Shock-induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the Ries impact crater (Germany) |
| |
| Authors: | Volker Stähle Rainer Altherr Mario Koch Lutz Nasdala |
| |
| Institution: | (1) Institute of Mineralogy, University of Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany;(2) Institute of Geosciences, University of Mainz, Becherweg 21, 55099 Mainz, Germany;(3) Present address: Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, 1090 Wien, Austria |
| |
| Abstract: | The microtextures of stishovite and coesite in shocked non-porous lithic clasts from suevite of the Ries impact structure
were studied in transmitted light and under the scanning electron microscope. Both high-pressure silica phases were identified
in situ by laser-Raman spectroscopy. They formed from silica melt as well as by solid-state transformation. In weakly shocked
rocks (stage I), fine-grained stishovite (≤1.8 μm) occurs in thin pseudotachylite veins of quartz-rich rocks, where it obviously
nucleated from high-pressure frictional melts. Generally no stishovite was found in planar deformation features (PDFs) within
grains of rock-forming quartz. The single exception is a highly shocked quartz grain, trapped between a pseudotachylite vein
and a large ilmenite grain, in which stishovite occurs within two sets of lamellae. It is assumed that in this case the small
stishovite grains formed by the interplay of conductive heating and shock reverberation. In strongly shocked rocks (stages
Ib–III, above ∼30 GPa), grains of former quartz typically contain abundant and variably sized stishovite (<6 μm) embedded
within a dense amorphous silica phase in the interstices between PDFs. The formation of transparent diaplectic glass in adjacent
domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and PDFs to diaplectic
glass in the solid state. Coesite formed during unloading occurs in two textural varieties. Granular micrometre-sized coesite
occurs embedded in silica melt glass along former fractures and grain boundaries. These former high-pressure melt pockets
are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite. The
latter is mostly replaced by amorphous silica. |
| |
| Keywords: | Impact metamorphism Stishovite Coesite Suevite |
| 本文献已被 SpringerLink 等数据库收录！ |
|
