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Shock effects on hydrous minerals and implications for carbonaceous meteorites
Shock effects on hydrous minerals and implications for carbonaceous meteorites MANFRED A. LANGE’+~, PESILIPPE LAMBERT’.~ and T~-~cx~AsJ. AHRENS’** ’ Division of Geological and Planetary Sciences, California Institute of Technology. Pasadena, CA 9 I I25 2SN6/NASA Johnson Space Center, Houston, TX 77058
(Received Augusr 21, 1984; arcepted in revised&m
May 1, 198.5)
Abstract--;New infrared absorption spectra, the~~~vimet~~ analyses and optical-and scanning dectron mieroseopy of shock-recovered specimens of amigo&e serpentine ~Mg~Si~U~~~~)~)from the pressure range between 25 to 59 CPa are reported. The infrared spectra show systematic changes in absorption peaks related to structural and molecufar surface absorbed water. HzG absorption peaks increase at the expense of OH peaks with increasing shock pressure. Changes in SiO bond ~bmtion~ modes with increasing shock pressure parallel those seen for other, non-hydrous minerals. The~o~vimet~c analyses of shock-recovered sampIes determine the amount of sh~k-indu~ water loss. For samples shocked in vented assemblies, the data define a relation between shock-induced water loss versus shock pressure. Results for samples shocked in seaied assemblies demonstrate a dependence of water loss on shock pressure and target confinement. For the vented assembly samples, a linear relation between shock pressure and both the length of dehydration interval and the effective activation energy for releasing postshock structural water in antigorite is found. Optical and scanning electron miscroseopy of shocked antigorite reveal a number of textures thought to be unique to shock laading of volatile-bearing minerals. Gas bubbles, which probably are the result of shock-released HzG appear to be injected into zones of partial melting. This process may produce the vesicular dark veins which are distributed throughout heavily shocked samples. The present observations suggest several criteria which may constrain possible shock histories of the hydrous matrix phases of carbonaceous coadrites. A model is proposed for explaining hydrous aiteration processes occurring on carbonaceous chondrite parent bodies in the course of their accretion. We speculate that shock loading of hydrous minerais would release and redistribute free water in the regoliths of carbonaceous chondrite parent bodies giving rise to the observed hydrous alterations. RVTRODUCXTON MAJUR MATRIX PHASESin Cf and CM carbc?naceous chondrites consist of complex mixtures of Mg- and Fe-bearing p~y~I~silicates (J~ARBER, 198 I). Clear identi~~tion of these minerais is often di~&ult,
however they strongly resembIe Mg- and Fe-bearing serpentine, amigo&e serpentine and brucite (NACY, 1975; MACKINN~N and BUSECK, 1979; BARBER, I98 1f. The presence of serpentinaceous phyllosilicates in carbonaceous chondrites implies substantial water budgets in CX carbonaceous chondrites of up to 10 wt.%, Hz0 (Bo~ro, 1954; WUK, 1956). Recent experiments to delineate the total H budget of carbonaceous chondrites demon~mte that the phyllo~li~te matrix contains some 5 to 10 times the amount available to form Hz0 relative to that found in the hydrocarbons (e.g., ROBERT and EPSTEIN, 1982). Previously direct condensation of phyllosilicates at low temperatures (
’ Permanent addressesz A~~~-Wrens-tnstitute for P&E Research, Columbus Center, D-2850 Bremerhaven, Fed. Rep. of Germany. ’ Centre d’Applications et de Recherches en Microscopic Electronique, 6, rue du Ma&ha1 Foch-33 260 La Teste de Buch, France.
* Correspondent.
fides) on a meteorite parent body provides a more pfausible explanation for the evidence of serpentinaeeous phy~Iosi~~cat~ in CI and CM carbonaceous chondrites (MA~KINNON and BUSECK, 1979; KERRIDCZEand BUNCH, 1979). CoIlisional intemctions in the earIy evolutional stages of the solar system and processes of shock compaction and shock iithification comprise major events in the evolution of meteorite parent bodies (e.g., WASSON, 1974). Although the terrestrial meteorite collection contains many objects which appear to have been subjected to shock pressures of up to 75 GPa (750 kbar) (BINNS, 1967; TAYLOR and HEYMANN, 1969; Sr&?%BB, 1972; BUCHWALD, 19X), few Cl or CM ~ar~n~~eous chondrites have previously been thought to have undergone a significant shock history, The effects of shock on volatile-bearing (hydrous) minerals may constitute a mechanism for alteration of phyllosilicates in carbonaceous chondrites. The hypothesis we examine is that shock elects on hydrous minerals in carbonaceous chondrites may have been important in ahering the volatile contents and textural characteristics of matrix phases fc$, BARBER, 198 1). However, pertinent data and investigations on shock loading of hydrous minerals are scarce. The first studies of changes in chemistry and mineralogy of hydrous minerals under shock, mainly represented by a (partial) loss of structural water and corresponding changes in the crystal lattices, were reported by BOSLOUGH d al. (1980) LANCE and AHRENS ( 1982a) and WELWN
et af. ( 1982).
1716 The
M. A. Lange, P. Lambert and T. J. Ahrens present
and analytical
study
summarizes
investigation
an experimental
of shock
effects in antigorite serpentine, over a wider range of shock pressures (29-59 GPa) than previously studied. Even though the material used in this study was of terrestrial origin and thus not completely identical to chondritic phyllosilicates, we believe that our basic results are applicable to meteoritic serpentinaceous phyllosilicates. Shocked antigorite was also studied via optical and scanning electron microscopy. New data from shock recovery experiments on antigorite serpentine and subsequent analysis of the shocked material by infrared spectroscopy and thermogravimetry are also reported. These data are used to characterize the major effects of shock loading on hydrous minerals. With these data we believe we can identify a number of features which constrain the shock history of carbonaceous chondrites. We discuss the observed shock effects in carbonaceous meteorites. Their occurrence suggests that these features will also be present on other planetary surfaces, such as on Mars (cl WELD~N d ul.. 1982), low albedo asteroids (possible carbonaceous chondrite parent bodies; cf. LEBOFSKY, 1980; FREIERBERGel al.. 1981), and possibly Europa (for which serpentine has been proposed as representing a major mineral; CL, RANSFORD et al., 198 1; FINNERTV et al., I98 I ). EXPERIMENTAL
I) and is close to the theoretical composition. The water content of the starting material (12.6 wt.%) was determined using a Mettler Thermoanalyzer (Model TA 2000C). Details of shock recovery experiments are given in LANGL
663 685 667 690
obtained from vented assemblies After each experiment, the sample disc was machrned ou: and split into two fractions. About half of the drsc. or the largest mechanically intact pieces, were potted rn epox]. sliced and mounted into two thin sections. one parallel tc$ the plane of the disc. one perpendicular to this plane. The remainder of the sample was used for infrared and therm+ gravimetric analysis. Absorption spectra were measured m a uavenumhcr region between 380 to 4000 cm-‘, correspondmg to wavelengths of 26 to 2.5 pm on a Perkin-Elmer grating spectrometer. Weight loss curves of the shocked samples, determming the amount of post-shock structural water and thus the amount of shock-induced water loss (LANGE and .~HRLNI. 1982a). were obtained by use of the Mettler Thermoanalyzer in the temperature range of 25 to 900°C (298 to I I73 K) Computing relative masses M/M, (where nr is the mass cli the sample as a function of temperature. 7_ &, is the rnaSb of sample at T = 1173 K) and plotting these ratios a\ .i function of reciprocal temperature. log,, (l/T), allows drtermination of the effective activation energy for dehydration in the heating experiments (N~JTTING. 1943: i AVGF an
Each sample thin section was photo-mapped and ther: by means of optical and scanning electron mrcroscop> (SEM). SEM was performed at NASA-JSC with a JEOL 35 CF, coupled with a PGT system IV energy dispersive system (EDS). EDS and backscattered imaging were used extensively in search for compositional variation. Microprobe analyses were obtained at Southern Methodist University (Dallas). with JEOL JXA-733 “superprobe”. Refractive indices were measured directly on polished thin section using a macroreflectometry technique described in LAMBERT! 1981t OBSERVATIONS ON SHOChRECOVERED MATERIALS
were carried out both at
the shock wave laboratory of the California Institute of Technology (CalTech), and at the shock wave facility of the Johnson Space Center (JSC) (Table 1). Experimental conditions at both laboratories differed insignificantly from those described by LANGE and AHRENS (1982a). Two different sample assemblies were employed. Sealed assemblies enclose the sample tightly and prevent most. if
1046
is thought to represent better the conditions of natural shock events. Aside from providing some results from sealed assemblies, most of the discusston will he devoted to darn
studied
DETAILS
The material used in the experiments is antigorite serpentine (Mg3SirOr(OH)4)from Thurman. New York, with an initial density of 2.540 f 0.004 g/cm3. The major oxide composition is given in LANGE and AHRENS (1982a; Table
and AHRENS (1982a). Experiments
not all, of the potential water vapor (produced as a con% quence of shock loading of serpentine) from leavmg thr assembly chamber. The vented assemblies have an escape path for shock-released water vapor (c$. LANGEand AHRENS, 1982a; Fig. I). The rationale for using different assemblies was the possibility for studying the effects of sample confinement on shock-induced water loss. The vented assembl!
Figure I shows absorption spectra for the shocked samples (shock-loaded in both vented Jnd scaled assemblies). together with a reference spectrum of an
SlCPI Alummum
27 2
\
I 28
Starnless Steel 31R
25 ”
\
I 55 1 89
Scamless srec1 Stsmless stre1
36 0 45 0
\’ \’
72.5
1 25
19 4 19.3 19 4 19 1
I 90
Sral”les.3
i
JSC CALTECH CALTECH CALTECH CALTECH
I
!
