The C, H2O and S budget of the deep crust (Ossola Valley, Ivrea-Verbano Zone)

The deep crust in the Ivrea-Verbano zone consists of underplated mafic and metasedimentary rocks. In the metasedimentary rocks, C in the form of residual carbonates and graphite, H₂O in hydrous phases and S in sulphides can potentially be stored for millions to billions of years. Yet, the budgets of these volatile elements in the deep crust are poorly quantified.

Drill cores obtained from the DIVE-project have been studied for the distribution of C-, H₂O- and S-bearing phases in the predominantly felsic metasedimentary upper portion of the lower crust. The drill core samples are mainly biotite-gneisses (75 vol-%, Qtz + Pl + Bt ± Grt ± Sil ± Kfs), metabasites (21 vol-%, Amp + Pl ± Qtz ± Grt ± Px ± Bt) and calcsilicate rocks (2 vol-%, Cc + Grt + Px + Ttn + Pl + Qtz ± Amp) which were metamorphosed at upper amphibolite facies conditions (720-770 ± 50°C at 7.4 ± 1.5 kbar). Varying degrees of partial melting have been observed in all lithologies. Graphite occurs in the matrix of biotite-gneisses and as inclusions in garnet. Moreover, carbonates are present in calcsilicate rocks and occasionally in metabasites. H₂O is hosted in biotite and amphibole and S occurs in sulphides.

C-N-S analyses revealed that the biotite-gneisses contain an average of 0.26 wt.-% C while the average values for the metabasites and calcsilicate rocks are 0.07 and 2.22 wt.-%, respectively. The majority of the water is stored in the biotite-gneisses (0.41 wt.-%) and metabasites (0.14 wt.-%). In contrast, the calcsilicate rocks contain negligible amounts of H₂O (< 0.004 wt.-%). S concentrations are highest in the biotite-gneisses (0.25 wt.-%) and metabasites (0.19 wt.-%) For the 578.7m deep borehole, overall concentrations reach an average of 0.23 wt.-% C, 0.55 wt.-% H₂O and 0.23 wt.-% S.

C mobilisation in the calcsilicate rocks likely occurred via garnet-forming decarbonation reactions (e.g. Zo + Hed + Cc --> Qtz + Grs + H₂O + CO₂) as well as via melt, which is indicated by the presence of calcite in leucosomes. FTIR spectra of apatite indicate varying concentrations of CO₂ which reach 1500 ppm in case of the calcsilicate rocks. Furthermore, apatites hosted by leucosomes contain several hundreds of ppm CO₂, indicating the presence of C-bearing hydrous melts. Therefore, devolatilisation reactions as well as partial melting result in a moderate transfer of C and H₂O from the deep crust to the upper crust. Nevertheless, metasedimentary rocks that reside at around 25 km depth at temperatures of ~750°C are an important reservoir for C, H₂O and S.