USE OF CARBONATE CLUMPED ISOTOPE THERMOMETRY TO STUDY INTERACTIONS OF STRUCTURES AND FLUID FLOW, MOAB FAULT, PARADOX BASIN, UTAH

Interactions among fluids, deformation structures, and chemical changes in sediments impact deformation of the shallow crust, influencing the preservation and extraction of the economic resources it contains. These interactions have been studied along the Moab Fault, in the Paradox Basin, Utah, where diagenetic cements, joints and cataclastic deformation bands developed during faulting are thought to control fault permeability. Previous fluid inclusion micro-thermometry and stable isotope data from calcite cements collected along segments of the Moab Fault suggest cements precipitated from hot, deeply circulating meteoric fluids ascending the fault. However, these methods do not measure temperature directly below ~50ºC and cannot rule out cement growth from lower-temperature fluids.

We measured calcite cement growth temperature directly using clumped isotope thermometry of samples collected at varying distance from fault segments dominated by joints and by deformation bands. While cement temperatures along one joint-dominated segment range from 57±10 to 101±2ºC (2 SE), similar to previous estimates from fluid inclusion micro-thermometry, nearby segments reveal cement temperatures between 12±4 and 78±4ºC, demanding precipitation from both basinal and surficial fluid sources. The spatial pattern of cement temperatures suggests that deformation bands effectively compartmentalize fluid flow, restricting fluid sources to warm waters thermally equilibrated with the country rock in some areas, whereas intensely jointed zones associated with fault intersections enable rapid down-fault migration of cool surface waters. This interpretation was not possible based on conventional stable isotopic, textural, and fluid inclusion data alone; nevertheless, the δ¹³C values of the cements, the δ¹⁸O values of the waters from which the cements grew (calculated from clumped isotope temperatures and measured δ¹⁸O values of the carbonate), cathodoluminescence patterns, and the few available fluid inclusion micro-thermometry data are consistent with it. Our data confirm that the relationship between faults and fluid flow can vary greatly over short length scales, and suggest that some fracture zones can be highly conductive to depths as great as 2 km.