Scales of preservation and root causes of mantle heterogeneities in the Iapetan Ocean convecting upper mantle

Although the terrestrial mantle comprises ~80 vol.% of our planet, its compositional architecture is not well understood despite the importance such knowledge holds for constraining Earth's thermal and chemical evolution over ~4.5 billion years of geological time. Our lack of detailed insight into the mantle stems in part from the fact that it is rarely exposed at our planets surface, making direct observation and study difficult. It is clear from recent study, however, that the mantle cannot be assumed to be compositionally homogenous or static over geological time. Peridotites from the ocean basins (abyssal peridotites) and from ophiolites preserve evidence for a convecting upper mantle that is chemically and isotopically heterogeneous at regional (100's km) and small (cm-to-m) scales. Complex formation and alteration upper mantle histories involving processes of melt-depletion, refertilisation (whereby originally refractory residues such as harzburgites become lherzolites again via melt addition) and melt-rock reaction have been held responsible, but the causes, timing and distribution of such processes are poorly resolved.

Ophiolites, which represent partially-to-wholly preserved slivers of obducted oceanic mantle, are particularly valuable resources for assessing the timing, causes and extent of mantle heterogeneity, as they allow field-based observation to be coupled with geochemical investigation on otherwise inaccessible mantle material. Furthermore, ophiolites preserve a range of oceanic mantle lithologies (e.g., harzburgites, lherzolite and dunite) and such variation allows detailed assessment of the distribution and relative timing of events acting upon the mantle that is preserved. A distinctive attribute of some ophiolites, which contrasts with abyssal peridotites, is the presence of podiform chromitite seams, typically in the region of the petrological Moho, which are often associated with Platinum-group element mineralization. The timing and genesis of ophiolite podiform chromitites is controversial, but it has been suggested that they represent zones of focused melt channeling in supra-subduction zone settings.

The Shetland (UK) and Leka (Norway) supra-subduction zone ophiolites comprise oceanic lithosphere separated at ~620 Ma on either side of a mid-ocean ridge and subsequently obducted over continental crust ~130 Ma later, each on opposite sides of the northern Iapetus Ocean. A pilot study already carried out on the Shetland ophiolite by the PI and Project Partner reveals that it preserves evidence for a complex sequence of melt depletion, percolation and refertilisation events that occurred over the lifetime of the Iapetus mantle. The critical observation made from the pilot dataset is that later mantle events only partially overprint the compositional heterogeneities developed from earlier mantle processes and that the relatively high degrees of partial melting associated with the supra-subduction zone are very effective at generating such heterogeneity. This important observation will be tested in the proposed research by 1) extending the Shetland study to greater levels of detail; 2) inclusion of a comparative study of carefully selected samples from the well-preserved Leka ophiolite; 3) drawing comparisons with existing geochemical and isotopic datasets from ophiolites that formed in other (e.g., mid-ocean ridge) tectonic settings. In order to achieve this, the powerful combination of the Re-Os isotopic system and highly-siderophile element (Os, Ir, Ru, Rh, Pt, Pd, Re, Au) abundance measurements will be utilised to discriminate between the processes responsible for generating mantle heterogeneities such as melt depletion, refertilisation and melt-rock reaction. Thus, profound insight will be gained into the chemical evolution of a piece of oceanic mantle and the development of compositional heterogeneity therein, from outcrop to oceanic plate scales, over much of the lifetime of the Iapetus Ocean.

1. Who will benefit from this research?
Aside from the academic community (discussed previously), this research will impact the metals industry, specifically that involved in exploration and extraction of the platinum-group metals (PGM). Other end-users who will benefit from the proposed research are Geography/Geology schoolteachers and pupils learning about Earth formation, natural heritage organizations/national trusts (particularly those responsible for the areas where the samples will be collected) and the general public.

2. How will they benefit from this research?
The PGM are amongst our rarest and most valuable natural resources and the detailed micro-scale studies of Pt- and Pd-bearing sulphides and arsenides proposed in this research will provide valuable new insight into the environments in which these metals are enriched. The current economic climate drives an ever increasing demand for mineral resources; there is every possibility that this research will generate knowledge that will inform industry policy in the exploration for and realization of precious metal resources. This expertise will help place the PI and Keele at the forefront of PGM-exploration studies, and its success will create opportunities for industrial collaboration and 'partnership-driven' NERC CASE proposals with Platinum exploration companies.
This project will provide general knowledge that impacts our understanding of Earth evolution, especially mantle evolution, and will contribute to informing scientific opinion on major paradigms such as mantle convection. Such topics are of great interest to much wider audiences than the specific academic field that carries out the research, as evidenced by frequent television 'documentary' broadcasts on subjects in the natural sciences/history in modern times. Major 'target' groups of the general public identified in the Impact Plan developed with this project are schoolteachers, pupils and tourists/visitors to the field localities studied.

3. What will be done to ensure that they have the opportunity to benefit from this research?
1. The project outcomes will be communicated to the metals industry at a major international forum: the 2013 International Precious Metals Institute Annual Conference to be held in Phoenix (USA). This meeting is the premier international forum for exchange of technical, financial, economic, environmental and other industry relevant information, and attendance of the PI at dedicated PGM sessions here will enhance the project impact on the industry sector. The PI's activity and interaction with the industry sector will be supported by informal biannual consultation with Dr P. du Pusain, (PGM Ore Geophysicist, Anglo American, Johannesburg, SA) so that dissemination of project outcomes to the industry sector is effective.
2. A website, hosted on the Keele Petrology Group's space on the Keele University server, will provide details to the general public about field localities, samples and project data and outcomes.
3. Insight gained into Earth evolution will be communicated to teachers and pupils at local Staffordshire schools via an Outreach Program organized by the PI. The program will run through the Staffordshire Science, Technology, Engineering and Mathematics Network (STEMNET), who provide professional assistance in the presentation of science to teachers and pupils at schools in a way that is interactive and 'curriculum-focused'. It is anticipated these activities will contribute to improving progression of children and teenagers to geoscience careers in the Stoke-On-Trent (North Staffordshire) area.
4. Posters and brochures will be designed for dissemination of geological knowledge and results (to tourists, the general public etc.) on site at the field localities. This will be achieved through liaising with natural heritage organisations (e.g., Scottish Natural Heritage), and will work particularly well for the Shetland Isles European Geopark.