Noble gases preserve history of retentive continental crust in the Bravo Dome natural CO₂ field, New Mexico

Budgets of ⁴He and ⁴⁰Ar provide constraints on the chemical evolution of the solid Earth and atmosphere. Although continental crust accounts for the majority of ⁴He and ⁴⁰Ar degassed from the Earth, degassing mechanisms are subject to scholarly debate. Here we provide a constraint on crustal degassing by comparing the noble gases accumulated in the Bravo Dome natural CO₂ reservoir, New Mexico USA, with the radiogenic production in the underlying crust. A detailed geological model of the reservoir is used to provide absolute abundances and geostatistical uncertainty of ⁴He, ⁴⁰Ar, ²¹Ne, ²⁰Ne, ³⁶Ar, and ⁸⁴Kr. The present-day production rate of crustal radiogenic ⁴He and ⁴⁰Ar, henceforth referred to as ⁴He^* and ⁴⁰Ar^*, is estimated using the basement composition, surface and mantle heat flow, and seismic estimates of crustal density. After subtracting mantle and atmospheric contributions, the reservoir contains less than 0.02% of the radiogenic production in the underlying crust. This shows unequivocally that radiogenic noble gases are effectively retained in cratonic continental crust over millennial timescales. This also requires that approximately 1.5 Gt of mantle derived CO₂ migrated through the crust without mobilizing the crustally accumulated gases. This observation suggests transport along a localized fracture network. Therefore, the retention of noble gases in stable crystalline continental crust allows shallow accumulations of radiogenic gases to record tectonic history. At Bravo Dome, the crustal ⁴He^*/⁴⁰Ar^* ratio is one fifth of the expected crustal production ratio, recording the preferential release of ⁴He during the Ancestral Rocky Mountain orogeny, 300 Ma.