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Magnesium Isotopes as a New Research Tool for Tackling the "Dolomite Problem"

Denis Lavoie¹, Simon Jackson², and Isabelle Girard²
¹Geological Survey of Canada, Quebec City, QB, Canada
²Geological Survey of Canada, Ottawa, ON, Canada

Dolomitization can result from early near synsedimentary to late in the deep burial realm. In many cases, conventional research tools cannot provide a definitive answer to the origin of the dolomite. The applicability of Mg isotopes to generate ideas for the origin of the dolomite and the source of Mg+2 is presented with an initial focus on hydrothermal saddle dolomite cements and potential Mg source rocks in the Lower Paleozoic successions in eastern Canada. The saddle dolomite cements occur in fault-bounded dolomite geobodies that overly diverse basements; the Ordovician cases occur over the Precambrian craton in southern Quebec and Hudson Bay (northern Canada) areas whereas the Silurian and Devonian cases of eastern Quebec are found over a complex succession of tectonically accreted sedimentary, volcanic and ultramafic successions of Cambrian to Ordovician age. Silurian dolomites have the most negative δ26MgDSM3 values of our dataset (-3.25 to - 1.13l). The isotopic values form two groups; a strongly negative group characterizes hightemperature (182°C) dolomites and a less negative group for lower temperatures (153°C) dolomites. The Ordovician dolomites were precipitated at lower temperature (105°C), their δ26MgDSM3 values range from -1.26 to -0.71l. Finally, the very high temperature (350°C) Devonian dolomites have δ26MgDSM3 values ranging from -1.29 to -0.78l. δ26MgDSM3 values in Lower and Upper Ordovician shales (-0.89 to +0.64l and - 1.68 to -1.04l, respectively) do not overlap even if their thermal history is fairly similar. Cambrian rift basalts and Ordovician back-arc volcanics have yielded statistically different δ26MgDSM3 values (-1.01 and -0.81l and –0.26 to 0.09l, respectively). Finally, Ordovician ultramafic (ophiolite) material has yielded δ26MgDSM3 values that overlap with the Ordovician basalts (–0.47 to –0.10l). No experimental data on high temperature (100-350°C) fluid-mineral Mg isotope fractionation factors for dolomite have been published; therefore, we use the recently published relationship for low temperature (4-45°C) Mg-calcite for a preliminary investigation of isotopic links between dolomites and potential Mg+2 source rocks. The Silurian dolomites were previously genetically linked with the adjacent ultramafics and from the δ26MgDSM3 values and fluid inclusions homogenization temperatures, different fluids are likely for the two temperature groups. The dolomite-ultramafic δ26MgDSM3 values for the high temperature dolomites would agree with an ultramafic source of Mg+2, although such direct connection cannot be made for the lower temperature dolomites. The source of Mg+2 for the Ordovician dolomites has been suggested to originate from the Precambrian crystalline basement; the isotope data for the dolomites do not support such a clear link, however, the calculated δ26MgDSM3 values for the hydrothermal fluid fall close to the heavy end of our Lower Ordovician shale dataset. Finally, the high temperature Devonian dolomites were suggested to be of magmatic origin, their δ26MgDSM3 values would require a fluid with δ26MgDSM3 values compatible to those reported for magmatic rocks. This preliminary study shows that the isotopic signature of the diagenetic fluid is the primary control for the δMgDSM3 values in high-temperature dolomite cement. The analyses of potential Mg sources have shown that significant δMgDSM3 differences exist between those sources.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013