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Early Diagenesis of Lacustrine Carbonates in Volcanic Settings: The Role of Magmatic CO2 (Lake Dziani Dzaha, Mayotte, Indian Ocean)
journal contribution
posted on 2020-02-24, 21:29 authored by Vincent P. Milesi, Mathieu Debure, Nicolas C. M. Marty, Manuela Capano, Didier Jézéquel, Carl Steefel, Virgile Rouchon, Patrick Albéric, Edouard Bard, Gérard Sarazin, François Guyot, Aurélien Virgone, Éric C. Gaucher, Magali AderLacustrine carbonates formed in rift
settings are increasingly
studied not only as archives of Earth chemical and climatic history
but also as potential hydrocarbon source rocks and/or reservoirs.
The role of magmatic gases in their formation and diagenetic evolution,
hence in their reservoir properties, remains unclear. We studied the
first meter of carbonate sediment of the Dziani Dzaha volcanic crater
lake (Mayotte Island) and developed a reactive-transport model with
CrunchFlow software that allows the quantification of diagenetic reactions
by considering depth-dependent burial rates and sediment compaction.
The model is constrained by the previously documented solid-phase
compositions of the lake sediment and new data consisting of 14C dating of plant macroremains to characterize the sediment
age model, chemical composition of sediment pore waters, and chemical
and isotopic composition of gases dissolved and bubbling through the
lake. These new data reveal a massive magmatic CO2 contribution
to the dissolved inorganic carbon of the lake, which fuels the primary
productivity and carbonate formation. A pH value of 9 in the surface
sediment pore waters induces supersaturation relative to aragonite,
hydromagnesite, and saponite. At 1 m depth in the sediment, our model
predicts that magmatic CO2 inflows and organic matter degradation
account for 22 and 2 mol % dissolved inorganic carbon, respectively.
The magmatic CO2 inflows result in a pH decrease at depth,
leading to the destabilization of hydromagnesite, while saponite and
aragonite remain stable. These results demonstrate the role of magmatic
CO2 in fueling carbonate production and controlling the
diagenetic evolution of sediment mineralogy.
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depth-dependent burial ratessediment age modelmatter degradation accountmagmatic CO 2 inflowssurface sediment pore waterscarbonatehydrocarbon source rocksLake Dziani Dzahasediment pore watersmagmatic CO 2magmatic CO 2 inflows result1 m depthmagmatic CO 2 contributionchemicaldiagenetic evolutionMagmatic CO 2
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