Long-term interaction between mid-ocean ridges and mantle plumes (original) (raw)
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Westward migration of oceanic ridges and related asymmetric upper mantle differentiation
Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying astheno-sphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions , themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at ~90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower as-thenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge.
PLUME-RIDGE INTERACTION: SHAPING THE GEOMETRY OF MID-OCEAN RIDGES
2000
First, I would like to thank my advisor, Garrett Ito, for providing numerous hours of discussions, help with problems, someone to bounce ideas off of, paper corrections, and most of all for his consistently positive and supportive attitude. Additionally, I would like to thank the members of my committee for all their help and encouragement over the past few years; Marcelo Kobayashi for insightful comments and discussions; Steve Martel for providing the 2-D boundary element code used in chapter 2 as well as checking my mechanical assumptions; Fernando Martinez for numerous talks during TG's and specifically for discussions about the evolution of the Mariana region; and John Sinton for his insights into the Galapagos and Iceland hotspots. Additionally, I would like to thank R. Hey for discussions on the evolution of the Galápagos Spreading Center, and E. Chapp for insights into the Mariana back-arc system. Maps were made using GMT version 3.4.2 by P. Wessel and W.F. Smith. Funding during work on my dissertation was provided by NSF grants OCE03-7051 and OCE03-51234 and Ito's startup money from SOEST. The computer cluster used for portions of the computations was funded with NSF grant OCE01-36793. iv Abstract Manifestations of plume-ridge interaction are found across the ocean basins.
Journal of Geophysical Research: Solid Earth, 1997
We investigate the three-dimensional interaction of mantle plumes and migrating mid-ocean ridges with variable viscosity numerical models. Numerical models predict that alongaxis plume width W and maximum distance of plume-ridge interaction Xma x scale with (Q/U) 1/2, where Q is plume source volume flux and U is ridge full spreading rate. Both W and Xma x increase with buoyancy number li b which reflects the strength of gravitational-versus plate-driven spreading. Scaling laws derived for stationary ridges in steady-state with near-ridge plumes are consistent with those obtained from independent studies of Ribe [ 1996]. In the case of a migrating ridge, the distance of plume-ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate. Given the mildly buoyant and relatively viscous plumes investigated here, the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow. From observed plume widths of the Galfipagos plume-migrating ridge system, our scaling laws yield estimates of Galfipagos plume volume flux of 5-16 x 106 km 3 m.y.-1 and a buoyancy flux of-2 x 103 kg s-1. Model results suggest that the observed increase in bathymetric and mantle-Bouguer gravity anomalies along Cocos Plate isochrons with increasing isochron age is due to higher crustal production when the Gal•pagos ridge axis was closer to the plume several million years ago. The anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly (50ø-100øC) and moderate radius (100-200 km). Predictions of the along-axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but, instead, may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source. plume-ridge distance x•,, and lithospheric thickening with age. 15,403 15,404 1TO ET AL.: PLUME-MIGRATING RIDGE INTERACTION While the above studies established scaling laws for plumes a•d stationary ridges, they did not investigate the effects of ridge migration. In the more realistic case of a migrating ridge, not only may thermal thinning of the lithosphere be important as envisioned by Schilling [1985; 1991] and Schilling et al. [1985], but also the plate trailing the migrating ridge typically moves slower relative to the plume than the plate leading the ridge axis thereby inducing less drag on the plume away from the ridge [Ribe, 1996; Ribe and Delattre, 1996]. We explore here the dynamics of plumes and migrating ridges with three-dimensional (3-D) numerical models that include thermal diffusion and fully pressure-and temperature-dependent mantle rheology. We will first establish scaling laws for along-axis plume width W and maximum plume-ridge interaction distance Xma x for steady state systems of stationary ridges. These results will be compared with those of the constant viscosity plume models of Ribe [1996] to quantify the importance of thermal diffusion and variable plume viscosity on the scaling laws. We will then quantify the effects of ridge migration on W and Xma x. Finally, we will compare model predictions with geophysical observations of the Galfipagos plume-migrating ridge system and discuss the implications for the dimensions, temperature anomaly, fluxes, and geochemical signature of the Galfipagos plume. Governing Equations and Numerical Method The mantle is modeled as a viscous Boussinesq fluid of zero Reynolds number and infinite Prandtl number. The equilibrium equations include conservation of mass V.u=O, (1) momentum and energy V * '• = Apg, (2) ø•T-n:V 2 T-u V T (3) (see Ito et al. [1996] for further details and Table 1 for defini
