Ridge Spreading Rates Research Articles

Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition.

Shifts in the MgCa ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 10(8) years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient MgCa ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO(3)). We grew three species of these algae in artificial seawaters having three different MgCa ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient MgCa ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO(2) for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low MgCa ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO(3)), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas.

Seafloor slopes at mid-ocean ridges from submersible observations and implications for interpreting geology from seafloor topography

Observations from 145 submersible dives are used to create a database of mid-ocean ridge scarp topography and lithology. Seafloor lithologies are classified into extrusives, basaltic talus, dykes, gabbros and serpentinites, and the dive locations are broadly classified according to whether they are close to transform valleys and as a function of ridge spreading rate. The database is used to determine whether there is any difference in the maximum slope for each rock type, which might relate to differences in rock jointing, cohesion or friction properties. There is a common perception that lower crustal rocks form steeper slopes than shallower crustal rocks but until now there has been little evidence to support or refute this idea. From our analysis we find a tendency for gabbro and dykes to form steeper slopes than serpentinite. The 90th percentile of each lithology slope distribution, used as a measure of limiting slope, is 43°, 39° and 32° for gabbro, dykes and serpentinite, respectively. Lithologic control on slope is weak, however, compared to overall slope variability in these mid-ocean ridge settings so seafloor relief is likely to be a poor guide to underlying geology. We speculate on the structure of eroding fault scarps, outline the implications for attempts to infer active faults from talus ramp activity and discuss more generally factors affecting the geomorphology of mid-ocean ridge slopes.

Vents at higher frequency

The oceanic hydrothermal vents found along mid-ocean ridges continue Indian ridge — which, according to predictions that vent frequency correlates with the spreading rate of ridges, should have a very low incidence of vents. The new study finds that incidence to be three times higher than expected.

Detailed mapping of a mantle diapir below a paleo‐spreading center in the Oman ophiolite

Detailed mapping of the well‐exposed Maqsad area in the Oman ophiolite addresses the question of accretion mechanisms at oceanic ridges. Steep peridotite lineations are interpreted as a three‐dimensional mantle diapir below a paleo‐fast spreading ridge. The uppermost part of the diapir reveals three heads covering 100 km2 in area. The main head (30 km2) displays warped vertical foliation trajectories. Cross sections show that the vertical flow rotated to the horizontal below the Moho within a 500‐m‐thick melt‐impregnated transition zone. Penetrative structures dip outward from the diapir. and the Moho bulges upward several hundred meters above this area. The ridge axis is ascribed to a 1‐ to 3‐km wide‐corridor of lineations parallel to the sheeted dike trend, which crosses the main diapir head. Mass balance calculations show that this diapir could account for the formation of a 25‐km‐long crustal segment, provided that the upward flow velocity is ≈10 times the ridge spreading rate. The narrow space left for the horizontal flow at the outskirts of the diapir implies active divergent flow from the diapir center with velocities 1 order of magnitude faster than the spreading rate. Accordingly, the horizontal foliations display the strongest fabrics. The calculations predict that the diverging flow should become passive 10 km past the diapir limits, as suggested by field data. The mechanisms making possible the sharp flow rotation at the top of the diapir are discussed.

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 Chile ridge: A tectonic framework

A new Chile ridge tectonic framework is developed based on satellite altimetry data, shipboard geophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.‐Chilean aeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), including two complex systems (the Chile FZ and Valdivia FZ systems), have been mapped between the northern end of the Antarctic‐Nazca plate boundary (Chile ridge) at 35°S and the Chile margin triple junction at 47°S. Chile ridge spreading rates from 23 Ma to Present have been determined and show slowdowns in spreading rates that correspond to times of ridge‐trench collisions. The Valdivia FZ system, previously mapped as two FZs with an uncharted seismically active region between them, is now recognized to be a multiple‐offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (∼12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earlier at the Pacific‐Nazca ridge. Evidence for this propagation event includes the Friday and Crusoe troughs, located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults. This propagation event led to the formation of the Friday microplate, which resulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500‐km northward stepwise migration of the Pacific‐Antarctic‐Nazca triple junction. Rift propagation, microplate formation, microplate extinction, and stepwise triple junction migration are found to occur during large‐scale plate motion changes and plate boundary changes in the southeast Pacific.

