Carbon Storage and Isotopic Variability in the Deep Crust of the Ivrea-Verbano Zone

The lower continental crust is a critical component of the deep carbon cycle, serving as a long-term reservoir for carbon (C) in the form of residual carbonates and graphite. Yet, the extent of C storage remains poorly understood. The Ivrea-Verbano Zone in northern Italy exposes lower crustal mafic and metasedimentary lithologies, providing a unique natural laboratory to investigate C retention. As part of the ICDP-funded DIVE project (Drilling the Ivrea-Verbano Zone), this study focuses on quantifying the C budget and exploring the isotopic composition of C phases.

The upper portion of the lower continental crust (borehole 5071-1_B, Ornavasso) consists primarily of felsic metasedimentary rocks (kinzigites, 73 vol-%) alongside amphibolites (13 vol-%) and calcsilicate rocks (11 vol-%), metamorphosed under upper amphibolite facies conditions (~750 ± 50°C, 7.5 ± 1.5 kbar). C is hosted in the form of graphite (Gr) and calcite (Cc). Gr occurs as inclusions in garnet and in the matrix of kinzigites, while Cc is observed in calcsilicate rocks and occasionally in amphibolites and leucosomes. Notably, a single marble layer has been identified.
The isotopic composition of C (Gr) and C-O (Cc) is being investigated to provide insights into the origin and evolution of C. Preliminary results range from -11.8 ‰ to -13.8 ‰ δ¹³C_Gr  in the kinzigites and -0.7 to ‰ to -5.6 ‰ δ¹³C_Cc as well as 11.0-15.0 ‰ δ¹⁸O_Cc in the calcsilicate rocks.The marble layer from the borehole exhibits δ¹³C- and δ¹⁸O-values of -8.22 ‰ and 12.5‰, respectively, while marbles from nearby outcrops show a broader range of -0.7 to 1.19 ‰ δ¹³C_Cc and 13.6 to 22.6 δ¹⁸O_Cc. The isotope data supports the field observations suggesting that the sequence formed at the surface before burial and metamorphism.

Two sampling approaches were employed to determine the average and the local variability of C concentrations. (i) A broad approach, where samples of 6-12 cm length were taken from each rock type at approximately 10-meter intervals throughout the entire borehole, providing a comprehensive overview of the C distribution across different lithologies. Carbon-Nitrogen-Sulfur analyses from this approach revealed that kinzigites contain an average of 0.26 wt.-% C, while amphibolites and calcsilicate rocks average 0.07 and 0.73 wt.-% C, respectively. For the 578.7m deep borehole, the overall carbon concentration reaches an average of 0.23 wt.-%. (ii) A complementary microbulk sampling approach was specifically designed compare the variability across different scales, which is especially relevant in heterogeneous rock types. This method involves extracting core segments perpendicular to the foliation and subdividing them into centimetre-scale slices to capture fine-scale heterogeneities. While detailed results from the microbulk approach are pending, preliminary observations reveal notable intra-rock variability in C content. For example, a single kinzigite segment analysed using the microbulk approach covers the range in which 63% of the total number of kinzigite samples from the more extensive broad approach dataset are contained (n=27).

Our findings underline the importance of metasedimentary rocks at lower crustal depths (~25 km, ~750°C) as C reservoirs which enhances our understanding of the carbon cycle in deep crustal environments.