Dolomites compositional zoning

Quantitative determination of elemental dolomite composition was carried out on five polished thin sections covered with a thin layer of carbon and using a CAMECA SX 51 electron microprobe at 15 kV, a 20 nA beam current and a 0.2 pm beam diameter. The BSE imaging system linked to the electron microprobe was used to detect zonation in the dolomite cement, and compositional analyses were carried out for each zone. Results were normalized to 100 mol% Fe, Mg and Ca. The precision of the analyses was 100% 2. 

The values of Fe and Mn reported for saddle dolomite are presented together in Fig. 3 because the geochemical properties of these elements are similar. Fe -i- Mn concentrations in saddle dolomite show wide variability, from <10 ppm to 30 mol% (Fig. 3), reflecting in part the compositional zoning in the saddle dolomite structure. In sandstones. 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

Reeder R.J. and Prosky J.L. (1986) Compositional sector zoning in dolomite. J. Sediment. Petrol. 56, 237-247. 

An important conclusion one can draw from the Sears study and other, similar studies is that in areas with well-developed soils, the chemistry of underlying shallow groundwaters is often chiefly determined by processes taking place in the soil and less by processes operating in the saturated zone below. In this case, groundwaters from the underlying dolomite are similar to the water from the soil 3 m deep. Similarly, the chemical composition of effluent streams (streams fed chiefly by ground-water) is also quickly determined by interactions with adjacent soils (see Section 7.8). 

The EMP analyses revealed that dolomite and ankerite vary considerably in chemical composition, particularly in terms of Fe content (FeCOj = 0.3-36 mol%) (Table 1 Fig. 9). The microcrystalline dolomite rims and crusts in dolocrete, as well as the small rhombs replacing clays and occurring within mica in the sandstones, are relatively Fe poor to moderately ferroan (0.3-7.8% FeCOs) (Table 1 Fig. 9). However, both the small rhombs and the large blocky crystals in sandstones are zoned, with Fe increasing outwards to an ankeritic composition (Fig. lOE). Furthermore, the dolomites are slightly to moderately calcian ( 53-62 mol% CaCOj) (Table 1) and have relatively low Mn content (0.2-1.9 mol%). 

Ward, W.C. Halley, R.B. (1985) Dolomitization in a mixing zone of near-seawater composition. Late Pleistocene, Northeastern Yucatan Peninsula. J. sediment. Petrol., 55, 407-420. 

Most of the dolomites (including the detrital cores) have calcium-enriched compositions (Table 6 Fig. 8). Zoning within the ferroan dolomite is clearly visible in back-scattered electron images, and the higher Fe contents tend to be characteristic of the later zones (Fig. 7A). The degree of Ca enrichment, however, is not strongly controlled by Fe content and more Fe-rich portions of the crystals display a range in Ca content that is nearly as broad as that of less Fe-rich portions. Mn content is correlated positively with Fe enrichment (Fig. 9). 

In marine turbidite sandstones of the central basin, following minor siderite precipitation, dolomites formed early in the zone of methanogenesis. These have Ca-rich compositions similar to dolomites reported from contemporaneous fine-grained rocks of the Monterey Formation, coastal California. The dolomites are an example of young (< 6 Ma) dolomite formation at shallow burial depth in marine pore water, and they may have undergone some recrystallization during shallow burial without resetting their initial Sr/ Sr values. 

Searl, A. Fallick, A.E. (1990) Geochemistry of some Dinantian dolomites from East Fife hydrothermal overprinting of early mixing-zone stable isotopic and Fe/Mn compositions. J. Geol. Soc. London, 147, 623-638. 

Lateral variations in the eogenetic association may be related to changes in sedimentary conditions or to the starting mineral composition. The development of nitrates was probably controlled in part by sedimentary rhythms. In the border zones of the Triassic Basin of the Sahara where the source of supply (Hoggar Massif) was probably closer by, calcretes were formed and the deposits were cemented by calcite with a heterogeneous structure. In the case of farther removed source areas and a more stable sedimentation the sandstones are characterized by a poikilitic nodular dolomite cement. Variations in the composition of the sandstone cements between calcite and dolomite may be explained by the ionic composition of the solutions in the... 

In the upper part of the succession in drill hole HM-i the carbonate carbon isotope composition fluctuates between well-defined limits of -1-4.5 and -7.4%o (Table 7.1, Fig. 7.2). In this case, recrystallization of carbonates and neoformation of calcite or dolomite have taken place under participation of CO which in certain instances contained a large portion of isotopically light carbon ( C) whereas in other instances there was a predominance of heavy carbon ( C). The results obtained, together with other geochemical data, show that in the zones in which the sedimentary rocks have been affected by infiltration waters the migrating hydrocarbons have been subjected to oxidation and other changes, to be discussed in detail below. 

Although lichens absorb and accumulate minerals, their mineral composition does not always accurately reflect the composition of their substrate. Dormaar (1968) found that the infrared spectra of the rocks differed from those of the minerals in the lichens growing on them, although the residues and weathered rock zones were more similar. He found, for example, that a specimen of Caloplaca growing on dolomite showed no evidence of Mg or carbonate in the peroxidized material, not even MgCOg. There was, however, a good deal of calcium in the thallus. It therefore seemed that Caloplaca picks up Ca in preference to Mg in dolomite. He concluded from his studies that It is likely that lichens obtain at least part of their minerals from the substrate. A similar conclusion was expressed more negatively by Jenkins and Davies (1966). They found close correlations between the minerals in lichen ash and ash from material deposited from the air, and... 

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