DOI: 10.1016/j.epsl.2021.116948
论文题名: Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting
作者: Kovaleva E. ; Kusiak M.A. ; Kenny G.G. ; Whitehouse M.J. ; Habler G. ; Schreiber A. ; Wirth R.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2021
卷: 565 语种: 英语
中文关键词: 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
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. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165550
Appears in Collections: 气候变化与战略
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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
Recommended Citation:
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]. Earth and Planetary Science Letters,2021-01-01,565