Redistribution of rare-earth elements during diagenesis of carbonate rocks from the Mid-Proterozoic Newland Formation, Montana, U.S.A.

Abstract

REE data for carbonate rocks from the Mid-Proterozoic Newland Formation, Belt Supergroup, show variable degrees of relative REE enrichment when normalized to a composite shale of the Newland Formation. Most conspicuous are relative enrichments in LREE's and positive Eu anomalies. Possible coprecipitation of REE's in carbonates from seawater is much too small to account for the REE enrichment, thus only diagenetic REE enrichment has to be considered. By far the strongest LREE enrichment is observed in carbonates with interbedded shales. During diagenesis REE apparently moved from the shale partings to the adjacent limestone beds. Massive limestone units have considerably smaller REE enrichments (except in proximity to shales), suggesting that REE addition from formation waters is less efficient for supplying REE's than is local redistribution between shale and limestone beds. Early precipitation of diagenetic silica in pore spaces reduces REE enrichment in limestone beds by reducing their permeability. Dolostones are cemented by pervasive early-diagenetic silica and lack significant REE enrichment except for Eu, probably because silica cement prohibited access of REE-bearing pore waters. Positive Eu anomalies indicate reduction of Eu to the divalent state during diagenesis and relatively large mobility of divalent Eu.