Controls on formation and alteration of early diagenetic dolomite: A multi -proxy delta Ca-44/40, delta Mg-26, delta O-18 and delta C-13 approach

摘要

The full potential of the dolomite Ca isotope proxy only unfolds when combined with data of the other main elements (C, O, Mg) in the crystal lattice of Mg -carbonates. Data presented here reveal the level of complexity inherent to dolomite pre- cipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation. Well -constrained Precambrian to Pleistocene dolomites were investigated, representing three characteristic formation and alteration environments: (i) sabkha, (ii) altered marine and (iii) lacustrine/palustrine dolomites. Primary sabkha dolomites with typically low cation ordering degree (COD), high delta C-13 and low delta Ca-44/40 values contrast with recrystallized sabkha dolo- mites with relatively high COD, low delta C-13 and high delta Ca-44/40 values. Both delta C-13 and delta Ca-44/40 values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium Ca isotope fractionation conditions. Primary sabkha dolomites display Delta Ca-44/40 (dolomite-fluid) ranging from -0.4 to -1.3 parts per thousand, whereas recrystallized dolomite is approaching isotopic equilibrium (Delta(44/40) Cadolomite-fluid similar to 0 parts per thousand). Using the fractionation factor deduced from the sabkha dolomite data set, recrystallized Precam- brian dolomite points to a Delta Ca-44/40(seawater/pore fluid) of about 1.2 parts per thousand (SRM 915a), a value that is distinctively lower compared to previously suggested ones and modern seawater. Altered marine dolomites display evidence for meteoric overprint, indicated by delta O-18 values as low as -4.62 parts per thousand. Both Mg and Ca isotope signatures correlate with delta 18 O values of altered marine dolomites, whereas delta C-13 values lack correlation with the other isotope systems. We propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and Ca, Mg and O isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint. Lacustrine/palustrine dolomites display a correlation between the isotope values of C, Mg and Ca. These dolomites are formed during pulses of marine ingression in swamp, playa and lake environments and are thus charac- terized by water-logged conditions (anaerobic) and saline, sulfate -rich fluids. Bacterial -sulfate reduction induces dolomite for- mation and leads to lower delta C-13 and Delta Ca-44/40 as well as to higher delta Mg-26 values. The Ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested. Taking COD and dolomite stoichiometry into consid- eration, the here documented multi -proxy isotope approach is promising and provides benchmarks against which proxy sig- nals can be calibrated and tested. Interestingly, even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research. (c) 2020 Elsevier Ltd. All rights reserved.