thickness along the axes of mid-ocean ridges, often referred to as segmentation1, are mainly observed at slow-spreading ridges2–4. This observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are low5,6. Here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust: segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow. Thermal models predict that thick crust at a slow-spreading ridge may be as hot as normal-thickness crust along fast-spreading ridges; we accordingly test our hypothesis at a slow-spreading ridge characterized by thick crust—the Reykjanes Ridge. Topography and gravity data along the Reykjanes Ridge axis indeed show an absence of segmentation, suggesting that the thermal state of the crust, rather than any mantle process, controls the development of this structure.

Inverse square-root dependence of mid-ocean-ridge flank roughness on spreading rate

THE topographic roughness of mid-ocean-ridge flanks is known to increase with decreasing spreading rate1–4, but the exact form of this relationship has not been established. As the topographic features that make up ridge flank roughness (abyssal hills) are created near the axes of mid-ocean ridges by faulting and volcan-ism5–7, the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres. Here I present measurements of ridge flank roughness on profiles crossing the world mid-ocean-ridge system, which show that roughness is proportional to the inverse square-root of the spreading rate. This result is consistent with some very simple inferences of how the topographic roughness of mid-ocean-ridge flanks scales with the lithospheric thickness near ridge axes, as determined by thermal models.