Download Shock-Induced Deformation in Olivine from Polycrystalline Dunites

SHOCK-INDUCED DEFORMATION I N OLIVINES FROM POLYCRYSTALLINE DUNITES
AND PARTICULATE SAMPLES. Jon F. Bauer, NASA Johnson Space Center, Houston, TX
Deformattonal features c h a r a c t e r i s t i c of shock have been reported i n
01i v i n e s from c h o n d r i t i c meteorites, explosively-shocked t e r r e s t r i a l basal t s
and also from l u n a r dunites, basalts, breccias, and r e g o l i t h materials. Experimental shock-loading o f o l i v i n e samples has shown t h a t many o f these inpact
features can be r e a d i l y simulated i n the laboratory. Recent i n v e s t i g a t i o n s o f
shock-loaded and recovered 01i v i n e samples have employed s i n g l e c r y s t a l s i n
comparatively 1it t l e work has
f i x e d o r i e n t a t i o n s as t a r g e t materials (1,2,3,4);
been done on randomly-oriented p o l y c r y s t a l 1i n e and p a r t i c u l a t e samples (5,6).
This r e p o r t presents some r e s u l t s o f a petrographic-mineralogic study on such
a s u i t e o f p o l y c r y s t a l l i n e dunite (from Twin S i s t e r s Peaks, Washington) and i t s
p a r t i c u l a t e equivalent o f known porosity, shock-loaded a t pressure increments
of approximately 50 kb up t o maximum pressures o f greater than 600 kb.
Dense, Twin S i s t e r s Dunite Samples: Diagnostic shock features observed
i n the dunite samples can be conveniently grouped i n t o 3 categories depending
on peak shock pressure. Samples shock-loaded a t pressures between 50 and
220 kb are c l a s s i f i e d as "weakly shocked,Ii those between 220 and 570 kb as
"moderately shocked," and those a t pressures greater than 570 kb as "heavily
shocked." This scheme i s s i m i l a r t o t h a t formulated by Carter e t a1 (5) i n
t h e i r study o f experimental ly-shocked Twin S i s t e r s dun1t e ; however, i n t h e
present study, the greater number o f experiments a t $50 kb increments allows
refinement o f c l a s s i f i c a t i o n c r i t e r i a and peak pressures marking the appearance
o f pressure-dependent, p e t r o g r a p h i c a l l y - i d e n t i f i a b l e shock features.
"Weakly shocked" dunites are characterized by b r i t t l e deformation o f the
01 i v i n e s r e s u l t i n g i n the formation o f i r r e g u l a r and l a t t i c e - c o n t r o l l e d (planar)
microfractures w i t h i n the i n t e r i o r s o f i n d i v i d u a l grains. The s t y l e o f deformation i s l a r g e l y mechanical and shock-induced thermal metamorphism i s n o t
s i g n i f i c a n t due t o low shock and post-shock temperatures a t P <220 kb. With
increasing pressure, the f o l l o w i n g shock features are observed: (1) i n t e r n a l
f r a c t u r e density increases; f r a c t u r e s become shorter i n length, f i n e r i n width
and l e s s sinuous, (2) i n t e r n a l fragmentation and comninution along g r a i n
margins becomes more prominant, and (3) e x t i n c t i o n becomes more s t r o n g l y undulatory; the percentage o f grains showing undulatory e x t i n c t i o n a l s o increases
w i t h pressure so t h a t by 220 kb, a l l grains observed i n t h i n section show some
e f f e c t o f residual s t r a i n .
"Moderately shockedi' dunites are characterized by the appearance o f mosaic
o r "fracture-boundedi' domains w i t h i n the g r a i n i n t e r i o r s . Such domains have
been created by microfragmentation i n which the i n d i v i d u a l segments have been
r o t a t e d o r displaced from t h e i r o r i g i n a l c r y s t a l l o g r a p h i c p o s i t i o n s by o n l y a
few degrees. I n the dunite samples, mosaicism i s f i r s t seen i n some grains a t
220 kb, and i s found i n a l l grains by 305 kb. Throughout the pressure range
220-570 kb, the average mosaic domain s i z e decreases w i t h increasing pressure.
