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.