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Shock-Induced Volatile Loss from a Carbonaceous

Chondrite: Implications for Planetary Accretion

James A. Tyburczy*, Benjamin Frisch, and Thomas J. Ahrens

Seismological Laboratory 252-21

California Institute of Technology

Pasadena, CA 91125

ABSTRACT

Solid recovery impact-induced volatile loss experiments on the

Murchison C2M meteorite indicate that for an impact of a given velocity,

H2O and total volatiles are driven from the sample in the same proportion

as present initially. Thus, the primitive surface volatile budget (volatiles

reacted with the solid surface plus atmosphere) of a planet growing by

accretion would have the same bulk elemental composition as the volatiles

in the incident planetesimals. Incipient devolatilization of Murchison

occurs at an initial shock pressure of about 11 GPa and complete devolatilization occurs at a pressure of about 30 GPa. For the Earth, incipient

and complete devolatilization of accreting planetesimals would occur when

the planet reached approximately 12% and 27%, respectively, of its

present-day radius. Thus, impact-induced devolatilization would profoundly affect the volatile distribution within the accreting planet. Prior

to metallic core formation and internal differentiation the growing planet

would have a very small core with the same volatile content as the

incident material, a volatile depleted 'mantle', and an extremely volatile

rich surface. In the case of the Earth, 99.4 wt% of the total incident

volatile material would end up on or near the planetary surface.

i

a.

-x .-..

''--'

•now at Department of Geology, Arizona State University, Tempe, AZ 85287
- 2-

Introduction

Impact-induced volatile release during planetary accretion may have played an

important role in planetary development and in atmospheric and oceanic formation [l, 2,

3, 4). Previous experimental results indicate that the minerals serpentine and calcite

begin to devolatilize (i.e., lose H2O and CO2) respectively) upon impact at initial shock

pressures of 10 to 15 GPa and completely devolatilize at shock pressures of 30 to 40 GPa

[5, 6, 7]. Assuming homogeneous accretion and symmetric impact, infalling

planetesimals could have achieved impact velocities sufficient to generate shock pressures

of these magnitudes when the Earth's radius was about 10 to 15% and 25 to 35%,

respectively, of its present value. Thus, extrapolation from single-mineral results suggests that impact-induced devolatilization would have been a major feature of planetary

accretion throughout most of the growth period of an Earth-sized planet.

We report here results of shock recovery experiments on Murchison carbonaceous

chondrite (C2M) performed to determine the conditions for loss of H2O and other volatiles upon impact from material more representative of an accreting planetesimal than

single minerals. One reasonable form in which the volatiles of the terrestrial planets may

have arrived is in a mineral assemblage similar to that found in carbonaceous chondrites.

It is important to study this material directly because shock propagation, and hence

shock-induced devolatilization, in a heterogeneous medium is complicated by local variations in mechanical properties that can lead to localized heating, chemical reaction, and

other types of anomalous behavior. Furthermore, the phyllosilicates that comprise the

bulk of the matrix of Murchison include a significant proportion of Fe-rich material [8],

whereas the serpentine shock recovery work performed to date is confined to the Mg end

members antigorite and lizardite [5, 7, 9). Volatile-containing minerals present in

Murchison are listed in Table 1.
- 3 -

Experimental

The experimental procedure used was similar to that employed in previous solid

recovery shock-induced devolatilization studies [5]. Murchison samples (discs 4.4 mm in

diameter, 0.5 mm thick) were polished until the surfaces were flat and held in vented

target assemblies that exposed the samples to the ambient atmosphere during the

impact. The assemblies and flyer plates were made of stainless steel, type 304. The

experiments were performed on the 20mm propellant gun at the California Institute of

Technology. Sample shock pressure was calculated using a one-dimensional impedance

match method [10]. The Murchison Hugoniot was assumed to be the same as that determined by R. G. McQueen for a 'serpentine' of similar density (see [11]). The shock wave

equation of state parameters used for Murchison were p0 = 2.85 g/cm3, C0 = 5.80 km/s,

and S = 0.62 (where UB = C0 + S up, Us is the shock velocity, and up is the particle

velocity) and were obtained from a linear fit to McQueen's Us-up data in the range 0

.£. up ^3 km/s [11].

