Concurrent mixing and crystallisation in MORB

Concurrent mixing and crystallisation in MORB over a range of length scales.

As MgO drops during crystallisation basalts become more mixed. This concurrent mixing and crystallisation is tracked by a measure of chemical variability at a given MgO (σ*), shown here for La/Yb in a global compilation of mid-ocean ridge basalts (Gale et al., 2013). In the plot to the left the amount of chemical variability present at a given MgO is represented by points coloured red for high variability and blue for low. Each horizontal band of colour indicates how variability changes during differentiation, for a suite of MORB geographically normalised over a particular length scale. Taken together, these σ*-MgO profiles record how coherent mixing patterns emerge from global datasets as the length scale of sample grouping becomes progressively more local. Plots to the right indicate the normalised data distribution of La/Yb at 10,000 km and 50 km. Figure modified from Shorttle (2015).

The basalts sampled at mid-ocean ridges and ocean islands provide critical insight into the chemical evolution of the Earth: with their isotope ratio and trace element abundances capturing processes as diverse as planetary accretion and the recent subduction of organic sediments.  Basalts are used in this way with the understanding that their composition reflects that of the solid mantle source from which they were derived. However, we can only use the chemical inventory of basalts to probe mantle geochemistry if we are confident that we can ‘see through’ the chemical signals that have been imparted to them in the tens to thousands of years preceding their eruption. During this interval basalts are produced by partial melting of the mantle, extracted by porous and focussed flow, aggregated in mantle and crustal magma chambers and cooled, leading to their crystallisation. At each of theses stages there is the opportunity for basalts to mix, leading to a reduction in the amount of information they carry about their source, and therefore limiting our ability to reconstruct mantle composition and history.

This paper focusses on the mixing basalts experience during crustal (and upper mantle) storage in magma chambers. I use Iceland as a case study, which nicely illustrates the chemical signature of mixing in whole rock and melt inclusion records, to then explore mixing systematics along the more sparsely sampled submarine portions of the mid-ocean ridge system. The significance of magma mixing in mid-ocean ridge basalts (MORB) has previously been recognised from global compilations of MORB (e.g. Rubin et al., 2001) and from micro-analytical studies of melt inclusions (e.g. Sobolev & Shimizu (1993) and Maclennan (2008)), amongst other observations. In this paper I focus on investigating the length scale at which coherent mixing relationships become visible in suites of basalts, and the spatial statistical methods we can use to understand the meaning of this length scale.

Applying geographic normalisation to the global dataset of Gale et al., (2013), I find that coherent mixing relationships are visible at length scales < 300 km – approximately the scale of first order ridge segmentation. Statistical tests repeatedly randomly repositioning samples demonstrate that this result is only apparent in the real dataset – i.e. spatial structure exists in the samples of natural MORB that statistically relates them to one another over ~100 km length scales. This observation is another hint at the geochemical provincialism present in basalts (e.g. Shorttle et al. 2013). Yet, it remains challenge to identify at which point in the system, from mantle to crust, that basalts erupting > 100 km apart gain similarity to one another. One possibility is that this relates to a decorrelation length scale in mantle compositional structure.

The challenge posed by magma mixing from a mantle geochemical perspective is even knowing that the information loss has occurred. If primitive basalts are erupted, containing early crystallising phases, then perhaps concurrent mixing and crystallisation (CMC) will be caught in the act and basalts will be only partially mixed. However, extensive mixing may occur before a crystal record is even produced, for example during transport out of the mantle (Rudge et al., 2013). In this case it is important to appreciate how mixing may bias the accessible geochemical record of basalts. In particular, developing an understanding of magma mixing is going to be important for understanding the relationship of ocean island basalt chemistry to that of MORB.  As their very different geodynamic settings will likely translate into different magma storage and transport histories, of which we are unable to fully predict the geochemical consequences.

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Reference: Oliver Shorttle. Geochemical variability in MORB controlled by concurrent mixing and crystallisation. Earth and Planetary Science Letters 424 (2015): 1-14.