1717
Shock effects on meteorite minerals
Wovelengt h. microns 2.5
3
4
5
6
7
8913
15 2030
S: - 0 Bond V~brotlonol modes
I------
59 GPO_
J-y_
#,‘L_-. -_%__5g _cI_--*
\,I
,I/
f-----
i\ I
I
.,=__
4000
45 ---t..___#_-~-
3000
_____/‘/
I
1 \
,i\
‘y”
1003
2ooo Wovenumber,
cm*’
FIG. 1. Infrared absorption spectra of unshocked and shocked antigorite serpentine. Numbers on each curve indicate shock pressure in GPa (I GPa = 10 kb). Solid and dashed lines give spectra of samples shock loaded, in vented, and sealed assemblies, respectively. Identification of absorption peaks is according to FARMER (1974).
Major fea-
tures of these spectra are:
(=a; Fig. 1) representing OH-stretching modes of the lattice (FARMER, 19741, decrease systematically in
(i) The absorption peaks, co~s~nding to structural water in serpentine, i.e.. peaks at 3 690 cm-’
We also observe a broadening of these peaks, which is indicative of non-un~fo~ local potentials. This
unshocked
antigorite
serpentine
specimen.
height with increasing shock pressure of each sample.
1718
M ,A. Lange. P. Lambert and 7. J. Ahrens
could be caused by increased internal disorder. Another interpretation of these results concordant with the thermogravimetry data. calls for a decrease in the amount of structural, post-shock water and thus an increase in shock-induced water loss with increasing shock pressure. (ii) Absorption peaks at 3 450 and I 630 cm ! (=b and c, respectively; Fig. 1). which represent H&I hydrogen bonding and Hz0 bending modes, respectiveiy, i.~., the presence of molecular, surf&e adsorbed water (FARMER, f 974) increase systematically (less clearly in the case of peak “c”) with increasing shock pressure. One possible interpretation of this observation is that fractions of the shock-released water become readsorhed and remain m the sample material. This is consistent with findings of BCXLOUGHd cr!. ( 1980) who investigated this effect more thoroughly. (iii) The absorption features in the I100 to 380 cm-’ region, corresponding in wave number approximately to Si-0 bond vibrational mode (FARMER. 1974). undergo a continuous change from a fourpeak to a two-peak absorption feature upon recovery from increasing shock pressure. This could signal ;1 decrease in structural order of the lattices. as well as the gradual onset of partial melting as a result of the shock loading (cl, ST~F?UR. 1974). The changes with respect to structural and molecular water described in (i) and (ii) are seen in exper”. iments carried out with both vented and sealed containers. However, for a given pressure these changes appear less pronounced for samples shocked in sealed as compared to those shocked in vented assemblies (Fig. 1). This can be most clearly seen in the spectra from the 59 GPa shots. Here. absorption peaks b and c appear much stronger when compared to the structural water peak in the vented assembl! shot versus those for the sealed assembly at the same pressure. These differences imply that the dehydration reaction is less efficient in the case of sealed assembl>shots. This is explained by a rapid increase in water vapor pressure in the sealed assembiies. leading to a drastic reduction of reaction rates in antigorite. Dehydration reactions are extremely sensitive IO thr level of partial water vapor pressure (BRINDLE\ t’f al., 1967; c$, LANGE and AHRENS. 1982a). Despite the differences with respect to structural and molecular water absorption features, the changes in the low wavenumber region are similar for both assembly types {Fig. 1). This suggests that changes with respect to Si-0 bond vibrational modes are only dependent on shock pressure and independent of sample confinement.
Major results of the thermogravimetric analyses ot’ the shocked antigorite samples are given in Table 2. Figure 2 shows the amount of shock-induced water loss as a function of shock pressure for both sealed and vented assembly shots. It should he noted that
the pressures in Table 2 correspond to those obtained in the stainless steel container enciosing thz sampit: material. Thus, shock loading of the samples to these pressures required multiple reverberations of the shock wave. and initial shock pressures fit,., prior to shock reverberation) lie below the final value given rn Table 2. Since most of the shock heating (entrap? r~sei takes place upon the initiaf compression of the sample material, shock pressures given in Table 2 and Fig. _” overestimate the critical pressures needed to ubtalrr ;+ certain amount of shock-induced dehydratron. ihr amount of shock-induced water loss is the Jifferencr. between initially present structural water and posrshock water. as determined by thermogram imete We note the good agreement of the data abtaincti in two different laboratories. These define % relatror~ between shock-induced water loss and shock pressure The resulting relation also agrees with theoretlcai estimates for total loss of structural water upon shoch loading (LANGE and AHRENS. 1980, 198%). T‘h? dependence of shock-induced water loss, WL. i wt.‘6.i. on shock pressure, P (GPa). as represented h\ thr solid line in Fig. 2, is given b\, WL = -160.440 t L.09 (LANGE
>
+ 10.88lIJ
i0 t Pz
for
ti.i77P
.‘i -‘ f’::. hi 5.iPa
::.
and AHRENS, 1982a). As can be SCCKrnc~p ient to complete water loss in antigorite srr’pentinc N :IrKf occurs in the pressure range between -65 CiPa. The data for the sealed assembly shots {Fig. Z; differ markedly from those for the vented assembl! shots. Shock-induced water loss appears to be less efficient in sealed assemblies as compared to vented shots for a given pressure. Also, there is nv clear relation between water loss and shock pressure a5 was seen in the vented experiments. This paraileis findings in similar experiments on brucite (LANCZ (9 111..1982). The significantly lower dehydration efh~. ciencies obtained in sealed assembties, as nored earlier. are thus quantitatively confirmed. We take this 3% evidence that the increased partial water vapor pressure in shock experiments influences the decomposition rate of antigorite to a comparable extenr 35 was observed in heating experiments I~RINDt k-k cl d. lY67).
Shock effects on meteorite minerals
I
8 3
80 -
I
ANTIGORITE
sealed
l
vented I
b “0 60-
I
I
l
assembly 0
Antigorite
\ Lange and Ahrens, 1980 (theoretical) 9
B u 8 405 5 f%j 20z
I
O0
I 20
1 60
I 40
Shock Pressure GPo RG. 2. Shock-induced water loss as a function of shock pressure in antigorite serpentine. The solid line represents a numerical fit to the vented-assembly data (Eqn. (I)). Open circle represents a theoretical prediction for complete dehydration of serpentine. (LANGE and AHRENS,1980).
The weight loss versus tern~mtu~ for each sample yields also a dehydration interval (i.e., the temperature interval over which loss of post-shock water occurs) and the effective activation energy for dehydration (i.e., the energy needed to decompose the remaining intact antigorite and to remove the post-shock water from the samples; cJ LANGE and AHRENS, 1982a). Table 2 gives numerical values for these quantities for each sample (except for sealed assembly shots) and Figs. 3 and 4 give dehydration interval and effective activation energy as a function of shock pressure, respectively. As can be seen, there is a clear relation between dehydration interval, AT (K), and effective activation energy, AAE (k~/mole), on shock pressure, P (GPa): 200
I
I
I
1
I
Id 0
I
I
I
I
20
30
40
50
I
IO
Shock Pressure,
t 60
GPO
FIG. 4. Apparent activation energy for dehydration as a function of shock pressure in shock loaded antigorite. Solid line represents tinear fit.
AT = 176 - 2.OP
(2)
log,, AAE = 0.47 - 0.02P.
(3)
While the length of the dehydration interval is a direct measure of the heat required to remove postshock structural water, the activation energy of dehydration is related to the bond strength of OH molecules within the crystal lattice. Both quantities are interrelated and show a comparable dependence on shock pressure. This indicates that shock loading not only leads to the removal of structural water in antigorite (and probably other hydrous minerals as well), but also weakens the bonds between remaining OH-molecules and other constituents of the crystal lattice.
I
ANTIGORITE
c) Optical microscopy
AT, K
~T~K~=176.17-2.0P~GPa~ w
I
Oo
I
IO
I
f
20
30
I 40
I 50
I
60
Shock Pressure, GPo FIG. 3. Length of the dehydmtion interval, AT, as a function of shock pressure in shock loaded antigorite (vented assemblies only). The solid line represents a linear fit.
Photomicrographs (Figs. 5 and 6) illustrate the petrographic characteristics of unshocked and shocked antigorite. Unshocked antigorite appears as equiaxial, polygonal grains displaying the typical mesh-texture of serpentine minerals (WICKS and W~~~AKER, 1977). Grains are typically 0.2 to f mm in size (Fig. 5a). When observed under crossed nicols, the grains appear to consist of various arrays of submicroscopic fibro-lamellae (Fig. 5b). Shocked antigorite (Figs. 5c and 5d) shows remarkably few deformation features or other traces of the shock loading compared to other silicates (cJ ST&FLER, 1972, 1974). When observed in more detail, antigorite recovered from 27, 45 and 59 GPa (shots 1046, 1007 and 1008 in vented and sealed assemblies, respectively; see Table 1) displays essentially no change in texture, grain mo~hology, birefringence, and extinction patterns with respect to unshocked samples. The refractive indices are un-
1720
M 4. Lange, P. lambert and T. J. Ahrens
FIG. 5. a) Photomicrograph of the unshocked antigorite specimen in transmitted Itght: hi same un& crossed nicoia; c) antigorite recovered of shot 1044 (45 GPa sealed assembly) in transmitted light. Arn~ points to black vein to be seen in greater detail in Fig. 7; d) same under crossed nicols iscale eqt~rralent in all Figs.)
changed too, except in the 59 GPa sealed shot where refractive indices are locally reduced from 1.5501.570 to about 1SO& A systematic though “volumetrically” minor difference with respect to the unshocked antigorite is the occurrence of black veining and irregular brownish coloration in the shock loaded
specimens. Although these features are seen in the sample from the 27 GPa experiment. the? arc best observed in material exposed to higher shock pressure% (Fig. 5~). Darkening occurs along fiber outi~nes. grain boundaries, cracks and intragranular fractures. In the sample from the 59 GPa scaled experimrn? WC a/~~
Shock effects on meteorite minerals
1721
FIG. 6. a) Thin section of a vesicular grain [shot 1045; 59 GPa, vented assembly) in transmitted light; same grain observed by SEM; b) SEM view of another grain showing both, vesicular glass (top) and crystalline rno~hoIo~~ (bottom) separated by a transitional zone with micron to submicron vesicles (arrow).
