The size and distribution of mantle heterogeneities

The source regions of mid-ocean ridge basalts (MORB) are heterogeneous, consisting of chemically and lithologically distinct domains of variable size. Partial melting of such heterogeneous mantle sources gives rise to diverse isotopic compositions of MORB and abyssal peridotites. An important observation is that the average 143Nd/144Nd in abyssal peridotites is higher than their spatially associated MORB. We show that this offset is a natural consequence of melt migration–induced mixing or smearing in the melting column. Observed Nd-Hf isotope variations in MORB and abyssal peridotites can be reproduced if these heterogeneities are on the kilometer scale and have similar isotope ratios to but less incompatible trace element abundances than recycled oceanic crust.

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Smearing in reactive porous flow

A chemical hetrogenity could be stretched and damped during its transit through the melting column. Such a process can be simulated in an advection-reaction system where the melt is in chemical disequilibrium with the solid. The effect of chemical disequilibrium is to add on top of the chromatography effect an element-dependent velocity reduction and to introduce an mechanochemical dispersion. In these two exaples shown below, enriched melt (with low 143Nd/144Nd) percolates ahead of the solid, leaving a halo of metasomatised solid. The melt flux in the right case is stronger than that in the left case. So, the smering effect is stronger in the right case.

143Nd/144Nd in the residue

The rectangle domain is 120km high and 80km wide. The parameter R and epsl desribe the melt extraction rate and the extent of chemical disequilibrium. For the demanding accuracy of these isotopes, I use an eighth order in space, four steps in time, Runge-Kutta Discontinuous Galerkin method.


Geochemical inversion using nonlinear method

Geological observation is always an integrated product of several physical and chemical processes which happened at depth. To see through the observation obtained at the surface, we will need inversion method that can utiilize a large number of data and handle nonlinear models for related geological processes. Markov chain Monte Carlo (MCMC) algorithm is such a powerful statistical method in solving inverse problems. In an application of geochemical inversion using MCMC, we utilize data of rare earth element abundances in clinopyroxene in abyssal peridotites from a number of mid-ocean ridges to investigate the degree of melting and the rate of chemical exchange relative to the rate of decompression melting. The inversion results elude to an incresing melting rate upward, providing support for theoretical estimate of isentropic melt productivity.

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Double-porosity ridge model

Several lines of evidence suggest that the melting and melt extraction region of the MORB mantle consists of an interconnected network of high-porosity dunite channels in a low-porosity harzburgite or lherzolite matrix. The fractal-shaped channel network is still a formidable task for numerical models. By treating the melting region as two overlapping continua occupied by the low-porosity matrix and high-porosity channels, we can test the effect of coexistence of sharply contrasted porous-media on the dynamics and distribution of melt beneath mid-ocean ridges. The key result is that shear-induce anisotropic permeability in the channel network could significantly increase the amount of focused melt.

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More to come...