Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Abstract

Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2-15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton, southern India: implications for Archaean mantle evolution and crustal growth

“…Vesicles are generally sparse in komatiite lavas (Arndt et al 2004) when compared to high vesicle density in their basaltic counterparts, possibly related to the low volatile content or their accumulation and preservation in high-temperature, low-viscosity lavas. The low percentage (~1%) of vesicular komatiite in most J.C. Pura sections is similar to that reported by Dann (2001) for komatiite sequences from the Barberton Greenstone Belt. Based on the disposition of vesicular horizons, it is envisaged that at least two distinct stages of vesiculation were associated with the J.C. Pura komatiite flows.…”

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

“…These samples were from the tumulus, a thick, differentiated, entirely serpentinized komatiite flow described by Dann (2001). Three of these latter ten samples are classified as hyaloclastite, one as olivine orthocumulate, one as olivine adcumulate, one as harrisite, two as spinifex textured komatiite, one as pyroxenite, and one as gabbro (Arndt et al, unpublished results).…”

Komatiites, kimberlites, and boninites

“…Grove and coworkers [ Grove et al , 1997, 1999; Parman et al , 1997] recently suggested that komatiites from the type section in the Barberton Greenstone Belt in South Africa [ Viljoen and Viljoen , 1969] were emplaced not as lava flows but as a series of high‐level intrusions. The situation has since become clearer, however, with the publication by Dann [2000, 2001] of detailed maps that show conclusively that these units are extrusive. Their morphological features and textures are very like those of lava flows in other regions and there is no reason to believe that they formed in a different manner.…”