## The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry

A comparison of previous estimates of mantle potential temperature at Iceland (red horizontal bars) to our new estimate based on combining crystallisation temperatures with crustal thickness and geochemical constraints (red/yellow histograms).  Estimates of mantle potential temperature for MORB are shown for reference in blue.

Variations in mantle temperature are a primary control on the melting behaviour of the mantle. Despite its importance for understanding present day volcanism and the thermal evolution of the Earth, mantle temperature has remained difficult to quantify. Proxies, such as crustal thickness, seismic velocity, and melt chemistry must be used; however, each suffers from its own uncertainties and trade-offs with other equally uncertain parameters. Melting anomalies, such as Iceland, have been variously linked to raised mantle temperature, unusually fusible mantle, or enhanced mantle flow.
Several studies have recently used olivine crystallisation temperatures, derived from olivine-spinel aluminium-exchange thermometry, as a proxy for mantle temperature. When offsets in olivine crystallisation temperatures are used to infer mantle temperature variation directly, it is implicitly assumed the method does not suffer from trade-offs arising from greater mantle fusibility or enhanced mantle flow.

Summary of the new crystallisation temperature estimates we made in this study.  Crystallisation temperatures were calculated from the composition of olivine-spinel pairs using the Wan et al. (2008) and Coogan et al. (2014) Al-exchange thermometer.

Using a new set of crystallisation temperatures determined for four eruptions from the Northern Volcanic Zone of Iceland, we demonstrate crustal processes, rather than mantle processes, are responsible for the crystallisation temperature variation within our dataset. However, the difference between Icelandic crystallisation temperatures and those from MORB, are most easily accounted for by substantial mantle temperature variations between the two locations.

The thermal structure of the mantle melting region will determine the chemical and thermal properties of the melts entering the crust. As lithological heterogeneity can exert a large effect on the thermal structure of the melting region, we assess its effect on crystallisation temperature using a forward thermal model of multi-lithology melting. Using crystallisation temperature estimates from Iceland and MORB as examples, we demonstrate that in the absence of further constraints on the thermal structure of the melting region (e.g. crustal thickness), crystallisation temperature provides only a weak constraint on mantle temperature.

By inversion of our thermal model, fitting for crystallisation temperature, crustal thickness, and fraction of bulk crust derived from pyroxenite melting, we demonstrate that a mantle temperature excess over ambient mantle is required for Iceland. We estimate a mantle temperature of $\mathsf{1480^{+37}_{-30}}$°C for Iceland, and $\mathsf{1318^{+44}_{-32}}$°C for MORB.

Online [publisher]: https://dx.doi.org/10.1002/2016GC006497

Reference: Simon Matthews, Oliver Shorttle, John Maclennan. The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry. Geochemistry, Geophysics, Geosystems (2016)

## A significantly hotter mantle beneath Iceland

A false color image of an olivine crystal (centre) found in the Borgarhraun eruption of north Iceland. The color picks out variations in the crystal’s composition. We estimated the temperature at which the crystal grew by comparing the composition of the olivine to that of the spinel crystal which has been trapped inside it (small red circle inside crystal) . Horizontal scale = 1.5mm.

We have shown that the Icelandic mantle is unusually hot, a result which has been featured in AGU’s Eos magazine.  We measured the chemistry of olivine and spinel crystals that grew from magmas sourced directly from the Icelandic mantle. These crystals recorded crystallisation at almost 1400°C, indicating that the underlying mantle must be at least this hot. By developing a model to account for how the temperature of the mantle changes during melting, we were able to show that this crystallisation temperature is consistent with a mantle temperature prior to melting of closer to 1500°C.  This is more than 160°C hotter than the mantle underlying most regions on Earth.

And head here for a more detailed summary of the science.

## Quantifying lithological heterogeneity

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.