Discriminating temperate carbonates and their diagenetic environments using bulk elemental geochemistry: A reconnaissance study based on New Zealand Cenozoic limestones

Use of the elemental geochemistry of carbonates for characterizing and distinguishing between temperate and tropical carbonate facies is in its infancy. In particular, apart from several Tasmanian examples, few elemental data exist for temperate carbonates. Analysis of the carbonate fraction of nearly 600 bulk samples of temperate Cenozoic limestones from New Zealand gives average values of Mg, 5500 ppm; Fe, 2500 ppm; Na, 1400 ppm; Sr, 550 ppm; and Mn, 500 ppm. Overall, the Mg and Sr contents are lower, and the Na, Fe, and Mn contents relatively higher, than is typical for tropical carbonates, whether ancient or modern. Bivariate plots involving combinations of these elements can thus be used to discriminate tropical from temperate carbonates, consistent with the temperature coefficient of the equilibrium constants for carbonate dissolution. The particular value of the Na-Sr plot in this discrimination is highlighted, the New Zealand limestones defining a "unique" temperate field having Sr values from 10-1000 ppm and Na values of 500-5000 ppm. From the slopes of regression lines for various element-element relationships expected in marine, meteoric, and burial processes during limestone diagenesis, it is possible to construct summary elemental matrices that enable an overall assessment of the extent to which individual limestone formations have been subjected to each of these processes. Consistent with the interpretation of diagenetic history from other evidence, the geochemical results suggest that burial or burial/marine processes dominate the diagenetic character of most major occurrences of New Zealand Oligocene-Miocene limestones, but that a strong meteoric signal is also evident in many of the Plio-Pleistocene limestones of eastern North Island.