Magnesium Isotopes Support a Seafloor Vent – Dolomite – Burgess Shale Genetic Link

The source of Mg-bearing fluids required for the formation of replacive and void-filling dolomite is subject to ongoing debate. Often, the fluids are attributed to post-evaporation modification of seawater. However, converging lines of evidence now indicate that some magnesium may be derived instead from magmatic sources.

Magnesium isotope analysis of dolomite and associated carbonates may provide evidence for and insight into the source of Mg-bearing fluids. The research program reported on here focuses on Mg isotopes and Cambrian-hosted hydrothermal dolomite and magnesite from locations in western Canada. The carbonates are spatially and possibly genetically linked with the World Heritage Burgess Shale of southeast British Columbia for which newly published mineralogical and field-based data show that Mg-enriched black shale of the Burgess Shale Formation formed in proximity to ancient submarine escarpments and passive-margin extensional faults. Seeping onto ancient seafloor, the hydrothermal fluids supported animal communities (the Burgess Fauna) much as modern communities flourish around black-smoker vents.

Preliminary laser ablation MC-ICP-MS Mg isotope values of dolomite from Yoho, Wapta, and Vermilion Pass vary 0.5 per mil and 2.6 per mil for magnesite from the Brussilof Mine, B.C. Two clusters of data for magnesite are consistent with fluid-inclusion studies that show two separate fluids in Brussilof deposit formation. Magnesite Mg isotope data in one cluster straddles all dolomite data, indicating a genetic link between Brussilof magnesite formation near Cambrian seafloor faults and more landward dolomitizing fluids. These preliminary data support the supposition that hot seafloor-vent fluids in the Cambrian of western Canada supported diverse and complex life, the Burgess Fauna, and were responsible for dolomitization of limestones that in other locations may be significant hydrocarbon producers.