Geochemistry of Redox-Sensitive Proxies for Early Earth Atmosphere-Hydrosphere Oxygenation in Archean Metasediments: Preliminary Results from the Cherry Creek Group, Montana, USA

Abstract:

Abundances of redox-sensitive trace elements in Archean marine sediments can provide evidence of the abundance of oxygen in early Earth’s atmosphere-hydrosphere. Analysis of low grade clastic and chemical metasedimentary rocks from the Gravelly Range of southwestern Montana has begun to better understand the region’s paleoclimate when the sediments of the Cherry Creek Group (CCG) were deposited ~2.7 Ga. Trace element analysis of bulk powder samples by solution ICPMS shows CCG clastic sedimentary samples are LREE enriched, have negative Eu anomalies, and lack Ce anomalies similar to the post Archean average shale pattern. Estimates of the authigenic component of redox-sensitive elements was determined through correlation with Al and shows a highly variable range from 0.4 to 88%. The CCG metasediments have a weak positive correlation of Zn, Mo, Ba, and U with Al, a weak negative correlation of Cr with Al, and moderately positive correlation of V with Al. These relationships represent varying degrees of authigenic contribution to CCG detrital sediments. Trace element data also show differences between phyllite, schist, and BIF samples whose association indicates the BIF is likely a Superior-type iron formation. Lack of strong positive Ce anomalies in CCG chemical sediments suggests that water column conditions were not as oxidizing as the present, yet not anoxic enough to keep all Ce in the Ce3+ state and prevent precipitation and deposition of Ce(IV). Examination of Enrichment Factors (EF) of 28 CCG clastic metasediment samples compared groups of elements sensitive to different conditions along an oxic to sulfidic gradient. Results indicate that 26 of the 28 clastic CCG samples analyzed were deposited in euxinic conditions. Of the 26 EFs that indicate euxinia, over half (14) have EFs of Cu and Ni suggestive of deposition by a relatively high organic material flux. Preliminary results of CCG black shale Mo/TOC values of 12.8–13.5 suggest deposition occurred in hydrographically restricted subchemoclinal anoxic waters. These results may indicate that some CCG metasediments were deposited in multiple hydrologically restricted areas or one area that fluctuated between anoxia/suboxia and euxinia periodically.