GEOSCAN, résultats de la recherche


TitreMagnesium isotopes as a new research tool for tackling the "dolomite problem"
AuteurLavoie, D; Jackson, S; Girard, I
SourceAAPG Hedberg Conference, abstracts; American Association of Petroleum Geologists, Annual Meeting Abstracts 2013 p. 1
LiensOnline - En ligne
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120256
RéunionAAPG Hedberg Conference; Beijing; CN; avril 21-24, 2013
Mediaen ligne; numérique
Sujetsisotopes; magnesium; géochimie du magnesium; rapports magnésium-calcium; dolomites; dolomitisation; Paléozoïque; Silurien; Dévonien; Ordovicien
ProgrammeBassins sédimentaires d'Hudson / Foxe Bay, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
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 ?26MgDSM3 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 ?26MgDSM3 values range from -1.26 to -0.71l. Finally, the very high temperature (350°C) Devonian dolomites have ?26MgDSM3 values ranging from -1.29 to -0.78l. ?26MgDSM3 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 ?26MgDSM3 values (-1.01 and -0.81l and –0.26 to 0.09l, respectively). Finally, Ordovician ultramafic (ophiolite) material has yielded ?26MgDSM3 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 ?26MgDSM3 values and fluid inclusions homogenization temperatures, different fluids are likely for the two temperature groups. The dolomite-ultramafic ?26MgDSM3 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 ?26MgDSM3 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 ?26MgDSM3 values would require a fluid with ?26MgDSM3 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 ?MgDSM3 values in high-temperature dolomite cement. The analyses of potential Mg sources have shown that significant ?MgDSM3 differences exist between those sources.