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<p>A case study is presented for high Sr<sup>2+</sup> concentration controls on an amorphous calcium carbonate-vaterite-calcite phase transformation as the dominant diagenetic process in sediments of a Canadian geothermal spring. This spring in Canadian Cordillera discharges onto bedrock of Proterozoic metasediments that overlie the Fry Creek batholith in southeastern British Columbia. The groundmass microfabrics of the unconsolidated orange-brown bottom sediment and an indurated white crust consist of  &gt; 100&#xa0;µm calcite flakes, each characterized by irregular edges and lack of crystal face outlines with one or more 1–2&#xa0;µm central holes that developed during phase transformation. A precursor amorphous calcium carbonate (ACC) phase transformed into an intermediate phase of vaterite as the ACC dehydration progressed toward endmember calcite. ACC nanospherules within biofilm domains rapidly expanded into hollow hexagonal vaterite mesocrystals, each characterized by a central hole. High Sr<sup>2+</sup> adsorption (4300–5500&#xa0;ppm) at the ACC-vaterite interface stabilized the central hole resulting from the dissolution of the precursor ACC nanospherule as the ACC-vaterite transformation proceeded toward the growth of endmember calcite flakes. The Sr<sup>2+</sup> concentration (8300–12000&#xa0;ppm) and low Mg<sup>2+</sup> concentration in the overlying crust resulted in the distribution of ACC nanospherules and calcite microspherules on the calcite flake surfaces. The ACC-vaterite-calcite phase transformation characterizing the overlying white crust microfabrics partially preserved ACC nanospherules, but many transformed into larger calcite microspherules, a process that did not occur in the bottom sediment because of lower Sr<sup>2+</sup> concentration. The transition from an ACC nanospherule to a calcite microspherule included an intermediate semi-spheroidal crystalline shape characterized by a dodecahedron outline and terraced growth surfaces as the crystal form evolved into a completely spheroidal shape. The strong strontium adsorption resulted in the interiors of calcite spherules consisting of mosaics of ACC-sourced anhedral crystallites, in contrast to Mg<sup>2+</sup> controls on spherulitic interiors characterized by radial-fiber or concentric growth layers in other deposits.</p>

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Spherulitic microfabrics of calcareous deposits along the Dewar Creek geothermal spring in the Canadian Cordillera: case study for Sr2+ controls on amorphous calcium carbonate-vaterite-calcite phase transformation

  • 期刊论文

  • 发表日期：

    2025年11月24日

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  • Paul L. Broughton

摘要

A case study is presented for high Sr2+ concentration controls on an amorphous calcium carbonate-vaterite-calcite phase transformation as the dominant diagenetic process in sediments of a Canadian geothermal spring. This spring in Canadian Cordillera discharges onto bedrock of Proterozoic metasediments that overlie the Fry Creek batholith in southeastern British Columbia. The groundmass microfabrics of the unconsolidated orange-brown bottom sediment and an indurated white crust consist of  > 100 µm calcite flakes, each characterized by irregular edges and lack of crystal face outlines with one or more 1–2 µm central holes that developed during phase transformation. A precursor amorphous calcium carbonate (ACC) phase transformed into an intermediate phase of vaterite as the ACC dehydration progressed toward endmember calcite. ACC nanospherules within biofilm domains rapidly expanded into hollow hexagonal vaterite mesocrystals, each characterized by a central hole. High Sr2+ adsorption (4300–5500 ppm) at the ACC-vaterite interface stabilized the central hole resulting from the dissolution of the precursor ACC nanospherule as the ACC-vaterite transformation proceeded toward the growth of endmember calcite flakes. The Sr2+ concentration (8300–12000 ppm) and low Mg2+ concentration in the overlying crust resulted in the distribution of ACC nanospherules and calcite microspherules on the calcite flake surfaces. The ACC-vaterite-calcite phase transformation characterizing the overlying white crust microfabrics partially preserved ACC nanospherules, but many transformed into larger calcite microspherules, a process that did not occur in the bottom sediment because of lower Sr2+ concentration. The transition from an ACC nanospherule to a calcite microspherule included an intermediate semi-spheroidal crystalline shape characterized by a dodecahedron outline and terraced growth surfaces as the crystal form evolved into a completely spheroidal shape. The strong strontium adsorption resulted in the interiors of calcite spherules consisting of mosaics of ACC-sourced anhedral crystallites, in contrast to Mg2+ controls on spherulitic interiors characterized by radial-fiber or concentric growth layers in other deposits.