Origin of chlorite enrichment and neodymium isotopic anomalies in Haltenbanken sandstones

Abstract

Thisis really two papers rolled into one. We have chosen to present them together because both topics derive from the same series of Nd isotopic analyses of some chlorite-coated, Jurassic-age sandstone reservoirs from the Norwegian continental shelf. The first topic is a comparison of the Nd isotopic composition of chlorite-rich vs chlorite-poor sandstones to determine whether systematic differences in sediment provenance have controlled the occurrence of grain-coating Fe-chlorite cement. The results show that both chlorite-rich and chlorite-poor sandstones have a wide range of 143Nd/ 144Nd, consistent with mixing of at least two main provenance terranes of differing mean crustal age in both 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 of chlorite enrichment by Fe-clay authigenesis on the sea-floor, localized by fluvial discharge into a nearshore marine setting. The implication is that studies of depositional setting rather than sand provenance should be the key to predicting occurrence and geometry of high- porosity zones in deeply buried sandstones with chlorite-related porosity preservation. The second topic deriving from this set of Nd isotopic data concerns the discovery of an association between anomalously high SmNd model ages and elevated contents of bulk-rock phosphorous. This sub-set of the samples is anomalous in that the SmNd model ages are too high to be interpreted as detrital provenance ages and also because SmNd model age correlates with Sm/Nd ratio. Furthermore, the anomalous SmNd signature can be removed by acid leaching, leaving a residue with apparently non- anomalous SmNd characteristics. Analysis of an apatite mineral separate from one sample confirms that the anomalous SmNd signature resides within diagenetic apatite. The anomalies are suggested 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 SmNd model age. The apatite probably 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 apatite indicates that Sm/Nd fractionation occurred within the past 100 million years and thus was not syndepositional. Formation of the Nd isotopic anomalies is therefore interpreted to be a burial diagenetic process involving local REE enrichment of apatite and complementary REE depletion of the surrounding rock mass. Experimental apatite:melt partition coe.cients determined at magmatic temperatures predict much lower Sm/Nd fractionation than is observed in the present samples, but we speculate that the degree of fractionation may increase greatly toward the lower, diagenetic temperatures relevant for the present samples. The discovery of these anomalies does not invalidate the use of SmNd model age profiling as a stratigraphic correlation tool and provenance indicator, but indicates that precautions should be taken when dealing with marine sandstones. Specifically, data sets should be examined for correlation between SmNd model age and Sm:Nd ratio, which may indicate the presence of anomalous fractionation effects. Even in such cases, the isotopic ratio 143Nd/ 144Nd can provide a reliable provenance signature. 1998 Elsevier Science Ltd. All rights reserved.