The origin of spinifex texture in komatiites

Shore, Mark; Fowler, Anthony D.

doi:10.1038/17794

Cited by 45 publications

(19 citation statements)

References 28 publications

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“…The position of spinifex zones within these inflation-induced structures implies that they formed during lifting of the upper carapace. The association of spinifex textures with submarine lava flows is commonly interpreted to indicate that rapid cooling caused spinifex growth (Shore and Fowler 1999). However, in these lavas, the spinifex textures occur beneath an insulating upper crust and must have cooled slowly during inflation.…”

Section: Introductionmentioning

confidence: 99%

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Dann

2001

Bulletin of Volcanology

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Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2-15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.

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Section: Introductionmentioning

confidence: 99%

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Dann

2001

Bulletin of Volcanology

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“…The tops of komatiite flows are commonly marked by a thin aphyric to olivine-phyric, originally glassy interval (A 1 ) or a random spinifex (A 2 ) zone of skeletal blades of olivine crystals (or acicular pyroxene crystals in komatiitic basalts and some differentiated komatiites). Both textural zones indicate supercooling (Donaldson 1982, Shore & Fowler 1999 and are analogous to the chilled surface crust on basalt pillow lavas and subaerial pahoehoe basaltic-type lavas. This upper A 1 -A 2 interval may be marked by a network of irregularly spaced fractures or, more rarely, a pervasive breccia horizon of clasts of the A 1 -A 2 zone.…”

Section: Textural Zonation and Textural Asymmetry: Distinguishing Kommentioning

confidence: 99%

“…This is a result of asymmetrical cooling and crystallization rates, with rapid cooling occurring from the surface into the ambient aqueous medium, with its high heat capacity (e.g., Shore & Fowler 1999), and slower loss of heat from the base to the substrate. By contrast, intrusions, especially those into deep-seated country-rock, will have an essentially symmetrical textural zonation because cooling rates around all margins will be similar.…”

Section: Textural Zonation and Textural Asymmetry: Distinguishing Kommentioning

confidence: 99%

Field Characteristics and Erosional Processes Associated With Komatiitic Lavas: Implications for Flow Behavior

Fernand

Beresford

2001

The Canadian Mineralogist

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Although komatiitic lavas have long been depicted as turbulent flows, especially near the vent, field characteristics indicate that many komatiitic lavas did not flow turbulently, or only initially so. The bases of komatiites are commonly conformable with their substrate, including fine pelitic sediments, and the margins of komatiites are overwhelmingly coherent, or marked by local quench-fragmented hyaloclastite breccia. Autobreccias are notably missing. These characteristics are not consistent with turbulent flow, but clearly indicate conditions of laminar flow. If komatiitic flows were turbulent, they should commonly have scoured into substrate sediments through a variety of physical erosion processes, including foundering into underlying seafloor sediments, because of density inversion, and turbulence-induced scouring of sediments. These features are not commonly developed, also indicating that generally komatiites were emplaced under tranquil, laminar-flow conditions. Trough-like structures that commonly host nickel sulfide mineralization have commonly been interpreted to originate by thermal erosion of substrate by the komatiitic lava. The evidence supporting thermal erosion is not strong, and commonly ambiguous. Trough structures at Kambalda, Western Australia, are fault bounded, as noted by several previous investigators. However, there is a common, but not universal, antithetic relationship between trough presence and sediment absence. Removal of sediment from troughs could be explained by physical erosion, with an initial narrow, turbulent flow-head scouring a channel in the underlying sediments. As the lava flows spread laterally, their flow-front velocity decreased, and flow became laminar, so explaining the conformable contacts with substrate and the presence of coherent crusts represented by the random spinifex textural zone. Thermal erosion was rare, and could only have resulted beneath sustained lava tubes, within the flow interior, not from the flow head.Keywords: komatiites, laminar flow, field characteristics, physical erosion. SOMMAIREQuoique les coulées de lave komatiitique ont maintes fois été considérées turbulentes, surtout près de l'évent, les caractéristiques de terrain indiquent que dans la plupart des cas, le mode d'épanchement n'était pas turbulent, ou s'il l'était, seulement au tout début. La base d'une coulée est en général conforme avec le substrat, y inclus les cas où ce sont des sédiments pélitiques à grains fins qui sont en contact, et les marges sont conformes dans la grande majorité de cas, ou bien localement marquées par une brèche d'hyaloclastite due au refroidissement brutal. Le phénomène d'autobréchification est complètement absent. Ces caractéristiques ne concordent pas avec une hypothèse de turbulence, mais elles indiquent plutôt des conditions d'épanchement laminaire. Si l'épanchement avait été turbulent, la lave aurait creusé dans les sédiments sousjacents selon une variété de mécanismes physiques d'érosion, par exemple affaissement dans les sédiments des fond...

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“…Volatile escape is therefore unlikely to be the explanation for the additional heat loss required to explain the Murphy Well textures. Strong radiative heat transfer along the c-axes of olivine crystals may enhance the rate of heat loss through the spinifex zone of some komatiites (Shore and Fowler 1999), but in the Murphy Well Flow, the olivine grains are small, randomly oriented and were initially separated by glassy matrix. Another possibility is that circulation of seawater through fractures in the solidified upper crust boosted the cooling rate, but this process again will act only after a significant thickness of crust had solidified and again the cooling rate cannot have been high.…”

Section: Crystallization Caused By Degassingmentioning

confidence: 99%

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Siégel

Arndt

Barnes

et al. 2014

Contrib Mineral Petrol

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pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30-38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2-5 vol% amygdales, a 110-120 m intermediate layer of olivine porphyry and a 20-30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38-40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H 2 O. This liquid degassed during Abstract Two Archaean komatiitic flows, Fred's Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred's Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred's Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick Communicated by J. Hoefs. Electronic supplementary materialThe online version of this article (

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The origin of spinifex texture in komatiites

Cited by 45 publications

References 28 publications

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Field Characteristics and Erosional Processes Associated With Komatiitic Lavas: Implications for Flow Behavior

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

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