A hole drilled through basaltic rocks on the west coast of Mauritius Island encountered 3 m thick, partly dolomitized reefal deposits of middle Pleistocene age, 120 m below mean sea level. Above this carbonate bed is a sequence, about 50 m thick, of strongly weathered basalts and paleosoils.

The degree of dolomitization increases toward the overlying basalt. Low-magnesian calcite and dolomite are distributed along the core as follows: (1) entirely dolomite 0.5 m downward from the volcanic rock, (2) from 0.5 to 2 m below the base of the basalt, both calcite and dolomite, and (3) below 2 m only calcite. High calcium/magnesium ratios (1.2 to 1.4) indicate that protodolomite has replaced micrites (as 0.5 to 3 µm crystals) or sparites (as 10 to 30 µm, subhedral to euhedral crystals) and fills pores created by the dissolution of calcite, demonstrating the near contemporaneity of calcite solution and dolomite precipitation. The isotopic composition of the dolomite (^dgrO¹⁸ = + 0.1 to +3.6^pmil, ^dgrC¹³ = -1.7 to + 2.00^pmil) and its relatively hig strontium content (> 500 ppm) suggest the system initially must have been partly closed in order to retain the O¹⁸, C¹³, and strontium of reef carbonates during dolomitization.

The calcium/magnesium ratio of the volcanic rocks decreases from the upper to the lower part of the series (0.66 to 0.075), confirming that higher magnesium contents present in the uppermost reef layers are derived from weathered basalts.

The chemistry of the dolomite and associated volcanic rocks strongly suggests that dolomitization was probably caused by leaching of magnesium out of the volcanic rocks and redistribution within carbonate sediments by descending waters.

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