*article written by Dr. Michael Broadley and Prof. Ray Burgess.
The Earth’s mantle makes up more than 80% of the Earth by volume and has controlling influence on the generation of the Earth’s crust, the oceans and the atmosphere. The mantle therefore plays a dominant role in sustaining the habitable environment of our Planet. Volatile element and compounds vital to life such as water, carbon and nitrogen are released through from the mantle to the surface through volcanic eruptions but what is less well known is whether these volatiles species are taken back down into the mantle through the subduction of old crustal material together with seawater and sediments.
Attempting to study the volatile composition of the Earth’s mantle is incredibly difficult and we therefore rely on the analysis of samples which originate from the mantle but have now been brought to the surface through volcanic eruptions, so called mantle xenoliths. Our study combined detailed noble gas isotopic and halogen elemental analysis of the volatiles trapped within mantle xenoliths erupted within Western Antarctica, to try and better understand the distribution of volatiles throughout the Earth.
Noble gases and halogens are concentrated on the surface of the Earth within the atmosphere, seawater and sediments and have unique signatures within each of these reservoirs, allowing them to be used as geochemical fingerprints for tracing the movement of volatiles throughout the Earth. Within the Western Antarctic xenoliths we have found signatures within these trapped fluids similar to air and marine sediments, as well as the background mantle signature. This indicates that the mantle beneath Western Antarctica has been infiltrated with volatiles from the Earth’s surface during a previous subduction event.
The new data confirm that the volatiles can be introduced to the mantle through subduction and can remain trapped there over millions of years. This implies that the mantle is a much larger reservoir for volatiles than was previously thought and needs to be taken in to account when considering the global volatile cycle.
MW Broadley, CJ Ballentine, D Chavrit, L Dallai, R Burgess (2016) Sedimentary halogens and noble gases within Western Antarctic xenoliths: Implications of extensive volatile recycling to the sub continental lithospheric mantle Geochimica et Cosmochimica Acta 176, 139-156 doi:10.1016/j.gca.2015.12.013
Fieldwork in Antarctica was supported by the Italian Programma Nazionale di Ricerche in Antartide, and laboratories and staff in the UK by the NERC and the ERC.