Rb-Sr and Sm-Nd systematics of cherts and other siliceous deposits

Abstract

We have analyzed a suite of marine and lacustrine cherts to determine the concentration of Rb, Sr, Sm and Nd and the isotopic composition of Sr and Nd. The samples ranged from Pliocene to Silurian in age and included nodular cherts in carbonates as well as bedded cherts. Massive samples were leached in acid to remove carbonate and both the leach and residue analyzed. The residues contained 0.1 to 17 ppm Rb and 0.5 to 20 ppm Sr. All samples have high Rb/Sr ratios (~0.1 to 0.8) and thus show enrichment of Rb relative to Sr of 6 to 60 times that of seawater. The Rb and Sr in pegmatitic quartz are much lower by factors of 10 to 10^3, although these also may have high Rb/Sr. The site of Rb and Sr in the cherts was not established but appears to be correlated with Al content. The high Rb/Sr ratios permit the use of cherts for dating in favorable circumstances. The Nd contents range from 9 to 1000 ppb with Sm/Nd ~ 0.16 to 0.19, typical of continental sources. The initial ^(87)Sr/^(86)Sr ratio for a Pacific deep sea chert is equal to that of associated carbonates and hence reflects the seawater environment. In shelf and marginal basin environments, the initial ^(87)Sr/^(86)Sr ratios of chert are distinctly more radiogenic than seawater. This is most obvious where a substantial clay component is present, but is observed even in samples with Al_2O_3 contents less than 0.1%. These results show the Rb-Sr system in chert to be rather isolated from the associated carbonate with very little Sr exchange. This separate behavior appears to persist through both early and late recrystallization stages. These data suggest that, in many cases, cherts are formed by "dissolved" SiO_2 nucleating on clay-like material which preserves the isotopic characteristics of the detrital source. The detrital component required is less than 1% of the total chert. In some cases, it is possible that the cherts were formed from silica-saturated waters from a nonmarine source, possibly in coastal environments or during uplift. These waters contained radiogenic Sr in solution that was not exchanged with the carbonate matrix. This latter mechanism is also indicated by oxygen isotopic studies (KNAUTH and EPSTEIN, 1976; KOLODNY and EPSTEIN, 1976). The Nd isotopic signature of the deep sea samples reflects the seawater composition in agreement with the initial ^(87)Sr/^(86)Sr data on the same samples. Samples with excesses of ^(87)Sr relative to seawater show low ϵ_(Nd), both sets of data thus indicating inheritance of an old detrital component. The results demonstrate that the Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts and that the Rb-Sr system may be used for dating, if consideration is given to the initial Sr composition and to diagenetic changes. This opens up numerous possibilities for sedimentological-geochemical studies.

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