TEXTURAL EVIDENCE THAT PRECAMBRIAN IMPACT MELT SPHERULES CRYSTALLIZED AND DEVITRIFIED IN FLIGHT

Five late Archean to early Paleoproterozoic formations in Western Australia and South Africa each contain one layer rich in splash-form spherules of former silicate melt which mainly consist of K-feldspar in radial-fibrous aggregates to lath-shaped crystals. Given their age, these textures are usually attributed to replacement, but microtektite glass dissolves instead of being replaced by crystalline phases and the textures of Phanerozoic impact spherules replaced by crystalline phases (e.g. in the K/T boundary layer) are different. The crystals in the Precambrian impact spherules have shapes that closely resemble plagioclase crystallites and spherulites in quenched basaltic magmas (the laths) and thermally devitrified terrestrial and lunar impact glasses (the radial-fibrous aggregates) respectively. Quench and thermal devitrification textures could only have formed in flight, not after deposition, because the spherules came to rest in hundreds of meters of seawater. Broken spherules provide further evidence in support of in-flight growth; radial-fibrous textures are present on the outer edges of most spherules but absent from surfaces formed via breakage in transit. This argues strongly against formation of the crystals via later replacement, as does the fact that virtually every spherule has a unique paragenesis. After deposition, all the plagioclase was slowly and precisely replaced by K-feldspar. Authigenic K-feldspar also replaced plagioclase crystallites in thin layers of basaltic tuff in these same formations, as well as spherules in the K/T boundary layer at a few locations. The textures in the Precambrian spherules suggest they were impact melt droplets rather than vapor condensates, and that they experienced complex and variable thermal regimes as droplets in flight. Specifically, some spherules cooled to glass then were reheated enough to devitrify, whereas others cooled slowly and continuously enough for partial crystallization in flight.