Asymmetric Electrical Structure in the Mantle Beneath the East Pacific Rise at 17°S

Evans, Robert; Tarits, Pascal; Chave, Alan D.; White, Antony; Heinson, Graham; Filloux, J. H.; Toh, Hiroaki; Seama, Nobukazu; Utada, Hisashi; Booker, J. R.; Unsworth, Martyn

doi:10.1126/science.286.5440.752

Cited by 125 publications

(127 citation statements)

References 30 publications

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“…Their results exhibited a broad region of anomalously slow velocities, centered to the west of the ridge axis, that they attribute to the presence of partial melt. A similar pattern of asymmetry was observed with teleseismic body wave tomography [Toomey et al, 1998;Hammond and Toomey, 2003], shear wave splitting [Wolfe and Solomon, 1998], and a magnetotelluric survey [Evans et al, 1999]. Dunn and Forsyth [2003] used short-period Love waves to probe S wave velocity to a depth of ∼50 km for up to ∼100 km on each side of the axis and found strong cross-ridge asymmetry in the slow anomaly.…”

Section: Introductionsupporting

confidence: 65%

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Katz

2010

Geochem Geophys Geosyst

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[1] Seismic tomography of the asthenosphere beneath mid-ocean ridges has produced images of wave speed and anisotropy that are asymmetric across the ridge axis. These features have been interpreted as resulting from an asymmetric distribution of upwelling and melting. Using computational models of coupled magma/mantle dynamics beneath mid-ocean ridges, I show that such asymmetry should be expected if buoyancy forces contribute to mantle upwelling beneath ridges. The sole source of buoyancy considered here is the dynamic retention of less dense magma within the pores of the mantle matrix. Through a scaling analysis and comparison with a suite of simulations, I derive a quantitative prediction of the contribution of such buoyancy to upwelling; this prediction of convective vigor is based on parameters that, for the Earth, can be constrained through natural observations and experiments. I show how the width of the melting region and the crustal thickness, as well as the susceptibility to asymmetric upwelling, are related to convective vigor. I consider three causes of symmetry breaking: gradients in mantle potential temperature and composition and ridge migration. I also report that in numerical experiments performed for this study, the fluid dynamical instability associated with porosity/shear band formation is not observed to occur.

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Section: Introductionsupporting

confidence: 65%

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Katz

2010

Geochem Geophys Geosyst

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“…For example, melt segregation and magma focusing beneath midocean ridges is an important magmatic process that may critically depend on carbonate melt migration. A fundamental geophysical observation of the mantle beneath mid-ocean ridges is that a low degree of melt is distributed over a broad region 42,43 . In addition, geochemical models of melting beneath mid-ocean ridges based on radioactive isotope disequilibrium predict that melt segregates from the matrix at very low porosities on the order of 0.1% and rises B100 km in 1,600-8,000 years (melt ascent velocity of 410 m per year) (refs 44,45).…”

Section: Article Nature Communications | Doi: 101038/ncomms6091mentioning

confidence: 99%

Ultralow viscosity of carbonate melts at high pressures

et al. 2014

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Knowledge of the occurrence and mobility of carbonate-rich melts in the Earth's mantle is important for understanding the deep carbon cycle and related geochemical and geophysical processes. However, our understanding of the mobility of carbonate-rich melts remains poor. Here we report viscosities of carbonate melts up to 6.2 GPa using a newly developed technique of ultrafast synchrotron X-ray imaging. These carbonate melts display ultralow viscosities, much lower than previously thought, in the range of 0.006-0.010 Pa s, which are B2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. As a result, the mobility of carbonate melts (defined as the ratio of melt-solid density contrast to melt viscosity) is B2 to 3 orders of magnitude higher than that of basaltic melts. Such high mobility has significant influence on several magmatic processes, such as fast melt migration and effective melt extraction beneath mid-ocean ridges.

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“…Third, seismic velocity anomalies extend to greater depth and change more rapidly with distance from the East Pacific Rise axis than expected if simple conductive cooling from above is the primary control on structure (Dunn and Forsyth, 2003;Forsyth, 1977;Hammond and Toomey, 2003). Finally, the presence of pronounced Earth and Planetary Science Letters 278 (2009) [96][97][98][99][100][101][102][103][104][105][106] asymmetries across spreading centers (Evans et al, 1999;Forsyth et al, 1998;Key and Constable, 2008;Toomey et al, 2007) suggests that flow in the mantle is more complex than is assumed in the simple models. As pointed out by many of these authors and others, the presence of water and/or melt in the asthenosphere and a dehydrated lithosphere following extraction of melt are probably necessary to explain the pattern of velocity and conductivity anomalies.…”

Section: Introductionmentioning

confidence: 99%

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

Harmon

Forsyth

Weeraratne

2009

Earth and Planetary Science Letters

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a b s t r a c t a r t i c l e i n f oDirect seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges. On the East Pacific Rise, at 17°S, there were two long-term broadband ocean bottom seismometer deployments, the MELT and GLIMPSE experiments. These arrays spanned young Pacific plate seafloor ranging in age from 0 to 8 Ma. Combining these two data sets, we describe the increase in Rayleigh wave phase velocities for 16-100 s period as function of distance from the ridge with two parameterizations: an arbitrary function of seafloor age and a simple polynomial function of age on the Pacific and Nazca plates. Although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the Pacific plate can be resolved at periods of ≤50 s, the three parameter polynomial model fits the data almost as well, indicating a simple pattern of the evolution of structure with age. To compare our observations to the predictions for the conductive cooling of the lithosphere, we convert a thermal half-space cooling model to shear velocity using anharmonic and anelastic contributions. The patterns in the velocities are not consistent with simple conductive cooling. The conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range. We observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend. The asthenospheric low velocity zone is asymmetric, dipping to the west with the lowest velocities beneath the Pacific plate west of the spreading center. The conductive cooling model also over predicts the shear velocities in the lithosphere by~0.2 km/s. These anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required.

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Asymmetric Electrical Structure in the Mantle Beneath the East Pacific Rise at 17°S

Cited by 125 publications

References 30 publications

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Ultralow viscosity of carbonate melts at high pressures

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

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