Experimental REE partitioning between calcite and aqueous solution at 25.DEG.C. and 1atm: Constraints on the incorporation of seawater REE into seamount-type limestones

Abstract

Partitioning experiments of rare earth elements (REEs) between calcite and aqueous (CaCl 2 + NaCl) solution at 25°C and 1 atm have been made in order to elucidate the incorporation process of seawater REE into marine limestones. Calcite-supersaturated solution doped with REE was constantly pumped into a reactor, in which calcite seeds and the solution were mixed and CO 2 + N 2 gas was bubbled to allow calcite overgrowths. Absolute values of REE partition coefficients, and REE analyses of upper Paleozoic Japanese Ishimaki and Tahara limestones, REE abundance patterns for seawater coexistent with the marine limestones have been calculated provided that pH, ∑CO 2 and salinity are the same in the ancient and present times. The calculated patterns are quite similar to those for relatively deep waters at around 400 to 1000 m in the modern Pacific. Ishimaki and Tahara limestones are of the seamount-type. Hence, the calculated seawater REE patterns with large negative Ce anomalies suggest that the incorporation of seawater REE into the limestones occurred in moderately deep water, probably because of subsidence of volcanic seamounts that were capped by Ishimaki and Tahara limestones.Keywords: rare earth element, partition coefficient, seamount-type limestone, tetrad effect, carbonate complex lies, negative Eu anomalies and higher Y/Ho ratios than the chondritic one (Kawabe et al., 1991;Bau et al., 1995;Nozaki et al., 1997). Masuda et al. (1987) classified the tetrad effects observed in chondrite-normalized REE patterns for geochemical samples into the two types: Wshaped (concave) and M-shaped (convex) ones. Seawater shows a representative concave tetrad effect of W-type, whereas evolved granitic rocks often indicate convex tetrad effects of M-type. Brief reviews of recent studies of tetrad effects in REE geochemistry are given in Kawabe and Masuda (2001) and Monecke et al. (2002).Japanese seamount-type Permian limestones also have seawater-like REE characteristics such as concave tetrad effects, negative Ce anomalies, negative Eu anomalies and higher Y/Ho ratios than the chondritic one in their chondrite-normalized REE abundance patterns (Kawabe et al., 1991;Tanaka et al., 2003). This strongly suggests that seawater REEs have been incorporated into seamount-

A new method for the study of trace element partitioning between calcium carbonate and aqueous solution: A test case for Sr and Ba incorporation into calcite

“…The constant addition system mentioned above (Zhong and Mucci, 1993) can maintain a steady state environment (i.e., constant saturation state and pH). However, the amounts of the crystals formed in this technique are rather small in normal experiments (e.g., Tanaka et al, 2004). Even though this method seems to have some advantages, it would be better to compare the experimental partition coefficients obtained by one method with those in other systems.…”

REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite and aqueous solution

“…REE partition coefficients between CaCO 3 and aqueous (seawater) solution have been reported (Terakado and Masuda, 1988;Zhong and Mucci, 1995), but they are inadequate to estimate REE abundances in ancient seawater. In our earlier work (Tanaka et al, 2004), we carried out partitioning rate of 0.26-0.29 ml/min. The temperature of the reactor was maintained at 25 ± 0.2°C.…”

“…The input solution (CaCl 2 + NaCl solution) was prepared in a similar manner to that of Tanaka et al (2004). The cation concentrations in the input solution were as follows: Ca: 0.2 mol/kg, Na: 0.05 mol/kg and individual REEs: 10 ppb (Pr,Sm,Tb,Ho and Tm: 20 ppb).…”

“…Before the ICP-AES determination, REEs in the experimental products were purified by a pre-concentration technique utilizing co-precipitation with Fe(OH) 3 and cation-exchange column chromatography (Kawabe et al, 1991;Tanaka et al, 2003;Tanaka et al, 2004). More detailed procedures of the REE analysis are described in Tanaka et al (2004). Calcium and Na concentrations in the input solutions were also determined by ICP-AES (SEIKO SPS-1500R).…”

“…Experimental conditions for respective runs are listed in Table 1. REE concentrations in the experimental products of separated seed crystals plus overgrowths and reacting solution (final solution) samples were determined by ICP-AES (SEIKO SPS-1500R). Before the ICP-AES determination, REEs in the experimental products were purified by a pre-concentration technique utilizing co-precipitation with Fe(OH) 3 and cation-exchange column chromatography (Kawabe et al, 1991;Tanaka et al, 2003;Tanaka et al, 2004). More detailed procedures of the REE analysis are described in Tanaka et al (2004).…”