Abstract
The observed variability of trace‐element concentration in basaltic lavas and melt inclusions carries information about heterogeneity in the mantle. The difficulty is to disentangle the contributions of source heterogeneity (i.e., spatial variability of mantle composition before melting) and process heterogeneity (i.e., spatial and temporal variability in melt transport). Here we investigate the end‐member hypothesis that variability arises due to source heterogeneity alone. We model the attenuation of trace‐element variability introduced into the bottom of a one‐dimensional, steady‐state melting column. Our results show that the melting column can be considered to be a filter that attenuates variability according to the wavelength of heterogeneity, the partition coefficient of the trace element, melt productivity, and the efficiency of melt segregation. We further show that while the model can be fit to the observations, this requires assumptions inconsistent with constraints on the timescales of magma assembly. Hence, we falsify the end‐member hypothesis and, instead, conclude that observed variability requires heterogeneity of melt transport. This might take the form of channels or waves and would almost certainly interact with source heterogeneity.
Highlights
Basaltic lava compositions can potentially constrain models of melting, melt transport, and the chemical character of the source mantle
The difficulty is to disentangle the contributions of source heterogeneity and process heterogeneity
Our results show that the melting column can be considered to be a filter that attenuates variability according to the wavelength of heterogeneity, the partition coefficient of the trace element, melt productivity, and the efficiency of melt segregation
Summary
Basaltic lava compositions can potentially constrain models of melting, melt transport, and the chemical character of the source mantle. Previous Melt Models Investigating the Transport of Source Heterogeneity Many previously published studies employ column models that assume porous magmatic ascent, with full or partial aggregation of the melts produced at different depths This is the basis on which McKenzie (1985, 1985) and Navon and Stolper (1987) developed theories for trace-element transport, showing that equilibration between liquid and solid phases leads to transport rates that depend on the partition coefficient. Our key finding is that melt transport attenuates chemical heterogeneity of the upwelling mantle, depending on partitioning of the element considered, its lengthscale of variation in the source mantle, and the vertical structure of melting rate This remains true for partial chemical equilibration. We conclude that source heterogeneity cannot fully explain the chemical diversity of basalts and that variability of melt transport (e.g., channelized flow) is required
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