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

摘要

The Neoproterozoic Hekkelstrand Marble from the North-Central Norwegian Catedonides 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, delta(18)O, delta(13)C and Sr-87/Sr-86 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 delta(13)C (+4 parts per thousand V-PDB), delta(18)O (27 parts per thousand V-SMOW) and Sr-87/Sr-86 (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. Sr-87/Sr-86 ratios range between 0.70725 and 0.71034. The least-altered delta(13)C of +6 parts per thousand and delta(18)O of -30 parts per thousand suggest a high degree of isotopic preservation of the dolomite. The dolomite marbles are isotopically heavier (both delta(18)O and delta(13)C) by -2 parts per thousand 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. The Sr-87/Sr-86 ratio was modified by post-depositional processes. The high delta(13)C and delta(18)O values and the low Mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration. The highest Sr-87/Sr-86 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 in where C and 0 isotope systems were reset by up to -4.5 parts per thousand and -9.7 parts per thousand, respectively, the Sr-87/Sr-86 ratio increased by up to 0.0012, whereas the Fe, Mn and Sr concentrations have increased by factors of similar to 80, similar to 15 and similar to 1.5, respectively. The dolomite marbles, despite amphibolitc-facies polyphase metamorphic alterations, have preserved high delta(18)O and delta(13)C values, which can be utilised for chronostratigraphic purposes, whereas Sr-87/Sr-86 ratios provide insights into the composition of the post-depositional fluids. (c) 2004 Elsevier B.V. All rights reserved.