Mercury efficiently volatilized but not completely removed from sediments around igneous intrusions

The impact of large igneous province (LIP) volatile emissions from magmatic and thermogenic sources has been widely studied using sedimentary mercury (Hg). Igneous sill emplacement in sedimentary basins was a key component of several LIPs, generating environmentally significant amounts of thermogenic gases (e.g., CH4, CO2, Hg) and fluids by heating sedimentary rocks. Understanding these processes is key to realizing the promise of Hg in sedimentary archives to understand LIP impacts and track LIP carbon outputs. Published data show sedimentary rocks in contact aureoles of such LIP-related intrusions retain some Hg despite exposure temperatures above the stability limit of sedimentary Hg phases (>300 °C). We examine this unexpected Hg retention using Hg thermal desorption profiles (TDPs). The TDPs reveal a remarkable pattern in Hg speciation related to intrusions: with increasing aureole temperature, Hg release is progressively dominated by a low-temperature phase that could have formed only during or after cooling of the intrusion. We calculate a temperature-dependent Hg volatilization efficiency and show that 70%−100% of Hg was mobilized above 350 °C. Despite the efficient volatilization, only ∼50% of the volatilized Hg was removed from the studied aureole. Mercury recapture in and around the thermal aureole likely extends into the associated hydrothermal vent systems, reduces the Hg:C ratios of emitted thermogenic gases, and may lead to underestimation of Hg-based LIP thermogenic CO2 and CH4 emissions estimates and their environmental impact.