Abstract

Herringbone calcite is a previously undescribed carbonate cement and sea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks. It is abundant in the nearly equal 2520 Ma Campbellrand-Malmani platform, South Africa, where field relationships, such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones, demonstrate that it precipitated from ambient marine water. This interpretation is supported by depositional relationships in the >= 2.6 Ga Huntsman Limestone of the Bulawayo greenstone belt, Zimbabwe; the 2.6 Ga Carawine Dolomite, Australia; the 1.90 Ga Rocknest Formation and the 1.8-1.2 Ga Dismal Lakes Group, Canada; the Ordovician Porterfield carbonate buildup, Virginia; and various Silurian carbonate buildups in the Midcontinent, United States. Each of these occurrences is associated with anaerobic depositional environments or organic-rich sediments. Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick. Microscopically, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement. The bases of the crystals are optically unoriented, but upwards in each crystal, the optical c axis rotates until it is perpendicular to crystal elongation. The tops of the elongate crystal are thus optically aligned and length slow. The light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals, whereas the dark bands correspond to the optically unoriented, lower parts of the elongate crystals. Microspar crystals are also present in some dark bands. A Mg-calcite precursor for herringbone calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of micro-dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low-Mg calcite. Precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and diffusion of a precipitation inhibitor away from the crystallization surface. Since herringbone calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water. Thus, the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time.

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