Low to Negligible Water Contents in Komatiites: Evidence from Pyroclastic Komatiitic Tuffs in the 3.5-3.2 Ga Barberton Greenstone Belt, South Africa

The 3.5-3.26 Ga Onverwacht Group in the southern part of the Barberton greenstone belt contains many inter-flow sedimentary layers that include pyroclastic komatiitic tuffs. Most of these tuffs have been altered to impure cherts by nearly syndepositional silicification that has resulted in exquisite textural preservation. The ultramafic pyroclastic debris is virtually all aphyric, with abundant ash, armored and accretionary lapilli, and rare chromites and silica pseudomorphs after olivine or pyroxene. Only two of the many tuff units contain common crystalline particles. One is microphyric, with over 95% of the crystals < 200 microns across, indicating a significant population of quench crystals. The other contains sparse olivine phenocrysts that range up to 1.2 mm. In all tuff beds, ash and lapilli are poorly to non-vesicular, with a few beds containing substantial bubble wall fragments. These units are regionally widespread, with many beds continuous for over 10 km. This suggests large komatiitic eruptions that generated plumes which dispersed ash over broad areas.

The mechanical difficulties in explosively fragmenting highly fluid ultramafic magmas are considered a major problem in forming pyroclastic komatiites. Fluid-fluxed melting in a subduction zone and the assimilation of hydrated crust within a dry magma are two processes that would result in elevated water contents and potentially violent degassing. Both mechanisms, however, would induce cooling and crystallization in the melt. This is inconsistent with the low vesicularity and nearly aphyric nature of most Barberton komatiitic tuffs. Instead, these tuffs likely were generated by the eruption of superheated or near liquidus magmas, which, due to the interaction with external water at low pressures, explosively fragmented to produce fine ultramafic glass at extremely high quench rates. This is similar to processes, such as those that occur at Kilauea, where the interaction of near-surface mafic melts with surface-derived water have produced explosive eruptions.