Tracing structural relicts of the ikaite-to-calcite transformation in cryogenic cave glendonite
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Abstract Ikaite is a calcium carbonate hexahydrate that forms at temperatures close to the freezing point of water; thus, its occurrence is associated with cryogenic conditions. This mineral is metastable and quickly transforms to calcite at temperatures above 5 °C. Pseudomorphs of calcite after ikaite are known as glendonite. The nanostructure of 25 000–43 000 year old glendonite from Victoria cave (Southern Ural, Russia) was investigated in search of structural features indicative of the ikaite-to-calcite transformation. Scanning electron microscope images display several micrometer- to submicrometer-size pores and indicate high intergranular porosity among the loosely aggregated grains. Transmission electron microscopy (TEM) data show evidence of 10–20 nm nanotwins [twin law (1014)] and 10–40 nm overlapping nanograins. Scanning TEM images reveal that the individual grains contain 5–10 nm long and 2–4 nm wide mesopores (sizes between 2 and 50 nm), which are aligned parallel to [1010] of calcite and might be associated with a crystallographically oriented dehydration of the precursor ikaite. Fourier transform infrared spectroscopy reveals no evidence of structural water but absorption bands related to molecular water trapped in fluid inclusions are present. Nitrogen absorption/desorption measurements show that the specific surface area of 5.78 m2/g and the pore volume of ~0.07 cm3/g for calcite, the constituent of glendonite, are comparable to those of a common natural calcite. We suggest that the aligned mesopores, frequently occurring twins, small grain size, presence of aqueous inclusions and the high micrometer- to submicrometer-size intergranular porosity arise from the ikaite-to-calcite transformation and thus may be used as criteria for the former presence of ikaite and hence for cold paleotemperatures. However, since similar features might also be common in biogenic carbonates, the diagnostic macroscopic pseudomorphs after ikaite are equally important for identifying glendonites and inferring cryogenic conditions.Cite
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ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ 18 O values of 19·6‰ (SMOW), whereas platform limestones have mean δ 18 O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar / appenninica zone of the Late Albian.
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Abstract Resolving the timing of brittle tectonism is a challenge. We examined intra‐cratonic Middle Ordovician limestone bedrock that overlies Mesoproterozoic crystalline basement, which are both cut by ENE‐WSW trending fault zones that have historic M4‐5 earthquakes along their trace. Calcite‐filled mode I joints are ubiquitous within the strata, and are parallel to the modern stress field. Calcite δ 18 O and δ 13 C values are analogous to the bulk composition of Middle Ordovician limestones, and suggest vein formation from a source dominated by connate fluids of the host rock. U‐Pb geochronology of six calcite veins sealing ENE‐WSW oriented joints produced similar Cretaceous dates and were regressed together yielding a date of 96.8 ± 1.1 Ma (MSWD: 2.9). A Cenomanian age is consistent with the most recent reorientation of North America's compressive stress field, which is attributed to a change in spreading direction along the northern Mid‐Atlantic Ridge and is linked to a global plate reorganization event at ca. 105–100 Ma. These results suggest that the most recent redistribution of lateral stress was not a passive event, and generated brittle deformation and fluid flow at least 500 km inland of North America's Atlantic passive margin.
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ABSTRACT The lower Eocene Rus Formation in Qatar reflects carbonate deposition in a semirestricted to fully restricted marine setting on a shallow ramp. Petrographic, mineralogical, and geochemical evidence from three research cores show early diagenesis has extensively altered nearly every petrological attribute of these rocks despite not having been deeply buried. In southern Qatar, the lower Rus (Traina Mbr.) consists of fabric-retentive dolomite intervals that preserve mudstone, wackestone, and packstone textures that are interbedded with depositional gypsum beds. In northern Qatar, the same member is dominated by fabric-destructive planar-e dolomite, and evaporites are absent. In both northern and southern Qatar, the upper Rus (Al Khor Mbr.) is composed of fabric-retentive dolomite intervals as well as limestone intervals rich with Microcodium textures that display evidence of dedolomitization. Geochemical analysis reveals that the limestones have an average δ18Ocal of –10.73‰ VPDB and δ13Ccal of –7.84‰ VPDB, whereas average dolomite δ18Odol is significantly higher (–1.06‰ VPDB) but δ13Cdol values (–3.04‰ VPDB; range –10 to 0‰) overlap with δ13Ccal values. Additionally, δ13Cdol trends toward normal marine values with depth away from the calcite–dolomite contact in all three cores. Petrographic observations demonstrate that dolomite crystals are commonly included in calcite and partially to completely replaced by calcite in these intervals and suggests that dolomite formed before calcite in the Microcodium-bearing intervals. Furthermore, the dolomites are commonly cemented by gypsum in the Traina Mbr. in southern Qatar, suggesting that dolomitization may have also occurred before, or concurrent with, bedded gypsum formation and indicates that dolomitization occurred early. Early dolomites were subsequently replaced by Microcodium-bearing limestones at and immediately below paleo-exposure surfaces, and at greater depths recrystallized in mixed marine–meteoric fluids, producing a negative δ13Cdol signature that trends toward more positive values away from the limestone–dolomite contact. Lastly, the dolomites underwent another phase of recrystallization in either marine-dominated fluids or possibly a well-mixed aquifer setting, resulting in a near-0‰ δ18Odol signature but retaining the negative δ13C signature. These findings thus have implications for reconstructing the diagenetic history of carbonate rocks, as they suggest that early diagenesis of carbonates can be extremely complex, resulting in multiple stages of mineral replacement and isotopic exchange in meteoric and shallow marine fluids before significant burial. Furthermore, this study shows that dolomitization of a limestone does not necessarily prevent additional early diagenesis and multiple recrystallization events. Lastly, it emphasizes the importance of incorporating petrographic observations with geochemical data when interpreting the diagenetic history of carbonate rocks.
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The Gambier Limestone is one of several extensive, shallow water, shelf carbonates of Eocene to Miocene age exposed along the southern margin of Australia. It is a muddy to grainy bryozoan calcarenite, with accessory benthonic foraminifers and echinoids. The sediments, originally composed almost entirely of calcite or Mg-calcite, have been in vadose and phreatic environments for over 10 m.y., yet are virtually unlithified. The only cements of any consequence are epitaxial on echinoids. Numerous karst features, dolines, caves, speleothems, and surface karren attest to prolonged residence in the meteoric zone. The Gambier is presently one of the best freshwater aquifers in Australia, with most flow intergranular and through sediments with over 30% porosity. Cementation is by minor intergranular pressure solution, which has developed under overburden of less than 100 m. The overlying Naracoorte Limestone (Miocene), a calcarenite of warmer water aspect, contains numerous aragonite molds and is cemented marble-hard. The authors propose that such temperate-water calcite limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than aragonite-rich tropical sediments. It is probable that many similar early and middle Paleozoic calcite limestones may have been in the meteoric zone for prolonged periods, yet contain little more » or no petrographic or geochemical record of such exposure. « less
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