New evidence has emerged for a different type of platy spinifex texture that has not previously been documented in the existing literature, in this case from 2·8 Ga high-Mg basalts in the Murchison Domain of the Yilgarn Craton, where petrographic and geochemical evidence shows that the dominant platy mineral is pyroxene, rather than olivine. In our samples, two scales of plates are evident. Larger plates have lengths and widths that are approximately equal and range from ∼1000 to 15 000 µm, with thicknesses typically ≲120 µm. These plates have ≲25 µm thick augite rims, and cores that are now a mixture of low-temperature hydrous alteration minerals. They occur in sets of similarly oriented crystals, and typically intersect other sets of crystals at oblique angles. A second population of smaller augite-only plates occur within the interstices of the larger plates; they have lengths and widths that range from 200 to 1500 µm, and thicknesses that are typically ≲50 µm. Pyroxene dendrites are also a typical component of this texture and represent a third scale of crystal growth, which probably crystallized shortly before the remaining liquid quenched to glass. All scales of pyroxene contained within this texture exhibit skeletal features and are considered to have crystallized rapidly. We discuss possible conditions that led to the crystallization of platy habits instead of the typical acicular ones. The exposed volcanic sequence in our study area is volcanologically similar to other Archean komatiites, such as those from the 2·7 Ga Abitibi greenstone belt, for example, and has probably experienced a similar cooling history; however, apart from having similar textures, we cannot demonstrate a komatiitic association. Liquid compositions, estimated from chilled flow margins, are distinctly lower in MgO (14·4–15·8 wt %) and higher in SiO2 (50·9–52·1 wt %) than those for most platy olivine spinifex-textured komatiites; from these compositions, we calculate dry liquidus temperatures of 1312–1342°C and mantle potential temperatures of 1440–1480°C. On the basis of these temperatures we question whether a mantle plume is a necessary element of their petrogenesis. 'Platy olivine spinifex' is an igneous texture that characterizes komatiites and its observation in outcrops or drill core (typically prior to, or in lieu of chemical analysis) leads geologists to classify a rock as a komatiite. Field descriptions may therefore drive assumptions and interpretations surrounding the prevailing tectonic or geodynamic setting at the time of emplacement. We emphasize the importance of careful discrimination between a variety of spinifex textures within a local volcanological framework and caution against the habit of making direct interpretations of rock type based on the existence of spinifex textures alone.
Layered mafic-ultramafic intrusions are among the largest igneous bodies on Earth, and represent aggregations of large volumes of mantle- and some crustal-derived melts. Melts are emplaced over time-intervals of less than one million years, predominantly through multiple pulses of injections into pre-existing melt-crystal slurries. The dynamic interaction of physical processes, including density-driven separation and mixing of different components, within a solidifying magma chamber leads to such extreme chemical diversity between cumulate rock units, that no unified model currently explains all aspects of the genesis of these intrusions. Here we present whole-rock stable Fe isotope data (expressed in ‰ variations as 57Fe relative to IRMM-014) for samples of drill core taken from the stratified paleo-magma chamber of the Upper Zone of the late-Archean Windimurra Igneous Complex, Western Australia. Variations from near chondritic (57Fe ~ +0 ‰) to heavy (57Fe~ +0.2 ‰) values show a co-variation with initial radiogenic Hf isotope data that is unique to the Windimurra Upper Zone. The systematic isotopic variations from the roof to the base of the Upper Zone are best explained by an intricate sequence of events that included fractional crystallisation and physical mixing. We propose that melt freshly sourced from the mantle was injected into and inflated a pre-existing crystal-melt mush comprising the upper Middle Zone. Re-establishment of crystal layering after replenishment introduced a chemical stratification with the formation of what became the Upper Zone and a crystal-interstitial melt ratio decreasing from roof to base. Basal, vanadiferous magnetitite horizons crystallised through enhanced fO2 during liquid replenishment. Variable degrees of perturbation and chaotic stirring of crystals with imperfect mixing of new and old components was followed by rapid crystal settling and subsequent cumulate stratification. Such melt rejuvenation, proposed here to be the cause for a newly established Upper Zone, leaves no unique petrologic fingerprint but can explain not only the observed coupled Fe-Hf isotope systematics but also mineral disequilibria and cryptic layering in layered intrusions.
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