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Mercury is a significant volcanic volatile species from effusive and explosive activity1. Understanding its emission to the atmosphere from volcanic activity, aids our understanding of the global mercury cycle and its environmental impacts. Sedimentary and ice core records can be archives of these mercury enrichments2,3.The Laki 1783-84 AD fissure eruption in Iceland was significant due to its voluminous outpouring of basaltic lava, copious sulphur emissions and widespread environmental effects locally and across the Northern Hemisphere4,5. Extreme weather events were recorded in Europe and North America, owing to a veil of sulphur dioxide that remained at the tropopause for over a year5. Due to the phreatomagmatic and thus explosive nature of Laki, a significant eruption plume was produced4. As such, cryptotephra shards have been located at distal locations from Iceland including ice cores in Svalbard and Greenland6,7 and in an Irish woodland peat at Brackloon Wood, Co. Mayo8. There is evidence to suggest significant heavy metal emission to the atmosphere during the Laki eruption, however these records are currently restricted to Greenland ice cores9. Previous heavy metal findings linked to Laki do not include mercury, despite its significance as a volcanic volatile, and a potentially environmentally damaging heavy metal. Therefore, to expand our knowledge of the Laki 1783-84 AD eruption plume, its associated emissions, and environmental consequences we returned to the woodland peat site in Brackloon Wood, Co. Mayo, Ireland.Analysis of a 50 cm peat core using an AMA 254 mercury analyser was combined with a novel technique to find tephra using BSE (back scattered electron) imaging and geochemical discrimination using SEM-EDX (scanning electron microscopy-energy dispersive x-ray). The Laki tephra is successfully located using this method and coincides with a visible enrichment in mercury relative to background concentrations and organic matter. An age-depth model developed using the tephra layer and two radiocarbon dates indicate a strong likelihood that such enrichments are a product of volcanic emission. Such a finding can expand our understanding of heavy metal deposition during Laki 1783-84 AD away from the poles and to our knowledge, demonstrates the first direct exploration of mercury enrichment in distal peat for this eruption. As a secondary test of volcanic volatile enrichment, trace element analysis of the same bulk peat will be conducted to explore enrichments in other volcanic volatiles such as sulphur, cadmium, lead, copper and zinc. 1. Pyle, D. M. & Mather, T. A. Atmos. Environ. 37, 5115–5124 (2003).2. Schuster, P. F. et al.,. Environ. Sci. Technol. 36, 2303–2310 (2002).3. Roos-Barraclough, F. et al.,. Earth Planet. Sci. Lett. 202, 435–451 (2002).4. Thordarson, T. & Self, S. Bull. Volcanol. 55, 233–263 (1993).5. Thordarson, T. & Self, S., J. Geophys. Res 108, 4011 (2003).6. Kekonen, T. et al., Polar Res. 24, 33–40 (2005).7. Wei, L. et al., Geophys. Res. Lett. 35, L16501 (2008).8. Reilly, E. & Mitchell, F. J., Holocene 25, 241–252 (2015).9. Hong, S. et al., Earth Planet. Sci. Lett. 144, 605–610 (1996).

The persistence of Archaean cratons for >2.5Ga was aided by thick, mechanically strong, and cool lithospheric mantle keels up to 250km deep. It is widely accepted that the cratonic mantle, dominated by depleted harzburgite, lherzolite and dunite, was formed by extensive melt extraction from originally fertile mantle peridotite. Models seeking to explain the formation of deep cratonic mantle in the garnet and diamond stability fields, initially sought to answer how such rocks could form in-situ at high temperatures and pressures and envisaged large-scale thermochemical plume upwellings. More recently, mineralogical and geochemical observations, namely the high Cr content of garnet and low whole rock HREE concentrations in cratonic harzburgites, have led to the conclusion that the deep cratonic mantle couldn’t have originally melted in the garnet stability field. Mechanical stacking of shallowly depleted oceanic lithosphere was therefore proposed to have thickened the depleted lithosphere cratonic roots. In this process, the spinel facies minerals are envisaged to transform into the garnet stability field.Here we present the first results of combined thermodynamic and geochemical modelling at temperatures high enough to reconcile the very refractory residues. We found that the requirement for initially shallow melting is no longer supported. Deep (150-250km), ultra-hot (>1800°C), incremental melting can produce the mineralogical and geochemical signatures of depleted cratonic harzburgites. The modelling also implies a link between areas of extreme depletion in the deep lithospheric mantle and the genesis of Earth’s hottest lavas (Al-enriched komatiite) by re-melting depleted harzburgite. Diamond inclusion minerals have a well-documented skew to the most refractory compositions found in cratonic peridotite. We propose that these ultra-depleted, highly reducing regions of the lithospheric root possess the highest diamond formation and preservation potential.