Janet Watson Meeting 2019: From core to atmosphere- Deep carbon

As part of the Year of Carbon, the Geological Society of London are hosting a meeting for Early Career Scientists centred around Deep Carbon. Together with Lotta Purkamo (St Andrews) I am chairing the meeting. The first two days of the meeting will consist of oral and poster presentations, and mentoring activities run by prominent senior Deep Carbon scientists. The final day of the meeting will be given over to workshops about the future of Deep Carbon research.

More info, registration, and information about abstract submission.

Goldschmidt 2018

I will be in Boston for the entire week of Goldschmidt.

On Friday morning I will present a talk about my new dataset of CO2 and trace element concentrations in Icelandic melt inclusions, and what this data implies about identifying CO2-rich mantle domains both in Iceland and globally.
02c 310 Friday 11:15-11:30 abstract

At the Monday poster session Helen Williams and I will present our new Fe-isotope analyses on material from Iceland, and how these observations may be reconciled in terms of magma oxidation state, mantle melting and melt transport processes.
02b board 6 Monday 17:15-19:15 abstract

Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing

The mantle is an important, yet poorly understood, part of Earth’s carbon cycle; interacting with Earth’s surface through volcanism and subduction. The CO2 flux balance in to and out of the mantle regulates the mass of CO2 in Earth’s crust and hydrosphere, exerting control over the evolution of Earth’s climate and carbon availability for life. However, carbon’s volatility, and therefore tendancy to degas from magmas and emanate at Earth’s surface diffusely, has made identifying the present-day mantle carbon distribution difficult.

Droplets of magma trapped within crystals as they grow deep in the crust offer a chance of observing CO2 concentrations in magmas prior to degassing. The behaviour of CO2 during magma evolution is encoded in the covariation of CO2 and trace element concentrations. In a small number of datasets, a correlation between CO2 and either Ba or Nb has been reported; consequently identical behaviour, in particular a lack of degassing, has been inferred. These, apparently undegassed, datasets underpin our understanding of carbon distribution in the mantle.

In this paper, we argue that many of the melts supplied from the mantle should be oversaturated in CO2 vapour at the pressure of magma storage, whilst others will be sufficiently depleted in CO2 that they should be strongly undersaturated. Such a population of melts will tend to partially degas at the earliest stages of melt evolution, before subsequent mixing and fractionation. We show that positive correlations between CO2 and both Ba and Nb, are a natural consequence of this process. Furthermore, our new model makes specific predictions about the covariance of CO2 with a gamut of trace elements, if partial degassing and mixing has taken place.

Since we demonstrate that positive correlations between CO2 and trace element concentrations are arise from partial degassing and mixing, we cannot use this as a criterion for identifying whether a dataset has been affected by degassing. Mantle carbon contents, derived by assuming such melts preserve primary CO2 concentrations, are likely to be underestimates. We find the maximum CO2/Ba ratio in a dataset is the best proxy for mantle carbon content.

Matthews, S., O. Shorttle, J. F. Rudge and J. Maclennan (2017), Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing, Earth and Planetary Science Letters, 480, 1-14. doi:10.1016/j.epsl.2017.09.047

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.