How did primordial and recycled geochemical signatures come to coexist in the Earth's deep mantle?

The transport and cycling of volatile elements between the solid Earth, oceans and atmosphere has shaped the evolution of our planet. Chemical fluxes of volatiles to and from long-term mantle reservoirs are maintained by convective and tectonic processes. However, the mechanisms that control the volatile budgets of distinct mantle reservoirs remain uncertain. Major questions include: How did the Earth acquire its initial volatile inventory? What was the closure age of the mantle to atmosphere loss? How has crustal recycling modified mantle volatile reservoirs? What is the spatial distribution of primordial and recycled volatiles in the mantle? The key objective of this project is to provide a new understanding of spatial and temporal relationships between primordial and recycled sources in the Earth's mantle, underpinned by high-precision geochemical data and detailed statistical modelling. We will achieve this by interrogating combined noble gas isotope compositions and halogen contents in an exceptional suite of subglacially erupted basalts from Iceland that map a high-resolution transect across a mantle plume. The samples preserve geochemical signatures of melts from both ancient sources and the shallow convecting mantle, which has been extensively modified by melt extraction and recycling over geologic time. The unparalleled high-resolution, spatially continuous sample of the mantle afforded by erupted basalts from Iceland's neovolcanic zones makes Iceland an ideal natural laboratory for investigating the history of accretion, differentiation, outgassing and recycling in the mantle.

The inert noble gases (He, Ne, Ar, Kr, Xe) provide key geochemical tracers of primordial mantle domains that have remained largely unmodified since the Earth's formation. In contrast, fluid-mobile halogens (F, Cl, Br, I) are reintroduced to the mantle by the subduction of seawater, sediment and altered oceanic crust at destructive plate boundaries, and are therefore key tracers of recycled material in the mantle. There is at present an unresolved dichotomy between existing interpretations of noble gases, halogens and other geochemical tracers such as lithophile isotopes (Sr, Nd, Pb). Lithophile isotopes and halogens provide abundant evidence that mixing, stretching and recycling in the mantle has created highly heterogeneous lithological and spatial structure beneath Iceland. However, noble gas isotopes indicate that some parts of the Icelandic mantle retain primordial, unprocessed geochemical signatures. A crucial limitation of previous work is that lithophile elements and isotopes, major volatiles and noble gases have been analysed in different sample sets, such that we cannot combine the sensitivities of different elements to obtain robust constraints on mantle sources. By bringing these data into a single coherent framework, this project provides the first opportunity to interrogate simultaneously multiple geochemical proxies to probe the spatial and temporal relationships between primordial and recycled mantle sources.

We will use state-of-the-art mass spectrometry techniques to provide new independent constraints on halogen and noble gas decoupling in the mantle, and the extent of halogen loss from different mantle reservoirs in the early Earth. We will provide the first high-precision analysis of the Kr-isotopic composition of the Earth's primordial mantle, and a new, precise determination of iodine abundance in the mantle. We will perform the first combined spatial statistics analysis of noble gas, halogen and lithophile isotope data to determine the location and distribution of primordial and recycled reservoirs in a mantle plume, providing key insights into the structure and dynamics of the mantle. Our results will provide a guide to interpreting mantle geochemical and spatial structure at other ocean islands on Earth, and will feed into ongoing efforts to model volatile budgets and volatile cycling on other planetary bodies.

The proposed project focuses on volatile accretion, transport and cycling between distinct mantle source reservoirs on Earth. This project will make significant advances in our understanding of Earth's initial volatile inventory; the formation of Earth's early atmosphere; the role of crustal recycling in controlling noble gas and halogen budgets in the mantle; and the spatial distribution of distinct geochemical and lithological mantle domains.

WHO will benefit from this research, and HOW?

(a) The scientific instruments and analytical sector will benefit from the technical advances made in this project, particularly with respect to heavy noble gas mass spectrometry, which should lead to improvements in existing instrument and software design. With effective dissemination, our initial results should be able to impact existing instrument and software specifications after about 2 years. On a 5-10 year timescale, the outcomes of this project will impact the development of the next generation of noble gas mass spectrometers.

(b) The wider public are expected to benefit through dedicated family- and adult-orientated events run in conjunction with the Manchester Museum, and further development of the outreach activities in which we already engage (school visits, public lectures, radio and TV presentations). It is clear from the extensive news and documentary coverage of recent and ongoing space exploration missions that there is a substantial and growing public interest in planetary science and in gaining a greater understanding of Earth and its place in the solar system. The wider public naturally turn to academia for informed opinions and to enhance their knowledge.

(c) Academic beneficiaries: We will present at UK and international conferences in both geosciences and planetary sciences disciplines to ensure that our results are communicated effectively and have wide impact. Access to all geochemical data will be provided through the GEOROC and PetDB databases.

(d) Planetary science and space exploration: Our results should impact the design of experiments and technologies for future space exploration missions, with potential long-term benefits to the UK economy.

(e) The PDRA will develop practical skills in a wide range of mass spectrometry techniques and theoretical skills in volatile, isotope and mantle geochemistry. This will transfer into his/her future career and employment potential in a range of earth and planetary science disciplines.