HERRINGBONE CALCITE FROM FRACTURES FEEDING EOCENE TUFA DEPOSITS, CHALLIS, IDAHO: DO THEY PROVIDE POSSIBLE GEOCHEMICAL CONSTRAINTS ON EARTHS EARLIEST LIFE?

Herringbone calcite (HC) is a unique carbonate that precipitated in association with some of Earth’s earliest evidence for life. Archean (2.9-2.5 Ga) HC occurs as widespread sheets that formed across much of the seafloor and commonly encrusted microbial communities, although it rarely, if ever, contains microbialites. Association of seafloor HC with in situ aragonite precipitation indicates Earth’s earliest oceans were likely oversaturated with respect to calcium and alkaline. By 1.8 Ga and throughout most of the Phanerozoic HC was restricted to marine Neptunian dikes or terrestrial caves.

Large (up to 8 m thick) sheets of flat-lying carbonate tufa cap local ridges, deposited directly on Eocene (~35-40 Ma) volcanics near Challis, Idaho. HC formed perpendicular to the walls of near-vertical faults that provided conduits for the fluids that formed the overlying, tufa deposits. HC ranges from fill in fractures (5 to 50 cm wide) to rinds (up to 20 cm thick) cementing large volcanic breccia clasts. Domal HC structures growing from the sides of the fractures indicate that at the time of their deposition open space was available for the domes to grow. Large HC blades (up to 3 cm tall) occur throughout the site, and commonly these grade into more typical HC textures (< 1 cm tall) and then into muddy, very fine grained textures.

Oxygen stable isotope analyses of the Challis HC and tufa are both highly depleted, and using the temperature equation of Hays and Grossman (1991) indicate they formed in waters ranging from 120° to 150° C. δ¹³C values range from –0.1 to –1.9 indicate there was little or no biological activity during precipitation of this fabric, in accordance with plane light petrography. The HC has elevated Sr (499-2041 ppm), Zn (3-66 ppm), and Zr (5-11 ppm). The Sr enrichment is likely derived from the dissolution and reprecipitation of CaCO₃ from underlying Paleozoic carbonates, whereas Zn and Zr enrichment is interpreted to be a volcanic/igneous source associated with emplacement of the volcanic rocks. No evidence of life occurs in the Challis HC, but the overlying Challis tufa has ample evidence of microbial life, indicating the waters that formed the HC were capable of supporting life. However, a change in their physical or chemical characteristics (temperature?) was necessary before life was supported.