Sm-Nd in marine carbonates and phosphates: Implications for Nd isotopes in seawater and crustal ages

Abstract

This study explores the possibility of establishing Nd isotopic variations in seawater over geologic time. Calcite, aragonite and apatite are examined as possible phases recording seawater values of ϵ_(Nd). Modern, biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have Nd concentrations of from 0.2 to 70 ppb, showing that primary marine CaCO_3 contains little REE and that Nd/Ca is not greatly enhanced relative to seawater during carbonate precipitation. Very young marine limestone and dolomite containing no continental detritus have ~200 ppb Nd. All the carbonates are LREE enriched (-0.16≤ƒ^(Sm/Nd) ≤-0.45). Modern and very young Atlantic and Pacific carbonates have ϵ_(Nd) in the range of shallow Atlantic and Pacific seawater respectively, implying that they derive their REE from local seawater. The Nd in well preserved carbonate fossils is ≤4 × 10^4 ppb, much greater than in their modern counterparts but like the high values found for carbonates in other studies. We believe the high REE contents (at the 500 ppb level) in some detritusfree carbonates are due to REE-rich Fe-hydroxide in/on the carbonate. In favorable cases, such material may record seawater ϵ_(Nd) values, however introduction of extraneous REE may obscure the original isotopic composition of pure CaCO_3 because of its very low intrinsic primary REE abundance. Modern biogenic apatite is also shown to have very low REE content (<150 ppb Nd) but appears to quickly scavenge REE from seawater. Inorganically precipitated apatite from phosphorites has high concentrations of seawater-derived REE. Young phosphorite apatite from the Atlantic and Pacific oceans has ϵ_(Nd) in the range of the seawater from these oceans. Older apatite samples of similar age from different localities bordering common oceans record similar values of ϵ_(Nd)(T). Sedimentary apatite has ϵ_(Sr)(T) values in good agreement with the curves for ^(87)Sr/^(86)Sr of seawater as a function of time. Individual conodonts from a single formation yield the same ϵ_(Sr)(T) and ϵ_(Nd)(T). Other workers have shown that sedimentary apatite preserves seawater REE patterns. These characteristics suggest that sedimentary apatite can be used to determine ϵ_(Nd)(T) in ancient seawater. The seawater values so inferred range between −1.7 and −8.9 over the last 700 my and lie in the range of modern seawater, showing no evidence for drastic changes. High values of seawater ϵ_(Nd)(T) in the Triassic and latest Precambrian may correlate with the breakup of large continental landmasses. The initial ϵ_(Nd)(T) =−15.0 of a 2 AE old phosphorite implies the presence of ~ 1.5 AE old continental crust at 2 AE ago. The approach outlined here can be used to constrain the age of the exposed crust as a function of time.

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