Geochemical characterisation of Neoproterozoic marine habitats: Evidence from trace elements and Nd isotopes in the Urucum iron and manganese formations, Brazil

Abstract

The Urucum iron and manganese formation (IF and MnF) in the Neoproterozoic Santa Cruz Formation, Brazil, provides unique insights into Neoproterozoic seawater and the habitat for the evolution of the Ediacaran fauna. Pure Urucum IF drill core samples, i.e. chemical sediments that are devoid of any syn- or post-depositional alteration, are reliable and robust archives of Neoproterozoic seawater proxies. They display low concentrations of rather immobile elements and shale-normalized rare earths and yttrium (REYSN) patterns similar to those of modern seawater. Positive LaSN and GdSN anomalies, enrichment of heavy relative to light REYSN, negative CeSN anomalies and super-chondritic Y/Ho ratios indicate a depositional environment dominated by open ocean water masses and oxic atmosphere–hydrosphere conditions. The REYSN patterns of manganese-rich chemical sediments from the Mn1 and Mn2 horizons, however, show significantly different REY distributions between and within these units. While REYSN patterns from the Mn2 unit are similar to those of the IF, the REY systematics of the Mn1 horizon can be subdivided into two different groups. Fe-poor group A shows rather flat REYSN patterns with large negative CeSN anomalies, while Fe-rich group B shows insignificant to positive CeSN anomalies, YbSN/PrSN ratios below unity and sub-chondritic Y/Ho ratios, suggesting that trace element distributions are controlled by the mineralogical composition rather than by sedimentation rate.Pure IF samples, reflecting Urucum seawater, yield εNd0.635Ga values between −4.56 and −4.08, and are more positive than those of the MnF (−5.52 to −4.66) and associated siliciclastic rocks (−8.35 to −7.69), and are considerably more radiogenic than the crystalline Rio Apa Basement (−13.7). While clastic sediments originated from the Amazonia Craton, the dissolved REY budget of Urucum seawater was derived from terrigenous material of the nearby Neoproterozoic Brasília Belt. There is no evidence for any REY input via high-temperature, hydrothermal fluids or for REY input from a mantle source.