Geochemical consequences of increased Late Cenozoic weathering rates and the global CO2 balance since 100 Ma

Large imbalances in the relative net CO2 flux over the last 100 m.y. are obtained from independently derived estimates of CO2 uptake by weathering and organic carbon burial and of CO2 outgassing assumed proportional to the mean mid‐ocean ridge (MOR) spreading and mantle plume production (MPP) rates. Increasing the river flux of Ca, Mg, P, and C by 50% since 30 Ma to parallel the proposed increase in the Sr river flux needed to explain the Sr isotopic evolution of seawater [Richter et al., 1992] has a large impact on the global carbon cycle by reducing the excess net CO2 imbalance in the late Cenozoic. CO2 uptake is calculated by assuming that 19% of the Ca river flux and 60% of the Mg river flux are derived from weathering silicates and that Mg is removed in the oceans by 1:1 molar exchange reactions with calcsilicates. An increase in the Mg river flux combined with an overall decrease in the MOR spreading rate predicts as much as a factor of 2 increase in the seawater Mg concentration since 100 Ma. The net organic carbon burial flux (burial minus weathering) largely reflects changes in the bulk carbonate δ13C record and, to a lesser extent, variations in the net carbonate burial flux and mean δ13C value of the carbonate weathering flux. Organic carbon burial efficiency is markedly less than that of P from the middle middle Miocene to the present because of early diagenetic transfer of organic P to francolite and oxidation of organic carbon during reworking of margin sediments by sea level fluctuations. The large CO2 imbalance of the Late Cretaceous requires significantly less carbonate subduction or greater weathering rates. A CO2 balance can be achieved in the Miocene by decreasing the proposed increase in river fluxes by up to half or by variably increasing the amount of carbonate subducted. Variations in shallow water carbonate burial, and hence the global carbonate record, are currently too poorly known to differentiate between the above possibilities. The decrease in the excess CO2 flux generally parallels the trend to cooler climate inferred from the δ18O record, but periods of large CO2 imbalance generally precede δ18O shifts by several million years.

The relationship between near-axis hydrothermal cooling and the spreading rate of mid-ocean ridges

Recent seismic observations and new crustal thermal models suggest that near-axis (± 1 km from the spreading axis) hydrothermal cooling increases linearly with increasing spreading rate. We present here the first corroborating evidence from near-axis hydrothermal plume observations. Comprehensive surveys of plume distributions have now been conducted on multi-segment portions of ridge crest with full-spreading rates from 20 to 150 km/Myr. These surveys find that p h , the fraction of ridge crest length overlain by hydrothermal plumes, and full-spreading rate, u s , are related by p h = αu s , where α = 0.004 Myr/km. We recast this large-scale spatial variability into long-term temporal variability by postulating that any vent field site is hydrothermally active for an αu s fraction of geologic time. We then use this relationship and the mean of eight studies that have determined instantaneous heat flux, H a , at vent field scales, 75 ± 45 MW/km, to estimate the time-averaged heat flux, H a , as H aαu s . Applying these scaled values of H a to the distribution of mid-ocean ridge spreading rates yields a global near-axis heat flux, ∑H a , of 8.8 × 10 11 W. This value is ∼ 10% of the total oceanic hydrothermal heat flux, and agrees with a crustal cooling model-derived value of ∑H a of 9 × 10 11 W. Our estimate of ∑H a implies a mean global 3He/heat ratio of 2.4–8.3 × 10 −13 cm 3/J (STP), and high-temperature hydrothermal fluid flow of 1.8 × 10 13 kg/yr.

