Master of Science (MS)
Geology and Geophysics
Over a dozen komatiitic flows in the 3.2-3.5 Ga Barberton Greenstone Belt are topped with unique green silica- and carbonate-rich rocks, which likely formed from interaction of the cooling lava flow with seawater shortly after deposition. This study focuses on an alteration zone in the Mendon Formation. Two styles of alteration are observed in this alteration zone: silicification and carbonation. The goal of this study is to characterize the carbonates in the alteration zone, constrain the timing of alteration, test whether the alteration zones are due to interaction with seawater after deposition, and estimate the elemental fluxes between the rock and the altering fluid. If the zones of silica and carbonate rich rocks are alteration zones due to near-surface interaction with seawater, then studying them will provide constraints on Archean near-surface environments. Cross-cutting relations indicate that after eruption of the komatiitic lava, serpentinization then wholesale replacement of the komatiite by quartz, carbonate, and Cr-muscovite took place before quartz and carbonate veins formed. After vein formation, a sedimentary chert cap was deposited and vertical black chert dikes were emplaced. Carbonates in the alteration zone are compositionally and isotopically homogeneous in veins and the host rock. They are ferroan dolomites, Ca1.0Mg0.7Fe0.3(CO3)2 with d18O ~20‰ and d13C ~2‰. The chemical and isotopic homogeneity of the carbonates suggests that they formed in equilibrium with a large aqueous reservoir, probably seawater. Using the isotopic composition of the carbonates within the samples, a seawater of ~86°C is calculated.
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Stoute, Judith E. F., "Carbonated Sections from Komatiite Flow- Top Alteration Zones in the Barberton Greenstone Belt: Implications for Near-surface Archean Environments" (2007). LSU Master's Theses. 2984.
Gary R. Byerly