Effects of diagenesis on the Nd-isotopic composition of black shales from the 420 Ma Utica Shale Magnafacies

Abstract

The Utica black shales were deposited in the Taconic Foreland basin ∼ 420 Ma ago. The organic matter in these shales is of marine origin and the timing of deposition of these shales has been constrained by graptolite biostratigraphy and bentonite chronostratigraphy. Rare earth element (REE) concentrations and Nd-isotopic ratios were measured in whole rock black shales with different grades of thermal maturity from the Utica Shale Magnafacies of Quebec (immature), Ontario (mature), and New York (post mature). These measurements were also made in the organic, carbonate, and sulfide fractions of the shales, that were isolated by sequential leaching. The purpose of this study is to understand how diagenesis (thermal maturation) may affect the Nd-isotopic compositions of different components as well as the whole rock black shales. Bulk rock REE concentrations of the Utica black shales are similar to average shales. However, these rocks show large range in 147Sm/ 144Nd (0.08–0.19) indicating diagenetic mobilization of the light-REE (LREE). Although thermal maturation of black shales may mobilize all LREE, our data indicate that mobility of La relative to Sm is greater in the mature shales compared to the post mature shales. In post mature shales, Nd is more mobile relative to Sm. Immature shales, however, show wide ranges in Sm/Nd and La/Sm that cannot be explained solely by diagenesis, implying source heterogeneity. Whole rock black shales and their isolated components show overlapping but characteristically different Nd-isotopic compositions. If the present day Nd-isotopic compositions of black shales are mostly an artifact of diagenesis (thermal maturation), which alters the Sm/Nd ratio of the rock, it can be argued that the different components of black shales are affected to different degrees for a given diagenetic process or different diagenetic processes affect the different components. Alternatively, the varying Nd-isotopic compositions of the shale components may reflect differences in provenance of the different shale components. Nd-isotopic composition of mature and post mature black shales of the Utica Magnafacies are controlled by thermal maturation of these rocks, as indicated by the correlation of ε Nd(0) with various LREE ratios such as, La/Sm and Sm/Nd. However, the observed variations in the Nd isotopic composition of immature shales dominantly reflect source heterogeneity. This is also supported by the correlation of ε Nd(0) and immobile heavy REE ratios, such as Dy/Yb, which reflect source compositions. The average ε Nd(0) compositions of the whole rocks and separated components vary with increasing maturity, the post mature samples showing the least radiogenic Nd and the mature samples with the most radiogenic Nd isotopic compositions. The initial Nd-isotopic compositions at 420 Ma of the Utica black shales overlap with the global seawater Nd-isotopic composition at that time. This similarity is observed for the samples with Sm/Nd ratios similar to average shales, suggesting no diagenetic imprint, and that the Taconic Foreland basin hosting the Utica Shale Magnafacies was probably connected to the global ocean during the time of deposition. Depleted mantle Nd model ages of the Utica black shales and their different components increase with increasing Sm/Nd. However, these model ages range from 1.6 to 2.3 Ga (average 1.9 Ga) for the samples with Sm/Nd similar to average shales. These ages indicate that the Taconic basin sediments of the Utica Shale Magnafacies were sourced from pre-Grenville-age rocks.