Calcium-carbonate cementation in the meteoric-marine mixing zone: Processes and criteria for recognition.

摘要

This study investigates the processes and products of mixing-zone diagenesis using integrated stratigraphic, petrographic, and geochemical data from the Cretaceous carbonates of Monte Camposauro, Italy, the Plio-Pleistocene Hope Gate Formation of Jamaica, and the Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, Spain. In contrast to the perception that dolomitization and dissolution are the major processes in meteoric-marine mixing zones, the results show that calcite and aragonite cements are common in mixing zones. Mixing-zone cements include bladed, radial-fibrous, and overgrowth low-Mg calcite, bladed and dendritic high-Mg calcite, microporous calcite with variable Mg-content, and acicular aragonite. It is well known that cements with similar morphology and mineralogy form in other diagenetic environments; this study shows that morphology and mineralogy are not particularly useful in identifying mixing-zone cements. The distribution of mixing-zone δ18O and δ13C data are virtually indistinguishable from those of marine cements, that have partially recrystallized in meteoric water, and physical mixtures of meteoric and marine cements. Thus, δ18O and δ13C analyses are not particularly reliable in identifying mixing-zone cements. Likewise, 87Sr/86Sr, Sr, Mg, Fe, Mn, and SO₄2− are similarly complex in mixing-zone precipitates. Fluid inclusion Tm ice is a more reliable tool for identifying mixing-zone cements.; Fluid inclusion Tm ice data show that precipitation took place from a wide range of salinities. Thus, mixing ratio does not determine if there is dissolution or precipitation. Mixing-zone speleothems of Sa Bassa Blanca cave have distributions, δ18O, and δ13C indicating that they precipitated because of CO₂ degassing at and just below the water-air interface of cave pools. Petrography, Sr, and Mg content of mixing-zone precipitates of the Hope Gate Formation indicate that aragonite dissolution caused precipitation in the mixing zone.; Secular variation of seawater chemistry has affected the mineralogy of mixing-zone precipitates. The mixing-zone precipitates of Monte Camposauro are low-Mg calcite, which is in accordance with the mineralogy expected during the Cretaceous “calcite seas”. In contrast, the mixing-zone precipitates of the Hope Gate Formation and Sa Bassa Blanca cave are mostly high-Mg calcite and aragonite, which is in accordance with the mineralogy expected during Pleistocene aragonite seas.