RELEASE OF MERCURY DURING CONTACT METAMORPHISM OF SHALE: IMPLICATIONS FOR UNDERSTANDING THE IMPACTS OF LARGE IGNEOUS PROVINCE VOLCANISM (Invited Presentation)

Elevated mercury (Hg) in sedimentary strata are a widely used tracer for assessing the relationship between large igneous province (LIP) activity and global environmental change. A key unknown in applying this proxy is the extent to which Hg was sourced from contact metamorphism of sedimentary rocks during sill emplacement or from gaseous emissions of the magmas. We investigate the Hg behavior during contact metamorphism of shales. We show loss of 80–99% of the sedimentary Hg in contact aureoles in four case studies covering the interactions around dykes, sills and plutons associated the High Arctic LIP (Sverdrup Basin, Canada), the Karoo LIP (South Africa) and the Skagerrak-centred LIP (Oslo Rift, Norway). A combination of geochemical data and thermal modelling around a dyke shows 33% Hg volatilization in the aureole at 265-300 °C. The other cases show similar behaviours with significant lowering of organic-bound Hg, more significantly in the innermost 60% of the contact aureoles. A small fraction of the Hg remains in the aureoles, and thermal desorption characteristics or ‘profiles’ (TDPs) for Hg during pyrolysis provide insight on sample-level Hg speciation. Our results demonstrate that differences in Hg speciation can be detected and quantified with TDPs. We hypothesize that gaseous Hg is transported out of aureoles during metamorphism, together with CH₄ and CO₂. Furthermore, we estimate the thermogenic Hg mobilization from Karoo LIP aureoles as 72-192 t per km³ of aureole, which is between 1-3 times higher than the estimated volumetric Hg release from Karoo magmas. When scaling our results to the size of the shale portions of the Karoo Basin affected by the LIP and a timescale of 100 kyr of sill emplacement, the average Hg flux is calculated to have been 78-207 t/y with maximum values up to ~300 t/y. The pulsed nature of intrusive volcanism suggests that this thermogenic Hg flux could have dominated LIP Hg emissions during periods of their life span. Our results demonstrate that the global Hg cycle can be significantly perturbed following LIP-scale sill emplacement into organic-rich sedimentary rocks and our quantification of the emissions based on source-rock analysis provides important information for independent interpretation of the sedimentary Hg record.