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dc.contributor.authorLowrey, Jack R.
dc.contributor.authorIvanic, Tim J.
dc.contributor.authorWyman, Derek A.
dc.contributor.authorRoberts, Malcolm P.
dc.date.accessioned2019-11-21T02:54:16Z
dc.date.available2019-11-21T02:54:16Z
dc.date.issued2017-09-01
dc.identifier.citationLowrey, J. R., Ivanic, T. J., Wyman, D. A., & Roberts, M. P. (2017). Platy Pyroxene: New Insights into Spinifex Texture. Journal of Petrology, 58(9), 1671–1700. https://doi.org/10.1093/petrology/egx069en_AU
dc.identifier.issn00223530
dc.identifier.urihttps://hdl.handle.net/2123/21394
dc.descriptionJeremy Shaw, Andrew Mehnert, Stefano Caruso and Scott Heywood-Smith are thanked for analytical, technical and visualization assistance. Fawna Korhonen is thanked for providing help with the interpretation of alteration assemblages. Joyce Peng is thanked for drafting figures. We thank reviewers Nick Arndt, Steve Parman and a third anonymous reviewer for their comments. This paper has also benefited from helpful discussions with Bob Nesbitt, Steve Barnes, Stephen Wyche, Hugh Smithies, Paul Morris, Sandra Romano and Roman Teslyuk. Staff at the Geological Survey of Western Australia are thanked for their assistance with fieldwork and sample handling at Carlisle labs. The Geological Survey of Western Australia is thanked for substantial in-kind support. We acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation and Analysis, University of Western Australia, a facility funded by the University, State and Commonwealth Governments. J.R.L. conducted the research presented in this paper as part of a PhD research degree at The University of Sydney. T.J.I. publishes with the permission of the Executive Director, Geological Survey of Western Australia. This work was supported by Australian Research Council Grant LP130100722 and by the Geological Survey of Western Australiaen_AU
dc.description.abstractNew 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 mm, with thicknesses typically ≲120 mm. These plates have ≲25 mm 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 mm, and thicknesses that are typically ≲50 mm. 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 (14.4-15.8wt %) and higher in SiO2 (50.9-52.1wt %) 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. © The Author 2017.en_AU
dc.description.sponsorshipBritish Geological Survey, Australian Research Council, University of Sydney, University of Western Australia, Western Universityen_AU
dc.language.isoen_AUen_AU
dc.publisherOxford University Pressen_AU
dc.relationARC LP130100722en_AU
dc.rightsThis is a pre-copyedited, author-produced version of an article accepted for publication in the Journal of Petrology following peer review. The version of record is available online at: https://doi.org/10.1093/petrology/egx069en_AU
dc.subjectArcheanen_AU
dc.subjectDendriteen_AU
dc.subjectMicro-tomographyen_AU
dc.subjectPlatyen_AU
dc.subjectPyroxeneen_AU
dc.subjectSpinifexen_AU
dc.titlePlaty pyroxene: New insights into spinifex textureen_AU
dc.typeArticleen_AU
dc.subject.asrc040304en_AU
dc.identifier.doi10.1093/petrology/egx069
dc.type.pubtypePost-printen_AU


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