There and back again – the 2.5 billion year journey recorded by lavas.

Lavas which are erupted at the Earth’s surface show the chemical fingerprint of their source magma. These fingerprints suggest that the Earth’s mantle is heterogenous in terms of composition with different reservoirs of elements coexisting, and geologists battle to unravel these complexities and mysteries by the careful observation of a wide range of rocks. Volcanic rocks form due to the extrusion of magma onto the surface and the type of lavas produced are dependent on the tectonic setting. Plate tectonics is the theory which describes the large-scale motions of the Earth’s crust, and involves the production of crust at divergent plate boundaries and the destruction of crust at convergent plate boundaries, for example by subduction. Subduction is the process of one tectonic plate moving underneath another and the crustal material (igneous and sedimentary) entering the Earth’s mantle where material can be recycled by melting processes.

20 million year old lavas from the Cook Islands have recently been analysed for their sulphur composition, and yielded some very interesting results published in Nature http://www.nature.com/nature/journal/v496/n7446/full/nature12020.html. There are four stable isotopes of Sulphur, and the difference in how they are incorporated (or fractionate) into different rocks and minerals can be described by two methods. Mass dependent fractionation, describes the relative proportions of each isotope incorporated into a phase dependent on the mass of the isotope (for example some minerals preferentially incorporate the heavier isotopes) and mass independent fractionation, whereby the different isotopes are incorporated in different amounts in a way which isn’t dependent on their mass.

The sulphur isotope signature of these hotspot lavas exhibits an anomalously low isotope signature (there is a greater proportion of the lighter isotopes of sulphur compared with the heavier elements) than is observed in other igneous rocks at the Earth’s surface.

Processes which cause this mass independent fractionation of sulphur isotopes include photochemical reactions between the intense beam of the sun, and different rocks. These photochemical reactions are not observed to occur at the Earth’s surface today due to the ozone rich layer of the atmosphere which provides a shield from some of the sun’s intensity. Earth is believed to have formed this oxygen rich atmosphere around 2.5 billion years ago and so the source of these lavas must include material which formed at the Earth’s surface and was subducted before this time.

These results are significant for placing time constraints on the initiation of plate tectonics on Earth, the recycling rate within the Earth’s mantle,  the interaction between different mantle reservoirs as well as mantle processes, for example the formation of diamonds.