Volatile sources, sinks and pathways: A helium‑carbon isotope study of Baja California fluids and gases

The Baja California Peninsula is located within a complex geodynamic setting, marked by continental rifting immediately to the east in the Gulf of California, and abundant geothermal manifestations. We report helium and carbon isotopic and relative abundance data sampled from gas and fluid localities (n = 12) across northern Baja California, Mexico. Samples were collected in three distinct regions of Baja: Pacific Borderlands (PB) in northwestern Baja, Peninsula Ranges and Inland Valleys (PRIV) in central Baja, and the Gulf Extensional Province (GEP) in northeastern Baja. Helium isotopes (3He/4He), carbon isotopes (δ13CO2), and CO2/3He values all are highest in GEP samples, and lower in samples from the PB and PRIV. In the GEP, helium isotopes reach moderately mantle-like values (1.74 RA) (where RA = air 3He/4He), while in the PRIV and PB provinces values are more radiogenic (~0.11 RA). 3He/4He values suggest mixing between mantle-derived and crustal-derived radiogenic components, with a maximum mantle contribution of ~21% in the GEP samples. High He isotopes correlate with low mantle velocity zones at a depth of 25–40 km, suggesting a mantle source of He in the GEP, due to extensional crustal thinning. Carbon isotopes (δ13CO2) vary widely between −17.6 and 4.5‰ (vs. PDVB) and CO2/3He values vary over several orders of magnitude (2.0 × 105 to 1.1 × 1013). Variable δ13CO2 and CO2/3He values in the PRIV and PB localities are consistent with calcite precipitation in shallow-level (upper 5 km) hydrothermal systems. The bimodal nature of the He-CO2 data suggests fundamentally different transfer mechanisms in each of the three distinct regions, indicating complex interplay between regional mantle degassing, faulting-assisted migration of crustal fluids, and volatile sequestration via secondary processes (e.g., calcite precipitation). These results, along with literature data, allow for systematic assessment of both mantle sources and secondary processes, providing new insights into how volatiles are transported through the Baja California peninsula.