Rare earth element and strontium isotopic study of seamount-type limestones in Mesozoic accretionary complex of Southern Chichibu Terrane, central Japan: Implication for incorporation process of seawater REE into limestones.

Abstract

Ishimaki and Tahara limestones occur as exotic blocks juxtaposed in the Mesozoic (Jurassic) accretionary complex of Southern Chichibu Terrane in eastern Aichi Prefecture, central Japan. They are supposed to be of the seamount-type limestone, since they have no terrigenous materials and are intimately associated with greenstones. REE (rare earth elements) and Sr isotopic studies for the limestones have been made in order to know their geochemical characteristics, ages and origins. Their 87 Sr/ 86 Sr ratios, when referred to the seawater 87 Sr/ 86 Sr curve and relevant geological data, suggest that Ishimaki and Tahara limestones are the late Permian and the Carboniferous to the Early Permian, respectively. Two greenstone fragments found inside the Ishimaki limestone block and one greenstone sample associated with Tahara limestone block, resemble the Hawaiian alkali basalt in their REE and Y patterns. This is supporting the idea that the limestone blocks may be parts of reef limestones on ancient volcanic seamounts. All the limestone samples, except three unusual Tahara ones, show seawater REE and Y signatures in their chondritenormalized patterns. Their REE/Ca ratios, however, are 10 2 -10 4 times as high as those ratios of modern biogenic carbonates like corals and the seawater. Accordingly, seawater REE and Y were incorporated into the limestones, when originally biogenic carbonates transformed into inorganic calcite and its secondary growths occurred in diagenesis in contact with sufficient seawater. This view is favored by the reported REE partition experiment between calcite overgrowths and seawater solution. The seawater Ce anomaly as a function of water depth in the modern ocean is a key to infer the water depth of the REE and Y incorporation. The Ce anomalies given by log(Ce/Ce*) for about a half of Ishimaki samples and most of Tahara ones are between -0.5 and -0.2, which are compatible with the shallow water origin. Another half of Ishimaki samples, however, have log(Ce/Ce*) values between -0.7 and -1.0, suggesting moderately deep waters (ca. 500-1000 m deep or more). This may reflect such a situation that water depths of REE incorporation into the seamount-type limestones are generally greater than the depositional water depths of original biogenic carbonates because of the fate of limestone-capped volcanic seamounts decided by the oceanic plate motion.

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

“…Modern coral has the same REE/Ca ratio as seawater (Sholkovitz and Shen, 1995;Akagi et al, 2004;Wyndham et al, 2004), whereas REE/ Ca ratios of seamount-type limestones are 10 2 -10 4 times higher (Tanaka et al, 2003;Miura et al, 2004). Hence, seawater REEs were incorporated into the limestones when the original biogenic carbonate was recrystallized to inorganic calcite through dissolution and reprecipitation during early diagenesis and secondary growths of calcite occurred in contact with sufficient seawater (Tanaka et al, 2003). Therefore, high permeability and porosity of carbonate sediments during early diagenesis are important.…”

“…Seamount-type limestone is thought to have formed on volcanic seamounts in the open ocean environment away from continents (Sano and Kanmera, 1988). Such limestones could therefore have incorporated seawater REEs without the influence of terrigenous materials (Tanaka et al, 2003;Miura et al, 2004). Hence, Japanese Permian seamount-type limestones in Ishimaki, Tahara (Aichi Prefecture), Kuzuu (Tochigi Prefecture) and Ohnogahara (Ehime Prefecture) areas (Kawabe et al, 1994;Miura and Kawabe, 2000;Tanaka et al, 2003), were used to estimate REE abundances in Permian seawater.…”

“…Such limestones could therefore have incorporated seawater REEs without the influence of terrigenous materials (Tanaka et al, 2003;Miura et al, 2004). Hence, Japanese Permian seamount-type limestones in Ishimaki, Tahara (Aichi Prefecture), Kuzuu (Tochigi Prefecture) and Ohnogahara (Ehime Prefecture) areas (Kawabe et al, 1994;Miura and Kawabe, 2000;Tanaka et al, 2003), were used to estimate REE abundances in Permian seawater. Non-carbonate impurities constitute less than 4% of the limestone samples.…”

“…Hence, REE studies of marine carbonates and limestones have been carried out (Kawabe et al, 1991;Webb and Kamber, 2000;Kamber and Webb, 2001;Tanaka et al, 2003;Miura et al, 2004). REE abundances in ancient seawater, however, have not been inferred successfully yet.…”

“…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).…”