Mantle volatiles: processes, reservoirs and fluxes

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences


We have brought together a consortium of UK investigators and international partners with the key objective of providing a new process based understanding of volatile element (e.g. H2O, C, S, noble gases and halogens) fluxes into the deep mantle at subduction zones and out of the mantle at mid ocean ridges and ocean island settings. The mantle is by many orders of magnitude the largest silicate reservoir for carbon, nitrogen and sulphur on Earth and the input and output of volatiles (e.g., H2O, C, N, S, P, and halogens) at plate boundaries provides long-term controls on the climate and the biosphere. Nevertheless, our understanding of the deep-Earth volatile cycle is crude. In part because we have a very poor understanding of the relative contribution of recycled to primordial volatiles in the mantle system and how this might vary in different mantle reservoirs. In part this is because volatile elements are extensively lost during the eruptive process from many sample types making it hard to identify the controlling processes necessary to develop coherent models.
To address our objective the consortium combines several advances in new sample resources and analytical tools:
i) The recognition that rapidly quenched melt inclusions (MIs) within erupted material often preserve mantle-source volatile compositions;
ii) The ability to determine sulphur and boron isotopes in addition to major volatiles in the MIs;
iii) The discovery that boron isotopes can track the extent of volatile loss to the surface from subducting slabs and preserve this signal in the deeper mantle;
iv) The innovations in noble gas isotope determination that allow us to resolve recycled volatiles from those trapped during accretion and provide links to halogens, H2O and C;
v) The development of non-traditional stable isotopes such as Fe, Cu and Se to identify system oxidation state (a key variable in understanding sulphur) and chalcophile trace element determinations;
vi) The advances in computing power and techniques that allow better representation of mantle-like systems.

By coordinating the combined consortium expertise and analytical resources on the same sample suites in two thermally contrasting subduction regimes (Kamchatka (cool) and Southern Chile (hot)) we plan to investigate how both the processes and thermal setting control the efficiency and geochemical character (isotopic composition and relative abundance to other volatiles) of volatile subduction into the deep mantle. This allows us to take into consideration changes in subduction temperature as the Earth cools in the development of flux models that run for the age of the Earth. At mid ocean ridges and ocean island settings with different geochemical provenance (e.g. HIMU, EMI, EMII, FOZO) we will determine the proportion and character of volatile elements that have been recycled compared to those that were incorporated into the mantle during its formation (primitive volatiles). This is an essential component in building our understanding of the volatile flux into the mantle required to support the signals in the mantle today. New experimental partitioning developed within the consortium and our ability to track oxidation state will allow us to make a step change in understanding the sulphur cycle - barely understood to date but critical in understanding climate and commercial mineral deposit formation. Numerical simulations of mantle transport for suites of geochemical elements, iterating the geophysical parameters to approach matches for the geochemical observables, will allow us to identify the key geophysical processes in subduction zones and during whole mantle convection that control the geochemical distribution of subducted vs. primordial volatiles in the mantle. Together, these will lead to a significant advance in reconstructing the deep Earth volatile fluxes over Earth history - a grand science challenge.

Planned Impact

Specific beneficiaries
- The mineral resources industry: Sulphide minerals host a large fraction of the crustal budget of economically important elements, including many of the critical raw materials identified by both NERC ('Sustainable use of Natural Resources' theme) and the EU ('EU-14 critical raw materials') as those on which the European economy depends upon, but which might be at risk of supply disruptions. Fundamental understanding of partitioning control of S within arc and mid ocean ridge systems will be a key science output of the consortium changing our understanding of where and in what terrain S and related elements are deposited to form commercial resources.

- The instrumentation and analytical technique development industry: The development of new analytical skills and instrument capabilities is vital for both academia and industry as evidenced by collaborations between scientific instrument companies and universities (e.g. between Durham, Oxford and Bristol and Thermo Scientific) which aim to develop the capabilities of specific techniques or instruments, resulting in open access published technical reports and contributing to product development and sales growth.

- The education sector (e.g., schools): The cycling of volatiles in different tectonic settings is fundamental to planet habitability but is also important in terms of natural hazards, mineral resources and Earth processes. We believe there is an opportunity to use this research as the perfect tool for demonstrating the interconnectedness of Earth processes and how they can affect us.

