Carbon isotopes in OIB Mantle source

The Atlantic Ocean hosts a number of intraplate volcanic islands that provide crucial information on mantle plume dynamics, volatile budgets, and the role of recycled components in the deep Earth. Among these islands, Sal (Cape Verde archipelago) and Madeira represent contrasting examples of ocean island basalt (OIB) magmatism. Both are situated above mantle domains associated with plume activity, yet they differ in age, geochemical signature, and eruptive history. Investigating volatile contents, specifically CO₂ concentrations and carbon isotope ratios (δ¹³C), is essential to constraining mantle source characteristics and understanding processes of melt generation and ascent. Analyses of olivine separates from Madeira, including wehrlite, dunite, and harzburgite, reveal CO₂ concentrations in the range of 4.1 × 10⁻⁷ to 9.8 × 10⁻⁷ mol/g, averaging approximately 6 × 10⁻⁷ mol/g. These values are high compared to other Atlantic islands such as the Canary Islands, underscoring Madeira’s volatile-rich character. The δ¹³C values are tightly clustered between –1.3‰ and –2.9‰, well above the canonical mantle range of –4‰ to –8‰. Together, these results indicate a mantle source with abundant carbon and minimal isotopic heterogeneity. By contrast, Sal samples yield distinctly lower CO₂ concentrations and more variable isotopic compositions. The reduced CO₂ contents highlight a volatile-poor character, while the δ¹³C signatures deviate from mantle norms, suggesting the influence of recycled crustal carbon components. Although CO₂ values are not as elevated as those of Madeira, they nonetheless record measurable volatile release. Madeira’s volatile-rich nature, coupled with its narrow isotopic range, points to a mantle source containing abundant primordial or recycled carbonates. The heavy δ¹³C values, less negative than typical mantle, may suggest carbonate recycling or limited isotopic fractionation during degassing. The relatively high CO₂ concentrations and pCO₂ values further support the interpretation of a fertile, volatile-bearing mantle beneath Madeira. Sal, in contrast, reflects a more depleted mantle environment. The lower volatile concentrations may be the result of prior melt extraction, leaving behind a refractory source, or efficient degassing during magma ascent. Despite the above, the isotopic heterogeneity, with excursions beyond the canonical mantle range, also suggests the contribution of recycled crustal carbon. This aligns with geochemical models of the Cape Verde archipelago, where variable plume components and lithospheric interactions are thought to generate the wide isotopic range observed across islands. This publication results from work conducted under the transnational access/national open access action at INGV-Palermo- Stable Isotope laboratory supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005.