Magnesium isotopes in high-temperature saddle dolomite cements in the Lower Paleozoic of Canada

Mg isotopes are used to better understand the genesis of hydrothermal saddle dolomite cements in Lower Paleozoic successions in Canada. These cements occur in fault-bounded dolostones that overlay lithologically diverse basement rocks; Ordovician dolomite lies over the Precambrian craton, whereas the Silurian and Devonian dolomites overlay a succession of tectonically accreted sedimentary, volcanic and ultramafic units of Cambrian to Ordovician age.

Lower Silurian saddle dolomites have the most negative δ²⁶Mg_DSM3 values of our dataset (-3.25 to -1.13‰), and plot in two distinct groups: a strongly negative subset that characterizes higher temperature (175°C) dolomites, and a less negative subset for lower temperature (153°C) dolomites. Upper Ordovician saddle dolomites precipitated at significantly lower temperatures (102°C), and their δ²⁶Mg_DSM3 values range from -1.26 to -0.71‰. Lower Devonian saddle dolomites formed at very high temperature (350°C) and have δ²⁶Mg_DSM3 values ranging from -1.29 to -0.78‰.

No experimental data on high temperature (100-350°C) fluid-dolomite Mg isotope fractionation factors have been published, and recent research suggests that no significant fractionation occurs between diagenetic fluids and dolomites at high temperatures in closed to semi-closed diagenetic systems. Our results indicate that the isotopic signature of diagenetic fluid is the primary control for the δ²⁶Mg_DSM3 values in these high-temperature dolomites.