Marine Low-Magnesian Calcite Cement from Periphery of Mururoa Atoll

Djafar M. Aissaoui

Reef-associated rudstones and grainstones from the periphery of Mururoa atoll (Pacific Ocean 21°50^primeN-138°50^primeW) are significantly more cemented than their lagoonal counterparts. With increasing depth, the centrifugal distribution of the marine lithification is associated with a change in the cement composition. Peripheral facies above 500 m are cemented by three types of high-magnesian calcite (HMC) with distinct mole % MgCO₃:14.4-16.9 for fibrous crusts, 7.6-13.5 for bladed cement, and 4.3-11.2 for sparitic, stubby forms. Large pores favor fibrous HMC; small, isolated pores are preferential sites for sparitic HMC, the bladed HMC occurring in association with the two end members of this morphological and geochemical variation. Below 500 m, ma ine HMCs are progressively replaced by acicular crystals forming fringes less than 100 µm thick around original grains. Individual crystals are 10-80 µm long and 2-10 µm wide. These acicular crystals are low-magnesian calcite (LMC) with less than 3.9 mole % MgCO₃ and up to 1,470 ppm strontium. The marine origin of LMC from the deep periphery of Mururoa atoll is deduced from (1) continuous occurrence over more than 200 m within a major shallowing-upward sequence; (2) absence of emergence within this sequence; (3) strontium content similar to that of marine HMC; and (4) deep infiltration of cold marine waters that may have favored the precipitation of magnesium-depleted calcite along the periphery of the atoll.

Recognition of marine LMC cementation at Mururoa atoll indicates LMC is not an exclusive indicator of meteoric diagenesis. Marine LMC cementation may have been more common than previously reported in ancient carbonates, and all acicular LMC cements are not necessarily secondary.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.