Research ArticleEARTH SCIENCES

Discovery of natural MgSiO3 tetragonal garnet in a shocked chondritic meteorite

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Science Advances  25 Mar 2016:
Vol. 2, no. 3, e1501725
DOI: 10.1126/sciadv.1501725
  • Fig. 1 Backscattered electron image of a shock-induced melt vein in Tenham chondritic meteorite.

    Fragments of minerals of the host rock are captured in the shock vein. Olivine and plagioclase (in a fragment at the center of the image) were transformed into the spinel phase (Rw, ringwoodite) and diaplectic glass (Msk, maskelynite), respectively. Low-Ca pyroxene (En) is partly transformed into the garnet phase (Maj, majorite). The rectangular hole is a portion extracted by a focused ion beam (FIB) apparatus for ATEM analysis.

  • Fig. 2 Transmission electron micrograph of an aggregate of (Mg,Fe)SiO3 tetragonal majorite in Tenham.

    The ATEM sample consists of monomineralic aggregates of euhedral or subhedral grains. No interstitial phases are present among the majorite grains.

  • Fig. 3 SAED patterns from single crystals of (Mg,Fe)SiO3 tetragonal majorite.

    (A) Along the [Embedded Image] zone axis. (B) Along the [010] zone axis. Weak {h0l} reflections, where both h and l are odd [indicated by triangles in the magnified pattern (right)] are diagnostic reflections for the tetragonal I41/a symmetry.

  • Fig. 4 Calculated cooling history on the wall of a 400-μm-thick shock vein based on the one-dimensional thermal conductivity model.

    The initial temperature just after the shock wave passage was assumed to be 2000°C, corresponding to the liquidus temperature of the bulk chondritic composition at 20 GPa (37). The solid lines represent temperature paths of shock veins cooled by a shock-temperature increase in the host rock. The broken line represents the temperature path in high-pressure synthesis in the Kawai-type multianvil apparatus (29). The gray background corresponds to the temperature range where (Mg,Fe)-Si ordering occurs in the octahedral sites in the majorite structure (28, 30). When cation ordering in the octahedral sites in meteoritic majorite is hindered owing to the higher cooling rate (as compared to that of synthetic majorite), the shock-temperature increase in the host rock must be lower than ~900°C.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/3/e1501725/DC1

    Fig. S1. Pressure-temperature phase diagram of MgSiO3.

    Fig. S2. Polarized optical micrographs of a fragment of host rock captured in a shock vein.

    Fig. S3. Raman spectrum of majorite at the rim of a shock vein in Tenham.

    Fig. S4. Transmission electron micrograph of the entire ultrathin foil sample of a tetragonal majorite aggregate processed by an FIB.

    Fig. S5. Schematic diagrams of electron diffraction patterns of cubic (Formula) and tetragonal (I41/a) majorites along the <001> and <010> zone axes.

    Fig. S6. One- and two-dimensional electron diffraction profiles of natural and synthetic majorites.

    Fig. S7. One-dimensional thermal conductivity model used for estimating the temperature paths of a shock vein cooled by the host rock of a meteorite.

    Table S1. Chemical composition of (Mg,Fe)SiO3 tetragonal majorite.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Pressure-temperature phase diagram of MgSiO3.
    • Fig. S2. Polarized optical micrographs of a fragment of host rock captured in a shock vein.
    • Fig. S3. Raman spectrum of majorite at the rim of a shock vein in Tenham.
    • Fig. S4. Transmission electron micrograph of the entire ultrathin foil sample of a tetragonal majorite aggregate processed by an FIB.
    • Fig. S5. Schematic diagrams of electron diffraction patterns of cubic ( Ia3d ) and tetragonal (I41/a) majorites along the ‹001› and ‹010› zone axes.
    • Fig. S6. One- and two-dimensional electron diffraction profiles of natural and synthetic majorites.
    • Fig. S7. One-dimensional thermal conductivity model used for estimating the
      temperature paths of a shock vein cooled by the host rock of a meteorite.
    • Table S1. Chemical composition of (Mg,Fe)SiO3 tetragonal majorite.

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