| Komatiites and variolitic basalts are widespread in Archean volcanic sequences. Spinifex is a spectacular bladed olivine or pyroxene texture that characterizes komatiite, a rock almost exclusively restricted to the Archean; varioles are cm-scale leucocratic globular structures abundant in many Archean basalts. These striking textures provide valuable information about conditions during emplacement of the host magmas, particularly about how the magmas crystallized. Many komatiite flows have spinifex textures consisting of arrays of numerous subparallel olivine blades that extend 10's of cm to m's from the flow tops. The habit of the strongly anisotropic crystals is suggestive of fast cooling near the flow margin, yet the crystals form deep within the flows. The large temperature difference between solidus and liquidus of komatiites provides a partial explanation for their formation. In addition, the crystals are sharp-tipped and aligned so that their fastest growing faces were normal to the cooling contacts, suggesting that they grew in a strong chemical-potential gradient, in part created by the crystals themselves, as they modified the composition and temperature of the liquid from which they crystallized. |
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