Ordovician low‐ to intermediate‐Mg calcite marine cements from Sweden: marine alteration and implications for oxygen isotopes in Ordovician seawater

Abstract

ABSTRACTPetrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO3, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low‐ to intermediate‐Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light‐brown turbid fibrous calcite has low MgCO3 (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ13C and δ18O values of all fibrous calcite form a tight field (δ13C=1·7 to 3·1‰ PDB, δ18O= − 2·6 to − 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ18O values are larger than adjacent meteoric or burial cements, which have δ18O δ − 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First‐generation equant and bladed calcite cements, which pre‐date fibrous calcite, are interpreted as unaltered, low‐Mg calcite marine cements based on δ13C and δ18O data (δ13C = 2·3 to 2·7‰ PDB, δ18O= − 2·8 to − 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first‐generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ18O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ18O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.