CENOZOIC DOLOMITES OF CARBONATE ISLANDS - THEIR ATTRIBUTES AND ORIGIN

Dolomites found on and below carbonate islands, atolls and oceanic pla tforms provide useful insights into the origin of dolomite; insights t hat may not be attainable from the study of more ancient cratonic dolo mites. For this reason they have been the subject of study for decades . A critical mass of case studies now exist and some significant concl usions can be drawn from the cumulative data. In most cases the associ ation with an island is casual and not genetic, nevertheless, these do lomite occurrences are referred to herein as island dolomites. One typ e of dolomite on carbonate islands is penecontemporaneous dolomite, a phase that forms while the host sediment is in its original deposition al setting. Island examples are Holocene in age, occur in Holocene sed iments and originate as a direct precipitate from either normal or eva porated seawater. These are microcrystalline, poorly ordered, Ca-rich O-18-enriched and geochemically unstable phases that are susceptible t o recrystallization in the setting in which they formed. Post-depositi onal dolomite is the other type of dolomite associated with carbonate islands. This type of dolomite replaces older precursors and forms as cement. Distinctive characteristics include a dominance of fabric-pres erving texture, pore-lining cement rims that may exhibit micron-scale banding with low-Mg calcite, and formation in association with precurs or dissolution. Sr-isotopic dating indicates that all examples are Neo gene or Quaternary in age. Those formed during the Middle Miocene thro ugh Pliocene are massive, laterally continuous, and often multigenerat ional. In contrast, younger examples tend to be localized partial repl acements of a single generation. Sr-isotope ages also suggest global s ynchroneity in many dolomitization events, which suggests a connection between dolomitization, global eustacy and/or global climatic factors . Geochemical attributes of post-depositional island dolomites are Ca enrichment positive delta(18)O and delta(13)C, low Sr contents (150-30 0 ppm) and low Fe(<300 ppm) and Mn(<35 ppm) concentrations. Exceptions to these characteristics occur, but are in the minority and can be ex plained by local enrichment of C-12 from sulfate reduction or oxidatio n of methane, excess Sr from aragonite precursors, and allochthonous F e and Mn from nearby siliciclastics or volcanics. Global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin. Inferred origins, however, depend primarily on how delta(18)O and Sr data are interpreted. Values for Delta(18)O and D-Sr must be assumed due to uncertainties in oxygen isotope fractionation and Sr partitioning. There is no consensus or uniformity in those assu mptions, thus interpretations can vary and be biased to a desired resu lt. Covariant trends in delta(13)C, and delta(18)O, some negative delt a(13)C values, and high Sr all favor a mixing-zone origin, but example s with these attributes are few. Lack of covariance in the isotopes, m ean delta(18)O of +2.0 parts per thousand to +3.5 parts per thousand a nd low Sr (< 300 ppm) favor an origin from normal or slightly evaporat ed seawater. Examples with these attributes are in the vast majority, and dolomite geometries and ages relative to overlying limestones indi cate that seawater derived laterally is the parent fluid for most of t hese dolomites. Many interpreted as mixing-zone or hypersaline reflux products are probably misinterpretations based on unreasonable or extr eme assumptions about D-Sr, Delta(18)O or the chemistry of hypothetica l end-member dolomites. Future studies should standardize the type of data collected and the analytical techniques employed. Multiple geoche mical attributes should be measured on microsampled components and qua ntitative modeling should be employed in order to constraint interpret ations as much as possible. Also needed are a better understanding of the kinetic processes that form these dolomites, more careful assessme nt of their recrystallization status and an improved understanding of D-Sr and Delta(18)O at low temperature.