find flow-like features. Under crossed nicois, these dark regions always disptay a characteristic bright glow of diffuse light which is similar to that observed in shock melted micas (LAMBERTand MACKINNON, 1983). The relative proportion of dark veins increases with increasing pressure and is relatively higher in vented than in sealed assembly experiments. The sample of shot 1045 (59 GPa, vented assembly) shows the strongest shock-induced modifications compared to unshocked antigorite or compared to samples 1046, 1007 and 1008. As a consequence of mechanical fracturing this sample was total disrupted. Although no large pieces were recovered, the study of a large number of grains mounted in polished thin sections show major changes compared to the unshocked antigorite. The majority of the grains are locally to completely opaque, depending on their relative thickness, and are vesicular (Fig. 6a). Vesiculation is responsible for the dominance of internal
reflections observed under crossed nicols. The remaining grains are characterized by the lack of optically resolvable vesicles. These are sometimes locally clear and apparently isotropic, though commonly brownish and birefringent. The latter display abundant internal reflections under crossed nieols resulting in a bright glow of diffuse light, similar to that observed in the dark veins of material shocked at lower pressure. d) Scanning elecrron microscopy (SE&f)
SEM combined with optical observations on thin sections confirms the vesicular character of the dark and opaque grains recovered from the 59 GPa vented experiment (Fig. 6b). This material is interpreted as glass on the basis of morphological and textural characteristics. Size and morphology of these vesicles vary from 0.1 to about 10 pm, and from spherical to elongated respectively (Figs. 6b and 7). The presence
1722
M. A, Lange. P. Lambert and T. 3. Ahrens
FIG. 7. a) SEM view of polished thin section of antigotite, shocked to 45 GPa (shot 1044); detailed view of dark vein seen in Fig. SC);b) detail of same vein; c) detail of similar vein of shot 1007 (59 GPa. sealed assembly) numbers above solid bars give length of bar in pm.
of these features and their shape (rapid change in the orientation of elongated vesicles) suggest turbulent flow of gas and liquid mixture. Locally, relatively large bubble-free areas are observed (with widths of 10 Mm or more) which correspond to the clear fields observed optically. Backscattered imaging x-ray energy dispersion and electron microprobe- analyses demonstrate that the glass is chemicalty homogeneous and the major non-volatile element composition is similar to unshocked antigorite. The texture and composition of the shockinduced glass is significantly different from that of the incongruent melt which would result from static heating (EVANS et al., 1976: CARUSO and CHERNOSKY, 1979; CHERNOSKY, 1982). The petrographic. SEM and chemical characteristics of shock-melted antigorite parallel those of experimentally shockmelted micas (LAMBERT and MACKINNON, 1983). From these characteristics and by analogy with shockproduced mica glasses, the vesicular glass of antigorite is interpreted as quenched liquid, produced by congruent mehing of antigorite. We speculate that the vesicles result from exsolved Hz0 upon pressure release (LANGE ef al.. 1982; LAMBERT and MACKINNON, 1983). Infrared and thermogravimetric data of the 59 GPa sample (shot 1045, vented assembly) suggest that the glass has lost most of its structural water. The limited amount of water still retained in the sample is probably associated with the remaining crystal fraction. Although this hypothesis could not be verified by thermogravimetry due to the difficulty of separating these phases, we suggest that Hz0 acts
like K in shocked micas where it is found to be in the fused material reiative to the remaining crystalline portions (LAMBERT and MA~KINN~~. 1983). Micron and sub-micron vesicles are resolved by SEM in the brownish, otherwise crystalline fragments recovered from the 59 GPa vented experiment. Scattered submicron vesicles might be responsible for the low refractive index measured by microre~ectomet~. Microvesiculation is the cause of the internal retlections seen under the optical microscope. The same observation and interpretation apply to the dark veins seen optically in the lower pressure shots (Fig. 7). Local vesicuiation seen by SEM in the 27 GPa sample (shot 1046, vented assembly) indicates inctpient melting. The number of vesicular zones and thus the relative amount of iOcd1 melting increase with increasing shock pressure and is also more abundant in vented than in sealed assemblies. The most obvious examples are the 59 GPa experiments where the sample in the sealed assembly is apparently stifi largely crystalline while that in the vented assemhlv has essentially melted. From the features observed with SEM, we conclude that shock melting of antigorite begins locally upon recovery from shock pressures as low as 27 GPa and is almost complete upon recovery from 54 CPa These values correspond to the range of pressure for the shock-induced dehydration of antigorite. However, the amount of Hz0 released vra shock loading of antigorite can by no means he accounted for by the vesicles observed in the shocked samples. Hence. :z depleted
Shock effects on meteorite minerals is concluded that shock-induced dehydration involves not only regions of local melting but also large fractions of the sample which remain unmelted. IMPLICATIONS, OBSERVATIONS AND POSSIBLE CONSEQUENCES FOR CARBONACEOUS CHONDRITES
a) Shock criteria in carbonaceous chondrites
The above results allow definition of the identification and possibly calibration shock effects in serpentine-bearing rocks bonaceous chondrites containing copious serpentinaceous phyllosilicates.
criteria for of natural and in caramounts of
I) Weight loss record. The temperature range required for complete dehydration and hence the ap parent activation energy for dehydration of shocked antigorite decrease systematically with increasing shock level. Since this behavior is distinctly different from that of unshocked but dehydrated hydrous minerals in regular heating experiments (c$, FARMER, 1974), we believe this dehydration behavior provides a quantitative shock indicator. This may also be true for cases where phyllosilicate minerals cannot be separated from other matrix phases (i.e., non-hydrous minerals in carbonaceous chondrites). Using appropriate calibrations such as given in Fig. 3 for antigorite, the dehydration interval could also yield semi-quantitative estimates of the shock pressures in natural samples. 2) Infrared spectra. Changes in the absorption spectra of shocked serpentine minerals constitute a potential shock indicator. The absorption patterns as observed for shock-loaded antigorite samples, and specifically the observation of simultaneous alteration in the OH-, HZ0 and SiOz peaks would appear to be characteristic of shock deformation. 3) Amount of structural water. Because shock is very efficient in releasing structural water from hydrous silicates, an anomalously low content of water in phyllosilicates may indicate a previous shock history. DZICZKANIEC et al. (1982) have shown that separation of matrix phases of Murchison by physical separation methods is feasible. These separates have been shown (by SEM/EDS- and x-ray diffraction analysis) to contain -80 to 98 vol.% se~ntinaceous material (consisting of betthierine, cronstedtite. chrysotile, and antigorite in decreasing proportions (G. LUMPKIN, pers. commun., 1982). Using such matrix separates, their water content (as obtained through thermogravimetry) could be interpreted as a maximum estimator (because some rehydration might have occurred) of the amount of water in a meteorite which may have had an impact history. Water associated with different hydrous minerals and surface adsorbed water can, in principle, be identified via weight loss versus temperature data. A shock-induced dehydration-versus-sh~k pressure relation such as Fig. 2, would then allow a quantitative estimate of the shock pressure. However this methodology has
1723
the difficulty that the peak shock pressure experienced by the sample cannot be uniquely determined for as complex a group of minerals as are present in carbonaceous chondrites. Clearly other endogenic heating processes, e.g., radiogenic, might be responsible for less than the nominal amount of structural water in hydrous matrix minerals. In addition. we do not know the stability of the shock-produced antigorite glass and/or that of the dehydrated antigorite. 4j Petrographjc characteristics. Although petrography is easily carried out on meteorites. it appears to be less useful to delineate shock effects. Antigorite shocked up to 45 GPa lacks significant petrographitally observable shock indicators. This contrasts with the extensive range of shock metamorphic features observed over the same pressure range, in shocked nonhydrous silicates (FRENCH and SHORT. 1968; H~Rz, 197 1; ST~FFLER, 1972). As noted above. melting and vesiculation provide the most reliable petrographic shock indicators for phyllosilicates, Although observed upon shock recovery over a shock pressure range from 27 to 60 GPa, signi~cant amounts of glassy and vesicular areas within shock loaded antigorite occur upon recovery from shock pressures well above 40-50 GPa. In summary, the above described shock indicators, when used individually, will probably not suffice to identify the detailed shock history of natural phyllosilicates. What is needed instead is a comprehensive approach employing several techniques. b) ~b~e~ation~ on carbonaceous chondrite.~
Figure 8 shows the infrared absorption spectrum of the present 59 CPa antigorite sample (vented assembly shot) together with a spectrum obtained for a matrix separate of Murchison, a CM chondrite containing copious amounts of hydrous phyllosilicates as a major matrix component (FUCHS et al.. 1973: KERRIDGE and BUNCH, 1979). Even though the similarity between the two spectra might be fortuitous, based on the arguments presented above, we suggest that shock loading of these matrix phases could be responsible for the observed spectrum. Whether a single shock event with shock pressures close to 60 GPa, or multiple shock loading led to the alteration of the hydrous matrix phases cannot be
FIG. 8. Infrared absorption spectra of shock loaded antigorite (59 GPa, vented assembly) and Murchison matrix material.
