The melting column as a filter of mantle trace-element heterogeneity

TL;DR

This paper models how a melting column filters mantle trace-element heterogeneity, concluding that observed variability in basaltic lavas is primarily due to heterogeneity in melt transport rather than source heterogeneity alone.

Contribution

It introduces a model showing the melting column as a filter for trace-element variability and falsifies the end-member hypothesis that source heterogeneity alone explains observed variations.

Findings

Melting column attenuates variability based on heterogeneity wavelength and other factors.

Model fitting requires assumptions inconsistent with magma assembly timescales.

Observed variability indicates heterogeneity in melt transport mechanisms.

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.