CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
EVIDENCE FOR KINETIC EFFECTS ON SHOCK WAVE
PROPAGATION IN TECTOSILICATES
Paul S. DeCarli u, Emma Bowden 2, Thomas G. Sharp 3, Adrian P. Jones 2, and
G. David Price 2
1
SRI International, Menlo Park, CA 94025, USA, 2 Dept of Geological Sciences, University College London,
Gower Street, London WC1E 6BT,3 Department of Geology, Arizona State University, Tempe, AZ 85287, USA
Abstract: The question of whether phase transition kinetics can affect shock wave propagation has been
around for about 50 years. Some workers have speculated that shock compression is fiindamentally different from static compression; others cite evidence that static and dynamic transitions follow the same
rules. Metastable high-pressure phases that are found in large (long-duration shock) impact craters constrain the post-shock temperature histories of the neighboring rock. The post-shock temperature is a
function of the area enclosed by loading and unloading paths. We use petrographic evidence to constrain the unloading paths. The presence of metastable phases then serves to constrain possible loading
paths. The limited range of possible loading paths indicates that there must be a large kinetic effect on
shock wave propagation in tectosilicates.
pothesis by looking for experimental evidence of
kinetic effects. We have found indirect evidence of
kinetic effects on Hugoniot measurements on rocks
and minerals. Kinetic effects are also inferred from
the results of shock recovery experiments and from
studies of natural impact craters.
INTRODUCTION
It has long been accepted that Hugoniot discontinuities can be interpreted in terms of dynamic
phase transitions. To account for the rapidity of
these transitions, some workers have speculated
that shock wave compression was fiindamentally
different from static compression. (1) Thus, one
could accept the possibility that the reconstructive
phase transitions of silicates, sluggish under static
high-pressure conditions, could occur rapidly under
shock compression.
Jeanloz presented evidence in support of a contrary view, that the Hugoniot data on olivines, pyroxenes and quartz did not represent evidence for
extraordinarily rapid reconstructive phase transitions. (2) Based on shock recovery studies, he inferred that a mineral could deform to a volume
nearly equal to that of a high-pressure phase without undergoing a reconstructive phase transition.
As a working hypothesis, we assume that phase
transitions under shock loading are governed by the
same thermodynamic and kinetic factors as phase
transitions under static loading. We test this hy-
LABORATORY STUDIES
We note that in laboratory shock experiments,
practical considerations limit the duration of peak
shock pressure to a few microseconds. However, it
is possible to study kinetic effects via variation of
the initial temperature of a sample. Two independent sets of shock recovery experiments on preheated quartz showed that the conversion to
diaplectic glass (formed by an inferred solid-solid
transformation) increased with increasing preheat
temperature. (3,4) Subsequent work confirmed the
quartz results and showed a similar initial temperature effect on the conversion of feldspar to
diaplectic glass.
Note that the conventional wisdom, among
those who have compared Hugoniot and release
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adiabat measurements with the results of shock
recovery experiments, is that diaplectic glass represents material that transformed at pressure to a
dense phase and expanded to glass on release of
pressure. In the case of quartz, this dense phase is
presumed to be stishovite-like, with 6:3 O:Si coordination, but not necessarily having long-range
crystalline order. Observed steep release adiabats
indicate that the dense material does not begin to
revert to lower coordination until pressure falls below about 7 GPa. (5,6)
One may also infer evidence of kinetic effects
from a comparison of Hugoniot data on an initially
porous material with data on the solid. The initially
porous material is much hotter, at a given shock
pressure, than the solid. One would expect the initially porous material to have a higher volume (because of thermal effects), at a given pressure, than
the initially solid material. We compare data on
Coconino sandstone (porous polycrystalline quartz,
2.0 g/cm3) with data on Arkansas novaculite (nonporous polycrystalline quartz, 2.65 g/cm3). (7) The
predicted relationship, initially solid denser than
initially porous, holds up to about 10 GPa. At
higher pressures, the sandstone becomes the denser
material. At 10 GPa, the internal energy content of
the sandstone is about 700 J/g higher than the novaculite. One may infer that the sandstone begins
to transform to 6:3 coordination at a lower pressure
than the novaculite.
small-scale experiments, relate the crater size to the
kinetic energy of the impactor. (9) One can thus
bracket likely ranges of density and impact velocity
for asteroidal (meteorite-like) objects to infer the
approximate size of the impacting object. The effective duration (in the region of maximum pressure duration) of the high pressure shock produced
by the impact is of the order of the shock wave
transit time through the impactor.
