Thermochronology of sandstone-hosted secondary Fe- and Mn-oxides near Moab, Utah: Record of paleo–fluid flow along a fault
Authors
Secondary Fe- and Mn-oxides are locally common near faults and fractures within sandstones of the Colorado Plateau in the form of cements, concretions, and fracture-fill material. However, little is known about how and why these oxides formed, and less is understood about when they formed. In this study, we integrated field observations, detailed scanning electron microscope and petrographic observations, and (U-Th)/He and 40Ar/39Ar dating to better understand the formation of Fe- and Mn-oxides from three fault zones in Flat Iron Mesa, Utah. Most Fe- and Mn-oxide (U-Th)/He ages range from 0.50 to 3.4 Ma, which are much younger than 40Ar/39Ar ages of 25–20 Ma from a previous study. Current 40Ar/39Ar ages on a Mn-oxide sample from this study yielded a plateau age of 3.6 ± 0.08 Ma. 4He/3He diffusion data from Fe- and Mn-oxides are consistent with the presence of multiple diffusion domains with varying He retentivity. Predicted fractional radiogenic He retention over 3.6 m.y. at near-surface temperatures for the bulk samples, with domain proportions of the diffusion experiments, is ∼90% for Fe-oxide and ∼45% for Mn-oxide. The multidomain behavior exhibited by the oxides and the variability observed in (U-Th)/He ages among aliquots are consistent with variable amounts of diffusive loss of He within dated aliquots. Using 4He/3He data and He bulk ages, step-age plots indicate a high fractional release plateau at ca. 3.6 Ma, concordant with 40Ar/39Ar dating and most of the oldest (U-Th)/He ages observed. Taken together, these results are most consistent with formation of Flat Iron Mesa Fe- and Mn-oxides near the surface (
Using bulk ages of 3.29 ± 0.03 Ma and 2.73 ± 0.02 Ma for Fe- and Mn-oxide, respectively, and He diffusion parameters, we created step age vs. cumulative 3He release fraction plots. (A) Fe-oxide plot shows more consistent 3He release patterns in which ∼75% of 3He released is ca. 3.6–3.0 Ma. (B) Fe-oxide best-fit effective diffusion temperature solution shows as minima in the mean square of weighted deviates (MSWD) misfit. (C) Mn-oxide plot shows variable 3He releases: ∼40% of 3He released has ages from 3.0 to 0.5 Ma, ∼50% of 3He released has age of 3.6–3.0 Ma, final ∼10% of 3He released has a more complicated release pattern from 5 to 0 Ma. (D) Mn-oxide best-fit effective diffusion temperature solution shows as minima in the MSWD misfit.
