Late diagenetic trace element remobilization in organic-rich black shales of the taconic foreland basin of Quebec, Ontario and New York

We report measured concentration values of 32 trace elements, including the fourteen rare earth elements (REE), in organic-rich black shales of the Late (Caradocian) Ordovician Utica Shale magnafacies of Quebec, Ontario and New York by an inductively coupled plasma mass spectrometer (ICP-MS). We report these analyses for selected shale samples from the three localities. These representative samples have similar mineralogical composition and organic carbon content. The time correlative shale units are of different thermal grades: Quebec (immature-T max 20° to 50°C), Ontario (mature and oil-bearing-T max ∼ 50° to 140°C) and New York (post-mature-T max greater than 200° C). The results show significant variation in trace element concentration (Sr, Th, U, Pb and the light and middle rare earth elements) with respect to the post-Archean Australian Shale (PAAS). The PAAS-normalized trace element abundance patterns, but not individual element concentrations, of these shales are similar to PAAS including negative Nb and Ta anomalies. There is a progressive decrease in several trace element ratios (Th/U, Th/Pb, Nb/U, Ce/Pb and Ce/Yb) with increasing thermal maturity, indicating that, during late diagenesis, there is light rare earth element (LREE) and Th depletion and U-enrichment. The trace element ratios of the immature samples are similar to NASC and PAAS values. The detailed results, however, suggest that U, Th, Pb and some of the REE mobilize during diagenesis. For example, (La/Sm) N and (La/Yb) N show progressive light REE (LREE) depletion with increasing thermal grade. The PAAS-normalized REE patterns of these shales show distinct differences such as a characteristic light REE depletion (ΣLREE average: 57 ppm) with increasing thermal grade. We relate the characteristic LREE depletion observed in these shales to the extent of burial diagenesis. The results of this study indicate that trace elements, particularly the LREE, U, Pb and Th can be remobilized during diagenesis. The trace elements migrate from the shales into the pore waters during or subsequent to illitization and related decomposition of organic matter. Those elements most affected by this interstitial diffusion such as the LREEs show characteristic depletions attributable to this process.