For a ca. 1 km diameter crater, such as the Arizona Meteor Crater, the impactor will have a diameter of a few tens of meters. The shock wave (or
peak release wave) velocity will be about 10 km/s,
and the maximum pressure duration in the material
directly below the impact point will be in the millisecond regime. For a 100 km diameter crater, such
as the Popigai crater, the impactor will be of the
order of 10 km diameter, and the maximum pressure duration will be of the order of a second.
SHOCK METAMORPHISM IN LARGE
IMPACT CRATERS
Numerous studies of shock metamorphism reveal differences between field observations and
laboratory calibration studies. (10, 11) It is customary to ascribe these differences to the longer duration of the natural events. However, quantitative
interpretations of these differences have hitherto
not been attempted.
We have been impressed by the relative abundance of "fragile" metastable high-pressure phases
in large impact craters. These phases include diamond, stishovite, coesite, and the alpha-PbO2
structure polymorph of TiO2. Simple estimates of
post-shock temperature, based on laboratory calibrations of shock metamorphic effects in surrounding rock, imply that the most fragile phases,
stishovite and the alpha-PbO2 structured TiO2,
should definitely have inverted to low-pressure
forms. Diamond and coesite can withstand much
higher post-shock temperatures but are nevertheless
found in environments in which their survival appears problematic. The possibility of quenching is
ruled out in most cases by simple heat flow calculations. In the absence of other possible explanations, one must postulate that the post-shock temperatures of the rock were actually much lower than
the laboratory calibrations would imply.
LONG-DURATION SHOCKS IN NATURE
Although long-duration laboratory shock experiments may be impractical, we have another
source of information in the form of large natural
impact craters. (8) These craters are generally identified by the presence of impact-metamorphosed
rocks and minerals. Impact metamorphosed rocks
have been extensively studied during the past 40
years. The question we address in this paper is
whether the study of impact-metamorphosed material found in large natural craters reveals anomalies
that might be ascribed to the operation of kinetic
effects during a relatively long-duration pressure
pulse. We begin by reviewing the relationship between crater dimensions and shock pressure duration.
Various scaling rules, based on calculations of
large cratering events and on extrapolations of
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account for the observed mineralogy. However, we
assumed dry Coconino sandstone. Kieffer has inferred that the sandstone was water-saturated at the
time of impact. (13)
In the absence of experimental data, we approximate the Hugoniot and release of watersaturated Coconino by the well-known and generally successful technique of volume addition of the
components. We assumed the release behavior of
the water to be the same as the compression behavior, neglecting the large expansion as the water
flashes into steam at low pressure on release. The
peak pressure of the sample is determined to be 32
GPa; the waste heat estimate is shown in Figure 2.
A BOOTSTRAP ESTIMATE OF KINETIC
EFFECTS ON SHOCK WAVE
PROPAGATION
Here we present the hypothesis: Kinetic effects
on shock wave propagation can account for the
survival of a fragile high-pressure mineral in a
shock-metamorphosed rock. To test this hypothesis
we examine a sample of shock-metamorphosed
Coconino sandstone from Meteor Crater, Arizona.
Optical and X-ray diffraction analyses of the
sample indicate that it consists of approximately 66
% diaplectic glass, 14 % crystalline quartz, 19 %
coesite, and 1 % stishovite, after correcting for 2 %
calcite that was deposited post-impact. We make
the customary assumption that the coesite formed
on release, as the pressure decayed into the coesite
stability field. Thus, at pressure, 86 % of the quartz
is presumed to have transformed to the 6:3 Sr.O
coordination as noted above. Using available Hugoniot release data (5) together with static compression data (12), we can approximate an appropriate
release adiabat for the observed post-impact mineralogy. The peak pressure in the sample is then determined to be 31 GPa, the intersection of the release adiabat with the Hugoniot. The waste heat,
the net internal energy increase after compression
and release, is the area between loading and release
paths, as shown in Figure 1.
Figure 2. Internal energy increase after release for watersaturated Coconino sandstone shocked to 32 GPa. The darker
hatched regions correspond to the area between the Rayleigh line
and the release adiabat.
Water saturation of the initially 25 % porous
Coconino sandstone reduces the waste heat by
about a factor of 2, to ~ 1.7 kJ/g. The corresponding post-shock temperature, -1400 K, is still too hot
for survival of either stishovite or coesite. Coesite
survival might be explained as indicative of about a
200 K uncertainty in our post-shock temperature
estimate combined with relatively rapid post-shock
cooling of the sample. However, the upper limit of
post-shock temperature for stishovite survival is
only about 900 K. The discrepancy seems much
too large to attribute to uncertainties in our approximations of the Hugoniot and release adiabat of
water-saturated Coconino sandstone.
