Successions of late Cenozoic platform dolomites distinguished by texture, geochemistry, and crystal chemistry; Niue, South Pacific

Abstract

ABSTRACT Niue Island, a former atoll in the South Pacific, contains unusually well-resolved successions of dolomites recording repeated episodes of Late Cenozoic dolomitization. The massive Upper Dolomite unit consists of shoaling-upward depositional cycles of Pliocene age which have been thoroughly replaced by corresponding repetitive successions of crystalline mimetic (CM), transitional (CMS), and microsucrosic (MS) dolomite textures. In contrast, the Lower Dolomite unit, of late Miocene age, has only partially been replaced by crystalline non-mimetic (CNM) dolomite texture. Porosity and permeability of the precursor facies were likely the chief controls of dolomite texture distributions. The CM-CMS-MS dolomite textures contain variable proportions of 1 to 3 diachronous dolomite crystal phases having distinct chemical and isotopic compositions. The CM dolomite is Ca, Sr, and Na rich and has relatively light 18O values (41.4 ± 0.6 Mg mole %, 234 ± 19 ppm, and 386 ± 39 ppm, 2.8 ± 0.9o/oo PDB, respectively; n = 36). The MS dolomite is Ca, Sr, and Na poor and has heavier 18O values (44.9 ± 0.9 Mg mole %, 142 ± 26 ppm, 241 ± 47 ppm, and 3.6 ± 0.3o/oo PDB, respectively; n = 29). Phase mixing causes linear trends in the elemental and 18O compositions of the CM-CMS-MS dolomites that have been interpreted in the Bahamas to represent kinetic effects. CNM dolomites lack linear geochemical trends because of single phase composition that is Ca, Sr, and Na rich and has heavy 18O values (40.9 ± 0.6 Mg mole %, 293 ± 67 ppm, 339 ± 59 ppm, and 4.2 ± 0.3o/oo PDB, respectively; n = 49). Isotopic and elemental compositions suggest that dolomites developed from normal seawater. Episodic dolomitization of the Upper Dolomite unit likely occurred in the marine phreatic zone beneath the meteoric-marine mixing zone whenever a glacio-eustatic fall subaerially exposed the platform. The 18O enrichments observed in the Lower Dolomite Unit may indicate dolomitization in deeper and colder seawater. The Niuean dolomites are remarkably similar to probably contemporaneous dolomites in the Mio-Pliocene section of the Bahamas, suggesting that glacio-eustasy may be important in dolomitization.