AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Magnesium Isotopes in Dolomites – Results for the Paleozoic of Eastern Canada and Significance for Exploration Models
(1) Natural Resources Canada, Geological Survey of Canada, Quebec City, QC, Canada.
(2) Natural Resources Canada, Geological Survey of Canada, Ottawa, ON, Canada.
The processes that form high temperature dolomites are controversial, with end-members of 1) tectonically controlled early processes with rapid upward migration of high temperature fluids, and 2) burial-dominated late processes with regional slow migration of high-temperature brines. Magnesium stable isotope ratios in saddle dolomites, ultramafics and shales are presented and offer critical new data in the ongoing debate.
Hydrothermal saddle dolomites in eastern Canada overly diverse Precambrian and Paleozoic basements, which may have acted as Mg sources. The dolomites and potential Mg sources were chemically characterized (ICP-ES) and their δ26MgDSM3 and δ25MgDSM3 ratios measured (MC-ICP-MS). Column chemistry was used to purify the Mg in the digested samples prior to isotopic analysis.
The Lower Silurian dolomites (Th of 150-200°C) are related to fluid flow along foreland faults. The Mg2+ was interpreted to originate from Ordovician ultramafic slivers. Near the Silurian occurrences, Lower Ordovician dolomitized slope carbonates are associated with a transpressional fault. These two dolomites have yielded negative δ26MgDSM3 values ranging from −3.2 to −1.5‰.
Middle Ordovician dolomites (Th of 90 to 120°C) are associated with foreland faults that reach the Precambrian metamorphic basement. They yielded δ26MgDSM3 ratios of −1 to −0.7‰. Lower Devonian reef with massive replacement dolomite of magmatic origin (Th of 300 to 350°C) occurs at the junction of two transpressional faults. Even though the reef neighbours the ultramafic slivers, the dolomite has δ26MgDSM3 ratios around −1‰.
Linear relationships between dolomite δ26MgDSM3 and 1) δ18OVSMOW of the fluid and 2) 87Sr/86Sr in the dolomite suggest a link with the nature of the fluid and its source. Linear relationships between δ26MgDSM3 and Th of fluid inclusions indicate a thermal kinetic effect on Mg2+ incorporation in the dolomite.
Data from potential Mg sources are being gathered. Lower Ordovician ultramafics are serpentinized; the altered material has a tight range of δ26MgDSM3 values of −0.4 to −0.2‰. Lower and Upper Ordovician shales abound in the Lower Paleozoic basin. The shales have Mg isotope ratios that differ with age; the Lower Ordovician has yielded δ26MgDSM3 values of −0.8 and +0.1‰ whereas the Upper Ordovician has given δ26MgDSM3 values of −1.2 and −1‰.