Plumes, subaxial pipe flow, and topography along the Mid-Oceanic Ridge
Earth and Planetary Science Letters, 1976
If plate thickness depends on crustal age, the region of extensive partial melting below the spreading axis will be wider around fast-spreading ridges. The melt region creates a subaxial conduit channeling partial melts away from ridge-centered hot spots. The channel is here modeled by an elliptical pipe of semiminor (vertical) axis 2 × 106 cm (20 kin) and semimajor (horizontal) axis KS, where S is spreading half-rate (cgs) and K is a constant of magnitude 1014 to 1015 seconds. This simple analytical model is used to explain the observation that maximum hot spot elevations on the Mid-Oceanic Ridge fall dramatically with increasing spreading rate (there are no lcelands or Afars on the East Pacific Rise!). A hot spot under a fast-spreading ridge has a broad pipe in which to discharge its partial melts; hence, only a slight topographic gradient and a low elevation is needed to discharge the mass flux rising out of the deeper mantle at the hot spot center. A second factor is that partial melts are "used up" faster in the accretion process on fast-spreading ridges. In the simple analytical model, both factors operating together explain the rapid fall of hot spot heights with increasing spreading half-rate. This result indirectly helps confirm the idea of horizontal pipe flow below the Mid-Oceanic Ridge. A theoretical topographic profile through a hot spot on the Mid-Oceanic Ridge is derived from the assumption that the pressure-i.e., topographic-gradient at a distance x from the hot spot is sufficient to supply all the accreting lithosphere downstream of x, out to x n, the limit of topographic hot spot influence. The predicted profile is quadratic in x and concave upward, and resembles several observed profiles where neighboring hot spots are not so close as to confuse the profiles. Some observed profiles are more nearly linear or even convex upward, This could be explained, for example, by downstream increases in viscosity or decreases in pipe dimensions, A hot spot on a ridge spreading at much less than 1 cm/yr half-rate would produce an enormous elevation of the ridge axis, according to our model, because the pipe would be very narrow. Such a large topographic high would create a large gravity potential which would cause the plates to move apart faster, thereby widening the pipe, and reducing the topographic high. The system of ridges and hot spots may thus be self-regulating with respect to plate speeds; this could explain why spreading half-rates on the Mid-Oceanic Ridge are in many areas as low as 1.0 cm/yr but very rarely as low as 0.5 cm/yr.
Ocean rises are products of variable mantle composition, temperature and focused melting
Nature Geoscience, 2014
Ocean ridges, where Earth's tectonic plates are pulled apart, vary from more than 5km depth in the Arctic to 750 m above sea level in Iceland. This huge relief is generally attributed to mantle plumes underlying mantle hotspots, areas of enormous volcanism marked by ocean islands. The plumes are thought to feed the mantle beneath adjacent ocean ridges. This results in thickened crust and ridge elevation to form ocean rises. The composition of mid-ocean ridge basalt, a direct function of mantle composition and temperature, varies systematically up ocean rises, but in a unique way for each rise. Here we present thermodynamic calculations of melt-evolution pathways to show that variations in both mantle temperature and source composition are required to explain rise basalts. Thus, lateral gradients in mantle temperature cannot be uniquely determined from basalt chemistry, and ocean rises can be supported by chemically buoyant mantle and/or by robust mantle plumes. Our calculations also indicate that melt is conserved and focused by percolative flow towards the overlying ridge, progressively interacting with the mantle to shallow depth. We conclude that most mantle melting occurs by an overlooked mechanism, focused melting, whereas fractional melting is a secondary process that is important largely at shallow depth.