1724
M. A.Lange. P.Lamhert and T. .i. Ahrens
discerned from the present level of knowledge; however, multiple shock events with small to moderate shock pressures would seem to be the more likely scenario (cJ, KERRIDGE and BUNCH, 1979;BUNCH and CHANG, 1980). Textural analysis via optical microscopy and SEM have been performed on a number of matrix phases of carbonaceous chondrites (e.g.. KERRIDGE and BUNCH, 1979; BUNCH and CHANG. 1980; BARBER, 198 1). Many of the features seen in matrix phases resemble those of the present shocked antigorite. Among these are vesicular, spongelike structures. resembling altered volcanic tuff, or tuff breccias (BUNCH and CHANG, 1980) vesicular zones, veinlike flow features, and indi~tions for material transport by a rapidly moving fluid or vapor phases (BARBER, 1981). Similar vesicular textures to those observed in shocked serpentine have recently also been found in shock-loaded calcite (LANGE and AHRENS, 1985). Whether similar textures observed in carbonaceous chondrites are related to shock processes remains to be demonstrated. LAMBERTand MACKINNON (1983) concluded that vesicular melt zones in shock loaded hydrous minerals are produced by congruent melting. Thus, microchemical analysis of texturally similar features in carbonaceous chonrdites might provide evidence as to whether these are related to shock loading or possibly to hydrous alterations of matrix material.
It is generally assumed that hydrous alteration of carbonaceous chondrite parent bodies (possibly identical with C-type asteroids; KERRIDGE and BUNCH. 1979) took place early in their evolution. It is also assumed that this process took ptace within the surface layers of the regolith of parent bodies. The hydrous alteration at low temperatures implies the presence of free water within the regolith. As originally proposed by DUFRIZSNEand ANDERS ( 1962), KERRIDCIEand BUNCH ( 1979) the hypothesized liquid water layer gradually moved outward in a meteorite parent body, as a result of heat produced by (possibly short-lived) radioactive elements in the interior. However, such a mechanism may not produce hydration of surhcial material because it is difficult to retain liquid water in the surface layer of a small. low escape velocity, object with low surface temperatures (9273 K). Referring to DUFRESNE and ANDERS'(1962) model, they propose that an icy surface layer could prevent escape of water. Such an ice layer, could heal itself after mechanical disruption because of the ratio of the heats of fusion and evaporation of water. How similar in operation a meteorite parent body with an icy crust and a mantle which contains &id water might be to a multiple impacted regolith layer is unclear. Thus, the ice-
water allows for hydration but not necessaniy shockinduced features. Repeated impacts, inducrng low to moderate shock pressures, could instead provide a means of producmg and transporting free water within the regolith layer of carbonaceous chondrite parent bodies. Such .! process would yield a relatively large amount of fret water which would reside within the ejecta blanker of an impact crater at relatively high te~~rat~~~~ (>I273 K) where hydration of unhydrous minerah could take place at fairly high reaction rates. We ser the major difference between the ice-water layer- and the present model in the fact that the present mode) does account for near surface hquid water which IF, in ciose contact with relatively warn% and reacu\r surface minerals, while the DuFresne and Anden hypothesis provides liquid water contained within ait icy crust. The present model accounts for both the generation of impact-induced textural characteristics of carbonaceous chondrites and the generation oi free water. Observations of mobilization of K VIGshock ret’leased (presumably interstitial and intracrystalline, water from various rocks have been reported at several terrestrial impact craters (MILTON 6'1 ui., 1971 and CURRIE and SHARQULLAH, 1968). We envision that sh~k-induced dehydration &’ hydrous phases accompanies accretion (at low encounter velocities) of carbonaceous parent bodies. This would lead to the constant re-disposttion ~6 water and hydrous phases at the outer parts of the growing body (& LANCE and AHRENS, 1982b). Wc note again that sh~k-induct dehydration may not necessarily require shock pressures as high as indicated in Fig. 2 (i.e.. between 20 and 60 GPa), but could result from repeated low and intermediate shock loadings which lead to the weakening of bonds be tween structural OH-groups and other constituent!: of the iattice. Thus, much lower shock pressures might be needed to release portions of the structural water in hydrous minerals, after a number of previous shocks have broken some of the bonds between structural water and the crystal lattice. CONCLUSIONS Shock experiments in antigorite produce mcipieni devolatilization and incipient melting upon recover:, from above 20 GPa. Total devolatilization, accompanied by complete melting occurs upon recovery from above about 60 GPa. Shock melting of antigoritc proceeds congruently yielding a vesicular glass which is characteristic of shock. Upon exposure to inrermediate shock pressures, a significant amount of H& is driven out of antigorite probably in the solid state, without melting. The petrographic characteristics oi shocked serpentine are remarkably insensitive to shock pressures up to about 40 to 50 GPa. Water loss versus temperature data and infrared spectra appear to be the most useful shock rndicatori
Shock effects on meteorite minerals
which can be applied to shock studies in matrix phases of carbonaceous chondtites and in serpentinebearing rocks. A shock history is inferred for the hydrous matrix phases of Murchison on the basis of infrared absorption characteristics. Although the data are consistent with those found in experimental samples shocked to 60 GPa, the infrared spectrum of the matrix phases are hypothesized to represent repeated low to intermediate shock loading. Textural characteristics of shock-toaded antigorite bears similarities to textures seen in carbonaceous chondrite matrix phases, which might indicate that shock loading is also responsible for the origin of these textures in natural specimens. We suggest that shock processes could play an important role in the modification and evolution of carbonaceous chondrite parent bodies. Shock loading of hydrous phyllosilicates may provide an explanation for the mechanism of dist~buting free water within the surface layers of these parent bodies. Thus impact could lead to the hydrous alterations observed in samples of carbonaceous chondrites. Acknowledgments-We appreciate the skillful help of W. Ginn, E. Gelle, and M. Long in the experiments and the use of the spectroscopic and thermogravimetric analyzer and advice proffered by G. Rossman and R. Aines. The assistance of R. Schmidt and T. Thompson with the shock experiments at NASA-Johnson Space Center is gratefully acknowledged. We thank F. H&z for critically reading several versions of this paper and R. Brett, J. Kerridge, and H. McSween for helpful reviews. M. Lange was supported by a stipend from the Deutsche Forschungsgemeinschaft while at CalTech. P. Lambert was supported by the National Research Council as a Research Associate. The microphotometer and its attachments were used through the courtesy of Dr. C. B. Moore and the Center for Meteorite Studies. Arizona State Unive~ity. This work is supported under NASA grant NGLOS-002-105. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 9 1125, Contribution 3936. Editorial handling:
R. Brett REFERENCES
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BARBER D.
Cosmochim. Acta 45, 945-970. BINNS R. A. (1967) Stony meteorites hearing maskelynite. Nature213, llll-1112. BINNs R. A., DAVIS R. J. and REED S. J. B. (1969).
Ringwoodite, natural (Mg,Fe)$iO, spine1 in the Tenham meteorite. Nature 221, 943-944. BOAT0 G. (1954) The isotopic composition of hydrogen and carbon in the carbonaceous chondrites. Geochim. Cosmochim. Acta 6, 209-220.
BOSLOUGHM. B., WELDONR. J. and AHRENST. J. (1980) Im~ct-indu~d water loss from serpentine, nontronite, and kernite. Proc Lunar Planet. Sci. Co& 11th. 21452158. BRINDLEYG. W., NAHIRA B. N. and SHARP J. H. (1967) Kinetics and mechanism of dehydroxylation processes: II. Temperature and vapor pressure dependence of dehydroxylation of serpentine. Amer. Mineral. 52, 1697-l 705. BUCHWALDY. F. (1975) Handbook of Iron Meteorites, Vol. 7. Univ. of California Press. _ BUNCH T. E. and CHANG S. (1980) ~r~naceous chon-
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Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface ~ndjtions. G~chim. Cosm~him. Acta 44, 1543-t 577.
CARUSO L. J. and CHERNOSKYJ. V., JR. (1979) The stability of lizardite. Can. Mineral. 17, 757-7’69. CHERNOSKY1. V., JR. (1982) The stability of clinochrysotile. Can. Mineral. 20, 19-27.
CURRIE K. L. and SHAFIQULLAHM. (1968) Geochemistry of some large Canadian craters. Nature 218,457-459. DUFRESNE E. R. and ANDERS E. (1962) On the chemical evolution of the carbonaceous chondrites. Geochim. Cosrn~h~rn. Acta 26, 1085-l 114. DZICZKANIECM., LUMPKIN G. R. and Lux G. E. (1982) Physical separates from the Murchison meteorite (abstr.). In 45th Annual Meteoriticat Socieiy Meeting, The Lunar and Planetary Institute. Houston. TX. XIV-9. EVANSB. W., J~HANNESW., GTER&OOMH. and TROMMSDOR!=FV. ( 1976) Stability of chtysotile and antigorite in the serpentine multisystem. Schweiz. Mineral. Petrog. Mitt. 56, 79-93.
FARMER V. C. (1974) The Znfared Spectra of Minerals. Miner~~cal Society, London. FINNERTV A. A., F~AN.WORD G. A., PIERI D. C. and KOLLERSONK. D. (1981) Is Europa’s surface cracking due to thermal evolution? Nature 289, 24-27. FREDRICKSSONK. and KRAUT F. (1967) Impact glass in the Cachari eucrite. Geochim. Cosmochim. Acta 31, 17011704. FREIERBERGM. A., LEEWS.KY L. A. and LARSON H. P. (1981) Spectroscopic evidence for aqueous alteration products on the surfaces of low-albedo asteroids. Geochim. Cosm~him. Acta 45, 97 1-98 1. FRENCH B. M. and SHORT N. M. (1968) Shock Metamorphism ofNatural Materials.
Mono Book, Baltimore.
