ABSTRACT

Distribution coefficient data for the coprecipitation of Sr⁺² with aragonite and calcite at low temperatures enable the Sr⁺² concentrations of carbonate minerals and rocks to be interpreted in terms of the strontium/calcium ratio of the precipitating solution and the temperature of precipitation. Sea water and a variety of other natural waters have been analyzed to provide the necessary solution data. The best value of the ratio ^mSr⁺²/^mCa⁺² in sea water has been determined to be (0.86 ± 0.04) 10^-2. Predicted Sr⁺² concentrations of aragonite precipitated from sea water are 8290 ± 850 ppm for the Bahamas and 8200 ± 1110 ppm for the Persian Gulf. Bahaman and Persian Gulf oolitic aragonite contains 9800 ± 500 ppm and 9590 ± 500 ppm Sr⁺² respectively, some 17-18 percent higher than predicted values. Bahaman grapestone aragonite contains 9520 ± 600 ppm Sr⁺² It is suggested that organic complexing of cations at the sites of precipitation may account for these differences.

Reef coral aragonites from the Bahamas and Persian Gulf contain 7980 ± 300 and 7740 ± 300 ppm Sr⁺² respectively. Bahaman codiacean algal aragonites contain 8740 ± 600 ppm Sr⁺² The uptake of Sr⁺² by both corals and algae seems to be affected only slightly by biochemical fractionation, in comparison with most molluscan aragonites which show very strong biochemical fractionation of cations. The predicted Sr⁺² concentration of calcite precipitated from sea water is about 1200 ppm. Biogenic calcites of pelagic foraminifera and some molluscs have closely similar Sr⁺² concentrations and thus exhibit minimal biochemical fractionation.

Lagoonal aragonite muds from the Persian Gulf have a Sr⁺² concentration of 9390 ± 500 ppm, a value very similar to that of oolitic aragonite. This similarity and the lack of any obvious skeletal breakdown or abrasion source strongly suggest that the aragonite is a non-skeletal ("inorganic") precipitate, although a bacterial, or indirect algally induced precipitation mechanism, cannot be ruled out.

The Sr⁺² concentrations of diagenetically altered limestones are demonstrated to be of potential value in indicating the mechanisms of diagenesis. Two simple diagenetic analogues are discussed in detail, the system and the open-system recrystallization of carbonate sediments in the presence of an aqueous solution. In closed-the diagenetic alteration of typical aragonitic sediments the former process may give rise to calcites with 700-10,000 ppm Sr⁺² whereas calcites with much lower Sr⁺² concentrations may result from the latter process (350 ppm or less). The rather low Sr⁺² concentrations of ancient limestones and the relatively low solubility of CaCO₃ minerals dictate that an open system prevailed through which rather large volum s of pore fluid migrated during diagenesis (probably > 10⁵ pore volumes). These low Sr⁺² concentrations also suggest that even if diagenetic alteration does occur with a sea-water pore fluid, later fresh-water diagenesis largely masks these early changes.