Extreme chemical variability as a consequence of channelized melt transport

The spatial and temporal variability of chemical signals in lavas, residues and melt inclusions provides important constraints on source heterogeneity, the efficiency of convection, and melt transport processes in the mantle. The past decade has seen an impressive increase in the number, precision and spatial resolution of chemical analyses. However, as resolution has increased, the picture of variation that emerges has become increasingly difficult to understand. For example, mid-ocean ridge basalts can display large variations in trace-element concentration on scales from 1000 km of ridge to melt inclusions in 500 micron crystals. These observations suggest that melt transport processes do not readily homogenize partially molten regions. While some of the observed variability is due to source variations, a large proportion could be the consequence of magma transport in channelized systems. We present results of numerical models that calculate the trace element signatures of high porosity dissolution channels produced by reactive fluid flow. These models were originally developed to explain the organization of melt transport networks, based on observations of “replacive dunites” found in ophiolites. Channelized flow can produce orders of magnitude variation in the concentrations of highly incompatible elements, even for idealized systems with a homogeneous source, constant bulk partition coefficients and equilibrium transport. Most importantly, the full range of variability may be found in each channel because channelization can transpose the chemical variability produced by melting throughout the melting column into horizontal variability across the width of the channel. The centers of channels contain trace element enriched melts from depth, while the edges of the channels transport highly depleted melts extracted from the inter-channel regions at shallower levels. As dunite channels may be spaced on scales of 1–100 m in the mantle, this mechanism allows highly variable melt compositions to be delivered to the Moho on small length scales. The chemical variation produced in the models is consistent with that seen in melt inclusion suites, lavas and residual mantle peridotites dredged from the ridges and sampled in ophiolites.