Quantifying lithological heterogeneity

Melt production as a function of lithology

Combining constraints on melt production (crustal thickness, tc) and the fraction of melts supplied by pyroxenite melting (from geochemistry, Fpx) unique source lithologies can be identified beneath Iceland. In these triangular diagrams position denotes the proportion of each lithology in the source, whilst colour indicates the relative proportion of enriched and depleted melts produced. Figure modified from Shorttle et al. (2014).

The presence of lithological diversity in the mantle has major implications for solid Earth dynamics, the mantle melting process and potentially the environmental impact of eruptions. In this paper we consider the processes affecting the representation of lithological diversity in erupted basalts, such as the biasing effect of the melting process, in order to construct quantitative estimates of the abundance of different lithologies in the mantle. Being able to form such estimates is the first step for understanding how lithological heterogeneity is affecting melt production and ultimately influencing the solid Earth’s interaction with the surface environment.

We focus our study on Iceland, and first use high MgO basalts from Iceland’s neovolcanic zones to characterise the trace element enriched and depleted endmember melts entering the Icelandic crust. These compositions can then be used in a mass balance with the average Icelandic crustal composition (as represented by evolved, mixed, basalts) to calculate the proportion of melt on Iceland being supplied from enriched vs. depleted mantle domains. With this estimate of enriched and depleted melt proportions, and knowing that the lithologies contributing to melting in the Icelandic mantle (pyroxenite and lherzolite, Shorttle et al. 2011), it is possible to construct a melting model to determine how abundant each lithology must be in the Icelandic source. However, as can be seen from the figure above, using only the constraint on proportion of enriched and depleted endmembers does not uniquely constrain a valid source lithology. To achieve a unique result we add in the constraint that the total melt production must be consistent with the volume of melt production on Iceland (the crustal thickness).

Even using enriched-depleted melt proportions and crustal thickness, source lithology remains non-unique unless mantle potential temperature is also known. Specifically, there is a strong trade off in harzburgite abundance against lherzolite proportion in the source – all valid solutions have ~10% pyroxenite component. To obtain added constraints on source we consider the buoyancy of the possible Icelandic source lithologies, and whether they would be consistent with recent volume flux estimates (Jones et al. 2014). Considering this additional dynamical constraint, the Icelandic source is required to have significant (>20%) harzburgite component. Although there is this uncertainty on the maximum temperature of the plume source, our model nonetheless constrains its minimum temperature to be significantly above ambient mantle, by ~130ºC (see figure above).

Online [publisher, open access]: http://dx.doi.org/10.1016/j.epsl.2014.03.040

ReferenceShorttle, Oliver, John Maclennan, and Sarah Lambart. Quantifying lithological variability in the mantle. Earth and Planetary Science Letters 395 (2014): 24-40.