Carbonate precipitation eogenetic

Marine eogenetic magnesite is also known to precipitate in deep-sea sediments. Matsumoto (1992) described rhombic, microcrystalline (2-15 pm) Ca-Mn-Fe rich magnesite and Fe-Mn rich lansfordite (hydrous Mg-carbonate) in Miocene to Pliocene mudstones from ODP Site 799 in the Japan Sea. He concluded that on progressive burial and increase in temperature ( 435 mbsf, T x 43 °C), the metastable lansfordite is transformed into magnesite. 

When carbonate cements are subjected to physicochemical conditions that vary considerably from those under which they formed, they may dissolve and re-precipitate at various scales. Carbonate dissolution and the creation of secondary porosity may occur during eodiagenesis or telodiagenesis or in response to progressive burial. Eogenetic secondary pores may survive subsequent burial and compaction in sandstones that have been subjected to early overpressuring or hydrocarbon emplacement, or if dissolution is incomplete, and leave evenly distributed remnants of carbonate cement. 

Sandstones with potentially better porosity preservation are characterized by (i) coarser grain size and better sorting (ii) lower tendency to host extensive eogenetic carbonate cement than the finer sediments, which are more represented by well 34/4-1 samples and (iii) chlorite rims evolved from the infiltrated clay coatings, which are more abundant in coarse-grained sands which inhibited precipitation of pore-occluding quartz and carbonate cements. 

The positive correlation (r = + 0.85) between 5 C and 5 0 values (Fig. 22) of the eogenetic Lunde carbonates may be related to relatively rapid near-surface precipitation caused by evaporation and CO2 degassing (e.g. Salomons et al., 1978 Schlesinger, 1985 Salomons Mook, 1986 Spotl Wright, 1992), which increases the enrichment of C and 0 isotopes. However, it is believed that at depths of a decimetre the evaporation rate is substantially reduced, and there is thus very little opportunity for significant 0 enrichment in soil water before the next rainfall causes sufficient infiltration to obliterate this effect (Hellwig, 1973). 

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