Impact-induced phase transformations at 50-60 GPa in continental crust: an EPMA and ATEM study

Details Impact-induced phase transformations at 50-60 GPa in continental crust: an EPMA and ATEM study Impact-induced phase transformations at 50-60 GPa in continental crust: an EPMA and ATEM study

Aug 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..207m&link_type=abstract

Earth and Planetary Science Letters, Volume 119, Issue 1-2, p. 207-223.

Computer Science

9

Scientific paper

Strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 Ma old Haughton impact crater were studied by optical microscopy, electron probe microanalysis (EPMA) and analytical transmission electron microscopy (ATEM). The study is focused on the substantiation of shock-induced phase transformations in rock-forming minerals, and their implications for post-shock thermal regimes and impact age dating.
In the gneiss fragments, the original layering is largerly preserved, although all the minerals have experienced dramatic phase transformations. Chemically, the various shock-generated decomposition regions still reflect the pre-shock mineralogy. However, significant heterogeneities exist on the 10-100 μm scale. Four different regions of decomposition products can be distinguished (EPMA): (1) shocked biotite-rich regions with strong variations in Si, Al, K, Fe and Mg on the 10 μm scale, (2) shocked quartz with about 3% Al and K and traces of Fe and Mg, (3) a region with Al/>Si ~ 1, which does not correspond to any pre-shock mineral, and (4) shocked K-feldspar showing a significant deficit in K, and traces of Fe and Mg. On the nanometer scale, a uniform, bimodal reaction pattern emerges: all minerals are transformed to new crystals embedded in a silica-rich glass. The shock-generated crystals are: (1) spinel in shocked biotite, (2) α-quartz, cristobalite, and coesite in shocked quartz, (3) an unknown polymorph of corundum (Al2O3) in feldspar and in the Al/Si ~ 1 region, and (4) mullite in association with SiO2 glass in shocked sillimanite grains. All crystals range in size between 0.05 and 1.0 μm. Taking into account estimated durations and pressure-temperature conditions for shock-wave passage and post-shock temperatures, the reactions observed can be ascribed to high temperatures during and after pressure release. The different mineral-glass parageneses suggest temperatures in excess of 1400°C. Moreover, given the small size of the newly formed minerals, very rapid cooling down to 1200°C is required.

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