Geochemical preservation potential of high-grade calcite marble versus dolomite marble: implication for isotope chemostratigraphy

The Neoproterozoic Hekkelstrand Marble from the North-Central Norwegian Caledonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit, which underwent polyphase amphibolite-facies, Scandian metamorphism. Major and trace element geochemistry, δ18O, δ13C and 87Sr/86Sr ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post-depositional isotopic resetting. A high strontium concentration in the calcite marble (up to 1200 ppm) suggests an aragonitic lime mudstone precursor, and its least-altered δ13C (+4‰ V-PDB), δ18O (27‰ V-SMOW) and 87Sr/86Sr (0.70615 to 0.70624) ratios are consistent with 700-600 Ma seawater. The dolomite marbles contain minor relics of the calcite (aragonite?) precursor, despite pervasive dolomitisation. The average Mn (44 ppm) and Fe (1041 ppm) contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non-metamorphosed dolostones of a similar Neoproterozoic age. The dolomite marbles have an average Sr content of 115 ppm. An average Mg/Ca ratio of 0.56 is lower than that for stoichiometric dolomite (0.62) and is controlled mainly by remnant inclusions of precursor calcite. 87Sr/86Sr ratios range between 0.70725 and 0.71034. The least-altered δ13C of +6‰ and δ18O of ∼30‰ suggest a high degree of isotopic preservation of the dolomite. The dolomite marbles are isotopically heavier (both δ18O and δ13C) by ∼2‰ on average compared with the calcite marble, thus suggesting a pre-metamorphic, lower-temperature, isotopic equilibrium. The dolomite was apparently originally precipitated from 600 to 700 Ma seawater-dominated fluids. The87Sr/86Sr ratio was modified by post-depositional processes. The high δ13C and δ18O values and the low Mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration. The highest 87Sr/86Sr ratio of 0.70868 suggests that the earlier dolomite was apparently modified by ca. 560 Ma seawater or by meteoric water considerably buffered by ca. 560 Ma limestones. The most significant geochemical and isotopic modification of the dolomite took place during metamorphism. This has been observed across the dolomite marble-silicate rock contacts over a limited distance of <0.5 m where C and O isotope systems were reset by up to −4.5‰ and −9.7‰, respectively, the 87Sr/86Sr ratio increased by up to 0.0012, whereas the Fe, Mn and Sr concentrations have increased by factors of ∼80, ∼15 and ∼1.5, respectively. The dolomite marbles, despite amphibolite-facies polyphase metamorphic alterations, have preserved high δ18O and δ13C values, which can be utilised for chronostratigraphic purposes, whereas 87Sr/86Sr ratios provide insights into the composition of the post-depositional fluids.