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

Mixing and degassing systematics

The systematics of mixing and degassing magmas, shown schematically (top) and as seen in the melt inclusion record (bottom). Primary melts are generated with correlated C-trace element systematics (left). Degassing only affects carbon, reducing its concentration in saturated (high C) melts (middle). Subsequent mixing of this variably degassed suite of inclusions can generate the appearance of no degassing having occurred.

The mantle is an important, yet poorly understood, part of Earth’s carbon cycle, which interacts with Earth’s surface through volcanism and subduction. The CO2 flux 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 tendency 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 metric alone 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.


Online [publisher, open access]: https://doi.org/10.1016/j.epsl.2017.09.047

Reference: Simon Matthews, Oliver Shorttle, John F Rudge, John Maclennan. Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing. Earth and Planetary Science Letters (2017).

Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

Olivine hosted melt inclusions from the AD 1783 Laki eruption. These inclusions were trapped early in the magma’s life, preserving many chemical signals from the subsequent reprocessing, which occurred as the magma cooled and crystallised in the crust. However, our new study shows the susceptibility of tracers of magma oxidation state in the melt inclusions to resetting, even following eruption.

The amount of oxygen in magmas affects their physical and chemical properties, and ultimately their impact on chemical cycles linking planetary oceans and atmospheres to their deepest interiors.  A key archive of information on oxygen in magmas is the abundance of Fe2+, reduced iron, compared with Fe3+, oxidised iron. The abundance of these two forms of Fe evolves as magmas are stored in the Earth’s crust, meaning that the primary Fe2+/Fe3+ that magmas have when they enter the crust from the mantle will not be preserved at the point a magma comes to erupt.

An important information source for getting back at the chemical state of primitive magmas, before their crustal evolution, is in melt inclusions — small pockets of melt trapped as crystals grow.  However, in this study we show that even these archives of early magma history are susceptible to chemical resetting.  Firstly, in magma chambers at high temperature diffusion can occur, resetting all the melt inclusions to record the same activity of oxygen in the magma (oxygen fugacity).   Secondly, even after eruption, as the magma is flowing along the surface of the Earth, changes in the oxygen fugacity of the surrounding magma can propagate through to the melt inclusions changing theirFe2+/Fe3+ ratio.

These results mean that to reconstruct the oxygen fugacity of primitive magma we have to 1) select our samples very carefully, and 2) characterise the crustal and eruptive processes that could have reset the melt inclusions.


Online [publisher]: https://doi.org/10.1016/j.epsl.2017.09.029

Reference: Margaret Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam, Marie Edmonds. Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems. Earth and Planetary Science Letters (2017).

AGU 2014

Come and find me or any of my collaborators at AGU this year to discuss our latest results.  Margaret Hartley and I have some great new XANES data collected at Diamond Light Source probing fO2 in enriched mantle domains and tracking its evolution during magmatic processes. I have an invited talk in V038: The Geochemical Diversity of the Mantle Inferred from Hotspots: Five Decades of Debate, where I will present evidence for the ubiquity of concurrent mixing and crystallisation in destroying the primary chemical diversity leaving the mantle at mid-ocean ridges. With Mark Hoggard’s fantastic record of dynamic support in the world’s ocean basins, we have begun to reconstruct spatio-temporal variability in mantle potential temperature over the last 100Ma.

Controls on OIB and MORB Geochemical Variabilty


Authors: Oliver Shorttle & John Maclennan

Concurrent mixing and crystallisation is visible on a local scale looking at melt inclusion and whole rock suites. Here we show that this basic magmatic process extends not only off of Iceland onto the adjacent Reykjanes Ridge, but by spatial statistical analysis can be seen to be present in global MORB datasets. Homogenisation of primary mantle chemical diversity is therefore a ubiquitous phenomenon occurring in magmatic systems. Understanding how this operates is going to be key for reconstructing mantle compositional diversity.


Authors: Oliver Shorttle & Yves Moussallam, Margaret E Hartley, Marie Edmonds, John Maclennan and Bramley J Murton

Recent evidence from Cottrell and Kelley (2013) has indicated that the mantle heterogeneity sampled by MORB and typically identified from studying radiogenic isotope tracers, may also be associated with redox heterogeneity in the mantle. This compelling observation has major implications for the flux of redox sensitive elements throughout the Earth system, for mantle dynamics, and for the melting process itself. In this work we have characterised the changes in mantle fO2 that occur towards the Iceland plume using a suite of basalt samples.


Authors: Margaret E Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam and Marie Edmonds

Melt inclusions record the primary diversity of melts leaving the mantle in terms of their trace and isotopic compositions, and there is the potential for melt inclusions to also record redox heterogeneity of the source.  However, post entrapment processes such as diffusion and crystallisation may compromise the melt inclusion record, resetting melt inclusion fO2 during shallow level processes. To investigate the potential of the melt inclusion archive in terms of fO2 we have studied a suite of melt inclusions from the AD 1783 Laki eruption, Iceland.

A History of Global Mantle Potential Temperatures from Oceanic Crustal Thicknesses


Authors: Mark Hoggard, Nicholas J White and Oliver Shorttle

We know from geophysical observations of gravity anomalies and petrological measurements on primitive basalts that mantle potential temperature is likely to vary by several hundred degrees in the modern Earth.  A record of potential temperature variation in the past is preserved in the crustal thickness of old seafloor, which will be thicker if high potential temperatures during its formation increased melt production. Here, we use Mark’s extensive compilation of reflection and wide-angle seismic profiles to constrain crustal thicknesses throughout the oceanic realm. These observations when combined with a mantle melting model allow us to back out a unique record of spatio-temporal syn- and post-rift variations in mantle temperature.