Extensive distribution of hydrothermal plumes along the superfast spreading East Pacific Rise, 13°30′–18°40′S

A general model relating relative hydrothermal activity to the rate of plate creation requires data from the full spectrum of mid‐ocean ridge spreading rates. To obtain data from a superfast spreading environment, the Japanese/U.S. Ridge Flux Project used continuous hydrographic/optical tow‐yos in 1993 to map the distribution of hydrothermal plumes along the southern East Pacific Rise from 13°30′ to 18°40′S. Plume incidence, the fraction of the spreading axis length overlain by a significant plume, was 0.6, including a virtually continuous vent field stretching 150 km from 17°20′ to 18°40′S. Hydrothermal venting was most concentrated along portions of the ridge crest with an inflated cross‐sectional area and an observable axial magma chamber reflector. This pattern agrees with previous results from the northern East Pacific Rise. Such consistency implies that hydrothermal circulation on fast spreading ridges is vigorous where the relative volume of partial melt is high and meager where melt volume is low or undetectable; “hot rock” alone is generally insufficient to drive significant hydrothermal circulation. Using a mean temperature anomaly of 0.014 ± 0.010°C, we estimated the hydrothermal heat flux from the study area as (1.5 ± 1.1 × 107 MW)Ux, where Ux is the cross‐axis flow at the ridge crest. No direct measurements of Ux are yet available. Combining the plume distribution found here with prior data from slow, intermediate, and fast spreading ridges yields a significant linear correlation between plume incidence and spreading rate that extends across the full range of plate motion. This correlation, which mirrors that for near‐axis heat flux calculated by recent models, implies that magma supply rate is the principal control on the large‐scale distribution of axial hydrothermal venting.

Thin Crust On the Flanks of the Slow-Spreading Southwest Indian Ridge

SUMMARY The Southwest Indian Ridge marks an end-member of mid-ocean ridge spreading rates, with a current full rate of only 13-15 mm a-'. Most thermal models of mid-ocean ridges suggest a decrease in crustal thickness at such slow spreading rates, because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression. Seismic measurements of the thickness of crust formed at very slow-spreading ridges are sparse. We have reanalysed data from a twoship, split spread seismic refraction experiment conducted in 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variations, which were carefully recorded by the original investigators. 1- and 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity of -6.0 km s--' near the top of the crust, increasing to -7.0 km s-' just above the Moho, and a crustal thickness of 5 km. Converted shear waves yield a Poisson's ratio of 0.30+0.01 in the crust, which is intermediate between values for gabbros and serpentinized upper mantle. The crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid-ocean ridges. The unusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.

4He/³He dispersion and mantle convection

Histograms of the 4He/³He ratios analysed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged 4He/³He ratio. We interpret the form of these histograms in terms of stirring time (τstir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection.Using 4He/³He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ∼1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogeneisation of helium in the upper mantle, and the consequent uniformity of the 4He/³He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion.

Disequilibrium partial melting model and its implications for trace element fractionations during mantle melting

A model to describe trace element evolutions during partial melting is proposed by assuming surface equilibrium instead of complete equilibrium between mineral grains and melt and by considering the following factors: (1) chemical diffusion in minerals; (2) melting and melt extraction. For mantle melting rates of ∼ 10−7 yr−1, inferred from mid-ocean ridge spreading rates, elements with diffusivities (D) in minerals of > 10−15 cm2/s be at equilibrium between minerals of 0.5 cm in diameter and melt during mantle melting. However, the anomalous radioactive excess of 236Ra over 230Th (up to 300%) in young basalts which cannot be explained by equilibrium melting because of the highly incompatible nature of both elements can be accounted for by the present model only when melting rate is in the orders of 10−6 to 10−5 yr−1. The disequilibrium effect cannot be ignored for elements with D < 10−13 cm2/s if the melting rate is 10−5 yr−1. Disequilibrium melting lowers the incompatibility of an incompatible element; the high the incompatibility, the more obvious this effect is.Fractionation of trace elements during disequilibrium melting depends both on their equilibrium partition coefficients (k) and their diffusivities. This has two consequences: (1) fractionation of two highly incompatible elements (k ⩽ 0.01) by more than 20% is still possible at melting degree as high as 15% if their diffusivities differ by a factor of 5 or more, as compared to the requirement for less than 5% partial melting for the same extent of fractionation by equilibrium melting; and (2) the incompatibility order of two incompatible elements can be reversed under certain circumstances (i.e., k1 >k2, but D1 >D2). The Pb paradox, namely, that the U/Pb ratio in mantle sources inferred from Pb isotopes has increased during the differentiation of the Earth even though Pb is less incompatible than U, is explainable if the diffusivity of Pb is higher than that of U (e.g., by a factor of 10) and if the melting rate is in the order of 10−5 yr−1.

