Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

Abstract

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mgphyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0Á2 mol l À1 and pH values of ca 9Á3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon-hydrogen-nitrogen elemental analyzer, X-ray fluorescence spectrometry and X-ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X-ray diffraction analyses of the <2 lm clay fraction indicate the presence of a saponite-like mineral, a Mg-rich smectite. The saponite-like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite-like mineral instead of Al-free Mgsilicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic 1 Present address: School of Earth and Space Exploration, ASU Tempe Campus, ISTB4 -BLDG75, 781 E Terrace Mall, Tempe, AZ 85287-6004, USA 983 link may exist between saponite and the development of organic-rich carbonate rocks, which may be fuelled by the input of CO 2 -rich volcanic gases. This provides novel insights into the composition and formation of saponiterich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.

Geochemistry of an endorheic thalassohaline ecosystem: the Dziani Dzaha crater lake (Mayotte Archipelago, Indian Ocean)

“…The petrography of the lake catchment consists of levels of pumice and volcanic ashes in which many decimetric to centimetric fragments of clinkstone are found (Figure in Supplementary 1). Ashes and pumices are of phonolitic composition [Pelleter et al, 2014] with anorthoclase Na 0.7 K 0.3 AlSi 3 O 8 as the dominant feldspar [Milesi et al, 2019]. It is likely that the volcanic eruption also sputtered the underlying carbonate reef, since ashes and pumices contain numerous fragments of reef reaching up to one meter in size [Milesi et al, 2019].…”

Early Diagenesis of Lacustrine Carbonates in Volcanic Settings: The Role of Magmatic CO2 (Lake Dziani Dzaha, Mayotte, Indian Ocean)

“…Those two phases are thus essential to explain the silica balance of the lake. The saponite used for the calculation is theoretical and does not have the actual stoichiometry of the saponite of the Dziani sediment 5 (Table S7). The Si/Mg ratio in the theoretical formula is lower than in the measured one, such that more magnesium is needed to form saponite in the calculation than in the field.…”

“…Stoichiometry and equilibrium constants of dissolution reaction of saponite of the Thermoddem database 2 and of the Dziani sediment (DZ-saponite)5 The base components are: Al 3+ , Ca 2+ , H + , H 2 O, Mg 2+ , K + , Na + and H 4 SiO 4 .…”

Carbon isotope evidence for large methane emissions to the Proterozoic atmosphere

“…Dziani Dzaha is a shallow tropical volcanic crater lake on the Petite Terre Island of Mayotte (Comoros Archipelago, Indian Ocean) 28 , which is located 50 km west of a huge submarine volcano triggered in 2018 29 . Several surveys have been conducted to determine the lake biological, physical and chemical characteristics [30][31][32][33][34] (see details in Supplementary Information and Supplementary Fig. S1).…”

“…This is only higher than the maximum value estimated in the Black Sea, which is another euxinic system overlain with at least 60 m of oxic water, where organic matter burial was estimated to 0.5-1.8% 61,66 (0.1% in the present-day oceans). In Dziani Dzaha, the very high productivity, the very thin oxic surface layer, the low sulphate concentrations, the shallow water column and the high sedimentation rate (close to 1.2 mm year −1 ) [30][31][32][33][34] , all create highly favourable conditions for the preservation and burial of organic matter [67][68][69] . By analogy, the value of 2.9% could be thus considered as a maximum for the organic burial rate in Precambrian oceans that were also microbially dominated, anoxic and sulphate-poor.…”