Goldschmidt 2017

I will be attending the Goldschmidt meeting all week, and will talk about my work on mantle CO2 heterogeneity at 9am on Thursday.

The roles of Degassing and Mixing on Volatile-Trace Element Systematics: Implications for Global Carbon Budgets. (05g: Ampithéâtre Havane, Thursday 0900-0915).

Magma mixing has become an important process in understanding how the geochemical variability of primary mantle melts is expressed in erupted products. In this talk I will argue that mixing has been under appreciated in the interpretation of the datasets that underpin our understanding of CO2-distribution in the mantle, I will use a combination of simple models, existing datasets and new melt inclusion data from Iceland.


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

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.

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.

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 °C for Iceland, and °C for MORB.

Matthews, S., O. Shorttle, and J. Maclennan (2016), The temperature of the Icelandic mantle from olivine-spinel aluminum exchange thermometry, Geochem. Geophys. Geosyst., 17, 47254752, doi:10.1002/2016GC006497.