Consequences of diffuse and channelled porous melt migration on uranium series disequilibria

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

“…An alternate explanation for the discrepancies between the reactive flow forward melting results and the Kolbeinsey basalts is that the Kolbeinsey melt regime is a "two-porosity" system, similar to those previously suggested to be important in generating the inverse correlations between 226 Ra and 230 Th excesses in EPR basalts (e.g. Iwamori, 1994;Kelemen et al, 1997;Lundstrom, 2000;Jull et al, 2002;Sims et al, 2002;Waters et al, 2011 Th/ 238 U) ratios at the top of the 100 km melt column, as a function of residual porosity (with constant mean melting rate) or mean melting rate (with constant porosity). Some, but not all of these model runs produce results within uncertainty of the measured Kolbeinsey data, with the exception of the basalt from the Eggvin Bank segment.…”

“…Mckenzie, 1985;Bourdon et al, 1996a;Lundstrom, 2000;Spiegelman, 2000;Jull et al, 2002). This is a reasonable approximation given the inherent limitations produced by uncertainties in mineral/melt partition coefficients (see below).…”

Composition of the Oceanic Crust

“…Hewitt and Fowler (2009) suggest that the vast majority of melt could be expected to eventually flow into these channels, where the ascent velocity would be on the order of 100 m year À1 , leaving the residual matrix with melt fractions of <0.5%. On the other hand, Jull et al (2002) concluded that disequilibrium among U-decay series isotopes is best explained if $60% of the melt travels to the surface slowly via porous flow, and the remainder travels more rapidly through these high-porosity channels. The constraints imposed on melt segregation and flow by U-decay series isotopes are discussed in greater detail in Chapter 4.15.…”

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

“…Several U-series melting models have been proposed (e.g., McKenzie, 1985;Iwamori, 1993;Spiegelman and Elliott, 1993;Lundstrom et al, 1999;Jull et al, 2002). The difference in residence time between the parent and daughter nuclides causes in-growth of the daughter nuclide when the parent nuclide is retained preferentially in the solid residue during partial melting.…”

“…Proposed melting models range from 'dynamic melting' assuming rapid melt extraction with no equilibration between the partial melts and solid (McKenzie, 1985) to 'equilibrium porous-flow' with continuous melt-solid equilibration (Spiegelman and Elliott, 1993). More complex models suggest a so-called two-porosity regime during melt extraction, which implies different degrees of melt-solid equilibration at different depths in the mantle (Iwamori, 1994;Lundstrom et al, 2000;Lundstrom, 2001;Jull et al, 2002). These melting models can be used to explain the observed U-series isotope variation by variation in mantle upwelling velocity, which is directly proportional to the melting rate, and residual porosity during partial melting.…”