CONODONT APATITE: A GEOCHEMICAL AND ISOTOPIC TIME CAPSULE OF THE PALEOCEAN?

Interest in the chemical evolution of the paleocean has increased the search for minerals that reliably record chemical/isotopic composition of the ancient ocean. Conodont apatite has long been considered because of its low chemical reactivity and high concentration of elements suitable for isotope analyses. Nonetheless, the interpretation of conodont composition has been limited by uncertainty: Are chemical elements primary (in vivo) or incorporated as a result of postmortem recrystallization/exchange on the seafloor and during diagenesis? We attempt to reduce this uncertainty by thorough characterization of conodonts of different ages and thermal maturities using petrographic, (optical, SEM, CL), microanalytical (laser ablation ICP-MS, microprobe), X-ray diffraction (XRD), and isotopic (Sr, Pb, Nd) analyses.

Differences in crystallinity and chemical composition among morphologic phases of conodont elements are most simply explained as primary differences. Conodonts exposed to saline depositional waters or temperatures > 300˚ C (CAI 5) are excluded because of visible recrystallization. XRD of white and hyaline matter display well ordered, highly crystalline apatite patterns. Basal plates, however, are significantly less crystalline and similar to other biogenic/sedimentary apatite. Systematic chemical differences in Sr, transition, and REE elements between hyaline and white apatite and fine-scale chemical zoning within single elements are believed to reflect biologic growth. Moreover, the absence of chemical zoning from core to rim or around natural internal porosity in conodonts suggests elements were not introduced postmortem.

Variance in some samples can be shown to result from mixing of conodonts that are coeval with sedimentation and mechanically reworked older conodonts. Compositional changes between samples collected within 5 cm of vertical section emphasize the need for thin sample intervals. Distinct chemical and isotopic signatures contained in reworked conodonts and closely spaced samples, despite having undergone identical diagenetic histories, suggests retention of primary chemical composition. Taken together this evidence suggests that careful characterization of conodont apatite permits robust interpretations of paleocean chemical compositions.