气候变化领域集成服务门户(CCPortal): Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting

CCPortal

DOI	10.1016/j.epsl.2021.116948
	Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting
	Kovaleva E.; Kusiak M.A.; Kenny G.G.; Whitehouse M.J.; Habler G.; Schreiber A.; Wirth R.
发表日期	2021
ISSN	0012821X
卷号	565
英文摘要	Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200 °C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt. © 2021 Elsevier B.V.
关键词	baddeleyite cubic zirconia immiscible melts mullite yttria zircon filaments
英文关键词	Aggregates; Crystal orientation; Granulation; Melting; Nanocrystals; Silica; Textures; Yttrium oxide; Zirconia; Baddeleyite; Cubic zirconia; High-pressure polymorph; Immiscible melt; Impact structures; Mullite; Nano scale; Nanocrystallines; Zircon filament; ZrO$-2$; Silicate minerals; crystal structure; crystallization; granular medium; immiscibility; impact structure; melting; microstructure; nanoparticle; P-T conditions; shock metamorphism; zircon; South Africa
语种	英语
来源期刊	Earth and Planetary Science Letters
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/203172
作者单位	Department of Earth Sciences, University of the Western Cape, Robert Sobukwe Road, Bellville, 7535, South Africa; Institute of Geophysics, Polish Academy of Sciences, Księcia Janusza 64, Warsaw, PL-01452, Poland; Department of Geosciences, Swedish Museum of Natural History, Stockholm, SE-104 05, Sweden; Department of Lithospheric Research, University of Vienna, Vienna, 1090, Austria; Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, 3.5 Surface Geochemistry, Telegrafenberg, Potsdam, D-14473, Germany
推荐引用方式 GB/T 7714	Kovaleva E.,Kusiak M.A.,Kenny G.G.,et al. Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting[J],2021,565.
APA	Kovaleva E..,Kusiak M.A..,Kenny G.G..,Whitehouse M.J..,Habler G..,...&Wirth R..(2021).Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting.Earth and Planetary Science Letters,565.
MLA	Kovaleva E.,et al."Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting".Earth and Planetary Science Letters 565(2021).