Isotopic Tracers of Nonclassical Crystallization

The stable and clumped isotope compositions of crystalline (bio)minerals are routinely used for paleoclimate reconstruction and as tracers of elemental exchange during crystal growth and aging. Stable isotope tracers have also been shown in recent years to provide unique mechanistic insights into crystal growth pathways, potentially helping to differentiate classical and nonclassical growth. Despite this promise, relatively few studies have been performed to date investigating stable isotope fractionation or clumped isotope compositions of crystals formed nonclassically. In this chapter, we review what is currently known about isotopic signatures of nonclassical crystallization by two distinct types of pathway: amorphous-to-crystalline transformation and particle-mediated growth. For minerals, particularly carbonates, grown via an amorphous precursor, the preponderance of isotopic evidence points to crystallization by dissolution–reprecipitation. In this case, the solid isotopic (and trace element) composition is strongly influenced by how closed the fluid reservoir is to exchange with an aqueous solution reservoir. Growth by particle attachment or aggregation is an inherently multistep process, giving numerous opportunities for shifting the isotope composition through exchange with the fluid reservoir, and isotopic measurements can provide useful mechanistic constraints. We discuss these processes in the context of several important classes of minerals: phosphates, (oxy)hydroxides and oxides, sulfides, and sulfates. The isotopic signatures preserved in crystals is ultimately specific to the growth pathway, and it is critical to develop a better understanding of these relationships to accurately interpret (bio)mineral paleoproxy signatures.