Geochim. Cosmochim. Acta, 66, A225 ; invité à la Goldschmidt Conf.

F. Faure¹, N. Arndt² and G. Libourel³

The spinifex zone in a komatiite flow consists of a thin upper layer of fine, randomly oriented olivine grains underlain by a thicker layer (30 cm to several metres) made up large plates of olivine oriented perpendicular to the flow top. Random spinifex is readily synthesized in dynamic cooling experiments that reproduce conditions during rapid cooling at the flow top. The coarse-grained spinifex is more problematic because the morphology of the olivine plates resembles those of dendritic crystals that grow experimentally only at high cooling rates, 50 to 100ºC/hr. Cooling rates 1-3 m beneath the komatiite flow top, in contrast, are calculated to be <5ºC/hr. This paradox has led to the suggestion that komatiites are hydrous magmas that crystallized in mid-crustal intrusions; a suggestion refuted by field studies that demonstrate clearly that most komatiites are extrusive.

To help resolve the problem we undertook a series of experiments in which synthetic Fe-free charges (51.9 wt. % SiO₂, 17.4 wt. % MgO, 13.2 wt. % Al₂O₃, 17.3 wt. % CaO) were slowly crystallized in a temperature gradient, such as exists at the top of a komatiite flow. The charges were confined in 5-cm-long graphite capsules in the upper part of a 1-atm vertical furnace (argon atmosphere) where the gradient is ~20ºC/cm, like that during cooling of a komatiite flow top. At cooling rates between 2 and 5ºC/hr, we grew long parallel dendritic olivines whose morphologies resemble those of crystals that grow only at cooling rates >50ºC/hr in experiments on the same starting material but without a thermal gradient. Plumose spinifex-like pyroxenes grow between the olivine crystals. These experiments demonstrate that plate spinifex texture forms naturally during cooling of ultramafic lava flows. The presence of water is not required to explain the texture.