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ABSTRACT

Wholesale removal of the unstable carbonate phases aragonite and Mg-calcite, and precipitation of calcite and dolomite is currently taking place where phreatic waters (the modern water table) invade 120,000-year-old Pleistocene biolithites (Falmouth Formation), North Jamaica.

Pleistocene rocks presently in the vadose zone are relatively unaltered, and consist of mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a ‘water table cave’ is commonly encountered. Below the water table the rocks are invariably more highly altered than those above. Mg-calcites are very rare, and considerable dissolution of aragonite has commonly occurred.

Dolomite occurs as 8–25 μm, subhedral to euhedral crystals replacing micrite, or precipitated as void linings. The isotopic composition of the dolomite (δO18=-1·0 %0, δC13=-8·4 %0), and its high strontium content (3000 p.p.m.) suggest precipitation as CO2-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve Mg-calcites and Sr-rich aragonites, and de-gas. Because some dolomitized rocks are enriched in magnesium relative to unaltered biolithites, addition of magnesium to the system is necessitated, and is probably derived from sea water in the mixing zone.

Phreatic meteoric diagenesis is thus demonstrated to be a rapid process, and to be capable of dolomitization.