A Reactive Porous Flow Control on Mid-ocean Ridge Magmatic Evolution

Abstract

Mid-ocean ridge basalts (MORB) provide fundamental information about the composition and melting processes in the Earth’s upper mantle. To use MORB to further our understanding of the mantle, is imperative that their crustal evolution is well understood and can thus be accounted for when estimating primary melt compositions. Here, we present the evidence for the occurrence of reactive porous flow, whereby migrating melts react with a crystal mush in mid-ocean ridge magma chambers. This evidence comprises both the textures and mineral major and trace element geochemistry of rocks recovered from the lower oceanic crust, and occurs on a range of scales. Reaction textures include dissolution fronts in minerals, ragged grain boundaries between different phases and clinopyroxene–brown amphibole symplectites. However, an important finding is that reaction, even when pervasive, can equally leave no textural evidence. Geochemically, reactive porous flow leads to shifts in mineral modes (e.g. the net replacement of olivine by clinopyroxene) and compositions (e.g. clinopyroxene Mg–Ti–Cr relationships) away from those predicted by fractional crystallization. Furthermore, clinopyroxene trace elements record a progressive core–rim over-enrichment (relative to fractional crystallization) of more-to-less incompatible elements as a result of reactive porous flow. The fact that this over-enrichment occurs over a distance of up to 8mm, and that clinopyroxenes showing this signature preserve zoning in Fe–Mg, rules out a diffusion control on trace element distributions. Instead, it can be explained by crystal–melt reactions in a crystal mush. The data indicate that reactive flow occurs not only on a grain scale, but also on a sample scale, where it can transform one rock type into another [e.g. troctolite to olivine gabbro, olivine gabbro to (oxide) gabbro], and extends to the scale of the entire lower oceanic crust. Melts undergoing these reactive processes change in composition, which can explain both the major element and trace element arrays of MORB compositions. In particular, reactive porous flow can account for the MORB MgO–CaO–Al2O3 relationships that have previously been interpreted as a result of high-pressure (up to �8 kbar) crystal fractionation, and for over-enrichment in incompatible elements when compared with the effects of fractional crystallization. The finding of a significant role for reactive porous flow in mid-ocean ridge magma chambers fits very well with the geophysical evidence that these magma chambers are dominated by crystal mush even at the fastest spreading rates, and with model predictions of the behaviour of crystal mushes. Together, these observations indicate that reactive porous flow is a common, if not ubiquitous, process inherent to mushy magma chambers, and that it has a significant control on mid-ocean ridge magmatic evolution.