Vesicular komatiites: Origin of vesicular horizons in thick komatiite lava flows by endogenous growth, Kambalda, Western Australia.

Beresford* S. W., R. A. F. Cas*, D. D. Lambert* and W. E. Stone+

*Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia.

+Kambalda Nickel Operations, WMC Resources Ltd, Kambalda, WA 6442, Australia.

 

Traditionally, amygdales (vesicles) have been considered to be rare in komatiites, and of ambiguous origin. However, detailed field studies at Kambalda, Western Australia, and critical literature examination suggest that vesicle presence and content in komatiites are grossly underestimated. For example, at Kambalda, vesicles are observed in minor amounts (<1%) in flow tops and major amounts (up to 30%) in internal horizons within thick (>10 m) komatiite lava flows. The vesicles are filled by secondary minerals (amygdales), altered spinifex-textured material (segregation vesicles), and (or) magmatic sulphides.

The vesicles located within chilled flow tops must have formed during magma ascent and lava emplacement, whereas the internal vesicle horizons, trapped under flow top crusts, must have formed during prolonged crystallisation and bubble coalescence after eruption, similar to those recorded in modern basalt lavas and flood basalts. The vesicular horizons are inferred to represent upward migrating bubbles frozen in place during solidification or ponding below the upper skeletal olivine-phyric crust during endogenous emplacement.

The presence of spinifex-textured fill in segregation vesicles and abundance of vesicles within spinifex or crescumulate sub-zones suggests an association between supercooling induced textures and presence of volatiles.

The common presence of vesicles in komatiites worldwide indicates significant volatile contents, which has implications for komatiite emplacement models. Although an assimilation origin for the volatiles cannot be dismissed, an intrinsic origin for the volatiles is considered more plausible.