FUCHSL. H., OLDENE. and JENSENK. F. (1973) Mineralogy, mineral chemistry, and composition of the Murchison (C2) meteorite. Smithsonian Contributions to the Earth Sciences No. IO, 39 pp. GROSSMANL. (1972) Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta 36, 597-6 19. HEYMANND. (1967) On the origin of hypersthene chondrites: Ages and shock effects of black chondrites. Icarus 6, 189221. HC~RZF. (ed.) (1971) Meteorite impact and volcanism. Meeting at the Lunar Science Institute, Houston. TX.. Oct. 19-23, 1970. J. Geophys. Res. 76, 5381-5798. KERRIDGEJ. F, and BUNCHT. E. (1979) Aqueous activity on asterois: Evidence from carbonaceous chondrites. In Asteroids (ed. T. GEHRELS), pp. 745-764. University of Arizona Press. LAMBERTP. (1981) Reflectivity applied to peak pressure estimates in silicates of shocked rocks. J. Geophys. Res. 86,6 187-6204. LAMBERTP. and LANGE M. A. (1982) Processes of shock dehydration on accreting planets (abstr.). In Corzfirence on Planetary Volatiles. Lunar and Planetary Institute, Houston, TX., 104-105. LAMBERTP. and MACKINNONI. D. R. (1983) Micas in experimentally shocked gneiss, Proc. 14th Lunar Planet. Sci. Conj Part 2. J. Geophys. Res. 84, suppl., 685-699. LANGE M. A. and AHRENS T. J. (1980) The evolution of an impact generated atmosphere (abstr.). Lunar Planet. Set. XI, 596-598. LANGE M. A. and AHRENS T. J. (1982af Impact induced dehydration of serpentine and the evolution of planetary atmospheres. Proc Lunar Planet. Sci. Conf: 13th.. J. Geophys. Res. 87, suppl., A451-A456. LANGE M. A. and AHRENST. J. (1982b) The evolution of an impact generated atmosphere. Icarus 51,96-120. LANGEM. A. and AHRENST. J. (1985) Shock-induced CO, loss from CaCO,; implications for early planetary atmospheres. Earth Planet. Sci. Lett. (submitted).
1726
M. 4. Lange. P. Lamhert and 1. J. Ahrens
LANGEM. A., LAMBERTP. and AHRENS T. J. f 1982) Shockinduced dehydration and impli~tioRs for carbonaceous rnet~~t~ (a&r.). In Papers Presented to the 45th Annual .~~eteor~ticai Society Meeting, f 12. LEBoFsKY L. A. (1980) Infrared refiectance spectra of asteroids A search for water of hydmtion. ~stronum~~ui J. 85, 573-585. MACKINNON I. D. R. and BusI~rc P. R. (1979) New phyllosihcate types in a carbonaceous chondrite matrix. Nature t80,2 19-220. MILTON D. J., BARLOWII. C., BRETT R., BROWN A. R.. GLIKSONA. Y., MANWARINGE. A.. Moss F. J.. SEDMIK E. C. E., VAN SON J. and YOUNG G. A. (1972) Gosses Bluff impact structure, Australia. Science 175, 1199-1207 NAGY B. (1975) Carbonaceous Meteorites Elsevier. Amsterdam/New York. NUTTINGP. G. (1943) Some standard thermal dehydration curves of minerals. US. Geol. Surr Prof Paper 197-E. 197-217. RANSFORDG. A., FINNERTVA. A. and COLLERSONI(. D. (198 I) Europa’s petrological thermal history. Nature 289, 21-24. ROBERT F. and EPSTEINS. (1982) The concentration and isotopic composition of hydrogen” carbon and nitrogen
in carbonaceous meteorites. (ieochlm. C’i~.sm~k #urn .A?,i 46.81-95. ST&%LER D. (1972) Deformation and transfo~ation ni rock-forming minerah by natural and experimental shock processes. I. Behavior of minerals under shock compression. Forfschr. Miner. 49, 50-i 13. ST~FFLER D. (1974) Deformation and t~nsfo~atton of rock-forming minerah by natural and experimental shock processes. II. Physical properties of shocked miner& Fortschr. Miner. 51, 256-289. TAYLOR G. J. and HEYMANND. (1969) Shock, reheating, and the gas retention ages of chondrites. .Ear:h Planet Sci. Lett. 7, 15I- 16 1. WASSON J. T. (1974) Mett~orrrc:~ Springer. ?,e~ i ark 316 pp. WELDON R. J.. THOMAS W. M., BOSI.OLIC;~I hl. H. snd AHRENS T. J. (1982) Shock-induced color changes II? nontronite: Implications for the manian fines. .I (;(+~r~ Res. 87, 10102-10114. WICKS F. J. and WHI?TAKEH Is. .I. W. (19’77)Serpentine textures and serpentinization. Can. Mineral. 15.459-488. WIJK H. B. (1956) The chemical composition of some stem meteorites. Geothim. Cosmochim. ilcta 9, 279-289, W~LKENINGL. L. (1978) Carbonaceous chondritic material in the solar system. Die Natur~,issenschqTen65. 73. 74
(Received Augusr 21, 1984; arcepted in revised&m
May 1, 198.5)
Abstract--;New infrared absorption spectra, the~~~vimet~~ analyses and optical-and scanning dectron mieroseopy of shock-recovered specimens of amigo&e serpentine ~Mg~Si~U~~~~)~)from the pressure range between 25 to 59 CPa are reported. The infrared spectra show systematic changes in absorption peaks related to structural and molecufar surface absorbed water. HzG absorption peaks increase at the expense of OH peaks with increasing shock pressure. Changes in SiO bond ~bmtion~ modes with increasing shock pressure parallel those seen for other, non-hydrous minerals. The~o~vimet~c analyses of shock-recovered sampIes determine the amount of sh~k-indu~ water loss. For samples shocked in vented assemblies, the data define a relation between shock-induced water loss versus shock pressure. Results for samples shocked in seaied assemblies demonstrate a dependence of water loss on shock pressure and target confinement. For the vented assembly samples, a linear relation between shock pressure and both the length of dehydration interval and the effective activation energy for releasing postshock structural water in antigorite is found. Optical and scanning electron miscroseopy of shocked antigorite reveal a number of textures thought to be unique to shock laading of volatile-bearing minerals. Gas bubbles, which probably are the result of shock-released HzG appear to be injected into zones of partial melting. This process may produce the vesicular dark veins which are distributed throughout heavily shocked samples. The present observations suggest several criteria which may constrain possible shock histories of the hydrous matrix phases of carbonaceous coadrites. A model is proposed for explaining hydrous aiteration processes occurring on carbonaceous chondrite parent bodies in the course of their accretion. We speculate that shock loading of hydrous minerais would release and redistribute free water in the regoliths of carbonaceous chondrite parent bodies giving rise to the observed hydrous alterations. RVTRODUCXTON MAJUR MATRIX PHASESin Cf and CM carbc?naceous chondrites consist of complex mixtures of Mg- and Fe-bearing p~y~I~silicates (J~ARBER, 198 I). Clear identi~~tion of these minerais is often di~&ult,
however they strongly resembIe Mg- and Fe-bearing serpentine, amigo&e serpentine and brucite (NACY, 1975; MACKINN~N and BUSECK, 1979; BARBER, I98 1f. The presence of serpentinaceous phyllosilicates in carbonaceous chondrites implies substantial water budgets in CX carbonaceous chondrites of up to 10 wt.%, Hz0 (Bo~ro, 1954; WUK, 1956). Recent experiments to delineate the total H budget of carbonaceous chondrites demon~mte that the phyllo~li~te matrix contains some 5 to 10 times the amount available to form Hz0 relative to that found in the hydrocarbons (e.g., ROBERT and EPSTEIN, 1982). Previously direct condensation of phyllosilicates at low temperatures (
’ Permanent addressesz A~~~-Wrens-tnstitute for P&E Research, Columbus Center, D-2850 Bremerhaven, Fed. Rep. of Germany. ’ Centre d’Applications et de Recherches en Microscopic Electronique, 6, rue du Ma&ha1 Foch-33 260 La Teste de Buch, France.
* Correspondent.
fides) on a meteorite parent body provides a more pfausible explanation for the evidence of serpentinaeeous phy~Iosi~~cat~ in CI and CM carbonaceous chondrites (MA~KINNON and BUSECK, 1979; KERRIDCZEand BUNCH, 1979). CoIlisional intemctions in the earIy evolutional stages of the solar system and processes of shock compaction and shock iithification comprise major events in the evolution of meteorite parent bodies (e.g., WASSON, 1974). Although the terrestrial meteorite collection contains many objects which appear to have been subjected to shock pressures of up to 75 GPa (750 kbar) (BINNS, 1967; TAYLOR and HEYMANN, 1969; Sr&?%BB, 1972; BUCHWALD, 19X), few Cl or CM ~ar~n~~eous chondrites have previously been thought to have undergone a significant shock history, The effects of shock on volatile-bearing (hydrous) minerals may constitute a mechanism for alteration of phyllosilicates in carbonaceous chondrites. The hypothesis we examine is that shock elects on hydrous minerals in carbonaceous chondrites may have been important in ahering the volatile contents and textural characteristics of matrix phases fc$, BARBER, 198 1). However, pertinent data and investigations on shock loading of hydrous minerals are scarce. The first studies of changes in chemistry and mineralogy of hydrous minerals under shock, mainly represented by a (partial) loss of structural water and corresponding changes in the crystal lattices, were reported by BOSLOUGH d al. (1980) LANCE and AHRENS ( 1982a) and WELWN
et af. ( 1982).
