Low-Mg calcite

Florida Bay sediment + CO2 then degas by air bubbling (2 runs) low-Mg calcite, aragonite,quartz, organic matter -1 day 8 Winland (1969)... 

Figure 25 Sr concentrations in biological low-Mg calcite of brachiopods (dots), belemnites (crosses) and rudist bivalve (boxes). Mean values (bold curve) and two standard errors (thin curves) were calculated by moving a 20 Myr window in 5 Myr intervals across the data set. Ranges (vertical lines) and mean values (stars) of intrashell variations for concentrations in single rudist shells are also shown, but were not used in calculation of running means (source...

Figure 30 Carbon isotope composition of Phanerozoic low-Mg calcitic shells (source Veizer et al, 1999).

Figure 34 Phanerozoic trend for low-Mg calcitic and aragonitic shells. Note that this is a trend for CaC03, offset by the fractionation factor a from that of seawater. See Figure 32 for further explanations (after Veizer et al,...

Tufa and travertine mineralogy tends to be dominated by low-Mg calcite, although there are some examples of aragonite and disordered dolomite in lacustrine tufas (e.g. Great Salt Lake, Utah Pedone and Dickson, 2000). The rare calcium carbonate hydrate known as ikaite has also been found in tufa (Buchardt et al., 2000). Vaterite, a rare anhydrous polymorph of calcium carbonate, has also been found to be co-precipitated with calcite in pools within a travertine barrage system at Huanglong, Sichuan, China (Lu et al., 2000). 

Simulations of marine-meteoric mixing (e.g. Plummer, 1975 Wigley Plummer, 1976) predicted calcite oversaturation in waters with 20-70% sea water. However, the saturation degree of the mixed waters varies depending on the initial calcite saturation index, C02 and temperature. Nevertheless, predictive models constructed by Frank Lohmann(1995) for low-Mg calcite precipitation in... 

Hendry, J.P., Trewin, N.H. Fallick, T.E. (1996) Low-Mg calcite marine cement in Cretaceous turbidites origin, spatial distribution and relationship to seawater. Sedimentology, 43, 877-900. 

Carbonate rock fragments could also be a source of calcite cement if they are available in the source area and if they survive transport. In the Palaeogene sandstones of the North Sea we have found intervals containing up to 55 vol% chalk clasts, for instance, and carbonate rock fragments have also been reported from sandstones in other basins (e.g. Richmann et al., 1980 Dickinson, 1988 Taylor, 1990). However, carbonate rock fragments are probably more stable than biogenic carbonate because of their low-Mg-calcite composition and coarser crystal size. 

A second generation of low-Mg calcite cement was precipitated in primary and secondary pores in both lenses and host rocks. This cement is moder-... 

Fig. 10. Plot of the Mg/Ca and Fe/Ca ratios of sparry calcite cement, (replaced) carbonate grains and fringe cement of lenses and host rocks. All carbonate is low-Mg calcite. Despite the rather large range in chemical compositions, there is no significant difference in chemical composition between the various carbonate components.

Fig. 11. Plot of oxygen and carbon stable isotopic compositions of bulk carbonate samples. Note the small range in isotopic compositions of calcite in lenses despite variable ratios between early and late calcite cement. This points to similar physical and chemical conditions during replacement of early diagenetic CaC03 cement by low-Mg calcite and late calcite cement precipitation.

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