Experimental investigations of Si-isotope fractionation associated with Fe-Si co-precipitates in simulated Precambrian seawater
Friday, 19 December 2014: 2:25 PM
The Si cycle was dramatically different in the Precambrian ocean due to the absence of marine Si-secreting organisms. Precambrian Si isotopic compositions were largely controlled by chemical precipitation of Si, input of Si with different isotopic compositions (e.g., continental versus hydrothermal sources) and later alteration and diagenetic processes associated with silicification. In Precambrian banded iron formations (BIFs) and chert deposits there is an over 4‰ spread of Si isotopes (δ30Si), which stands in marked contrast to the narrow range (<0.5) measured in igneous rocks, highlighting the potential of using Si isotopes to reconstruct those processes that controlled the Precambrian marine Si cycle. However, unequivocal interpretations of Si isotope compositions measured in Precambrian Fe-Si rich sediments is hampered by a lack of understanding of Si-isotope fractionation factors associated with formation of these sediments and subsequent diagenetic processes. This study experimentally investigates Si isotope fractionation during the formation of Fe-Si co-precipitates, and between aqueous Si and Fe-Si co-precipitates. All experiments are conducted in an artificially prepared medium that mimics Archean seawater (e.g. Si: ~60 ppm), rather than in a simple Fe-Si solution, because previous studies have revealed distinct Fe isotope fractionation behaviors in artificial Archean seawater (AAS) compared to simple solutions. One set of experiments investigated oxidation of Fe2+ in the AAS at room temperature, which produced amorphous Fe-Si precipitates. Preliminary results show that δ30Si values of Fe-Si co-precipitates are ~2‰ lower than the initial AAS (Δ30Siprecip-AAS = -2.13 ± 0.18‰ (2σ)). A second set of experiments trace Si-isotope exchange between aqueous Si (AAS) and Fe-Si co-precipitates in an anaerobic chamber, using a 29Si spike (i.e. three-isotope method). The results of these experiments will form a basis for reliable interpretations of Si isotopes recorded in Precambrian sediments.