Spinifex Texture

One of the first names for spinifex texture in komatiite lavas was "crystalline quench texture". Viljoen and Viljoen (1969a,b) used this term when they first recognized komatiite as a separate rock type. They described spinifex as a texture characteristic of komatiite, and they noted that the morphology of the dominant minerals resembled those of crystals that form when molten olivine basalt is cooled rapidly.

Nesbitt (1971) formally introduced the term spinifex when he described and classified several different types of skeletal crystals in komatiites from Australia and Canada. He too noted the similarities between the skeletal or dendritic morphologies of olivine and pyroxene crystals in natural spinifex textures and those in experimental charges and silicate slags. He was also first to recognize what has come to be known as the spinifex paradox.

Spinifex texture is commonly found in the interior of komatiite flows, well below the upper chilled crust. In the thickest units, large dendritic crystals appear to have crystallized at depths 10 or more meters below the surface of the flow. Under such circumstances, the loss of heat from the interior of the flow is controlled by conduction through the upper solidified crust. In a typical 2-m-thick komatiite flow, the cooling rate during crystallization of the lower part of the spinifex layer is only 1 to 3°C per hour. In thicker flows the rate is far lower. In contrast, the morphologies of the olivine or pyroxene crystals in spinifex-textured lavas resemble those produced experimentally at cooling rates never less than about 30°C/hr.

A factor that has been mentioned in many papers on spinifex texture, but has not received sufficient attention, is the role of constrained crystal growth during solidification of the crust of a komatiite flow. Constrained growth refers to the crystallization of parallel grains of olivine or pyroxene in the downward-growing crust of a lava flow. The crystals compete with one another for nutrients, the atoms of Mg, Fe and Si that are essential components of their crystal frameworks. It is this competition that leads to the preferred, near-perpendicular orientation of the olivine crystals in spinifex textures.

Erupted komatiite contains a small proportion of olivine phenocrysts that grew either during ascent to the surface or during flowage on the surface. During cooling, some of these phenocrysts become trapped in the crust that forms right at the top of the flow, others settle towards to the base of the flow and become part of the lower cumulate layer. Olivine then nucleates in the layer of crystal-free liquid just beneath the crust.

komatiite(13).jpg

Fig.1: Three stages during the solidification of a spinifex-textured komatiite flow



The crystals that grow from these nucleii have highly skeletal morphologies, due to the high cooling rate in the crust of the flow, and they are randomly oriented. As cooling proceeds, these olivine grains continue to grow. Those with a near-vertical orientation are favoured because their tips extend downwards into unfractionated nutrient-rich liquid; those with orientations closer to horizontal find only nutrient-poor liquid or collide with other crystals, and they cease to grow (Fig. 1).

The crystallization of olivine produces a residual liquid with a composition different from that of the parental liquid, depleted in Mg and enriched in Si, Al, Ca and Na. Its density is less than that of the parental liquid. As downward growth proceeds, this liquid is expelled and it accumulates as a layer of low-density at the base of the crystal front. The growing tips of the spinifex crystals are bathed in a liquid depleted in the components they require to grow (Fig. 1).

Faure and other research, have suggested that this situation provides an explanation for the unusual habit of spinifex. They point out that the situation has certain parallels with the accumulation of a zone of nutrient-poor liquid that surrounds rapidly growing crystals in quenched liquids.

In such cases the rate of crystal growth exceeds the diffusion rate of the major elements within the silicate liquid, and the elements expelled by the growing crystal accumulate in a layer around the crystal. The skeletal or dendritic morphologies of such crystals result when the crystal sends out protuberances – fine needles or plates - that penetrate the nutrient-poor layer. Faure et al. suggest that an analogous phenomenon applies to the growth of spinifex crystals - their dendritic habit is a consequence of their growth into the accumulated layer of nutrient-poor liquid.

komatiite(6).jpg

Fig.2: Komatiite from Komati river (Africa). Note the Spinifex Texture. From James St. John.



komatiite(7).jpg

Fig.3: Spinifex Texture in a komatiite Sample from Munro, Canada. From Nick Arndt.



komatiite(5).jpg

Fig.4: Spinifex Texture in a komatiite Sample from Abitibi Greenstone Belt. From Ryan Anderson.



komatiite(11).jpg

Fig.5: Spinifex Texture in a komatiite Sample from Abitibi Greenstone Belt. From Ryan Anderson.






Bibliography



• Arndt, N.T. and Fowler, A.D. 2004. Textures in komatiites and variolitic basalts. In: The PreCambrian Earth: Tempos and Events. Pat Eriksson, Wlady Altermann, David Nelson, Wulf Muller, and Octavian Catuneanu (eds.). Developments in Precambrian Geology 12.
• Dann, J.C ., 2001, Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: Inflation of submarine lavas and origin of spinifex zones: Bulletin of Volcanology, v. 63, p. 462-481.


Photo
komatiite(4).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(5).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(6).jpg

Skeletal olivine crystals (Serpentinized), regularlyarranged, in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(7).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(8).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(9).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(10).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(11).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(12).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(13).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(14).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(15).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(16).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(17).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(18).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(30).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(31).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(32).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(33).jpg

Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(70).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(71).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(72).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(73).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(74).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(75).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(76).jpg

Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(77).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(81).jpg

Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(82).jpg

Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(84).jpg

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(48).jpg

Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(50).jpg

Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(51).jpg

Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(52).jpg

Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
komatiite(53).jpg

Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
komatiite(54).jpg

Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
komatiite(85).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
komatiite(86).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)
komatiite(88).jpg

Skeletal Pyroxene. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
komatiite(92).jpg

Skeletal Olivine (spinal column). Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
komatiite(93).jpg

Skeletal Olivine (spinal column). Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)
komatiite(95).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
komatiite(96).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)
komatiite(97).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)
komatiite(100).jpg

Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)