SPINIFEX TEXTURES AND SEGREGATION VESICLES IN 3.5 GA KOMATIITES, SOUTH AFRICA, AND SLAG FLOWS, LACKAWANNA

Komatiites, high-Mg lavas of a hotter Archean Earth, share many features with pahoehoe flows, so our observations of modern flow fields can help explain how these ancient flow fields developed, but without spinifex textures, the modern analogues have limited usefulness. Fortunately, a series of 'experimental flows', poured out as a byproduct of steel production, do have spinifex textures and provide a useful comparison. The textures produced by the interaction of the three phases--crystals, liquid, and bubbles--are exemplified here in two extreme examples: spinifex komatiites from 3.5 Barberton Greenstone Belt, South Africa, and slag flows from the Bethlehem Steel brownfields now host to Steel Winds.

The slag was poured from train cars down a steep slope (15-25 degrees), forming lava tubes (0.5 m thick) and thin sheets (15-20 cm thick), which moved the shore line of Lake Erie westward. A trench exposes cross sections of two types of lenticular lava tubes: full and empty. In the full tubes the core crystallized, and in the empty tubes the core drained out, leaving a gaping cavity. Some sheet flows have wrinkled flow-top surfaces like pahoehoe. Most slag flows are glassy with blue and green chills, surprisingly revealed in cut slab, but some flows and tubes display spinifex textures. One spinifex flow has a distinct upper zone or crust of elongate vesicles (aspect ratio, 3:1 to 10:1) aligned perpendicular to the flow top. Thin sections reveal that crystals, growing downward in response to cooling from the flow top, guided the coalescence of rising vesicles into their elongate shapes. In the lower zone, segregation vesicles formed where vesicles, trapped within a randomly oriented crystal framework, filled with melt.

Similar textures occur in the Barberton komatiites despite the different scale, composition, and emplacement conditions, and this paper focuses on that comparison.