Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence, Murchison Province: Oxygenated marine bottom waters

doi:10.1016/j.chemgeo.2010.11.012

Chemical Geology

Volume 280, Issues 1–2, 7 January 2011, Pages 232-241

https://doi.org/10.1016/j.chemgeo.2010.11.012 Get rights and content

Abstract

Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce^+ 3 in an oxic water column.

Research Highlights

► Extreme positive Ce anomalies in 2.9 Ga submarine volcanics, Western Australia. ► Positive anomalies balance negative Ce in Archean BIF. ► Collectively evidence for scavenging of Ce³⁺ in an oxic water column. ► Oxygenated Archean oceans 500 Ma before the putative great oxygenation event (GOE).

Section snippets

Introduction and scope

There are two principal contrasting models for the secular evolution of atmospheric oxygen. The first model, termed by Ohmoto, 1997, Ohmoto, 2004 the “Cloud–Walker–Holland–Kasting” (C–W–H–K) model after the four main researchers, and other workers, assumes an anoxic Archean atmosphere–hydrosphere where pO₂ changed from < 1000 ppm to pO₂ > 0.1 present atmospheric levels (PAL) at ~ 2.4 Ga, the putative great oxidation event [GOE] (Cloud, 1968, Walker, 1977, Holland, 1964, Holland, 2002, Rye and

Regional geology

The Yilgarn Craton is a large composite granite–greenstone terrane, divided into the Southwest Terrane, Youanmi Terrane, and Eastern Goldfields Superterrane based on lithotectonic associations and geochronology. In turn, the Youanmi Terrane has been divided into the western Murchison and eastern Southern Cross Domains formerly termed terranes (Fig. 1 inset; Nelson, 1997, Pidgeon and Hallberg, 2000, Cassidy et al., 2006, Ivanic et al., 2010). The Murchison Domain includes a number of greenstone

Drillcore samples, volcanic textures, mineralogy

Drillcore, located external to the hydrothermal feeder zone of stockwork vein mineralization, and subjacent to the stratabound Cu–Zn ore zone, was sampled over the interval of 350–580 m, well below the limits of weathering, in a subvertical core collared at the following coordinates): Easting 503 259.085 and Northing 6 836 329.187. Sample depths in meters are listed on Table 2. Several authors have drawn attention to the importance of collecting core material below the weathering horizon for

Magma series

Compositionally the volcanic sequence is bimodal basalts and dacites–rhyolites. Data for basalts and dacites–rhyolites cluster separately for the elements Ti and Al, which are stable during hydrothermal alteration, but there are large variations for the mobile elements Si, Mn, Mg, Fe, Ca, and K, in keeping with the regional alteration assemblage of quartz-chlorite-muscovite (Fig. 4; Sharpe and Gemmell, 2001). Notably, basalts are depleted in Mn, Mg, and Fe relative to average Archean upper

Recent evidence on the GOE issue

There has been polarized debate as to the oxidation state of Earth's atmosphere–hydrosphere prior to ~ 2.4 Ga. Based on several lines of evidence, including detrital uraninite in Archean sedimentary rocks, studies of paleo-regolith redox, and the absence of redbeds, several authors proposed that the Archean atmosphere had low levels of oxygen levels, which rose dramatically at ~ 2.4 Ga in the so-called Great Oxidation Event (GOE; Cloud, 1968, Holland, 2002, Holland, 2009 and references therein;

Conclusions

Based on observations of modern oceanwaters and sediments, several researchers have attempted to reconstruct the chemical evolution of the Precambrian oceans from the rare earth element budgets of banded iron formations (BIF), especially the oxidation state of the source thermal waters from Eu anomalies, and of the depositional site from Ce anomalies (Fryer, 1977, Kato et al., 1998, Kato and Nakamura, 2003). In a recent summary, Kato et al. (2006) document true negative Ce anomalies in many

Acknowledgements

This work was supported by a Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grant to R. K. We thank Marcel VanEck of OzMinerals for assisting with sample collection at the Golden Grove deposit. Two journal reviewers are thanked for incisive critiques that improved the manuscript.

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