We will deliver benefit by:
- The mineral resources industry: We will run a workshop in Y2 of the grant that brings key industry and consortium academics together with the objective of forming an industry/consortium advisory committee, defining a consortium mineral exploitation/exploration research program and developing the industry funding base. We will seek matching funds from industry for PDRA time to commercialise our research outputs relevant to the relationship between copper porphyry deposits and the fluxing of fluids through arc volcanoes.

- The instrumentation and analytical technique development industry: We will exploit existing collaborations and links with instrumentation companies as well as looking to develop new links to disseminate the new developments in analytical protocols and instrument capabilities that we will achieve. New data and techniques will be presented at international conferences, published in specialised online journals and also in open-access publications prepared by instrument companies (e.g. Thermo Scientific "Application Notes"), which are aimed at diverse target audiences in both academia and industry.

- The education sector: We are well placed to deliver this through structures within our institutions (e.g. the national online teaching tool Your Planet Earth - Bristol; Oxford Sparks, led by Co-I Pyle; the Scottish Earth Science Education Forum, Edinburgh). Via these we will provide resources for teachers delivering: (i) Educational activities for Key Stages 2-5 describing different aspects of the global volatile cycle, what its relevance is, and how they relate to the other topics in the National Curriculum (Natural Hazards, Plate Tectonics, Earthquakes, Volcanoes etc.); (ii) specific student exercises exploring the cycling of volatile elements, from chemical, geological and societal/natural hazard points of view. We plan to convene workshops for teachers to support integration of Earth Sciences into science teaching (e.g. the Earth Sciences Teaching Association Conference, Cardiff, Sept. 2015).

As a group we shall also continue and build upon our current activities engaging with the broader public, capitalising on the outreach frameworks in place at all 10 of our institutions. We will also develop new high profile activities specific to this TA such as participation in the Royal Society Summer Exhibition.
Description We have collected a high quality data set using in situ techniques that will help advance our understanding of when and how material has been subducted into the mantle and the subsequent effect on the composition of lavas erupted at the surface.

Presented results at conferences including AGU and Goldschmidt in 2017, 2018 and in 2019, including an invited talk on the topic at AGU.

Data collection:
We have collected the largest and highest quality boron isotope dataset for ocean island basalts to-date which includes:
- 38 different samples that represent the major ocean island basalt end-members and mid ocean ridge basalt
- 11 melt inclusion suites, at least one from each ocean island basalt end-member
- 9 different ocean island chains
This represents the largest, internally consistent, high-precision B isotope dataset from ocean island basaltic glasses and olivine-hosted melt inclusions measured by SIMS. The data help us to investigate heterogeneity in the mantle and how material is recycled and influences the composition of igneous rocks erupted at the Earth's surface.

In addition to the novel dataset produced, we have developed a new technique for the digestion and analysis of boron isotopes by MC-ICPMS and have analysed a variety of standards and unknowns that show the technique is working successfully.
The first major academic paper was published in 2019 with two more papers in various stages of production.
Exploitation Route Not clear as yet, however recent published paper establishes differences between different mantle reservoirs but that deep mantle plumes have similar ispotopic ratios to mid ocean ridge basalts suggesting limited recycling of volatile elements to the deep mantle.
Two further papers on technical developments and on the global overview boron provides regarding mantle heterogeneity are being submitted. Key results suggest the deeper mantle is becoming increasingly B-depleted over time because boron is effectively stripped from recycled lithologies during shallower upper mantle processes (e.g., subduction and dehydration). In addition, the results highlight the decoupling of B isotopes from traditional noble gas and radiogenic isotopes, providing a new perspective on volatile recycling and the physical processes that impact the compositional evolution of the mantle.
Sectors Environment

Description Pisa collaboration 
Organisation University of Pisa
Country Italy 
Sector Academic/University 
PI Contribution Project PDRA travelled to Pisa to run samples using multiple methods. Excellent correlation found between some standards. Publication is being planned.
Collaborator Contribution Samuele Agostini's boron isotope facility hosted project PDRA Andre Paul to do bulk digestion technique for comparison of standards.
Impact Planned publication in preparatipon
Start Year 2018