(U-Th)/He geochronology and chemical compositions of diagenetic cement, concretions, and fracture-filling oxide minerals in Mesozoic sandstones of the Colorado Plateau

摘要

Oxide minerals in diagenetic cements, concretions, and fracture fill reflect episodes of ancient groundwater flow that have the potential to record tectonic, geomorphic, and climatic changes through time. To better understand the ages, conditions of formation, and potential geologic significance of these diagenetic materials, we have measured (U-Th)/He ages and element concentrations in hematite, goethite, and Mn-oxide in Mesozoic sandstones from several locations in the Colorado Plateau. Most (U-Th)/He ages are Pliocene-Pleistocene, but some samples are as old as 25 Ma, the age of previously determined ~(40)Ar/~(39)Ar ages on Mn-oxide cement. In one region, texturally diverse Mn- and Fe-oxides yield relatively reproducible ages of ca. 2-3 Ma, and in another region, vitreous Fe-oxide fracture coatings yield ages of ca. 300 ka. Elsewhere, most cements and concretions show a wide range of ages among aliquots taken over millimeter length scales. Hematite-dominated samples show a broad positive correlation between age and U-Th concentrations, whereas most goethite-dominated samples show a negative correlation. Exterior portions of spherical goethite concretion rinds have higher U-Th concentrations and younger (U-Th)/He ages than interior portions. Taken together, the age-composition relationships of these samples suggest that wide ranges of ages in some samples largely reflect relatively recent (Pliocene-Pleistocene) U-Th addition, recrystallization, or later oxide growth that affected precursor cements that may have been oxides or other minerals. In some cases, these precursors may have formed as early as 25 Ma. In at least two areas, the (U-Th)/He dates (or in cases where samples show a wide range in ages, the minimum dates) are associated with major climate or local incision rate changes. We speculate that diagenetic oxides, and possibly other types of fine-grained secondary oxide minerals, remain open to U-Th uptake, recrystallization, or continued growth through contact with groundwater long after initial formation. This may provide opportunities to understand groundwater compositions and flow regimes as samples are exhumed through the shallow crust, or as surface conditions change through time in diagenetic systems in the critical zone of Earth's crust and on Mars.