ABSTRACT

Radiaxial calcite is abundant in Lower Miocene strata, 377-819 m deep on Enewetak Atoll. These radiaxial crystals have equant, bladed, and fibrous morphologies and occur as isopachous crusts 0.05-2.0 mm thick. Radiaxial terminations consist of fibrous subcrystals bundled together into larger acute, obtuse, or rounded shapes. Enewetak radiaxial calcite ranges from clear to very inclusion-rich. Most inclusions are microcavities caused by organic infestations (Videtich 1985). Alternating bands of inclusion-rich and inclusion-poor calcite mimic existing termination patterns. Radiaxial calcites fill intergranular pores, sponge borings, fractures, and aragonite molds. Aragonite has been pervasively dissolved from the Lower Miocene strata containing radiaxial calcite. Banded radiaxial calcit can be traced from intergranular pores into aragonite molds. Radiaxial calcites have a mean magnesium concentration of 3.2 mole % MgCO₃ with a range from 1.6 to 11.1 mole % MgCO₃. Inclusion-rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations. Enewetak radiaxial calcite has ¹³C values of +1.3 to +2.5 (PDB) and ¹⁸O values of -1.8 to +0.4 (PDB).

Petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater. Precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition, probably in a burial environment. Dissolution of aragonite by deep marine water is suggested by: 1) abundant marine cement associated with aragonite dissolution; 2) a lack of meteoric cements associated with aragonite dissolution; and 3) no evidence of subaerial exposure in Lower Miocene strata between 400 and 819 m. Modern Pacific Ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed (375-850 m). Tidal fluctuations and temperature profiles indicate that Lower Miocene strata containing radiaxial calcite are in open communication With modern Pacific Ocean water. Therefore, aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater (undersaturated with respect to aragonite) circulating through the atoll.

Variations in magnesium concentration in Enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater. The first alternative is preferred. Precipitation of radiaxial calcite at different rates (at different degrees of saturation) could cause variable magnesium concentrations. More deeply buried radiaxial calcite may have precipitated more slowly in deeper, less supersaturated water, resulting in lower magnesium concentrations. Likewise, slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion-rich bands. Enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric: 1) precipitation at fluctuati g rates, and 2) formation in waters undersaturated with respect to aragonite.