The magnesium isotope (δ26Mg) signature of dolomites

Dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments. Using well-established isotope systems (delta O-18, delta C-13, Sr-87/Sr-86), fluid inclusions and elemental data, as well as a detailed sedimentological and petrographic data set, we established the precipitation environment and subsequent diagenetic pathways of a series of Proterozoic to Pleistocene syn-depositional marine evaporative (sabkha) dolomites, syn-depositional non-marine evaporative (lacustrine and palustrine) dolomites, altered marine ("mixing zone") dolomites and late diagenetic hydrothermal dolomites. These data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios (delta Mg-26(dol)). Dolomite delta Mg-26 ratios documented here range, from -2.49 parts per thousand to -0.45 parts per thousand (delta Mg-26(mean) = -1.75 +/- 1.08 parts per thousand, n = 42). The isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites (-2.11 +/- 0.54 parts per thousand 2 sigma, n = 14). In comparing ancient compositions to modern ones, some of the variation is probably due to alteration. Altered marine (-1.41 +/- 0.64 parts per thousand 2 sigma, n = 4), and earliest diagenetic lacustrine and palustrine dolomites (-1.25 +/- 0.86 parts per thousand 2 sigma, n = 14) are less negative than sabkha dolomites but not distinct in composition. Various hydrothermal dolomites are characterized by a comparatively wide range of delta Mg-26 ratios, with values of -1.44 +/- 1.33 parts per thousand (2 sigma, n = 10). By using fluid inclusion data and clumped isotope thermometry (Delta(47)) to represent temperature of precipitation for hydrothermal dolomites, there is no correlation between fluid temperature (similar to 100 to 180 degrees C) and dolomite Mg isotope signature (R-2 = 0.14); nor is there a correlation between delta Mg-26(dol) and delta O-18(dol). Magnesium-isotope values of different dolomite types are affected by a complex array of different Mg sources and sinks, dissolution/precipitation and non-equilibrium fractionation processes and overprinted during diagenetic resetting. Further progress on the use of delta Mg-26(dol) as a proxy will require new theoretical and experimental data for Delta Mg-26(fluid-dol) that includes dehydration effects of the free Mg aquo ion versus fluid temperature. In ancient diagenetic systems, complex variables must be considered. These include fluid chemistry and physical properties, Mg sources and sinks, temporal changes during precipitation and post-precipitation processes including open and closed system geochemical exchange with ambient fluids. All of these factors complicate the application of delta Mg-26(dol) as proxy for their depositional or diagenetic environments. Nevertheless, the data shown here also indicate that delta Mg-26(dol) can in principle be interpreted within a detailed framework of understanding. (C) 2014 Elsevier Ltd. All rights reserved.