Shock wave experiments were conducted in San Gabriel anorthosite and San Marcos gabbra 10 11 GPa using a 40 mm-borne propellant gun. Particle velocities were measured directly at several points in each target by means of electromagnetic gauges. Hugoniot states were calculated by determining shock-transit time from the gauge records. Sound speeds indicate a loss of shear strength upon sock compression for both rocks, with the strength loss persisting upon release to zero stress om the anorthosite. Stress-density release paths in the anorthosite indicate possible transformation of albite to jadeite + (quartz or coesite), with the amount of material transformed increasing as a function of shock stress. Electrical interferene effects in the gabbro precluded the determination of accurate release paths for the rock

Anderson

Cowperthwaite

Grady

Liebermann

Marsh

Rice

English

Boslough, M. B.

Ahrens, T. J.

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T. I. S. LIBRARY
EXPERIMENTS IN A14ORTMOSITE AND GABBRO
Mark B. Boslough and Thomas J. Ahrens
Seismological Laboratory
California Institute of Technology,
Pasadena, California 91125
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Shock wave experiments have been conducted in San Gabriel anorthosite and San Marcos
gabbro to 11 GPa using a 40 mm-bore propellant gun. Particle velocities were measured
directly at several points in each target by means of electromagnetic gauges.
Hugoniot states were calculated by determining shock-transit times from the gauge
records. Sound speeds indicate a loss of shear strength upon shock compression for
both rocks, with the strength loss persisting upon release to zero stress in the
anorthosite. Stress-density release paths in the anorthosite indicate possible
transformation of albite to jadeite + (quartz or coesite), with the amount of material
transformed increasing as a function of shock stress. Electrical interference effects
in the gabbro precluded the determination of accurate release paths for that rocK.
1. INTRODUCTION
In most shock-wave experimental work, only one
state--the Hugoniot state--is determined per
experiment.	 By employing particle-velocity
gauges,	 a complete stress-strain	 history
subsequent to shock compression can	 be
determined,	 along with	 sound	 velocity
Information.	 Particle
	
velocity experiments
provide more information about rheology,
mechanical properties, and polymorphism than is
available with Hugoniot experiments alone.
Anorthosite and gabbro are _two rocks present in
the terrestrial and lunar crusts, and effects of
impacts into them are controlled by their
behavior under shock and rarefaction. The
purpose of the present study is to characterize
this behavior.
2. EXPERIMENTAL METHODS
Rock samples were shock-loaded by impact of
flat-faced polycarbonate (Lexan) projectiles
fired from a 40 mm-bore propellant gun at
velocities from 1.4 to 2.4 km/sec. Three
U-shaped copper gauges were sandwiched between
four 1.5 mm-thick slabs of rock with one gauge
on the free surface. A uniform magnetic field
of 1.8 kG--at right angles to each 1 cm-long
gauge and to its direction of motion--was
supplied by Helmholtz coils. The voltage across
each gauge element is directly proportional to
the velocity of the gauge (the particle velocity
of the surrounding medium), and was recorded ty
an array of oscilloscopes.	 The experiment is
shown schematically in Fig. 1.
associated with the Hugoniot state. After
reflection of the shock wave from the fret
surface, each gauge again accelerates as the
resulting rarefaction wave	 propagates	 back
through the sample.
3. RESULTS AND CONCLUSIONS
Digitized	 _xperimental data for anorthosite
shocked to 10 GPa are shown in Fig. 	 3.
Hugoniot	 states were determined by	 an
impedance-match solution s , where the shock
velocity is determined from the shock-transit
times taken from the particle velocity records,
and the known polycarbonate Hugoniot 2 and
projectile velocity are used. Eulerian sound
speeds were determined from the transit time of
the free-surface rarefaction front, and the
Hugoniot density.	 Hugoniot states and sound
speeds for both rocks are given in Tables 1 and
2.
The observed release waves are nonsteady simple
waves, and can be inverted to yield the
stress-density release paths by numerically
integrating the equations for conservation of
mass and linear momentum3
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The geometry and time history of a typical
experiment is illustrated by means of an x-t
diagram in Fig. 2. Each gauge is stationary
until overtaken by the shock w° e, at which time
it begins moving at the
	 particle
	 velocity
where a is the density, oo is the initial
density, a is the stress, up is the particle
velocity,	 and h is the Lagrangian	 space
coordinate. C(u ) is the
	 Lagrangian	 sound
speed, approximated by
3Rarefaction
characteristics
I.
I,
5
0
Shock intarget
X (mm)
.St
o 'o.
5
N
i
Projectile	 Gauge
torget
interface
	