1716 The
M. A. Lange, P. Lambert and T. J. Ahrens present
and analytical
study
summarizes
investigation
an experimental
of shock
effects in antigorite serpentine, over a wider range of shock pressures (29-59 GPa) than previously studied. Even though the material used in this study was of terrestrial origin and thus not completely identical to chondritic phyllosilicates, we believe that our basic results are applicable to meteoritic serpentinaceous phyllosilicates. Shocked antigorite was also studied via optical and scanning electron microscopy. New data from shock recovery experiments on antigorite serpentine and subsequent analysis of the shocked material by infrared spectroscopy and thermogravimetry are also reported. These data are used to characterize the major effects of shock loading on hydrous minerals. With these data we believe we can identify a number of features which constrain the shock history of carbonaceous chondrites. We discuss the observed shock effects in carbonaceous meteorites. Their occurrence suggests that these features will also be present on other planetary surfaces, such as on Mars (cl WELD~N d ul.. 1982), low albedo asteroids (possible carbonaceous chondrite parent bodies; cf. LEBOFSKY, 1980; FREIERBERGel al.. 1981), and possibly Europa (for which serpentine has been proposed as representing a major mineral; CL, RANSFORD et al., 198 1; FINNERTV et al., I98 I ). EXPERIMENTAL
I) and is close to the theoretical composition. The water content of the starting material (12.6 wt.%) was determined using a Mettler Thermoanalyzer (Model TA 2000C). Details of shock recovery experiments are given in LANGL
663 685 667 690
obtained from vented assemblies After each experiment, the sample disc was machrned ou: and split into two fractions. About half of the drsc. or the largest mechanically intact pieces, were potted rn epox]. sliced and mounted into two thin sections. one parallel tc$ the plane of the disc. one perpendicular to this plane. The remainder of the sample was used for infrared and therm+ gravimetric analysis. Absorption spectra were measured m a uavenumhcr region between 380 to 4000 cm-‘, correspondmg to wavelengths of 26 to 2.5 pm on a Perkin-Elmer grating spectrometer. Weight loss curves of the shocked samples, determming the amount of post-shock structural water and thus the amount of shock-induced water loss (LANGE and .~HRLNI. 1982a). were obtained by use of the Mettler Thermoanalyzer in the temperature range of 25 to 900°C (298 to I I73 K) Computing relative masses M/M, (where nr is the mass cli the sample as a function of temperature. 7_ &, is the rnaSb of sample at T = 1173 K) and plotting these ratios a\ .i function of reciprocal temperature. log,, (l/T), allows drtermination of the effective activation energy for dehydration in the heating experiments (N~JTTING. 1943: i AVGF an
Each sample thin section was photo-mapped and ther: by means of optical and scanning electron mrcroscop> (SEM). SEM was performed at NASA-JSC with a JEOL 35 CF, coupled with a PGT system IV energy dispersive system (EDS). EDS and backscattered imaging were used extensively in search for compositional variation. Microprobe analyses were obtained at Southern Methodist University (Dallas). with JEOL JXA-733 “superprobe”. Refractive indices were measured directly on polished thin section using a macroreflectometry technique described in LAMBERT! 1981t OBSERVATIONS ON SHOChRECOVERED MATERIALS
were carried out both at
the shock wave laboratory of the California Institute of Technology (CalTech), and at the shock wave facility of the Johnson Space Center (JSC) (Table 1). Experimental conditions at both laboratories differed insignificantly from those described by LANGE and AHRENS (1982a). Two different sample assemblies were employed. Sealed assemblies enclose the sample tightly and prevent most. if
1046
is thought to represent better the conditions of natural shock events. Aside from providing some results from sealed assemblies, most of the discusston will he devoted to darn
studied
DETAILS
The material used in the experiments is antigorite serpentine (Mg3SirOr(OH)4)from Thurman. New York, with an initial density of 2.540 f 0.004 g/cm3. The major oxide composition is given in LANGE and AHRENS (1982a; Table
and AHRENS (1982a). Experiments
not all, of the potential water vapor (produced as a con% quence of shock loading of serpentine) from leavmg thr assembly chamber. The vented assemblies have an escape path for shock-released water vapor (c$. LANGEand AHRENS, 1982a; Fig. I). The rationale for using different assemblies was the possibility for studying the effects of sample confinement on shock-induced water loss. The vented assembl!
Figure I shows absorption spectra for the shocked samples (shock-loaded in both vented Jnd scaled assemblies). together with a reference spectrum of an
SlCPI Alummum
27 2
\
I 28
Starnless Steel 31R
25 ”
\
I 55 1 89
Scamless srec1 Stsmless stre1
36 0 45 0
\’ \’
72.5
1 25
19 4 19.3 19 4 19 1
I 90
Sral”les.3
i
JSC CALTECH CALTECH CALTECH CALTECH
I
!
1717
Shock effects on meteorite minerals
Wovelengt h. microns 2.5
3
4
5
6
7
8913
15 2030
S: - 0 Bond V~brotlonol modes
I------
59 GPO_
J-y_
#,‘L_-. -_%__5g _cI_--*
\,I
,I/
f-----
i\ I
I
.,=__
4000
45 ---t..___#_-~-
3000
_____/‘/
I
1 \
,i\
‘y”
1003
2ooo Wovenumber,
cm*’
FIG. 1. Infrared absorption spectra of unshocked and shocked antigorite serpentine. Numbers on each curve indicate shock pressure in GPa (I GPa = 10 kb). Solid and dashed lines give spectra of samples shock loaded, in vented, and sealed assemblies, respectively. Identification of absorption peaks is according to FARMER (1974).
Major fea-
tures of these spectra are:
(=a; Fig. 1) representing OH-stretching modes of the lattice (FARMER, 19741, decrease systematically in
(i) The absorption peaks, co~s~nding to structural water in serpentine, i.e.. peaks at 3 690 cm-’
We also observe a broadening of these peaks, which is indicative of non-un~fo~ local potentials. This
unshocked
antigorite
serpentine
specimen.
height with increasing shock pressure of each sample.
1718
M ,A. Lange. P. Lambert and 7. J. Ahrens
could be caused by increased internal disorder. Another interpretation of these results concordant with the thermogravimetry data. calls for a decrease in the amount of structural, post-shock water and thus an increase in shock-induced water loss with increasing shock pressure. (ii) Absorption peaks at 3 450 and I 630 cm ! (=b and c, respectively; Fig. 1). which represent H&I hydrogen bonding and Hz0 bending modes, respectiveiy, i.~., the presence of molecular, surf&e adsorbed water (FARMER, f 974) increase systematically (less clearly in the case of peak “c”) with increasing shock pressure. One possible interpretation of this observation is that fractions of the shock-released water become readsorhed and remain m the sample material. This is consistent with findings of BCXLOUGHd cr!. ( 1980) who investigated this effect more thoroughly. (iii) The absorption features in the I100 to 380 cm-’ region, corresponding in wave number approximately to Si-0 bond vibrational mode (FARMER. 1974). undergo a continuous change from a fourpeak to a two-peak absorption feature upon recovery from increasing shock pressure. This could signal ;1 decrease in structural order of the lattices. as well as the gradual onset of partial melting as a result of the shock loading (cl, ST~F?UR. 1974). The changes with respect to structural and molecular water described in (i) and (ii) are seen in exper”. iments carried out with both vented and sealed containers. However, for a given pressure these changes appear less pronounced for samples shocked in sealed as compared to those shocked in vented assemblies (Fig. 1). This can be most clearly seen in the spectra from the 59 GPa shots. Here. absorption peaks b and c appear much stronger when compared to the structural water peak in the vented assembl! shot versus those for the sealed assembly at the same pressure. These differences imply that the dehydration reaction is less efficient in the case of sealed assembl>shots. This is explained by a rapid increase in water vapor pressure in the sealed assembiies. leading to a drastic reduction of reaction rates in antigorite. Dehydration reactions are extremely sensitive IO thr level of partial water vapor pressure (BRINDLE\ t’f al., 1967; c$, LANGE and AHRENS. 1982a). Despite the differences with respect to structural and molecular water absorption features, the changes in the low wavenumber region are similar for both assembly types {Fig. 1). This suggests that changes with respect to Si-0 bond vibrational modes are only dependent on shock pressure and independent of sample confinement.
Major results of the thermogravimetric analyses ot’ the shocked antigorite samples are given in Table 2. Figure 2 shows the amount of shock-induced water loss as a function of shock pressure for both sealed and vented assembly shots. It should he noted that
the pressures in Table 2 correspond to those obtained in the stainless steel container enciosing thz sampit: material. Thus, shock loading of the samples to these pressures required multiple reverberations of the shock wave. and initial shock pressures fit,., prior to shock reverberation) lie below the final value given rn Table 2. Since most of the shock heating (entrap? r~sei takes place upon the initiaf compression of the sample material, shock pressures given in Table 2 and Fig. _” overestimate the critical pressures needed to ubtalrr ;+ certain amount of shock-induced dehydratron. ihr amount of shock-induced water loss is the Jifferencr. between initially present structural water and posrshock water. as determined by thermogram imete We note the good agreement of the data abtaincti in two different laboratories. These define % relatror~ between shock-induced water loss and shock pressure The resulting relation also agrees with theoretlcai estimates for total loss of structural water upon shoch loading (LANGE and AHRENS. 1980, 198%). T‘h? dependence of shock-induced water loss, WL. i wt.‘6.i. on shock pressure, P (GPa). as represented h\ thr solid line in Fig. 2, is given b\, WL = -160.440 t L.09 (LANGE
>
+ 10.88lIJ
i0 t Pz
for
ti.i77P
.‘i -‘ f’::. hi 5.iPa
::.
and AHRENS, 1982a). As can be SCCKrnc~p ient to complete water loss in antigorite srr’pentinc N :IrKf occurs in the pressure range between -65 CiPa. The data for the sealed assembly shots {Fig. Z; differ markedly from those for the vented assembl! shots. Shock-induced water loss appears to be less efficient in sealed assemblies as compared to vented shots for a given pressure. Also, there is nv clear relation between water loss and shock pressure a5 was seen in the vented experiments. This paraileis findings in similar experiments on brucite (LANCZ (9 111..1982). The significantly lower dehydration efh~. ciencies obtained in sealed assembties, as nored earlier. are thus quantitatively confirmed. We take this 3% evidence that the increased partial water vapor pressure in shock experiments influences the decomposition rate of antigorite to a comparable extenr 35 was observed in heating experiments I~RINDt k-k cl d. lY67).
Shock effects on meteorite minerals
I
8 3
80 -
I
ANTIGORITE
sealed
l
vented I
b “0 60-
I
I
l
assembly 0
Antigorite
\ Lange and Ahrens, 1980 (theoretical) 9
B u 8 405 5 f%j 20z
I
O0
I 20
1 60
I 40
Shock Pressure GPo RG. 2. Shock-induced water loss as a function of shock pressure in antigorite serpentine. The solid line represents a numerical fit to the vented-assembly data (Eqn. (I)). Open circle represents a theoretical prediction for complete dehydration of serpentine. (LANGE and AHRENS,1980).
