LOW GRADE ARCHEAN METASEDIMENT GEOCHEMISTRY OF REDOX-SENSITIVE PROXIES FOR EARLY EARTH ATMOSPHERE-HYDROSPHERE OXYGENATION: PRELIMINARY RESULTS FROM THE CHERRY CREEK GROUP, MONTANA, USA

Redox-sensitive trace element abundances 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 in southwestern Montana can help understand the regional paleoclimate when sediments of the Cherry Creek Group (CCG) were deposited after 2.7 Ga, the youngest detrital zircon in the sequence. Interlayered metavolcanic rocks give similar ages. Lower greenschist facies quartzite, phyllite, pelitic schist, marble, and iron formation represent the lowest metamorphic grade in the Gravelly Range and the lowest grade among metasupracrustal sequences designated CCG in southwestern Montana. The phyllite-schist-BIF association indicates CCG BIF is likely a Superior-type iron formation. Trace element analysis by solution ICPMS shows CCG clastic sedimentary rocks are LREE enriched, have negative Eu anomalies, and lack Ce anomalies, similar to average post Archean shale. Correlations of redox-sensitive elements with Al show a weak positive correlation of Zn, Mo, Ba, and U, a weak negative correlation with Cr, and a moderately positive correlation with V. Lack of strong positive Ce anomalies in CCG BIF suggests 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). Enrichment Factors (EF) calculated as ratios of Al-normalized elements in samples versus average shale were used to determine their relative enrichment or depletion. EFs of 28 CCG clastic metasediments compared groups of elements sensitive to oxidizing, reducing, and sulfidic conditions. Results indicate 26 of the 28 clastic CCG samples were deposited in euxinic conditions. Of the 26 EFs that suggest euxinia, over half (14) also have Cu and Ni EFs suggestive of deposition with a relatively high organic material flux. Preliminary results of black shale Mo/TOC of 12.8–13.5 imply deposition occurred in hydrologically restricted subchemoclinal anoxic waters. These results indicate that some CCG sediments were deposited in multiple restricted basins or in one area that fluctuated between anoxia/suboxia and euxinia.