Trace element and isotopic effects arising from magma migration beneath mid-ocean ridges

Kenyon, P. M.

doi:10.1016/0012-821x(90)90166-u

Cited by 29 publications

(26 citation statements)

References 15 publications

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“…Well before that degree of mixing is achieved, however, it is unlikely that the blob could be identified by geochemical studies of, for instance, mid-ocean ridges [Gurnis, 1986c, Kenyon and. Partial melting beneath a mid-ocean ridge will reset the clock for heterogeneities [Kenyon, 1990] We are primarily interested here in the fate of heterogeneities introduced into the mantle by subduction. We used the following "rules" for introduction and removal of particles (Figure 1 2.…”

Section: Geochemical Tracersmentioning

confidence: 99%

Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth‐dependent viscosity

Hunt

Kellogg

2001

J. Geophys. Res.

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Abstract. Using a two-dimensional finite element model of mantle convection containing over a million tracer particles, we examine the effects of depth-dependent viscosity on the rates and patterns of mixing. We sinrelate the processes of recycling crust at subduction zones and the homogenization of recycled material (by dispersion and by melting at mid-ocean ridges). Particles are continually introduced at dowmvellings and destroyed when they either are so thoroughly dispersed that it would be impossible to measure their presence in the geochemical signature of mid-ocean ridges or oceanic islands, or when they are close to spreading centers, at which point melting would "reset" the geochemical clock. A large number of factors influence the flow pattern and thus the rate at which heterogeneities are dispersed by convection. We examine the effect of increasing the viscosity with depth, and determine how both the residence time of heterogeneities and the extent of lateral mixing and exchange between the upper and lower mantle vary with the viscosity profile of the mantle. We determine the particle distribution resulting from convection models with three viscosity profiles: uniform viscosity, a smooth increase of viscosity with depth, and an abrupt jump in viscosity between the upper and lower mantle. We characterize the resulting distribution of heterogeneities in space and time by examining the age distribution of particles and their locations relative to others introduced into the flow at separate downwellings. Mixing rates in the three models are calculated as a function of the number of particles removed from the flow through time. We fbund that an increase of viscosity at depth does not induce age stratification in which older particles stagnate in the lower mantle, and it does not produce an upper layer (the source of mid-ocean ridge basalt) that is well-mixed compared to the deeper regions. However, pronounced lateral heterogeneity is evident in the distribution of particles of different ages and starting locations that is not apparent from the particle positions alone.

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Section: Geochemical Tracersmentioning

confidence: 99%

Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth‐dependent viscosity

Hunt

Kellogg

2001

J. Geophys. Res.

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“…Im. Equilibrium During Melt Transport A number of authors have studied various aspects of chemical equilibrium and fractionation during melt transport in the mantle (9,(32)(33)(34)(35)(36)(37)(38). Most of these studies dealt with melt migration in either simple one-dimensional systems or in homogeneous porous networks.…”

Section: Introductionmentioning

confidence: 99%

Equilibration during mantle melting: a fractal tree model.

Hart

1993

Proc. Natl. Acad. Sci. U.S.A.

127

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Many basalts from oceanic islands, ridges, and arcs show strong trace element evidence for melting at great depths, where garnet is a stable phase in mantle peridotites. If partial melts ascend to the surface by porous (intergranular) flow processes, the high-pressure garnet signature will be obliterated by diffusive reequilibration at shallower depths in the mantle. Spiegelman and Kenyon [Spiegelman, M. & Kenyon, P. (1992) Earth Planet Sci. Left. 109, 611-620] argued that partial melts must therefore be focused into a coarser transport network, for high-speed delivery to the surface. Numerous natural network systems, such as rivers and the human vascular and bronchial systems, have fractal structures that are optimal for minimizing energy expenditure during material transport. I show here that a fractal magma "tree" with these optimal properties provides a network in which magma rapidly loses diffusive chemical "contact" with its host matrix. In this fractal network, magma conduits combine by twos, with the radius and flow velocities scaling as (2)"/3, where n is the generation number. For reasonable values of volume diffusivities, viscosities, and aspect ratios, melts wili experience only limited diffusive reequilibration once they have traveled some hundreds ofmeters from their source. Melts thus represent rather local mantle domains, and there is little problem in delivering melts with deep (<100 km) geochemical signatures to the surface.

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“…Kenyon (1990) shows that a periodic trace element signature is damped during disequilibrium melt transport (see also Spiegelman & Kenyon 1992). This effect should also be enhanced if solitary waves make a significant contribution t o melt transport.…”

Section: Discussionmentioning

confidence: 96%

Geochemical Effects of Magmatic Solitary Waves-I. Numerical Results

Watson¹,

Spiegelman

1994

Geophysical Journal International

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S U M M A R YNumerical studies have suggested that solitary waves may be a natural consequence of melt transport in the mantle. However, the likely physical properties of magma solitary waves mean that they are unlikely to be imaged by conventional geophysical techniques. This study aimed to determine whether solitary waves leave a detectable signature in the geochemistry of the melts they transport.The major effect seen during the interaction between a solitary wave and a concentration anomaly is a steepening of the concentration gradients by a factorthe maximum porosity in the solitary wave, &, is the background porosity and k,) is the partitidn coefficient. The solitary wave transports the tracer a distance proportional to S, a completely incompatible tracer (k, = 0) being transported a distance equal to the volume of the solitary wave. Considerable spreading of the concentration anomaly occurred in many experiments because the solitary waves enhance diffusion by steepening the concentration gradients. For a constant value of S, the increase in the width of the concentration anomaly is proportional to the square root of the diffusivity of the tracer, D"2. At constant D it depends on V V T where D,, is the effective diffusivity of the tracer and T is the length of time that the interaction takes.A single solitary wave passing through a set of tracers with different k, values will cause an apparent 'chromatigraphic' effect that causes the chemical peaks to separate much further than would be predicted using a constant porosity model. Approximately S times more diffusion than is predicted by the constant porosity model occurs. Moreover, because of enhanced steepening and dispersion, solitary waves may provide a mechanism for mixing melts normally separated by several kilometres.

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Trace element and isotopic effects arising from magma migration beneath mid-ocean ridges

Cited by 29 publications

References 15 publications

Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth‐dependent viscosity

Quantifying mixing and age variations of heterogeneities in models of mantle convection: Role of depth‐dependent viscosity

Equilibration during mantle melting: a fractal tree model.

Geochemical Effects of Magmatic Solitary Waves-I. Numerical Results

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