2026 Theses Doctoral

Reconstructing Primary Mantle Carbon Isotopes: Methodological Developments and Applications to Intraplate Volcanism

Lee, Hyun Joo

Deep carbon cycling between Earth's surface and its interior is a fundamental regulator of long-term climate stability and mantle evolution. While volcanic systems act as essential geochemical windows into these deep carbon reservoirs, the primary carbon isotopic signatures (δ¹³C) of mantle-derived magmas are significantly overprinted by extensive CO₂ degassing during ascent. This process induces substantial gas-melt isotopic fractionation, masking the original source signals and leaving the efficiency of carbon recycling across different tectonic settings poorly constrained. Although melt inclusions trapped in early-formed minerals can preserve pre-eruptive volatile budgets, accurately restoring their primary δ¹³C values requires both a precisely constrained gas-melt fractionation factor and high-spatial-resolution analytical techniques.

This dissertation establishes an integrated experimental and analytical framework to overcome these obstacles and reconstruct primary mantle carbon signatures. First, the carbon isotope fractionation factor was constrained across a wide range of silicate melt compositions through high-temperature and high-pressure experiments. Second, the development of 31 new silicate glass reference materials for Secondary Ion Mass Spectrometry (SIMS) enabled high-precision (±1 ‰) in-situ δ¹³C analysis of micro-scale samples, providing the necessary resolution for melt inclusions.

Applying these advancements to natural systems reveals the significant impact of recycled carbon on mantle heterogeneity. In the Ross Island volcanic system, reconstructed primary δ¹³C signatures (-2.7 ± 0.5 ‰) indicate that recycled carbonates from ancient Gondwana subduction account for at least 46% of the parental melt's carbon budget. Furthermore, integrating data from the 2021 La Palma eruption with a compilation of global ocean island basalts (OIB) reveals that the primary δ¹³C of the OIB mantle source is variable, ranging from typical mantle values to isotopically heavier compositions. Together, these methodological developments provide the critical quantitative tools required to correct for degassing effects and accurately recover primary source signatures, enabling a more accurate assessment of the global deep carbon cycle.