UNUSUAL CARBONATE CRYSTAL FABRICS AS INDICATORS OF OCEAN CHEMICAL EVOLUTION (Invited Presentation)

Crystal fabrics in precipitated carbonate provide fundamental information regarding he relative rates of crystal nucleation and crystal growth. Such information is critical to inferring potential mineralogy of ancient carbonate rocks (e.g. calcite vs high-Mg calcite vs aragonite), interpreting chemical environments of growth (e.g. marine vs meteoritc waters), and deciphering pathways of early diagenetic recrystallization.

There are, however, some carbonate crystal fabrics that remain relatively poorly understood. Herringbone carbonate, first described from Archean rocks (cf. Sumner and Grotzinger, 1996), consists of elongate lathes that show an unusual rotation of the c-axis from parallel, to perpendicular, to the growth axis of the crystal. Such rotation of the c-axis suggests that herringbone carbonate represents the neomorphic recrystallization of spherulitic, or curved, crystal growth. Curvature of crystals during growth commonly results from: (1) the presence of impurities (including organic macromolecules) that inhibit crystal growth along specific faces, (2) the presence of ions that favors secondary nucleation along the primary crystal growth front, or (3) conditions that favor rapid crystal growth and associated lattice defects.

Since its original description, documentation of the geologic distribution of herringbone carbonate has been ongoing. Original findings in such varied environments as Archean marine rocks and Eocene travertine place our primary constraints on the origin of herringbone carbonate. Here I describe occurrences from Mesoproterozoic, Early Paleozoic, and Late Paleozoic rocks, and suggest that marine environments containing herringbone carbonate broadly track the position of a marine chemocline, thereby providing a potential link between carbonate crystal morphology and ocean redox.