The importance of diffusion, advection, and host-rock lithology on vein formation: A stable isotope study from the Paleozoic Ouachita orogenic belt, Arkansas and Oklahoma

摘要

More than 600 stable isotope analyses from veins and their metasedimentary host rocks from the Ouachita orogenic belt of Arkansas and Oklahoma provide an opportunity to study fluid-rock interaction processes associated with vein formation during deformation and low-grade regional metamorphism. The 18O values of vein quartz vary from 16.0 to 26.4, whereas coexisting host rocks have a greater range from 12.9 to 27.4. The oxygen isotopic compositions of quartz vein versus those of the coexisting host rocks follow an array described by 18O_{vein quartz} 18O_{whole rock} + , where 8–0.3(18O_{whole rock}). This relationship emphasizes the dependence of 18O values of vein quartz on host-rock oxygen isotopic composition. The term empirically monitors the difference between the quartz-water fractionation factor and the compositional dependence of the bulk-rock–water fractionation factor. Vein-quartz–host-rock 18O fractionations are 0 in chert, novaculite, quartzite, and siliceous shale and typically between 1 and 4 in sandstones and shales. In quartzite and sandstone units that are bounded by shales and associated with significant quartz-crystal deposits, vein-quartz–host-rock fractionations are often unusually large, near 7. Quartz-calcite oxygen isotope geothermometry indicates that veins from the Ouachita Mountains formed over a temperature interval of 100 °C, consistent with fluid-inclusion temperatures previously obtained from quartz crystals. Individual quartz veins are homogeneous, with <0.4 variation, for all vein orientations at all scales, even though vein formation occurred over a temperature interval in which quartz-water fractionation varies by 5. This homogeneity highlights the insensitivity of vein-quartz 18O values to temperature when veins form under rock-buffered conditions. The similarity between vein and host-rock 18O values in quartz-rich lithologies, and between vein and host-rock 13C values in calcite-bearing rocks, indicates that diffusion was an important mass-transport mechanism. The variability in 18O values between calcite-bearing veins and host rocks and large vein-quartz–whole-rock fractionations in some sandstones and quartzites indicates that advection also played a major role in mass transport associated with vein formation. This inference leads to the interpretation that veins from the Ouachita Mountains formed by a combined diffusion-advection process, whereby 18O and 13C from the host rock was transported into the veins with the assistance of a rock-buffered fluid on outcrop scales of 10–100 m.