CHARACTERIZING FAULT ZONE FLUIDS FROM CARBONATES USING STABLE ISOTOPES AND CLUMPED ISOTOPE PALEOTHERMOMETRY IN NORMAL FAULTS OF CENTRAL ITALY

Fluid-rock interactions and the record of fluid migration in fault zones can be observed in the study of carbonate precipitates using stable isotopes. Fault zone architecture constrains fluid migration, in some cases enhancing permeability parallel to the fault in the damage zone, but inhibiting fluid migration across the fault core. Carbonate veins, cements, and cataclasites are the result of the coevolving relationship between faulting and fluids in fault zones, preserving information on spatial reach of specific fluids as well as compositions and temperature during mineralization. We analyzed carbonate fault rocks from three normal fault zones (Fucino, Roccacasale, and Campo Imperatore) in central Italy, where faulting has juxtaposed Mesozoic-Cenozoic carbonate rocks with Quaternary basin sediments. Prior work showed evidence for isotopically distinct carbonate cements in the fault damage zone vs. the fault core of normal faults in this region and predicted distinct temperatures for the fluids, with warm groundwater in the footwall damage zone partitioned from cooler meteoric water in the fault core and hangingwall. We sampled calcite veins, vugs, cements, and cataclasites from polished samples from the fault core and damage zones of each fault. Carbon and oxygen isotope analyses on these new samples are consistent with the findings of earlier studies: Cements from the fault core have a range of isotopic values (δ¹³C -5 to +2 ‰ VPDB; δ¹⁸O 23 to 30 ‰ VSMOW) that overlaps at the lower end with published values for cements from within the Quaternary basin sediments, suggesting precipitation from meteoric water. Carbonate from the fault damage zones and calcite from all sampled veins have a range of isotopic values similar to the host rock (δ¹³C 0 to +3 ‰ VPDB; δ¹⁸O 28 to 31 ‰ VSMOW), suggesting precipitation from a fluid in isotopic equilibrium with the host rock. Initial findings from clumped isotope (∆47) analyses of samples from the Campo Imperatore fault suggest that the carbonate in the damage zone precipitated from warmer fluids (~39°C) than those in the fault core (~22°C), lending support to the earlier hypothesis.