Recrystallization versus neomorphism, and the concept of ‘significant recrystallization’ in dolomite research

Abstract

The term ‘neomorphism’, originally introduced in 1965 as a substitute for the term ‘recrystallization’ in limestones, is now in use also for dolostones. However, only very few dolomite researches use the term neomorphism (even though it is well and strictly defined), many do not know what this term means, and using the term commonly leads to confusion among carbonate researchers, as well as between them and other geologists. Partially or largely for these reasons, most dolomite researchers use the term recrystallization rather than neomorphism. In addition, the term neomorphism does not encompass all the important characteristics of recrystallization known today. It is recommended, therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones: textural changes (size, shape), structural changes (ordering, strain), compositional changes (stoichiometry, isotopes, trace elements — including zoning, fluid inclusions), and changes in the paleomagnetic properties. For practical applications, the absence, presence and/or degree of recrystallization is often important for a genetic interpretation of dolostones. If changes via recrystallization in texture, structure, composition, and/or paleomagnetic properties results in data ranges that are larger than the original ones (which must be known for reference), a dolomite/ dolostone is said to be ‘significantly recrystallized’ and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization. However, not all measurable properties must be reset during recrystallization. In this case, inherited properties represent the event of dolomitization, whereas reset properties represent the fluids and process of recrystallization. In recrystallization resets only one of 10 properties (e.g., 87Sr/ 86Sr ratios) beyond the range of the pristine samples (within the analytical errors), this dolomite is significantly recrystallized with respect to Sr isotopes, yet insignificantly recrystallized with respect to the other nine properties. In either case, values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization. The concept of ‘significant recrystallization’ is of great use in genetic interpretations of dolomites/dolostones. In particular, a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization. Rather, it is sufficient to recognize that a dolomite is insignificantly recrystallized. Application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized. Finally, the concept of significant recrystallization is independent of mineralogy. Hence, it can and should be applied also to limestones.