Discrepancy Between Thin-Section and X-Ray Estimates of Dolomite in Limestone

Lumsden, David N.

doi:10.1306/212f7761-2b24-11d7-8648000102c1865d

Cited by 58 publications

(32 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The percent dolomite (relative to calcite) was calculated following the approach of Goldsmith and Graf (1958). Dolomite stoichiometry (mol% MgCO 3 ) was calculated using the position of the corrected (104) dolomite reflection (Lumsden, 1979). Cation ordering, which describes the proportions of Ca and Mg correctly positioned within alternating cations layers, was calculated following Goldsmith and Graf (1958).…”

Section: Methodsmentioning

confidence: 99%

Evaluating reflux dolomitization using a novel high-resolution record of dolomite stoichiometry: A case study from the Cretaceous of central Texas, USA

Manche

Kaczmarek

2019

Geology

View full text Add to dashboard Cite

Refluxing brines are routinely invoked to explain dolomitization in peritidal settings. One such example is the Cretaceous Upper Glen Rose Formation (UGR) of central Texas, United States. The UGR is characterized by high-frequency cycles of variably dolomitized limestones interpreted to reflect periodic deepening and shallowing in a peritidal setting. Previous work suggests that reflux commenced near the top of each depositional cycle when evaporative brines seeped downward, causing dolomitization of the underlying sediments. To test this model, a novel high-resolution lithological, mineralogical, and geochemical data set is presented. These data show that transgressive facies successions exhibit a pattern of decreasing dolomite abundance, dolomite stoichiometry, and δ 18 O (relative to Vienna Peedee belemnite [VPDB]), and increasing dolomite crystal size. In contrast, regressive facies successions are characterized by a pattern of increasing dolomite abundance, dolomite stoichiometry, and δ 18 O (VPDB), and decreasing dolomite crystal size. No such variability is observed laterally. The patterns suggest that fluids did not migrate and evolve downward or basinward during reflux, which would have produced a systematic decrease in dolomite abundance and dolomite stoichiometry within each cycle. The data instead support a model of syndepositional dolomitization whereby each facies was dolomitized before deposition of the overlying sediments. The mineralogical and geochemical trends thus reflect temporal changes in water depth as well as a combination of the Mg/Ca, temperature, and salinity of the formative solutions. These results suggest that syndepositional dolomitization in peritidal environments may be a more common and widespread occurrence than previously recognized, particularly during greenhouse conditions.

show abstract

Section: Methodsmentioning

confidence: 99%

Evaluating reflux dolomitization using a novel high-resolution record of dolomite stoichiometry: A case study from the Cretaceous of central Texas, USA

Manche

Kaczmarek

2019

Geology

View full text Add to dashboard Cite

show abstract

“…All other mineral phases were adjusted accordingly. Where dolomite was present in a sample, the MgCO 3 content of dolomite was calculated based on the d-value of the [104] peak (Lumsden, 1979).…”

Section: Xrd Analysismentioning

confidence: 99%

Expedition 356 methods

Gallagher¹,

Fulthorpe²,

Bogus³

et al. 2017

Proceedings of the International Ocean Discovery Program

View full text Add to dashboard Cite

show abstract

“…In particular, the origin of ionic solutions conducive to dolomite formation is still unclear. Comparison with modern environments shows that ionic solutions may either be seawater-derived, as shown for the sabkhas along the Persian Gulf coast, where several hydrological mechanisms have been discussed (Adams and Rhodes, 1960;Hsü and Siegenthaler, 1969;McKenzie et al, 1980, McKenzie, 1981; see Machel, 2004, for an overview; see also Teal et al, 2000), or derived from continental groundwater, as shown for the coastal ephemeral lakes of the Coorong area (Australia; Alderman and Skinner, 1957;Von der Borch et al, 1975;Rosen et al, 1989;Warren, 1990). While both types of fluid become concentrated during evaporation and are, perhaps, modified by the precipitation of carbonates and evaporites, it remains unclear which source prevailed during deposition of the Travenanzes Formation.…”

Section: Introductionmentioning

confidence: 96%

“…Chilingar (1956) suggested that the portion of dolomite in carbonates increases with geological age, implying replacement during burial. However, burial dolomitization requires a mechanism to pump large volumes of Mg-rich water through porous rock (Machel, 2004), and is not always a viable process. There is evidence that large amounts of dolomite could have formed under near-surface conditions (penecontemporaneous dolomite) at certain times in Earth's history, and several studies linked the abundance of dolomite to secular variation in seawater chemistry, with primary dolomite preferentially forming during times of "calcite seas" (Given and Wilkinson, 1987;Warren, 2000;Burns et al, 2000).…”

Section: Introductionmentioning

confidence: 99%

Precipitation of dolomite from seawater on a Carnian coastal plain (Dolomites, northern Italy): evidence from carbonate petrography and Sr isotopes

et al. 2019

View full text Add to dashboard Cite

Abstract. The geochemical conditions conducive to dolomite formation in shallow evaporitic environments along the Triassic Tethyan margin are still poorly understood. Large parts of the Triassic dolomites in the Austroalpine and the southern Alpine realm are affected by late diagenetic or hydrothermal overprinting, but recent studies from the Carnian Travenanzes Formation (southern Alps) provide evidence of primary dolomite. Here a petrographic and geochemical study of dolomites intercalated in a 100 m thick Carnian sequence of distal alluvial plain deposits is presented to gain better insight into the conditions and processes of dolomite formation. The dolomites occur as 10 to 50 cm thick homogeneous beds, millimetre-scale laminated beds, and nodules associated with palaeosols. The dolomite is nearly stoichiometric with slightly attenuated ordering reflections. Sedimentary structures indicate that the initial primary dolomite or precursor phase consisted largely of unlithified mud. Strontium isotope ratios (87Sr∕86Sr) of homogeneous and laminated dolomites reflect Triassic seawater composition, suggesting precipitation in evaporating seawater in a coastal ephemeral lake or sabkha system. However, the setting differed from modern sabkha or coastal ephemeral lake systems by being exposed to seasonally wet conditions with significant siliciclastic input and the inhibition of significant lateral groundwater flow by impermeable clay deposits. Thus, the ancient Tethyan margin was different from modern analogues of primary dolomite formation.

show abstract

Discrepancy Between Thin-Section and X-Ray Estimates of Dolomite in Limestone

Cited by 58 publications

References 0 publications

Evaluating reflux dolomitization using a novel high-resolution record of dolomite stoichiometry: A case study from the Cretaceous of central Texas, USA

Evaluating reflux dolomitization using a novel high-resolution record of dolomite stoichiometry: A case study from the Cretaceous of central Texas, USA

Expedition 356 methods

Precipitation of dolomite from seawater on a Carnian coastal plain (Dolomites, northern Italy): evidence from carbonate petrography and Sr isotopes

Contact Info

Product

Resources

About