Models of U-series disequilibria generation in MORB: the effects of two scales of melt porosity

Abstract

Conflicting evidence exists about the extent of melt–mantle reaction as melt ascends beneath a mid-ocean ridge. On the one hand, uranium series (U-series) disequilibria in mid-ocean ridge basalts suggest that melts reactively flow towards the surface and continuously re-equilibrate with lower pressure mantle. On the other hand, trace elements in abyssal peridotites suggest fractional melting and isolation of ascending melt from surrounding lower pressure peridotite. Following a suggestion by Kelemen et al. (1997) [Kelemen, P.B., Hirth, G., Shimizu, N., Spiegelman, M., Dick, H.J.B., Philos. Trans. R. Soc. 355, 283–318], I construct a model for melting beneath mid-ocean ridges which consists of two materials having different melt porosity distributions in order to quantitatively assess whether the U-series and abyssal peridotite observations could actually be compatible. The melting column consists of a volumetrically dominant lherzolite which produces melt by decompression and a volumetrically minor dunite which serves as a conduit for fast melt ascent. A suction parameter S (after Iwamori (1994) [Earth Planet. Sci. Lett. 125, 1–16]) controls the transfer of melt between the lherzolite and dunite media. S can vary from 0 where none of the ascending melt flows through the dunite melt conduit (reactive porous flow melting) to 1 where the melt produced in the lherzolite is immediately transferred to the dunite (near-fractional melting). Transport times of U-series nuclides are accounted for in both porosity regimes based on calculated flux balances. Results show that the trace element patterns observed in abyssal peridotites require S to range from 0.5 to 0.8 depending on assumptions about the composition of the original source material. U-series models are evaluated in terms of three quantities: S, χ (the volumetric fraction of dunite), and a 2/ ηC which lumps together several constants relating to permeability. 226 Ra and 231 Pa excesses are most sensitive to a 2/ ηC (which affects the melt porosity within the melting region). 226 Ra excesses match the observed levels of 226 Ra excess in mid-ocean ridge basalts (MORB) at all values of S. 231 Pa excesses favor lower values of S as models with high S produce significantly less ( 231 Pa)/( 235 U) than that observed. There remains some disparity between the values of S required by the abyssal peridotite data and the observed 231 Pa excesses although further refinement of partition coefficients could resolve the difference. Within the models presented, the two geochemical observations are close to being reconciled if S=0.5 and the mantle permeability is at least 1×10 −14 m 2. However, a factor of two higher bulk partition coefficient for U ( D U=0.003) would allow the observed ( ( 231 Pa)/( 235 U) ) to be matched at S=0.5 while dropping the required bulk permeability to less than 1×10 −15 m 2.