Dolomite Formation on Top of Salt Diapirs—Where Has All the Magnesium Come From?

Dolostones are frequently found on top of salt diapirs, but there are several mechanisms to explain their presence. They can be 1) deposited after the emplacement of the diapir body without any causal relationship to the salt; 2) derived from a layered evaporite sequence (LES), entrained by the salt and then exhumed as an accreted part of the caprock of the salt body; 3) the result of geochemical transformations during salt movement; or finally, 4) a consequence of the subaerial exposure of a salt diapir. From a salt tectonics perspective, the latter two options are interesting, as they create a causal relationship between salt diapirism and dolomite formation, but simultaneously present problems with respect to the conditions required. Models for dolomite formation must address both thermodynamic and kinetic constraints, the former requiring a sufficiently high magnesium to calcium ratio, the latter conditions conducive for disruption of the stable hydration sphere around the magnesium ion, which prevents interaction with the surface of a growing dolomite crystal. Whereas debates about dolomite formation, i.e., the ‘dolomite problem’ often focus on the latter issue, in the case of salt diapirs, the former may pose an insurmountable issue: a lack of magnesium. Here, we explore what conditions enable salt-diapir induced dolomite formation, namely the accumulation of interlayered magnesium-rich potash salts, such as bischofite or carnallite, or authigenic high-magnesium clay minerals, such as magnesium-smectites or sepiolite, in the LES, as well as the timing and circumstances under which these minerals become sources of magnesium in dolomite.