High-Mg calcite

Figure 25 Predominant skeletal mineralogy, calcite (shaded) versus aragonite + high-Mg calcite (blank), in Phanerozoic reefs. Mineralogy from Kiesshng (2002), Sr data from Steuber and Veizer (2002) and Lear et al. (2003)...

Carbonate precipitation from mixed waters is enhanced by an increase in alkalinity due to the oxidation of organic matter and methane (Lunde-gard, 1994). Gas pockets are common in Holocene sediments rich in organic matter (e.g. McMaster, 1984). Nelson Lawrence (1984) and Simpson Hutcheon (1995) reported the formation of Holocene, high-Mg calcite nodules (6 Cs -49%o to -7%o) in hybrid, bioclastic deposits of the modem Fraser River delta ( 49° N) due to methane oxidation close to the seafloor. Early diagenetic, ex-... 

Nelson, C.S. Lawrence, M.F. (1984) Methane-derived high-Mg calcite submarine cement in Holocene nodules from the Fraser Delta, British Columbia, Canada. Sedi-mentology, 31, 645-654. 

Generally the calcite cements are characterized by relatively low average mol% of Fe (1.0%), Mg (0.6%) and Mn (0.2%) (Table 2). Cements in the turbiditic arenites have somewhat more Fe than those in the shelf hybrid arenites (av. 1.4% and 0.6%, respectively) (Table 2). Bioclasts such as echinoderms, originally composed of high-Mg calcite, show low Mg contents, which confirms their pseudomorphic replacement. Calcite which replaced feldspar grains is occasionally Sr rich (up to 2.1% SrC03) (Fig. 6F Table 2). 

Ciyptociystalline, equant high-Mg calcite First, ciyptocrystaUine second, acicular aragonite Similar to previous, but lengths of acicular crystals may be more than 30 fim. Crystals extend to center of pores... 

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