A statistical description of concurrent mixing and crystallisation during MORB differentiation: implications for trace element enrichment

Melt formation, mixing, and transport schematic

A cartoon of the processes operating from mantle to crust, which give rise to the appearance of trace element over-enrichment in mid-ocean ridge basalts. Diverse melts are produced in the mantle, potentially from chemically heterogeneous sources (left). These melts are transported out of the mantle by channelised flow, which preserves some primary heterogeneity of the melts supplied to the crust (centre). In the crust melts fractionate crystal phases and mix (centre and right).  The supply of heterogenous melts to crustal magma chambers, and their subsequent mixing, gives rise to apparent trace element over-enrichment in the final erupted basalt.

Basalts are our window into Earth’s chemical structure, and being a product of mantle melting and transport, constrain the processes by which it continues to differentiate.  However, to interpret the basalt record in terms of mantle source and melting conditions requires our being able to see through the processes that affect basalt chemistry following melting, during their transport through the mantle, and their storage in the crust.  The extent to which basalts reflect mantle processes vs. crustal processes has been a topic of long-running debate, with workers such as Klein and Langmuir (1987) and Gale et al. (2014) arguing for the importance of mantle processes, whereas more recently O’Neill and Jenner (2012) and Coogan and O’Hara (2015) have emphasised the role of crustal magma chambers in modifying the trace and minor element chemistry of basalts.  A key observation in this debate has been the presence of trace element over-enrichment in mid-ocean ridge basalts, whereby differentiated basalts (those with low MgO) appear to have higher concentrations of incompatible trace elements than can be accounted for by simple fractional crystallisation (see figure below).

Trace element enrichment gradients.

The rate of enrichment of trace elements during magmatic differentiation (expressed as the gradient in trace element – MgO space). More negative numbers indicate stepper gradients, which when they fall below the orange line, cannot be accounted for by simple fractional crystallisation.

In this article we demonstrate how apparent trace element over-enrichment during differentiation can result simply from the chemically heterogeneous melts being supplied to the crust.  The key aspect of the model is that once resident in crustal magma chambers, the probability of melts mixing is proportional to their degree of differentiation (a proxy for their residence time).  Therefore, as melts differentiate, they progressively interact with other melts, until at low MgO (~5  wt%), magmas have a composition close to that of the mean melts being supplied from the mantle.  A consequence of this process, which we call concurrent mixing and crystallisation (CMC; Maclennan, 2008), is that the overall trend of trace element enrichment during differentiation is steeper than what it would be predicted in the absence of mixing.  What’s more, the most incompatible elements have the most variable abundance in melts supplied to the crust, and therefore exhibit the greatest degree of over-enrichment.  Thus, this model reproduces the observations presented in the figure above, whereby highly incompatible elements such as Th and U have the steepest enrichment gradients.

The importance of these results is that in our model, although mixing destroys much mantle-derived chemical variability and therefore information on the melting and melt transport process, the mean composition of the magmas supplied from the mantle is not affected.  As such, magmas retain bulk information on their sources and conditions of formation.  This means that global correlations like those established by Klein and Langmuir (1987) and Gale et al. (2014) will be valid, and the elemental ratios forming the basis for isotope evolution models of the Earth’s mantle (e.g., Sm/Nd for the <sup>143</sup>Nd isotope system) will not have been perturbed.

Online [publisher]: https://dx.doi.org/10.1093/petrology/egw056

Reference: Oliver Shorttle, John F. Rudge, John Maclennan, and Ken Rubin. Journal of Petrology (2016): 1-35, doi:10.1093/petrology/egw056.