The weight loss versus tern~mtu~ for each sample yields also a dehydration interval (i.e., the temperature interval over which loss of post-shock water occurs) and the effective activation energy for dehydration (i.e., the energy needed to decompose the remaining intact antigorite and to remove the post-shock water from the samples; cJ LANGE and AHRENS, 1982a). Table 2 gives numerical values for these quantities for each sample (except for sealed assembly shots) and Figs. 3 and 4 give dehydration interval and effective activation energy as a function of shock pressure, respectively. As can be seen, there is a clear relation between dehydration interval, AT (K), and effective activation energy, AAE (k~/mole), on shock pressure, P (GPa): 200
I
I
I
1
I
Id 0
I
I
I
I
20
30
40
50
I
IO
Shock Pressure,
t 60
GPO
FIG. 4. Apparent activation energy for dehydration as a function of shock pressure in shock loaded antigorite. Solid line represents tinear fit.
AT = 176 - 2.OP
(2)
log,, AAE = 0.47 - 0.02P.
(3)
While the length of the dehydration interval is a direct measure of the heat required to remove postshock structural water, the activation energy of dehydration is related to the bond strength of OH molecules within the crystal lattice. Both quantities are interrelated and show a comparable dependence on shock pressure. This indicates that shock loading not only leads to the removal of structural water in antigorite (and probably other hydrous minerals as well), but also weakens the bonds between remaining OH-molecules and other constituents of the crystal lattice.
I
ANTIGORITE
c) Optical microscopy
AT, K
~T~K~=176.17-2.0P~GPa~ w
I
Oo
I
IO
I
f
20
30
I 40
I 50
I
60
Shock Pressure, GPo FIG. 3. Length of the dehydmtion interval, AT, as a function of shock pressure in shock loaded antigorite (vented assemblies only). The solid line represents a linear fit.
Photomicrographs (Figs. 5 and 6) illustrate the petrographic characteristics of unshocked and shocked antigorite. Unshocked antigorite appears as equiaxial, polygonal grains displaying the typical mesh-texture of serpentine minerals (WICKS and W~~~AKER, 1977). Grains are typically 0.2 to f mm in size (Fig. 5a). When observed under crossed nicols, the grains appear to consist of various arrays of submicroscopic fibro-lamellae (Fig. 5b). Shocked antigorite (Figs. 5c and 5d) shows remarkably few deformation features or other traces of the shock loading compared to other silicates (cJ ST&FLER, 1972, 1974). When observed in more detail, antigorite recovered from 27, 45 and 59 GPa (shots 1046, 1007 and 1008 in vented and sealed assemblies, respectively; see Table 1) displays essentially no change in texture, grain mo~hology, birefringence, and extinction patterns with respect to unshocked samples. The refractive indices are un-
1720
M 4. Lange, P. lambert and T. J. Ahrens
FIG. 5. a) Photomicrograph of the unshocked antigorite specimen in transmitted Itght: hi same un& crossed nicoia; c) antigorite recovered of shot 1044 (45 GPa sealed assembly) in transmitted light. Arn~ points to black vein to be seen in greater detail in Fig. 7; d) same under crossed nicols iscale eqt~rralent in all Figs.)
changed too, except in the 59 GPa sealed shot where refractive indices are locally reduced from 1.5501.570 to about 1SO& A systematic though “volumetrically” minor difference with respect to the unshocked antigorite is the occurrence of black veining and irregular brownish coloration in the shock loaded
specimens. Although these features are seen in the sample from the 27 GPa experiment. the? arc best observed in material exposed to higher shock pressure% (Fig. 5~). Darkening occurs along fiber outi~nes. grain boundaries, cracks and intragranular fractures. In the sample from the 59 GPa scaled experimrn? WC a/~~
Shock effects on meteorite minerals
1721
FIG. 6. a) Thin section of a vesicular grain [shot 1045; 59 GPa, vented assembly) in transmitted light; same grain observed by SEM; b) SEM view of another grain showing both, vesicular glass (top) and crystalline rno~hoIo~~ (bottom) separated by a transitional zone with micron to submicron vesicles (arrow).
find flow-like features. Under crossed nicois, these dark regions always disptay a characteristic bright glow of diffuse light which is similar to that observed in shock melted micas (LAMBERTand MACKINNON, 1983). The relative proportion of dark veins increases with increasing pressure and is relatively higher in vented than in sealed assembly experiments. The sample of shot 1045 (59 GPa, vented assembly) shows the strongest shock-induced modifications compared to unshocked antigorite or compared to samples 1046, 1007 and 1008. As a consequence of mechanical fracturing this sample was total disrupted. Although no large pieces were recovered, the study of a large number of grains mounted in polished thin sections show major changes compared to the unshocked antigorite. The majority of the grains are locally to completely opaque, depending on their relative thickness, and are vesicular (Fig. 6a). Vesiculation is responsible for the dominance of internal
reflections observed under crossed nicols. The remaining grains are characterized by the lack of optically resolvable vesicles. These are sometimes locally clear and apparently isotropic, though commonly brownish and birefringent. The latter display abundant internal reflections under crossed nieols resulting in a bright glow of diffuse light, similar to that observed in the dark veins of material shocked at lower pressure. d) Scanning elecrron microscopy (SE&f)
SEM combined with optical observations on thin sections confirms the vesicular character of the dark and opaque grains recovered from the 59 GPa vented experiment (Fig. 6b). This material is interpreted as glass on the basis of morphological and textural characteristics. Size and morphology of these vesicles vary from 0.1 to about 10 pm, and from spherical to elongated respectively (Figs. 6b and 7). The presence
1722
M. A, Lange. P. Lambert and T. 3. Ahrens
FIG. 7. a) SEM view of polished thin section of antigotite, shocked to 45 GPa (shot 1044); detailed view of dark vein seen in Fig. SC);b) detail of same vein; c) detail of similar vein of shot 1007 (59 GPa. sealed assembly) numbers above solid bars give length of bar in pm.
of these features and their shape (rapid change in the orientation of elongated vesicles) suggest turbulent flow of gas and liquid mixture. Locally, relatively large bubble-free areas are observed (with widths of 10 Mm or more) which correspond to the clear fields observed optically. Backscattered imaging x-ray energy dispersion and electron microprobe- analyses demonstrate that the glass is chemicalty homogeneous and the major non-volatile element composition is similar to unshocked antigorite. The texture and composition of the shockinduced glass is significantly different from that of the incongruent melt which would result from static heating (EVANS et al., 1976: CARUSO and CHERNOSKY, 1979; CHERNOSKY, 1982). The petrographic. SEM and chemical characteristics of shock-melted antigorite parallel those of experimentally shockmelted micas (LAMBERT and MACKINNON, 1983). From these characteristics and by analogy with shockproduced mica glasses, the vesicular glass of antigorite is interpreted as quenched liquid, produced by congruent mehing of antigorite. We speculate that the vesicles result from exsolved Hz0 upon pressure release (LANGE ef al.. 1982; LAMBERT and MACKINNON, 1983). Infrared and thermogravimetric data of the 59 GPa sample (shot 1045, vented assembly) suggest that the glass has lost most of its structural water. The limited amount of water still retained in the sample is probably associated with the remaining crystal fraction. Although this hypothesis could not be verified by thermogravimetry due to the difficulty of separating these phases, we suggest that Hz0 acts
like K in shocked micas where it is found to be in the fused material reiative to the remaining crystalline portions (LAMBERT and MA~KINN~~. 1983). Micron and sub-micron vesicles are resolved by SEM in the brownish, otherwise crystalline fragments recovered from the 59 GPa vented experiment. Scattered submicron vesicles might be responsible for the low refractive index measured by microre~ectomet~. Microvesiculation is the cause of the internal retlections seen under the optical microscope. The same observation and interpretation apply to the dark veins seen optically in the lower pressure shots (Fig. 7). Local vesicuiation seen by SEM in the 27 GPa sample (shot 1046, vented assembly) indicates inctpient melting. The number of vesicular zones and thus the relative amount of iOcd1 melting increase with increasing shock pressure and is also more abundant in vented than in sealed assemblies. The most obvious examples are the 59 GPa experiments where the sample in the sealed assembly is apparently stifi largely crystalline while that in the vented assemhlv has essentially melted. From the features observed with SEM, we conclude that shock melting of antigorite begins locally upon recovery from shock pressures as low as 27 GPa and is almost complete upon recovery from 54 CPa These values correspond to the range of pressure for the shock-induced dehydration of antigorite. However, the amount of Hz0 released vra shock loading of antigorite can by no means he accounted for by the vesicles observed in the shocked samples. Hence. :z depleted
Shock effects on meteorite minerals is concluded that shock-induced dehydration involves not only regions of local melting but also large fractions of the sample which remain unmelted. IMPLICATIONS, OBSERVATIONS AND POSSIBLE CONSEQUENCES FOR CARBONACEOUS CHONDRITES
a) Shock criteria in carbonaceous chondrites
The above results allow definition of the identification and possibly calibration shock effects in serpentine-bearing rocks bonaceous chondrites containing copious serpentinaceous phyllosilicates.
