Abstract
Abstract. In the Barberton Greenstone Belt, South Africa, a 100–250 m thick complex of carbonaceous chert dikes marks the transition from the Mendon Formation to the Mapepe Formation (3260 Ma). The sub-vertical- to vertical position of the fractures, the abundance of highly shattered zones with poorly rotated angular fragments and common jigsaw fit, radial structures, and multiple injection features point to repetitive hydraulic fracturing that released overpressured fluids trapped within the shallow crust. The chemical and isotopic compositions of the chert favour a model whereby seawater-derived fluids circulated at low temperature (< 100–150 °C) within the shallow crust. From the microscopic structure of the chert, the injected material was a slurry of abundant clay-sized, rounded particles of silica, carbonaceous matter and minor clay minerals, all suspended in a siliceous colloidal solution. The dike geometry and characteristics of the slurry concur on that the chert was viscoelastic, and most probably thixotropic at the time of injection: the penetration of black chert into extremely fine fractures is evidence for low viscosity at the time of injection and the suspension of large country rock fragments in the chert matrix provides evidence of high viscosity soon thereafter. We explain the rheology by the particulate and colloidal structure of the slurry, and by the characteristic of silica suspensions to form cohesive 3-D networks through gelation. Our results provide valuable information about the compositions, physical characteristics and rheological properties of the fluids that circulated through Archean volcano-sedimentary sequences, which is an additional step to understand conditions on the floor of Archean oceans, the habitat of early life.
Highlights
Siliceous sediments are common in many Palaeoarchean greenstone belts
Two main theories have been proposed to explain the formation of Barite Valley dikes, both based on extensive field mapping, analysis of dike geometry and structure, and petrographic observations
The main arguments are (1) the presence in some dikes of rock fragments from higher stratigraphic levels, (2) downward displacement of blocks from adjacent country rocks, (3) carbonaceous matter in the cherts that originated from biogenic processes on or near the ocean floor, and (4) the geometry of the entire dike complex, which reaches a maximum width of 50 m at the top and narrows downward in the Mendon volcanic units
Summary
Siliceous sediments are common in many Palaeoarchean greenstone belts. A recurrent pattern consists of conformable, bedded chert sequences overlying pervasively silicified volcanic units (e.g. de Wit et al, 1982; Paris et al, 1985; Lowe and Byerly, 1986a; Hofmann and Harris, 2008). Lowe and Knauth, 1977; Lowe and Byerly, 2003; Ueno et al, 2004; Hofmann, 2005; de Vries et al, 2006; Van Kranendonk, 2006; Hofmann and Bolhar, 2007; de Wit et al, 2011). Nijman et al, 1998; Brasier et al, 2002, 2005; Hofmann, 2005; Kiyokawa et al, 2006; Van Kranendonk, 2006) consider chert dikes as fossilized conduits of low-temperature hydrothermal circulation within sub-seafloor volcanic rocks; others (Lowe and Byerly, 2003; Lowe, 2013; Sleep and Lowe, 2014) envisage a meteorite impact that created open fractures on the seafloor subsequently filled by surface sediments. We depart from previous approaches by providing a dynamic view of the infilling process of Archean chert dikes, which offers a rare window to sub-seafloor hydrothermalism at that time
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