This observation c o r r e l a t e s q u i t e we1 1 w i t h a corresponding increase i n
f r a c t u r e density as observed i n plane p o l a r i z e d l i g h t .
E f f e c t s o f shock-induced thermal metamorphism were f i r s t seen i n the
dunite sample shock-loaded a t 368 kb. This metamorphism was manifested as a
p a r t i a l r e c r y s t a l 1i z a t i o n o f 01i v i n e a1ong (a) g r a i n boundaries and (b)
deformation lamel l a e (planar elements). I n t h i s sample, r e c r y s t a l 1i z e d
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SHOCK-INDUCED DEFORMATION IN OLIVINES
Jon F. Bauer
regions constituted <<I%of the total olivine. Contrary to the results of
previous studies on olivine single crystals (2, 7 ) , the percentage of recrystallized olivine was found to be inde endent of peak pressure, a t least
throughout this range. Recrystal 1ize areas were always confined to intergranular regions (or intragranular deformation lamellae) where dissipation of
thermal energy i s typically the greatest (8). The formation of recrystallized
zones appears to be more a function of i n i t i a l grain boundary energy and grain
orientation with respect t o the shock front than peak pressure of impact. This
generalization i s probably not valid for samples shock-loaded a t higher peak
pressures where large-scale intragranular recrystal 1ization might occur.
"Heavily shocked" dunites are characterized by the dominance of thermal
rather than mechanical phenomena. The onset of this pressure regime i s marked
by the appearance of a distinct zonation within the interiors of the olivine
grains. This zonation f i r s t appears in the sample shock-loaded a t 570 kb
where i t i s found in approximately 60% of the grains. The cores of these
grains are similar to the interior of olivine grains shocked a t slightly lower
pressures; they are colorless, highly strained, (containing numerous short,
thin planar fractures as well as planar "slip lamellae") and display distinct
mosaic extinction. The margins of these grains, however, are light brown i n
color, virtually free of planar microfractures, and apparently only weakly
strained. The abundance of such brown, "fracture-freen regions increases w i t h
peak pressure of impact; samples shock-loaded a t P > 600 kb frequently contain
grains which have been entirely transformed. Electron microprobe analyses of
b o t h colorless cores and brown-colored margins indicate that they are identical
in composition, implying that large-scale diffusion has not occurred during the
shock. These analyses also indicate that target metal contamination has not
been the cause of the brown coloration.
The decrease in fracture density observed in the brown 01ivines i s interpreted as the result of thermal annealing due to h i g h post-shock temperatures
generated during adiabatic decompression of the sample. The brown coloration
appears to be due to a valence change (probably Fe2+ -+ Fe3+) which has occurred
during this thermal event. Oxidation may have proceeded either a ) homogeneously w i t h the production of on.ly ~e~~ions and vacant cation MI or M2)
s i t e s or (b) heterogeneously with the formation of discrete submicroscopic
g
TEM studies of these regions are planned in
nuclei of ~ e ~ + - b e a r i noxide(s).
order to establish the existence o r absence of such domains.
In addition to these annealed and colored regions, there are other areas
found only in the 570 kb dunite samples which show a marked decrease i n birefringence as compared to other grains i n the t h i n section. These regions are
always associated with the colorless portions of the grains which exhibit the
highest degree of mechanical damage (and internal strain) as evidenced by very
high fracture densities and highly mottled extinction. In several areas,
birefringence i s so low that the 01i vine appears isotropic. U-stage examination of these regions has shown that (a) the "isotropic" zones typically occur
adjacent to either fractures or deformation lamellae which have been produced
by s l i p , and (b) the "isotropic" zones are never greater than about 10 yrn i n
cross section and appear t o grade into material of low b u t positive biref ri ngence.
These regions have been tentatively interpreted as zones containing diaplectic olivine glass which was probably produced by extreme shear
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