Quantifying geological CO2 accumulation in the subcontinental lithospheric mantle during post-rift evolution of a sedimentary basin (Bakony-Balaton Highland Volcanic Field, Pannonian Basin, Central Europe)

Abstract

The subcontinental lithospheric mantle (SCLM) is a potentially significant but poorly constrained carbon reservoir. In this study, the accumulation of CO2 in the SCLM beneath the Bakony–Balaton Highland Volcanic Field (BBHVF), Hungary, was quantified during the post-rift evolution of the Pannonian Basin (10 Ma - today). Using 164 upper mantle xenoliths with known sampling ages by the host basalts, and their equilibrium temperatures, their depth of origin was derived based on newly modelled paleogeotherms for 2, 4.5, and 8 Ma. Our findings demonstrate that peridotite xenoliths encompass the entire vertical range of the SCLM (34–69 km), indicating a depth dichotomy between equigranular textures, which are found at shallower depths (37–55 km), and protogranular textures, which occur at greater depths (52–68 km). In other words, the xenoliths proved to represent full-depth SCLM sampling. According to xenolith textures, a progressive deepening trend of protogranular xenoliths through time reflects lithospheric thickening during the post-rift phase. We estimate CO2 accumulation within newly accreted lithospheric mantle volumes using diverse density and CO2 content scenarios. The most probable scenario (2000 ppm CO2, 3.2 g/cm3 SCLM density) suggests an average CO2 accumulation rate of ∼3700–16,300 tons/year in the BBHVF area (25 × 40 km), with a total of ∼103 Gt over 10 million years. Our calculations showed that the contribution of deep mantle CO2 to carbon budget could be more significant than expected. Our results provide essential constraints on long-term carbon cycling within the evolving continental lithosphere, supporting a more accurate global carbon budget model.