Y-Ho fractionation and lanthanide tetrad effect observed in cherts

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The abundances of rare earth elements (REE) and Y in cherts were precisely determined by ICP-MS: the Archean Marble Bar Chert from the Pilbara Block, Western Australia, Triassic cherts from central Japan, and Cretaceous deep-sea cherts from central Pacific and the Caribbean Sea. Both of W-type and M-type REE patterns are observed for the studied cherts and, in particular, clear W-type tetrad effect is observed for the Archean Marble Bar banded cherts. Appearance of tetrad effect phenomenon is related to their Fe<sub>2</sub>O<sub>3</sub><sup>∗</sup> (total iron as Fe<sub>2</sub>O<sub>3</sub>) and Al<sub>2</sub>O<sub>3</sub> concentrations. The cherts relatively enriched in Fe<sub>2</sub>O<sub>3</sub><sup>∗</sup> show W-type tetrad effect, while that enriched in Al<sub>2</sub>O<sub>3</sub> tends to show M-type tetrad effect. Therefore, impure clastic detritus in the cherts might be responsible for M-type tetrad effect. It is worthwhile noting that, irrespective of their REE patterns of W-type and M-type tetrad and little tetrad effects, the cherts chiefly show non-chondritic Y/Ho ratios between 13 and 27, negative deviations from chondritic value around 28, in contrast with seawaters and limestones characterized by strongly higher Y/Ho ratios ranging from 50 to 80 than the chondritic value. Among the cherts, the lowest Y/Ho ratios between 13 and 18 are observed for the Marble Bar yellowish gray cherts, which suffered disturbance of geochemical features by thermal events at ca. 2.5 Ga. It has been widely recognized that both of Y-Ho fractionation and lanthanide tetrad effect are phenomena observed in the marine environment. However, Y-Ho fractionation is not closely associated with lanthanide tetrad effect. Y-Ho fractionation is suggested to be related to the ligand that dominates the chemical complex in aqueous system, from our results and the observation that silicate-rich part and carbonate-rich part in banded iron formations show lower and higher Y/Ho ratios than the chondritic value, respectively. On the other hand, tetrad effect in the cherts is related to impurity minerals such as Fe-oxides and clastic detritus, with which REE might well by associated. It is interesting that the Archean banded charts, not containing organic nor detrital materials, display clear W-type tetrad effect coupled with slightly lower Y/Ho ratios than chondritic ratio.



    GEOCHEMICAL JOURNAL 32(6), 405-419, 1998-12-01


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