We began this section with the hypothesis that
kinetic effects could account for the survival of
fragile high-pressure phases like stishovite. In the
absence of an a priori basis for inferring the details
of the kinetic effects, we take the bootstrap approach. We will assume an arbitrary but reasonable
kinetic effect and then calculate the post-shock
temperature. If we can find a combination of a rea-
Figure 1. Internal energy increase after release for dry Coconino sandstone shocked to 31 GPa. The darker hatched regions represent the waste heat, the area between the Rayleigh
line and the release adiabat.
The waste heat, 3.3 kJ/g, corresponds to a postshock temperature of -2800 K, corresponding to
superheated liquid and obviously much too high to
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sonable kinetic effect with a sufficiently low postshock temperature, we have effectively confirmed
the possibility that kinetic effects could account for
the survival of fragile high-pressure phases. As we
noted earlier, the observed mineralogy serves as a
basis for constraining the release path. One also
requires that the loading path intersect the release
path. The initial part of the loading path would
consist of a Rayleigh line connecting the initial
state to a peak stress state on the Hugoniot. This
state would be identical to that obtained in microsecond-duration loading. Over the millisecond duration of pressure in the Meteor Crater impact, the
stress would relax as transformation to 6:3 O:Si
coordination progressed. Figure 3 represents our
guess at the effects of kinetics at a peak stress of 15
GPa.
other quartz rocks. Second, the Meteor Crater impact is the smallest natural impact for which we
have inferred possible evidence of kinetic effects.
The impact that produced Meteor Crater was approximately equivalent in energy to about a 2 MT
thermonuclear explosion.
Further examination of possible kinetic effects
on wave propagation in tectosilicates (openstructured minerals like quartz and feldspars) appears warranted by the need for confidence in the
validity of calculations of ground motion produced
by large events. The kinetic effect that we have
inferred is so large that it may well be detectable in
appropriately designed large-scale high explosive
experiments.
REFERENCES
1.
2.
3.
4.
5.
6.
Figure 3. Effects of kinetics on the internal energy increase
of water-saturated Coconino sandstone after pressure release.
The darker cross-hatched regions represent the waste heat, the
area enclosed by the Rayleigh line, the stress relaxation line, and
the release adiabat.
7.
8.
The assumption of kinetic effects reduces the
waste heat to -1.14 kJ/g. If one assumes that the
water and quartz achieve thermal equilibrium, the
post-shock temperature is -900 K. This is about
the upper temperature limit for stishovite survival.
At this point, we conclude that we have adequately
tested our initial hypothesis: that the survival of
fragile high-pressure phases in the impactmetamorphosed rocks of large impact could indeed
be accounted for by the assumption of kinetic effects on dynamic phase transitions.
9.
10.
11.
12.
DISCUSSION
13.
We picked the example of Meteor Crater impact-metamorphosed Coconino sandstone for several reasons. First, we have adequate Hugoniot and
release adiabat data on Coconino sandstone and
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Alder, B. J., "Physics Experiments With Strong
Pressure Pulses", in Solids Under Pressure, edited
by Paul, W., and Warshauer, D. W. , McGraw-Hill,
New York, 1963, pp. 385-420
Jeanloz, R., J. Geophys. Res. 85, 3163-3176 (1980)
Langenhorst, F., et al, Atore 356, 507-509(1992)
Gratz, A. J., et al, Physics and Chemistry of Minerals 19, 267-288 (1992)
Grady, D. E., et al, J. Geophys. Res. 79, 322-338,
(1974)
Grady, D. E., and Murri, W. J., Geophys. Res. Lett.
3,472-474, (1976)
Ahrens, T. J., and Gregson, V. G., J. Geophys. Res.
69, 4839-4874 (1964)
Grieve, R A. F., and Shoemaker, E. M., "The Record of Past Impacts on Earth", in Hazards Due to
Asteroids and Comets, edited by Gehrels, T., Univ.
of Arizona Press, Tucson, 1994, pp. 417-462
Melosh, H. J., Impact Cratering:A Geologic Process, Oxford University Press, New York 1989,
Chapter VII
Stoffler, D., and Langenhorst, F., Meteoritics 29,
155-181(1994)
Huffman, A. R., and Reimold, W. U., Tectonophysics 256, 165-217(1996)
Knittle, E., "Static Compression Measurements of
Equations of State", in Mineral Physics and Crystallography AGU Reference Shelf 2, edited by
Ahrens, T. J., American Geophysical Union,
Washington, 1995, pp 98-142
Kieffer, S. W., et al, Contributions to Mineralogy
and Petrology 59,41-93 (1976)