This dataset includes ∂¹³C data of fluid inclusions hosted in lavas from St Helena.

St Helena, a small volcanic island in the South Atlantic Ocean, represents one of the classic examples of ocean island basalt (OIB) volcanism associated with intraplate mantle plumes. Although the island is geographically remote, it occupies a significant position in the geochemical classification of Atlantic OIBs due to its association with the so-called HIMU (high μ = high U/Pb) mantle reservoir. The isotopic and geochemical signatures of HIMU basalts are widely believed to reflect the long-term recycling of ancient oceanic crust, possibly subducted hundreds of millions of years ago and stored in the mantle before re-emergence as plume volcanism. One important aspect of understanding OIBs is the role of volatile components such as CO₂. Carbon is a critical tracer of mantle source regions because its concentration and isotopic composition (δ¹³C) can help distinguish between pristine mantle-derived volatiles and those derived from recycled sediments or altered oceanic lithosphere.

The present report examines six measurements of olivine separates from picritic samples collected on St Helena. These analyses, focusing on CO₂ contents, carbon isotopic ratios, and net pCO₂, provide a window into the volatile budget of the St Helena mantle source and allow comparison to global mantle values. The carbon isotopic composition shows significant variability. δ¹³C values span from –12.54‰ to –1.52‰. The canonical mantle δ¹³C range is considered to be around –4‰ to –8‰. The spread in δ¹³C values provides strong evidence for the involvement of recycled carbon in the mantle source. The most negative δ¹³C values (<8‰, which coincide with the lowest CO2 contents) fall well below the range expected for unmodified mantle, pointing to isotopic fractionation. Conversely, the least negative value (–1.5‰) suggests the presence of isotopically heavy carbon, potentially linked to marine carbonates recycled into the mantle. Such isotopic heterogeneity aligns with St Helena’s broader classification as a HIMU-type OIB, reflecting the long-term storage and reprocessing of subducted oceanic lithosphere. The HIMU reservoir sampled by St Helena is thought to be derived from ancient oceanic crust that has undergone extensive alteration and dehydration. The relatively low CO₂ contents observed in the olivines may therefore reflect a mantle source that is volatile-depleted relative to fertile peridotite or enriched plume1 sources. The isotopic spread suggests that while the bulk source is depleted, heterogeneities persist at the scale of melt production, allowing carbonate-rich components to contribute to magmatism.

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.