Physics
Scientific paper
Nov 1969
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969gecoa..33.1351b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 33, Issue 11, pp.1351-1352
Physics
6
Scientific paper
The grey hypersthene chondrite Ramsdorf--which has short and concordant 4 He-U/Th- and 40 Ar- 40 K-gas retention ages--shows unambiguous evidence for severe thermal and mechanical alterations. 1. (1) The distribution of metal and troilite is very inhomogeneous. Both are found together in rather large, millimeter-sized patches near slickenside "veins". 2. (2) The normal separation of the metal into individual grains of Ni-poor kamacite and Ni-rich taenite is lacking. Instead, it occurs as rounded globules ( d 1 mm ) with an interior Ni-content of about 11 per cent and a Ni-rich rim. We believe these rims not to be due to solid state lattice diffusion but to crystallization of metal from the melt. 3. (3) The silicate portion is extensively "recrystallized": only occasionally chondrule remnants can be discovered; the feldspar (12 per cent An) has been transformed into a clear isotropic glass, which must have been a liquid melt proper. 4. (4) The pyroxenes are clinopyroxenes and are high (up to 16·3 per cent) and variable in CaO, the highest values being found at the pyroxene-glass interfaces. In about 30 per cent of all cases the CaO-content falls into the immiscibility gap of the enstatite-ferrosilite-diopside-hedenbergite system. 5. (5) Chromite occurs as small euhedral crystals in the glassy silicate matrix and as larger badly "corroded" crystals at the silicate-troilite boundaries. 6. (6) Some pyroxene crystals have been broken and the fragments displaced over short distances. In some metal globules shearing forces have shifted portions of the globules over distances up to 300 microns. At these shearing planes the Ni-rich rim is missing indicating the mechanical deformation to be a late feature acquired after the thermal alterations. It is proposed that the observed features came about by the following sequence of events: Shock heating with a mean residual temperature of about 1200°C, homogenization and redistribution of a metal + troilite melt by acceleration forces, rapid cooling to about 900°C within less than one month, a secondary collision causing the mechanical deformations. The rapid cooling indicates Ramsdorf to have had a radius of less than 2 m--more probably less than about 30 cm--at that time. At present it cannot be decided unambiguously whether this sequence of events occured 4 Ma ago-the cosmic ray exposure age-or 400 Ma ago--the radiogenic gas retention age.
Begemann Friedrich
Wlotzka Frank
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