A global study of dolomite stoichiometry and cation ordering through the Phanerozoic

Abstract

Various geochemical proxies are used to constrain the diagenetic origin and evolution of ancient dolomites. Dolomite stoichiometry (mole % MgCO3) and cation ordering, two mineralogical attributes that define dolomite, have also been shown to demonstrate utility in this regard. Observations from laboratory experiments and field studies suggest that these attributes broadly reflect the fluid chemistry and temperature of the dolomitizing environment. The degree to which these parameters reflect global conditions during dolomitization (e.g., seawater chemistry, eustasy, atmospheric pCO2) and long-term geological processes is poorly understood, however. Here, a large dataset consisting of mineralogical data from over 1,690 Phanerozoic dolomites from various geographic locations, stratigraphic ages, platform types, and depositional environments are queried to investigate the broader geological controls on dolomite stoichiometry and cation ordering in dolomites formed by early, near-surface dolomitization. A suite of statistical analyses performed on the global dataset indicate: 1) despite wide ranges at the eon, period, and epoch level, dolomite stoichiometry and cation ordering broadly increase with geologic age; 2) significant variations in dolomite stoichiometry and cation ordering throughout the Phanerozoic do not correlate with global parameters, such as seawater chemistry, eustasy, orogenic events, and ocean crust production; 3) dolomites associated with restricted depositional settings, such as restricted lagoons, and the intertidal and supratidal zones, are more stoichiometric than dolomites associated with open marine settings, such as the deep-subtidal and shallow-subtidal zones; and 4) dolomites from shallow ramps and epeiric carbonate platforms are generally more stoichiometric than dolomites from open shelves and isolated carbonate platforms. These observations permit a number of inferences to be drawn. First, the principal signal observed in the data is that local environmental conditions associated with platform type and depositional setting are the strongest control on dolomite mineralogy. The observation that more stoichiometric dolomites correlate with shallow and restricted depositional environments is consistent with laboratory experiments that show environmental factors, such as higher Mg:Ca, temperature, and salinity of the dolomitizing fluids yield more stoichiometric dolomite. Second, a weaker secondary signal is also observed such that dolomite stoichiometry and cation ordering both increase with geologic age, suggesting that progressive recrystallization driven by mineralogical stabilization during burial is also occurring. Collectively, these data suggest that spatial and temporal variations in stoichiometry and cation ordering reflect the interplay between local dolomitizing conditions near the surface and long-term mineralogical stabilization during burial.

Extensive recrystallization of Cenozoic dolomite during shallow burial: A case study from the Palaeocene–Eocene Umm er Radhuma formation and a global meta‐analysis

“…The findings reported here are broadly consistent with the hypothesis that young, metastable dolomites are often replaced by more stable dolomite, particularly those dolomites now preserved in the rock record (e.g. Lumsden & Chimahusky, 1980; Sperber et al ., 1984; Manche & Kaczmarek, 2021), but suggest that the dolomite stabilization process can happen very early and rapidly in terms of geological time, and possibly continue with successive burial and uplift. This process can undoubtedly hinder interpretations about environmental conditions that are conducive to initial dolomitization, as well as possibly obscure trends in primary ancient seawater chemistry, a similar conclusion reached by many others (Land, 1980; Mazzullo, 1992), but seldom fully appreciated.…”

“…The diversity of reported petrological observations in ancient dolomites has also led to a common assumption that most, if not all, have been recrystallized to some degree during burial (Land, 1980, 1992; Kupecz et al ., 1993; Machel, 2004; Manche & Kaczmarek, 2021). The high temperatures and long residence times experienced in the burial setting, it is argued, promote dolomite recrystallization (Kupecz et al ., 1993; Machel, 2004).…”

Shortwave infrared hyperspectral imaging as a novel method to elucidate multi-phase dolomitization, recrystallization, and cementation in carbonate sedimentary rocks

“…We distinguished five phases of dolomite, ranging from 49.67 to 50.21%Ca, that are approaching a stoichiometric, well-ordered endmember following 100’s of millions of years of diagenesis and several kilometers of burial 23 – 26 . The progressive recrystallization of dolomite over geological time is driven by mineralogical stabilization during burial that increases dolomite stoichiometry and cation ordering 61 . Consequently, this methodology has profound transferability to other, younger, successions that have not been subject to such burial, recrystallization, and mineralogical stabilization.…”

The Effect of Stoichiometry, Mg-Ca Distribution, and Iron, Manganese, and Zinc Impurities on the Dolomite Order Degree: A Theoretical Study

“…Once dolomite structures with different Mg-Ca arrangements and impurity contents were constructed, diffractograms were calculated with the RIETAN-FP code [19] included in VESTA software [16]. Then, I 01.5 /I 11.0 ratios were measured in the calculated diffractograms using a standard protocol that can be found elsewhere (e.g., [9,10,13,14,20,21]). This method consists in calculating the intensity I 01.5 /I 11.0 ratios by measuring peak intensities in counts, and without conducting any previous treatment of the diffractograms (e.g., Rietveld method).…”

“…Figure 3b shows the non-linear increase of I01.5/I11.0 ratio as the s parameter The results presented here were obtained by considering the structure of dolomite only. As explained in the introductory section, diffractograms of dolomites can show very low I 01.5 /I 11.0 ratios, indicating a very low cation order [14,20]. However, the minimum order to distinguish between a dolomite and a VHMC, i.e., when its structure changes from an R3 to an R3c, has not yet been clearly defined.…”