Earth and Planetary Science Letters, 2013
Keywords: South China Sea basalts ancient mantle reservoir Hainan mantle plume subducted oceanic crust core-mantle boundary Whether or not mantle plumes and plate subduction are genetically linked is a fundamental geoscience question that impinges on our understanding of how the Earth works. Late Cenozoic basalts in Southeast Asia are globally unique in relation to this question because they occur above a seismically detected thermal plume adjacent to deep subducted slabs. In this study, we present new Pb, Sr, Nd, and Os isotope data for the Hainan flood basalts. Together with a compilation of published results, our work shows that less contaminated basaltic samples from the synchronous basaltic eruptions in Hainan-Leizhou peninsula, the Indochina peninsula and the South China Sea seamounts share the same isotopic and geochemical characteristics. They have FOZO-like Sr, Nd, and Pb isotopic compositions (the dominant lower mantle component). These basalts have primitive Pb isotopic compositions that lie on, or very close to, 4.5-to 4.4-Ga geochrons on 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram, suggesting a mantle source developed early in Earth's history (4.5-4.4 Ga). Furthermore, our detailed geochemical and Sr, Nd, Pb and Os isotopic analyses suggest the presence of 0.5-0.2 Ga recycled components in the late Cenozoic Hainan plume basalts. This implies a mantle circulation rate of >1 cm/yr, which is similar to that of previous estimates for the Hawaiian mantle plume. The identification of the ancient mantle reservoir and young recycled materials in the source region of these synchronous basalts is consistent with the seismically detected lower mantle-rooted Hainan plume that is adjacent to deep subducted slab-like seismic structures just above the core-mantle boundary. We speculate that the continued deep subduction and the presence of a dense segregated basaltic layer may have triggered the plume to rise from the thermal-chemical pile. This work therefore suggests a dynamic linkage between deep subduction and mantle plume generation.
Geochemistry, Geophysics, Geosystems, 2021
Lavas that have erupted at near-axis seamounts provide windows into mid-ocean ridge mantle heterogeneity and melting systematics which are not easily observed on-axis at fast-spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near-axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near-ridge mantle using a ∼200 km-long chain at ∼8°20′N. High-resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near-axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid-ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid-ocean ridge (MOR) mantle. Normal mid-ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid-ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub-ridge mantle is much more heterogeneous than is commonly observed on-axis and heterogeneity exists over small spatial scales.
Ridge-Hotspot Interactions: What Mid-Ocean Ridges Tell Us About Deep Earth Processes
Oceanography, 2007
Earth is a thermal engine that dissipates its internal heat primarily through convection. The buoyant rise of hot material transports heat to the surface from the deep interior while colder material sinks at subduction zones. Mid-ocean ridges and hotspots are major expressions of heat dissipation at Earth's surface, as evidenced by their abundant volcanic activity. Ridges and hotspots, however, could differ significantly in their origins. Ridges are linear features that wind more than 60,000 km around the globe, constituting the major diverging boundaries of Earth's tectonic plates. Hotspots, on the other hand, are localized regions of abnormally robust magmatism and distinctive geochemical anomalies (Figure 1). The causes of hotspots and their depths of origin are the focus of an intense debate in the scientific community. The "plume" model hypothesizes rising of buoyant mantle plumes as the primary cause of prominent hotspots such as Iceland and Hawaii (Morgan, 1971). In contrast, the "anti-plume" school argues that many of the observed "hotspot" volcanic and geochemical anomalies are simply due to melts leaking through tensional cracks in Earth's lithospheric plates-in other words, hotspots reflect only where the lithospheric plate is cracked, allowing melts to pass through, and not where the underlying mantle is hotter (see www.mantleplumes.org). A hybrid notion is that only a relatively small number of hotspots, especially those of enormous magmatic volumes, have their origin in buoyant thermal plumes rising from the deep mantle (e.g., Courtillot et al., 2003). Regardless of its specific depth of origin, however, when a hotspot is located close enough to a mid-ocean ridge, the two volcanic systems will interact, resulting in unique volcanic, geochemical, and hydrothermal features. In this paper, we discuss major features of hotspot-ridge interactions. riDge-HotSpot iNteractioNS what mid-ocean ridges tell us about Deep earth processes