ORDOVICIAN LOW-MG TO INTERMEDIATE-MG CALCITE MARINE CEMENTS FROM SWEDEN - MARINE ALTERATION AND IMPLICATIONS FOR OXYGEN ISOTOPES IN ORDOVICIAN SEAWATER

Petrography demonstrates the presence of three types of fibrous calcit e cement in buildup deposits of the Kullsberg Limestone (middle Carado c), central Sweden. Translucent fibrous calcite has intrinsic blue lum inescence (CL) indicative of pure calcite. This cement has 2-5 mol% Mg CO3, low Mn and Fe (less than or equal to 100 p.p.m.), and is consider ed to be slightly altered to unaltered, primary low- to intermediate-M g calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with M n and Fe concentrations up to 1200 and 500 p.p.m., respectively. The h eterogeneous characteristics of this variety of fibrous calcite are ca used by diagenetic alteration of a translucent fibrous calcite precurs or. 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.) concentratio ns, with an abundance of bright luminescent patches, which formed duri ng alteration caused by reducing diagenetic fluids. The delta(13)C and delta(18)O values of all fibrous calcite form a tight field (delta(13 )C = 1.7 to 3.1 parts per thousand PDB, delta(18)O = -2.6 to -4.1 part s per thousand PDB) compared with fibrous calcite isotope values from other units Fibrous calcite delta(18)O values are larger than adjacent meteoric or burial cements, which have delta(18)O less than or equal to 8 parts per thousand PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the ma rine diagenetic realm. First-generation equant and bladed calcite ceme nts, which pre-date fibrous calcite, are interpreted as unaltered, low -Mg calcite marine cements based on delta(13)C and delta(18)O data del ta(13)C = 2.3 to 2.7 parts per thousand% PDB, delta(18)O = -2.8 to -3. 5 parts per thousand PDB). Unlike fibrous cement, which reflects globa l sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restrict ed settings. Kullsberg abiotic marine cements have larger delta(18)O v alues than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg delta(18)O 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.