2
Shock in	 - `fprojectile
i
projectile f I
	 Rock target
San iabriel Anorthosite
Free surfacegauge
Ror efoction
Shock (loading)
Gouge* 1	 3 4	 wove
0	 1	 2	 3
Figure 1:
Particle velocity experiment
A. Polycarbonate projectile
B. 40 mm gun barrel
C. Timing laser
D. Photodetector
E. High-power switch (ignitron)
F. Capacitor bank
G. Helmholtz coils
H. Rock target
I. Self-shorting trigger pins
J. Fiducial pulse generator
K. Particle velocity gauge
elements
K
Time (µs)
Figure 2: x-t diagram representing experimental
event. Projectile approaches stationary target 	 Figure 3: Gauge records for experiment 40-571:
from left and impacts at tn 0.	 anorthosite shocked to 10 GPa.
Table 1
San Gabriel Anorthosite Hugoniot Data
Eulerian
Shot Pressure Density Sounds	 ad
number (GPa) (Mg/m3) (km/sr
40-572 5.9 2.86 6.9
40-570 7.5 2.90 7.6
40-571 10.2 2.97 8.4 a
c^
Table 2
a
d
San Marcos Gabbro Hugoniot Data cn
Eulerian
Shot Pressure Density Sounds	 ed
number (GPa) (Mg/ma) (km/s
40-569 5.4 3.04 6.5
40-573 8.4 3.13 8.5
40-555 9.9 3.14 ---
40-556 10.4 3.17 7.0
40-574 11.2 3.18 6.7
i2	 Son Gabriel Anorthosite
Hupmat state
Release path
Coalz5z0$ Glass
10
	 (8oslough et al., 1983)
o Huganiot state
e Partially released
8	 state
i
6
ii4-
i
i
2 i
ii
i
21	 78	 23	 3A
Density (Mgfm3)
ah
C ( up) , -
at
where ah is the initial distance between gauges,
and at is the transit time for a disturbance
with particle velocity u
	 Eulerian sound
speeds, equal to (p / p ) Nu ), were calculated
for the release pat	 REsults of these
calculations for the two rocks are plotted in
Figs. 4-6. The anorthosite data are
significantly better than those for gabbro
because of the relative noisiness of the gabbro
Particle velocity records, presumably due to
piezoelectric interference from quartz grains in
the gabbro. The gabbro contained -1.4% quartz
by volume, whereas only trace quantities were
found in the anorthosite.
Measured longitudinal velocities4,5 in
anorthosites of similar composition (An40 ) are
in the range 6.91 to 7.47 km/sec, and shear
velocities are 3.87 to 4.09 km/sec. Calculated
bulk sound speeds are therefore in the range
5.04 to 5.99 km/sec. All three anorthosite
experiments release to zero stress with sound
speeds in this range, indicating that shear
strength is lost upon shock above 6 GPa and
never regained upon release.
Stress-density release paths for 	 shocked
anorthosite show a net densification from the
mean initial density of 2.654 Mg/m3. The
density change is an increasing function of
shock stress. Because strength effects are not
important according to sound-speed data, a
possible interpretation is polymorphic phase
transition of albite to jadeite and quartz or
coesite. This is analogous to the behavior of
quartz under shock pressure,	 which loses
Figure 4: Hugoniot states and release paths of
San Gabriel anorthosite. Included are two
Hugoniot states of anorthite glass $ , with
respective partial release states.
Son Gabriel Anorthosite
E
c
o'
Stress (GPa)
Figure 5: Eulerian sound speeds along release
paths of shocked San Gabriel anorthosite.
Son Marcos Gabbro
Hu90niot state
	
Rekau path	 f
r
r0a
N	 r
r
V)
	
Ir,
r
r
r
r
r
r
r
r
Density (Mg,/M3)
Figure 6: Hugoniot states and release paths of
San Marcos gabbro. Noisy particle velocity
records precluded determination of release to
zero stress.
strength and transforms to stishovite6.7.
Similar densification upon release has been
observed in shocked anorthite glassy (Fig. 4),
for which the composition is Ant^^pp. and the
density increase must be attributed - to strength
effects, annealing. or irreversible compaction
of the amorphous material.
Acknowledgments. Supported under NASA Grant
NGL05-002-105. We appreciate the assistance of
M. Long and E. Gelle. Contribution 3949,
Division of Geological and Planetary Sciences,
Pasadena, California 91125.
4. REFERENCES
[1] Rice, M. H., McQueen, R. G., and Walsh,
J. M., Compressibility of solids by strong
shock waves, Solid State Phys. 6 (1958)
1-63.
[2] Marsh, S. P., LASL Shock Hugoniot Data
(University of California Press, Berkeley,
1980).
[3] Cowperthwaite, M., and Williams, R. F.,
Determination	 of	 constitutive
relationships with multiple
	 gauges	 in
nondivergent flow, J.
	
Appl.
	 Phys. 42
(1971) 456-462.
[4] Anderson, 0. L., and Liebermann, R. C..
Sound velocities in rocks and minerals:
experimental methods, extrapolations to
very high pressures, results, in Mason,
W. P. ked.),	 Physical	 Acoustics	 1VB
(Academic Press, New York. 1968).
[5] Liebermann, R. C., and Ringwood, A. E.,
Elastic properties of anorthite and the
nature of the lunar crust, Earth Planet.
Sci. Lett. 31 (1976) 69-74.
[6] Grady, 0. E., Murri, W. J., and Fowles,
G. R., Quartz to stishovite: wave
propagation in the mixed phase regfan,
J. Geophys. Res. 79 (1974) 332-338.
[7] Grady, 0. E., Murri, W. J. and DeCarli,
P., Hugoniot sound velocities and phase
transformations
	
in
	
two	 silicates,
J. Geophys. Res. 80 (1975) 4857-4861.
[8] Boslough, M. B.,	 Rigden,	 S. M..	 and
Ahrens, T. J., Hugoniot equation of state
of anorthite glass and lunar anorthosite,
in preparation (1983).


Particle velocity experiments in anorthosite and gabbro

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