60th Annual Report on Research 2015

Carbonate cement, vein fill, and concretions along fault zones hold a record of geologic fluids and their paths through time. Our work applies the new carbonate clumped-isotope paleothermometer along with more traditional stable isotope and petrologic analyses to describe the source fluids and map the distribution of ancient fluid temperature in sandstones in exhumed fault zones in the Paradox Basin, southeast Utah. By making observations at selected sites along three faults, we are testing hypotheses regarding the origin of diagenetic fluids and the details of fault architecture in the transmission and compartmentalization of fluids.

Field work. An early start date permitted us to include two excursions in this first reporting period. In March 2014, we conducted a reconnaissance and sampling trip to the three Paradox Basin fault zones targeted by the proposal. In March and April 2015, Ph.D. candidate Keith Hodson returned for detailed sampling and mapping at key sites on the Moab Fault, with particular emphasis at Courthouse Junction fault intersection zone.

Samples were collected for petrographic analysis in thin section and for bulk- and clumped-stable isotope analyses. Samples from the 2014 season were selected for broad spatial coverage. Samples from 2015 were selected to represent the range of cementation styles associated with four classes of fault-related deformation structures at Courthouse Junction: thick and thin deformation bands, sheared thin deformation bands, and joints.

Analyses. Thin section observations were made under standard cross polarized light and cathodoluminescence (CL), in order to describe the sedimentological and structural context of carbonate cements . Both luminescent and non-luminescent cements are observed, suggesting two fluid sources with distinct trace element compositions. Both appear as pore-filling cements and vein fills, but only luminescent cement was observed in concretions. Coarsely crystalline luminescent cement commonly fills fractures and joints, infiltrating pore space in the surrounding host rock. Patches of thin deformation band material can be observed along the margins of some luminescent veins, suggesting the vein intruded a fractured thin deformation band. Non-luminescent carbonate is typically finely crystalline. Fractured thin deformation bands commonly contain non-luminescent carbonate veins. Non-luminescent cement also occurs as pore filling cements around thick and thin deformation bands.

We measured bulk carbon and oxygen stable isotope compositions on 41 individual samples. Measured δ¹³C values range from -5.24 to 3.96 ‰ PDB, and measured δ¹⁸O values range from -23.50 to 2.77 ‰ PDB (6.68 to 33.77 ‰ SMOW). δ¹³C and δ¹⁸O are positively correlated. Luminescent and non-luminescent carbonate cements have distinct δ¹³C and δ¹⁸O values. Twenty-one samples were analyzed for clumped isotope thermometry: Δ₄₇ values, representing averages of 2 to 3 repeat measurements, range from 0.587 to 0.775 ‰. The corresponding carbonate precipitation temperatures (TD₄₇) range from 1°C to 69°C, with standard errors between 1.4 and 7.5°C. The temperature ranges for luminescent and non-luminescent carbonate are distinct. Using the temperature dependence on δ¹⁸O fractionation between carbonate and parent fluid, luminescent carbonate source fluids average ~-15 ‰ and non-luminescent carbonate source fluids average ~-5 ‰.

Findings. Building on prior work by our group and others, we have unraveled the temporal and spatial connections between deformation structures and the carbonate cements they host. The geochemical analyses reveal two distinct carbonate cements: the first luminesces orange in CL, has lower δ¹³C and δ¹⁸O values and cooler TD₄₇. These cements are found in discrete fractures and joints at Courthouse Junction and other sites along the Moab Fault. The second cement is non-luminescent, has higher δ¹³C and d18O values and warmer TD₄₇. This cement fills pores in the sandstone and is associated with deformation bands. Precipitation temperatures for the two groups of carbonate cement are distinct, with little to no overlap. Textural evidence in the rock suggests that the temperatures reflect ambient rock temperature at the time of cement precipitation, thereby linking the structural and diagenetic history to the burial history of the rocks.

 Our data reveal approximate isotopic compositions for the two source fluids: a δ¹⁸O of -15 ‰ (SMOW) and δ¹³C of -3 ‰ (PDB) for luminescent carbonate and a δ¹⁸O of -5 ‰ and δ¹³C of -1 ‰ for non-luminescent carbonate. These observations support the conclusions of earlier studies at Courthouse Junction, which concluded that two fluid sources produced the observed carbonate cements, but for the first time we are able to identify the isotopic signatures of each source fluid.

Our ongoing work considers possible fluid source scenarios under the new isotopic constraints. For example, with temperature information from Δ₄₇, we find δ¹³C variations in the carbonate cements are consistent with contrasting hydrocarbon decomposition processes. Methanogenesis of organic matter, which is generally limited to temperatures below ~45°C, is produces carbonate with enriched δ¹³C signatures. Above ~45°C , thermocatalytic decarboxylation of the same organic carbon source can produce relatively depleted δ¹³C values. Positively correlated trends in δ¹⁸O complement these processes, reflecting changes in ambient temperature corresponding to each decomposition process.