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Reconstructing conditions during dolomite formation in evaporative Triassic environments

Final Report Summary - TRIADOL (Reconstructing conditions during dolomite formation in evaporative Triassic environments)

Final Publishable Summary Report:
Reconstructing conditions during dolomite formation in evaporative Triassic environments

The formation of dolomite [MgCa(CO3)2], one of the major rock-forming carbonate minerals occuring in the geological record, under earth-surface conditions is still poorly understood and remains difficult to reproduce under controlled laboratory conditions. The abundance of dolomite formed during certain time periods, such as in the Alpine realm during the Triassic, are not reflected in any modern analogues. This phenomenon has remained one of the great non-actualistic enigmas in Earth Sciences, despite 225 years of research.
This project named TRIADOL (for Triassic dolomite) addressed the problem of dolomite formation from a somewhat different perspective than previous studies. In fact, we combined the analysis of petrographic features and palaeo-environmental reconstructions with state-of-the-art mineralogical and geochemical techniques and used numerical models to evaluate different formation processes. Here, I briefly summarize the major outcomes of this study.

Microfacies analysis and reconstruction of Triassic palaeo-environments

Dolomite beds and laminated dolomites were collected from the Carnian Raibl Group (Travenanzes Fm.) and similar units in the Eastern Alps for petrographic analysis. The dolomites of the Travenanzes Fm. have been previously interpreted as primary dolomite, which has been preserved due to efficient shielding from diagenetic fluids by clay minerals (Preto et al., 2015). The dolomites are intercalated in thick beds of clay and silt, which resulted from a substantial terrigenous input.
Lack of fossils and sometimes fine lamination indicate a quiet environment that was largely restricted from open seawater. Moreover, several laminae rich in celestine [SrSO4] and barite [BaSO4] may have formed from ancient evaporites (e.g. gypsum), suggesting that the depositional environment was, at least temporarily, hypersaline. Rip-up clasts, densely packed peloidal grainstones lacking a fine fraction, and soft sediment deformation indicate episodically high water energy due to storms (tempestites) or strong tidal currents. Overall, these observations match well with a coastal sabkha/ephemeral lake situation as observed today in Abu Dhabi or Coorong Lakes (Australia).

Penecontemporaneous dolomite

Soft sediment deformation and brittle deformation showing dolomicrite infill between the clasts, flat pebble breccias and fluid escape structures (dish structures and convolute bedding) indicate that the sediment was largely unlithified. This finding is inconsistent with pronounced precipitation of dolomite within the sediment as this would result in cemented crusts. Instead, the unlithified sediment probably resulted from flocculation and slow settling of dolomite nano-crystals from a concentrated brine. The lamination can then be explained by (presumably) seasonal variation in deposition of clay and carbonate. The cohesiveness of some rip-up clasts and flat pebbles can be explained by cohesive forces between the clay-sized dolomite crystals.
Some cementation and partial recrystallization has occurred later during early diagenesis. Spheroidal dolomite crystal aggregates were recognized by backscatter electron imaging. These aggregates grew within the clay matrix, and they coalesce to form the adjacent microcrystalline dolomite. Further analysis by electron backscatter diffraction revealed that these aggregates consist of micron-scale sub-crystals. This structure was not obliterated by late diagenetic overprint, such that our observations concur with the interpretation of Preto et al. (2015) that dolomites in the Travenanzes Fm. are penecontemporaneous or even primary.

The microbial role

Since it was discovered that microbes can mediate dolomite formation in culture experiments, the microbial dolomite model has become widely accepted. However, the mechanism behind this process is still not understood in detail. At least one study (Rodriguez-Blanco et al., 2015, American Mineralogist, v. 100) has now shown that dolomite can also form via an abiotic pathway under earth-surface conditions by the aggregation of nuclei of poorly ordered dolomite (proto-dolomite). Similar aggregates have been observed in the dolomites of the Travenanzes Fm. (Preto et al., 2015).
In fact, a formation of dolomite in microbial mats would not be consistent with the petrographic structures described above. Rather, a precursor dolomite (or protodolomite) phase precipitated in a concentrated brine is compatible with observations. This interpretation is supported by geochemical modelling, showing that neither microbial sulphate-reduction nor other microbial processes can efficiently induce carbonate precipitation in phototrophic microbial mats. This geochemical model confirms previous studies (e.g. Meister, 2013, 2014). Furthermore, a lipid biomarker and isotope study in modern stromatolites of Lagoa Salgada (Brazil ; Birgel et al., 2015) shows that bacterial remains are abundant, but the bacterial activity is not inducing carbonate precipitation (cf. also Balci et al., in press).
Instead of indicating a microbial influence during dolomite formation, our findings are more consistent with a non-classical nucleation pathway as suggested by Rodriguez-Blanco et al., 2015), i.e. a pathway where nano-crystals are formed first and larger crystals form via oriented attachment of these nano-crystals. Thereby, a microbial factor could still be involved, e.g. by affecting the free-energy landscape and thus kinetic barrier for nucleation and aggregation of dolomite crystals.

The source of alkalinity

If the microbial metabolism does not induce carbonate precipitation, what does then? On the one hand, dolomite may precipitate from evaporating seawater as it is observed in the coastal sabkhas of Abu Dhabi. On the other hand, dolomite may precipitate from continental waters, which deliver alkalinity from continental carbonate and silicate weathering. The latter effect can indeed be observed in coastal ephemeral lakes (e.g. Coorong Lagoon, Australia) but also far inland in alkaline play lakes, such as Deep Springs Lake (California; Meister et al., 2011). Both models (the sabkha model for seawater and the mixing zone/Coorong model for continental water) have been intensively discussed in the literature as mechanisms for dolomite formation.
As an indicator for the origin of alkalinity (from continental weathering vs. Tethyan seawater), we performed a radiogenic strontium isotope analysis in dolomites of the Travenanzes Fm. We used a sequential leaching method to separate Sr from the celestine, dolomite and clay fraction. First results of the celestine fraction show 87Sr/86Sr ratios distinctly more radiogenic than Late Carnian seawater. The celestine is most likely a secondary phase, probably with sulphate derived from former gypsum and Sr derived from a potential aragonite phase or from clay minerals. Analysis of the dolomite fraction is currently underway and will provide further insight into the origin of alkalinity.
Moreover, alkalinity derived from silicate alteration may also play a role in diagenetic dolomite formation (e.g. Meister et al., 2011; Mavromatis et al., 2014). A further modelling project in the frame of a master thesis is currently in progress with the goal to better simulate alkalinity production and dolomite formation in early diagenetic systems.