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Evaluating the Relationship Between Stoichiometry and Cation Ordering in Ancient Dolomites

Abstract

Over the past century, geologists have used various proxy resources to help explain the mechanisms responsible for dolomitization. Recently, laboratory experiments examining the fundamental controls on dolomite stoichiometry and cation ordering suggest that these mineralogical parameters may provide useful diagenetic information about the conditions of dolomitization. To test the applicability of these parameters to natural systems, this study presents a newly compiled global dataset of stoichiometry and cation ordering from over 1,200 dolomite samples from various geographic locations, geologic ages, and depositional and diagenetic environments. A number of relationships are examined, including the degree to which cation ordering and stoichiometry vary spatially and temporally, as well as the local and global geologic factors controlling the observed trends. The principal findings of this study are that: (i) dolomites formed in evaporitic settings are generally more stoichiometric than those formed in normal marine settings; (ii) dolomites associated with higher formation temperatures are generally more stoichiometric and well-ordered than their lower temperature counterparts; (iii) stoichiometry and cation ordering, though not co-dependent, may covary under certain dolomitization conditions; (iv) stoichiometric variations in stratigraphically related dolomites reflect dolomitization by evolving fluids. Taken together, these observations are inconsistent with the hypothesis that older dolomites are more stoichiometric because they’ve undergone a higher degree of recrystallization. Rather than reflecting global scale conditions, the data suggest that dolomite stoichiometry and cation ordering are controlled by local dolomitizing conditions, and thus make valuable proxies for understanding the conditions and fluids responsible for dolomitization.