ORIGIN OF CHLORITE ENRICHMENT AND NEODYMIUM ISOTOPIC ANOMALIES IN HALTENBANKEN SANDSTONES

This is really two papers rolled into one. We have chosen to present t hem together because both topics derive from the same series of Nd iso topic analyses of some chlorite-coated, Jurassic-age sandstone reservo irs from the Norwegian continental shelf. The first topic is a compari son of the Nd isotopic composition of chlorite-rich vs chlorite-poor s andstones to determine whether systematic differences in sediment prov enance have controlled the occurrence of grain-Coating Fe-chlorite cem ent. The results show that both chlorite-rich and chlorite-poor sandst ones have a wide range of Nd-143/Nd-144, consistent with mixing of at least two main provenance terranes of differing mean crustal age in bo th groups. This does not support the model of chlorite enrichment due to localized influx of an unique Fe-rich provenance component, such as mafic igneous material, but is consistent with an alternative model o f chlorite enrichment by Fe-clay authigenesis on the sea-floor, locali zed by fluvial discharge into a nearshore marine setting. The implicat ion is that studies of depositional setting rather than sand provenanc e should be the key to predicting occurrence and geometry of high-poro sity zones in deeply buried sandstones with chlorite-related porosity preservation. The second topic deriving from this set of Nd isotopic d ata concerns the discovery of an association between anomalously high Sm-Nd model ages and elevated contents of bulk-rock phosphorous. This sub-set of the samples is anomalous in that the Sm-Nd model ages are t oo high to be interpreted as detrital provenance ages and also because Sm-Nd model age correlates with Sm/Nd ratio. Furthermore, the anomalo us Sm-Nd signature can be removed by acid leaching, leaving a residue with apparently non-anomalous Sm-Nd characteristics. Analysis of an ap atite mineral separate from one sample confirms that the anomalous Sm- Nd signature resides within diagenetic apatite. The anomalies are sugg ested to result from REE fractionation due to the crystal chemistry of apatite, which favors preferential incorporation of the intermediate- weight REE, thus increasing Sm/Nd and Sm-Nd model age. The apatite pro bably formed by recrystallization of phosphate that was present at the time of sand deposition, either as organic detritus or as the product of precipitation on the sea floor. The isotopic composition of the ap atite indicates that Sm/Nd fractionation occurred within the past 100 million years and thus was not syndepositional. Formation of the Nd is otopic anomalies is therefore interpreted to be a burial diagenetic pr ocess involving local REE enrichment of apatite and complementary REE depletion of the surrounding rock mass. Experimental apatite/melt part ition coefficients determined at magmatic temperatures predict much lo wer Sm/Nd fractionation than is observed in the present samples, but w e speculate that the degree of fractionation may increase greatly towa rd the lower, diagenetic temperatures relevant for the present samples . The discovery of these anomalies does not invalidate the use of Sm-N d model age profiling as a stratigraphic correlation tool and provenan ce indicator, but indicates that precautions should be taken when deal ing with marine sandstones. Specifically, data sets should be examined for correlation between Sm-Nd model age and Sm/Nd ratio, which may in dicate the presence of anomalous fractionation effects. Even in such c ases, the isotopic ratio Nd-143/Nd-144 can provide a reliable provenan ce signature. (C) 1998 Elsevier Science Ltd. All rights reserved.