FABRIC AND URANYL RETENTION OF NEOMORPHOSED ARAGONITE BOTRYOIDS: CONSTRAINTS ON THE DIAGENETIC PROCESS

Folk pioneered the recognition of aragonite precursors, speculation on elemental interactions with carbonates, and utilization of his white-card technique to reveal relict fabric and distribution of organics. We propose that the degree of fabric retention and trace-element behavior can be used to gain insight into processes of neomorphism of aragonite to calcite. Our examples are relict botryoidal aragonite cement fabrics preserved in mosaic calcites of the Upper Carboniferous Laborcita algal mounds (Sacramento Mts, N.M.). Their brown color and pleochroism have been attributed to ubiquitous fine-scale organic inclusions. The white-card technique reveals that the better the fabric preservation, the more uniform and brown the color. The fibrous fabric is mainly preserved by solid opaque organic and other non-carbonate inclusions. Inclusions of relict aragonite are oriented with growth direction. The calcite with the relict aragonite fabric has 4.1-4.6 ppm U and 1000-2000 ppm Sr. Preliminary synchrotron X-ray analyses of the intermediate energy U-M5 edge shows U is mainly in the oxidized state (uranyl ion). Modern marine aragonite cements have approximately 4 ppm U and 8,000-10,000 ppm Sr. If the Carboniferous aragonite had concentrations similar to the modern, then U was largely retained while Sr was decreased up to 90%. Paradoxically, based on distribution coefficients (Kd's), more U than Sr should have been lost during the replacement of aragonite by calcite (Kd for Sr+2 in calcite is 0.05 to 0.14; Kd for U+6 is 0.002-0.05). We propose that in the reaction zone during replacement U was either physically trapped or sequestered by another phase. It is unlikely that the uranyl ion alone was trapped because it is similar in size to CO3. However, if a uranyl-organic-matter complex was originally incorporated into the aragonite, then such a molecule may have been larger than the “sieve size” of the reaction zone. This scenario predicts homogeneous distribution of U. Alternatively, the uranyl ion may have been sequestered by the organics or oxides defining the relict fabric. This predicts a heterogeneous distribution with U concentrated in opaque inclusions. Both scenarios are consistent with U retention coupled with fabric preservation.