Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle

Mercury (Hg) is a toxic heavy metal with a well-studied surface geochemical cycle (atmosphere, hydrosphere, biosphere). However, its deep Earth cycling (crust and mantle) is less understood. Mercury ore deposits are primarily found in active continental margins, suggesting a link between Hg formation and plate subduction. Oceanic Hg could be transported into the mantle via subducting slabs and released during arc magmatism, forming Hg-rich deposits. A previous study indicated crustal Hg recycling into the lower mantle using Hg isotopes in OIBs. Hg exhibits both mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ199Hg, etc.). MIF of 199Hg and 201Hg is predominantly caused by photochemical processes and serves as a reliable source tracer. The primitive mantle is estimated to have near-zero Δ199Hg. Significant positive Δ199Hg in arc-related hydrothermal deposits suggests marine Hg recycling. The distinct isotopic compositions (Sr-Nd-Pb) of various basalts (MORBs, IABs, OIBs, CFBs) reflect mantle heterogeneity from crustal recycling. This study aims to investigate mantle Hg isotope heterogeneity and potential Hg recycling using a global-scale analysis of basalts.

Existing research extensively documents the surface geochemical cycle of mercury, highlighting its atmospheric transport, deposition into terrestrial and oceanic ecosystems, and its toxicity. However, the deep Earth cycling of mercury remains less well-studied. Studies focusing on mercury ore deposits have established a connection between their formation and plate subduction processes, suggesting the subduction of oceanic mercury into the mantle. Previous research utilizing mercury isotopes, particularly mass-independent fractionation (MIF) of 199Hg and 201Hg, has provided insights into mercury sources and transformations. The use of mercury isotope signatures in various basalt types (MORB, IAB, OIB, CFB) has helped to understand the degree of mantle heterogeneity due to recycled crustal material. This builds upon prior work showing positive Δ199Hg values in arc-related hydrothermal deposits as evidence of marine Hg recycling.

The study involved measuring the Hg isotopic compositions of a variety of basalts globally. Samples included MORBs from the Mid-Atlantic Ridge (MAR), Southwest Indian Ridge (SWIR), and East Pacific Rise (EPR); IABs from the Mariana Island Arc; HIMU-like and EM1-like OIBs from the Pako guyot; and CFBs from the Siberian Traps. The geological context of each sample is detailed in the supplementary materials. Total Hg (THg) concentrations were determined using an RA-915+ Hg analyzer. Hg isotope analysis was performed using a Neptune Plus multi-collector inductively coupled plasma mass spectrometry after sample preparation involving a double-stage tube furnace and aqua regia digestion. Mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ199Hg, Δ200Hg, Δ201Hg) were calculated using established conventions. Standard reference materials (GSR-2, BCR-2, NIST-3133, NIST-3177) were used for quality control. Loss on ignition (LOI) was measured to assess alteration effects. The analytical uncertainties for δ202Hg and Δ199Hg were 0.11‰ and 0.07‰ respectively. Analytical uncertainties for THg were <9%.

The study revealed significant variations in Hg concentrations and isotopic compositions across different basalt types. MORBs and IABs exhibited positive Δ199Hg values (up to 0.34‰ and 0.22‰, respectively), deviating from the primitive mantle estimate (0.00 ± 0.10‰). These positive Δ199Hg values strongly suggest the recycling of marine Hg into the mantle. In contrast, OIBs and CFBs showed near-zero Δ199Hg values, indicating that the recycling of oceanic Hg is predominantly observed in upper mantle-derived basalts. Significant variations in δ202Hg values were observed across all basalt types, ranging from -2.13‰ to 0.13‰. The variation in δ202Hg is attributed to mass-dependent fractionation during magmatic processes and/or mixing of Hg sources with different δ202Hg values. No significant correlation was observed between loss on ignition (LOI) and Δ199Hg, indicating that seawater alteration does not significantly affect Δ199Hg values. The Δ199Hg/Δ201Hg ratio of approximately 1 suggests photochemical reduction as the main cause of MIF in the studied basalts. The positive Δ199Hg values in MORBs and IABs are attributed to the incorporation of marine Hg during subduction. The study highlights the contribution of subducted marine sediments as a significant source of Hg in IABs. The relatively low Δ199Hg values in OIBs and CFBs suggests that the amount of recycled Hg in the lower mantle is likely small, compared to the total Hg pool.

The findings of this study provide compelling evidence for large-scale translithospheric Hg recycling via plate tectonics. The positive Δ199Hg values in MORBs and IABs directly support the recycling of marine Hg into the upper mantle, likely via subduction. The near-zero Δ199Hg values in OIBs and CFBs suggest a less significant role of recycled Hg in the lower mantle. The difference in Hg isotopic signatures between upper and lower mantle-derived basalts highlights the significant heterogeneity of Hg isotopes within the Earth's mantle. The use of Hg-MIF (Δ199Hg) proves to be a valuable tool for tracing crust-mantle interactions. The results reconcile previous conflicting observations about mercury cycling in the mantle. This study provides valuable insights into the complex interplay between surface and deep Earth processes related to Hg cycling and the use of mercury isotopes as powerful tracers in geodynamic studies.

This study demonstrates significant mantle Hg isotopic heterogeneity and provides robust evidence for large-scale translithospheric Hg recycling via plate tectonics. The positive Δ199Hg values in MORBs and IABs highlight the importance of marine Hg recycling into the upper mantle. While some recycling to the lower mantle is suggested, its contribution appears relatively minor. Hg-MIF (Δ199Hg) is shown to be a useful tracer for studying crust-mantle interactions. Future research should focus on expanding the dataset of OIBs and CFBs from diverse locations to further constrain the extent of Hg recycling in the lower mantle and refining the models of Hg cycling through Earth's interior.

The study’s interpretations rely on the assumption that the Hg isotopic composition of the samples accurately reflects their source regions and hasn't been significantly altered by post-eruptive processes. While efforts were made to minimize the impact of alteration, the possibility of subtle changes in isotopic ratios cannot be completely excluded. The number of OIB and CFB samples analyzed, while substantial, might not fully represent the entire range of compositional variability in these rock types, potentially affecting the conclusions about lower mantle Hg recycling. Further research involving more extensive sampling is recommended to enhance the robustness of the findings. The study's conclusions on the relative proportions of recycled Hg in the upper and lower mantle are tentative and could benefit from further investigations using refined models.