| Popis: |
Calcite pseudomorphs after ikaite (glendonite) are associated with coldwater depositional systems, including glaciomarine and deepwater settings, as dictated by the limited stability field of ikaite. Ikaite precipitation is favored by elevated alkalinity and dissolved phosphate, conditions encountered commonly in association with organic-rich marine sediments where methane oxidation is occurring. The rapid recrystallization of ikaite to calcite during slight warming or pressure release results in considerable solid volume loss, producing a highly porous crystal mesh. Preservation of the original ikaite crystal form requires precipitation of diagenetic calcite cement during early burial to prevent compaction and collapse of pseudomorph structures. During later burial diagenesis remaining pore space may be filled with deeper burial calcite cement. Glendonites from the Permian of the Sydney Basin occur in subtidal shelf facies containing glacial dropstones and a normal marine fauna. Stable oxygen isotope signatures of modern ikaite suggest carbonate precipitation in equilibrium with ambient seawater; carbon signatures are usually strongly negative relative to normal marine carbonate, consistent with derivation of carbonate from methane oxidation. Review of published data suggests that while Holocene glendonite may provide reliable isotopic records of the conditions of ikaite precipitation, precipitation of later calcite cement within the glendonite structure reduces the significance of the isotopic signature as an indicator of primary depositional conditions. Bulk glendonite samples from the Permian Sydney Basin, Australia, have a broad range of δ18 O and δ13C (δ18OPDB = −5 to −15‰; δ13C = −8 to −16‰), in contrast to the narrow range for brachiopod carbonate (δ18OPDB = +1 to −5‰; δ13C = +5 to +7‰) from the same strata. Handpicked separates of "primary" glendonite and secondary calcite also have a wide range of stable isotope values. The data from Sydney Basin glendonites indicate that diagenetic precipitation of calcite has blurred the isotopic signature of primary ikaite replacement calcite at the scale of micosampling done in this study. |