criteria for of natural and in caramounts of
I) Weight loss record. The temperature range required for complete dehydration and hence the ap parent activation energy for dehydration of shocked antigorite decrease systematically with increasing shock level. Since this behavior is distinctly different from that of unshocked but dehydrated hydrous minerals in regular heating experiments (c$, FARMER, 1974), we believe this dehydration behavior provides a quantitative shock indicator. This may also be true for cases where phyllosilicate minerals cannot be separated from other matrix phases (i.e., non-hydrous minerals in carbonaceous chondrites). Using appropriate calibrations such as given in Fig. 3 for antigorite, the dehydration interval could also yield semi-quantitative estimates of the shock pressures in natural samples. 2) Infrared spectra. Changes in the absorption spectra of shocked serpentine minerals constitute a potential shock indicator. The absorption patterns as observed for shock-loaded antigorite samples, and specifically the observation of simultaneous alteration in the OH-, HZ0 and SiOz peaks would appear to be characteristic of shock deformation. 3) Amount of structural water. Because shock is very efficient in releasing structural water from hydrous silicates, an anomalously low content of water in phyllosilicates may indicate a previous shock history. DZICZKANIEC et al. (1982) have shown that separation of matrix phases of Murchison by physical separation methods is feasible. These separates have been shown (by SEM/EDS- and x-ray diffraction analysis) to contain -80 to 98 vol.% se~ntinaceous material (consisting of betthierine, cronstedtite. chrysotile, and antigorite in decreasing proportions (G. LUMPKIN, pers. commun., 1982). Using such matrix separates, their water content (as obtained through thermogravimetry) could be interpreted as a maximum estimator (because some rehydration might have occurred) of the amount of water in a meteorite which may have had an impact history. Water associated with different hydrous minerals and surface adsorbed water can, in principle, be identified via weight loss versus temperature data. A shock-induced dehydration-versus-sh~k pressure relation such as Fig. 2, would then allow a quantitative estimate of the shock pressure. However this methodology has
1723
the difficulty that the peak shock pressure experienced by the sample cannot be uniquely determined for as complex a group of minerals as are present in carbonaceous chondrites. Clearly other endogenic heating processes, e.g., radiogenic, might be responsible for less than the nominal amount of structural water in hydrous matrix minerals. In addition. we do not know the stability of the shock-produced antigorite glass and/or that of the dehydrated antigorite. 4j Petrographjc characteristics. Although petrography is easily carried out on meteorites. it appears to be less useful to delineate shock effects. Antigorite shocked up to 45 GPa lacks significant petrographitally observable shock indicators. This contrasts with the extensive range of shock metamorphic features observed over the same pressure range, in shocked nonhydrous silicates (FRENCH and SHORT. 1968; H~Rz, 197 1; ST~FFLER, 1972). As noted above. melting and vesiculation provide the most reliable petrographic shock indicators for phyllosilicates, Although observed upon shock recovery over a shock pressure range from 27 to 60 GPa, signi~cant amounts of glassy and vesicular areas within shock loaded antigorite occur upon recovery from shock pressures well above 40-50 GPa. In summary, the above described shock indicators, when used individually, will probably not suffice to identify the detailed shock history of natural phyllosilicates. What is needed instead is a comprehensive approach employing several techniques. b) ~b~e~ation~ on carbonaceous chondrite.~
Figure 8 shows the infrared absorption spectrum of the present 59 CPa antigorite sample (vented assembly shot) together with a spectrum obtained for a matrix separate of Murchison, a CM chondrite containing copious amounts of hydrous phyllosilicates as a major matrix component (FUCHS et al.. 1973: KERRIDGE and BUNCH, 1979). Even though the similarity between the two spectra might be fortuitous, based on the arguments presented above, we suggest that shock loading of these matrix phases could be responsible for the observed spectrum. Whether a single shock event with shock pressures close to 60 GPa, or multiple shock loading led to the alteration of the hydrous matrix phases cannot be
FIG. 8. Infrared absorption spectra of shock loaded antigorite (59 GPa, vented assembly) and Murchison matrix material.
1724
M. A.Lange. P.Lamhert and T. .i. Ahrens
discerned from the present level of knowledge; however, multiple shock events with small to moderate shock pressures would seem to be the more likely scenario (cJ, KERRIDGE and BUNCH, 1979;BUNCH and CHANG, 1980). Textural analysis via optical microscopy and SEM have been performed on a number of matrix phases of carbonaceous chondrites (e.g.. KERRIDGE and BUNCH, 1979; BUNCH and CHANG. 1980; BARBER, 198 1). Many of the features seen in matrix phases resemble those of the present shocked antigorite. Among these are vesicular, spongelike structures. resembling altered volcanic tuff, or tuff breccias (BUNCH and CHANG, 1980) vesicular zones, veinlike flow features, and indi~tions for material transport by a rapidly moving fluid or vapor phases (BARBER, 1981). Similar vesicular textures to those observed in shocked serpentine have recently also been found in shock-loaded calcite (LANGE and AHRENS, 1985). Whether similar textures observed in carbonaceous chondrites are related to shock processes remains to be demonstrated. LAMBERTand MACKINNON (1983) concluded that vesicular melt zones in shock loaded hydrous minerals are produced by congruent melting. Thus, microchemical analysis of texturally similar features in carbonaceous chonrdites might provide evidence as to whether these are related to shock loading or possibly to hydrous alterations of matrix material.
It is generally assumed that hydrous alteration of carbonaceous chondrite parent bodies (possibly identical with C-type asteroids; KERRIDGE and BUNCH. 1979) took place early in their evolution. It is also assumed that this process took ptace within the surface layers of the regolith of parent bodies. The hydrous alteration at low temperatures implies the presence of free water within the regolith. As originally proposed by DUFRIZSNEand ANDERS ( 1962), KERRIDCIEand BUNCH ( 1979) the hypothesized liquid water layer gradually moved outward in a meteorite parent body, as a result of heat produced by (possibly short-lived) radioactive elements in the interior. However, such a mechanism may not produce hydration of surhcial material because it is difficult to retain liquid water in the surface layer of a small. low escape velocity, object with low surface temperatures (9273 K). Referring to DUFRESNE and ANDERS'(1962) model, they propose that an icy surface layer could prevent escape of water. Such an ice layer, could heal itself after mechanical disruption because of the ratio of the heats of fusion and evaporation of water. How similar in operation a meteorite parent body with an icy crust and a mantle which contains &id water might be to a multiple impacted regolith layer is unclear. Thus, the ice-
water allows for hydration but not necessaniy shockinduced features. Repeated impacts, inducrng low to moderate shock pressures, could instead provide a means of producmg and transporting free water within the regolith layer of carbonaceous chondrite parent bodies. Such .! process would yield a relatively large amount of fret water which would reside within the ejecta blanker of an impact crater at relatively high te~~rat~~~~ (>I273 K) where hydration of unhydrous minerah could take place at fairly high reaction rates. We ser the major difference between the ice-water layer- and the present model in the fact that the present mode) does account for near surface hquid water which IF, in ciose contact with relatively warn% and reacu\r surface minerals, while the DuFresne and Anden hypothesis provides liquid water contained within ait icy crust. The present model accounts for both the generation of impact-induced textural characteristics of carbonaceous chondrites and the generation oi free water. Observations of mobilization of K VIGshock ret’leased (presumably interstitial and intracrystalline, water from various rocks have been reported at several terrestrial impact craters (MILTON 6'1 ui., 1971 and CURRIE and SHARQULLAH, 1968). We envision that sh~k-induced dehydration &’ hydrous phases accompanies accretion (at low encounter velocities) of carbonaceous parent bodies. This would lead to the constant re-disposttion ~6 water and hydrous phases at the outer parts of the growing body (& LANCE and AHRENS, 1982b). Wc note again that sh~k-induct dehydration may not necessarily require shock pressures as high as indicated in Fig. 2 (i.e.. between 20 and 60 GPa), but could result from repeated low and intermediate shock loadings which lead to the weakening of bonds be tween structural OH-groups and other constituent!: of the iattice. Thus, much lower shock pressures might be needed to release portions of the structural water in hydrous minerals, after a number of previous shocks have broken some of the bonds between structural water and the crystal lattice. CONCLUSIONS Shock experiments in antigorite produce mcipieni devolatilization and incipient melting upon recover:, from above 20 GPa. Total devolatilization, accompanied by complete melting occurs upon recovery from above about 60 GPa. Shock melting of antigoritc proceeds congruently yielding a vesicular glass which is characteristic of shock. Upon exposure to inrermediate shock pressures, a significant amount of H& is driven out of antigorite probably in the solid state, without melting. The petrographic characteristics oi shocked serpentine are remarkably insensitive to shock pressures up to about 40 to 50 GPa. Water loss versus temperature data and infrared spectra appear to be the most useful shock rndicatori
Shock effects on meteorite minerals
which can be applied to shock studies in matrix phases of carbonaceous chondtites and in serpentinebearing rocks. A shock history is inferred for the hydrous matrix phases of Murchison on the basis of infrared absorption characteristics. Although the data are consistent with those found in experimental samples shocked to 60 GPa, the infrared spectrum of the matrix phases are hypothesized to represent repeated low to intermediate shock loading. Textural characteristics of shock-toaded antigorite bears similarities to textures seen in carbonaceous chondrite matrix phases, which might indicate that shock loading is also responsible for the origin of these textures in natural specimens. We suggest that shock processes could play an important role in the modification and evolution of carbonaceous chondrite parent bodies. Shock loading of hydrous phyllosilicates may provide an explanation for the mechanism of dist~buting free water within the surface layers of these parent bodies. Thus impact could lead to the hydrous alterations observed in samples of carbonaceous chondrites. Acknowledgments-We appreciate the skillful help of W. Ginn, E. Gelle, and M. Long in the experiments and the use of the spectroscopic and thermogravimetric analyzer and advice proffered by G. Rossman and R. Aines. The assistance of R. Schmidt and T. Thompson with the shock experiments at NASA-Johnson Space Center is gratefully acknowledged. We thank F. H&z for critically reading several versions of this paper and R. Brett, J. Kerridge, and H. McSween for helpful reviews. M. Lange was supported by a stipend from the Deutsche Forschungsgemeinschaft while at CalTech. P. Lambert was supported by the National Research Council as a Research Associate. The microphotometer and its attachments were used through the courtesy of Dr. C. B. Moore and the Center for Meteorite Studies. Arizona State Unive~ity. This work is supported under NASA grant NGLOS-002-105. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 9 1125, Contribution 3936. Editorial handling:
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