Mg

Next to calcium, magnesium (Mg) is the major cation involved in the carbon cycle and the reaction of atmospheric CO2 with Ca and Mg from silicate minerals is a dominant component of the global climate system. Magnesium is the... [ view full abstract ]

Next to calcium, magnesium (Mg) is the major cation involved in the carbon cycle and the reaction of atmospheric CO2 with Ca and Mg from silicate minerals is a dominant component of the global climate system. Magnesium is the eight most abundant element in the continental crust and the fourth most abundant species in seawater. Both, the modern Mg concentration in seawater of 53 mmol/l and the isotopic composition of delta26Mg of -0.82 permil are globally rather uniform and the residence time of Mg in seawater is considerable (about 13 Myr). Magnesium is derived from weathering on continents and reaches the oceans via riverine and coastal groundwater. At mid-ocean ridges, Mg is hydrothermally exchanged for Ca forming, in combination with dolomite precipitation and ion-exchange reactions with clay minerals, the main sinks in the global Mg cycle.
With reference to carbonates, Mg is a key element and particularly the fluid Mg/Ca molar ratio in marine and continental carbonate depositional environments represents one of the most important controls on carbonate mineralogy (aragonite, calcite, vaterite, ikaite, Mg-calcite, dolomite, magnesite, amorphous calcium carbonate), petrography and crystallography. Fluctuations of the seawater Mg/Ca molar ratio through Earth history resulted in calcite and aragonite seas, respectively, affecting abiogenic cement precipitation and the evolution of carbonate secreting taxa. In the context of biologically controlled mineralization, high fluid Mg/Ca ratios in vacuoles are used as a strategy to stabilize non-crystalline (amorphous) carbonates for extended periods of time. The Mg concentration of different calcite polymorphs represents the most important factor affecting carbonate diagenetic pathways with increasing calcite Mg contents leading to increasing diagenetic reactivity. The formation of non-stoichiometric and stoichiometric dolomites in the presence of Mg-rich fluids is a field of intensive research in applied and fundamental carbonate geology.
In order to quantitatively explore the significance of parent fluid Mg/Ca molar ratios in carbonate precipitation and alteration environments in a process-oriented manner, a series of field and laboratory experiments have been performed and some of the results are reported here. The applicability of this work and resulting findings to natural settings must be continuously tested. A first set of experiments deals with inorganic laboratory precipitation work with focus on the relation between fluid Mg/Ca molar ratio (and other parameters) and carbonate mineralogy, morphology and crystallography. This work is complemented by cave monitoring experiments documenting the relation between climate, fluid Mg/Ca molar ratios and speleothem mineralogy. Moreover, crystals precipitated on watch glasses are studied geochemically and crystallographically in context to their fluid chemistry. Along similar lines, gel growth experiments using seeds of island spar, calcite and belemnite rostrum fragments shed light on the significance of Mg/Ca molar ratios in the formation of radiaxial fibrous and fascicular-optical calcites. Data obtained represent an important step forward in our understanding of these poorly constrained fabrics. Finally, hydrothermal carbonate replacement experiments provide quantitative data on rates and mechanisms of biogenic and abiogenic aragonites reacting with Mg-bearing solutions during the transforming to dolomite and magnesite and have a direct bearing on processes in the subsurface.