Hydrothermal diagenesis of Paleozoic seamount carbonate components

We present geochemical results of Paleozoic carbonate components (brachiopods—biogenic low-Mg calcite: bLMC, crinoids—biogenic high-Mg calcite: bHMC and whole rock—admixture of aragonite and calcite: wrAC) from open ocean Permo-Carboniferous seamounts of Japan situated in the tropical mid-Panthalassic Ocean. The diagenetic history of these and other carbonate allochems was determined by visual/optical inspection of their external morphology and luminescence, by evaluation of their microstructures, and by assessment of their trace element and stable isotope distributions. The degree and trends of the diagenetic alteration tests were then reconciled with their concomitant Sr isotope values and compared to those of the established seawater–Sr isotope curve. The altered carbonate material from three of the four time slices support the novel concept of diagenesis in the presence of marine/hydrothermal fluids leading to less radiogenic Sr isotope values. In addition, Sr isotope values of unaltered bLMC from three of the four time slices do not concur with the values for the concurrent time of the established seawater 87Sr/86Sr curve. An adjustment of their stratigraphic position is untenable and unsatisfactory in light of their biostratigraphic assignment, which suggests that Sr isotope diagenesis is much more complex and complicated than is espoused in the literature. Consequently, the concept of less radiogenic Sr isotope values in diagenetic marine carbonates has profound implications for the veracity of the established seawater 87Sr/86Sr profile and curve, and it needs to be re-evaluated in light of the findings.

Research Highlights
► Diagenetic history of Paleozoic carbonate allochems is investigated using multiple screening tests.
► This is a first presentation of trace chemistry and C, O, Sr isotopes of unaltered and altered carbonates from seamount Japan in the Panthalassic Ocean during the Paleozoic.
► Detailed evaluation suggests that alteration of carbonate allochems occurred in the presence of mixed fluids/water.
► These mixed fluids were either: marine/meteoric water or marine water/hydrothermal fluids.
► This has important implications for constructing seawater–87Sr curves.