海洋性磁気異常と海底玄武岩中の磁性鉱物

The discovery of the magnetic anomaly lineations that can give ages of ocean floor is a very important role for establishment of the plate tectonics theory. The magnetic anomaly lineations also give us information of a history of movements of the oceanic plates. However, the origin of magnetic anomaly lineations still been obscured, that is, we can not clearly answer for the following questions: How thick is the source layer of magnetic anomaly lineations? How strong is the intensity of the magnetic source layer? In this paper we examined the relevant information concerning the magnetization of the oceanic crust from studies of observed marine magnetic anomalies and from rock -magnetism of oceanic basalts to get a goal of these questions in this paper.The skewness parameter that is deduced by precise magnetic anomaly lineations is important to identify marine magnetic anomalies. The magnetic polarity transition width is also important to do, though the parameter associated with this transition width has not almost utilized in the previous works. The anomalous skewness and the skewness discrepancy are often observed over the oceans. These observations might be explained not by a single-layer model but a two-layer model for magnetic source layer. The polarity transition width is defined the width which 95.4 % of the change from normal to reversed polarity occurs within. This width increases monotonically with spreading rates of ridges and/or with ages of ocean floors. This increasing is considered to be a manifestation of a more complicated crustal source consisting of two discrete layers. The analysis of the skewness parameter and transition width strongly supports that the sourc elayer of marine magnetic anomalies has a two-layer structure. The upper layer, consisting of surface lava flows of layer 2 A and possibly the sheeted dike complex, hasdi stinct and approximately vertical magnetic in the vicinity of opposite magn etized region boundaries. The lower layer, consisting of intrusive and gabbroic layers, has the boundaries gradually sloping downward away from the spreading center.Many detailed survey are carried out to reveal the structure of magnetic source layer by the multi narrow and the deep-towed magnetometer near active ridges. Inversion of magnetic layer using results of detailed surveys concluded that the magnetic source layer near the active ridges is less than 1 km in thickness. The polarity transition width of the relatively young layer is narrower than that of older oceanic floor, and the magnetic intensity of the relatively young layers higher (more than 10A/m) than that of older one. These conclusion indicate that the magnetic source layer near the active ridg es consists of a single layer structure. It is thought that the magnetic source layer grows with ages asoceanic crust by results of analysis of skewness and polarity transition width and inversion of magnetic source layer near active ridges.Several previous paleomagnetic studies indicate that intensity of natural remanent magnetization (NRM) of basaltic rocks composing the ocean crust rapidly decreases with ages in the past 10 to 20 Ma, and gradually increases older one. This change in NRM intensity is roughly proportional to the changes in intensity of saturation magnetization of the rocks and possibly due to sea-water alteration (low-temperature oxidation) of the primary ferromagnetic minerals contained in the rocks. NRM of the oceanic rocks is initially thermoremanent magnetization acquired at the time of formation of the oceanic crust. In accordance with progressive oxidation, fraction of TRM to bulk intensity decreases, while that of the secondary magnetization increases.

Formation of oceanic crust: Some thermal constraints

An energy-conserving analytical solution for heat flow at midocean ridge crests was obtained by assuming that the material accreting at the ridge acts as a known heat source along the ridge axis. Latent heat is included consistently in the adiabatic melting of the ascending mush and in the solidification of the basaltic crust. At spreading rates below about 1 cm/yr, lateral heat conduction away from the ridge precludes the existence of a steady state magma chamber. At higher spreading rates, material cannot cool, since dikes in the lower crust and a magma chamber of some type must exist. If most of the chamber were filled with mush, the chamber would be more mechanically stable than one filled with molten magma. It is most probable that new material from below ascends to the top of the magma and then either further ascends as a dike or spreads laterally along the top of the chamber. If crystal settling is important and accretion of material to the roof is minor, the magma chamber will be flat topped and will extend far enough laterally that incoming material will cool to a mostly crystalline cumulate mush. The geothermal calculations and kinematic models are testable from geologic observations on ophiolitic complexes and by planned drilling. It appears that the spreading rate of fossil ridges can be determined from these considerations.