In the recovery assembly, the sample is sandwiched between two steel surfaces.

During the shock event the sample is shocked first to a pressure-density point on the

sample Hugoniot, but subsequent reverberations bring the sample to a higher pressure

state with a density greater than that of the Hugoniot (see [12], for a description of

reverberations in shock recovery experiments). In this work we report both the initial

(Hugoniot) shock pressure Pi( and the peak reverberated pressure Pp for each shot. In

applying the results to models of planetary accretion, we use a formulation for devolatilization as a function of Pj (as opposed to Pp). This is because current theoretical models

of the impact-induced devolatilization process are generally based on shock entropy calculations [3, 13, 14], and because after the initial shock, the reverberations are nearly

isentropic. Thus, the entire entropy increase of a sample during a shock recovery experiment occurs during the first shock. Previous shock recovery experiments have often

been reported in terms of Pp [6, 15, 16]. Data used from previous studies have been
- 4-

corrected to indicate Pj (see discussion).

After the shock experiment, the impacted sample was recovered from the target

assembly and analyzed for H2O and total volatile content. Shock-induced H2O and volatile loss were obtained by comparison with analyses of unshocked material. Total volatile

content was measured by thermogravimetric analysis (TGA) using a Mettler Thermoanalyzer 2000C. A heating rate of 5°C/min, a maximum temperature of 1200 °C,

and a nitrogen flow rate of 20 ml/min were employed. The total volatile content of

unshocked Murchison released over the range 200 ° C-1200 ° C was 10.09 ± 0.17 wt %.

Water content was determined using a DuPont Moisture Evolution Analyzer with a

maximum temperature of 1000° using N2 as a carrier gas. The Moisture Analyzer was

operated manually; the temperature was increased in increments of 50-100 "C, whenever

the cell voltage indicated that the electrolytic cell was below the saturation level. This

procedure was adopted to allow minimum analysis time, thereby minimizing the baseline

correction and yielding the most precise total H2O values. H2O loss as a function of

temperature under constant heating rate conditions were not obtained as a result of

using this procedure. The H2O content of unshocked Murchison used for reference in

this work was 6.50 ± 0.20 wt % (200-1000° C).

Results and Discussion

Figure 1 shows representative TGA curves for shocked and unshocked Murchison.

Unshocked Murchison meteorite loses mass over a broad temperature range, about

260 ° C-1020 * C, and the minor structure in the curve indicates that different components are decomposing in different temperature intervals. The TGA curves of the

shocked material display much less structure. Mass loss begins at a higher temperature

than for the unshocked material, 650-750 ° C, indicating that the more labile materials

were decomposed during shock and release. No attempt was made to determine which

components of the matrix were decomposed during the shock.
- 5-

Total volatile loss and H2O loss results are listed in Table 2. These data are plotted in Figure 2. Incipient devolatilization occurs at an initial shock pressure Pj of about

11 GPa, and complete devolatilization occurs at an initial shock pressure of about 30

GPa. Aside from one apparently erroneous data point, there is no discernible difference

between H2O loss and total volatile loss as a function of initial shock pressure. Therefore, there is no fractionation of H2O relative to total volatiles caused by impact-induced

devolatilization of carbonaceous chondritic material. The atmospheric (or surficial) complement of volatiles in an atmosphere created by impact induced degassing of

planetesimals will have the same bulk elemental composition as the volatiles contained in

the incident material, and will also be the same as that of the buried volatile component.

Chemical reaction, both in the gas phase and with surficial materials, will change the

actual speciation.

It must be stressed that the above conclusion applies strictly only to those volatiles

released in an analysis carried out in a N2 carrier gas, at temperatures up to 1200 ° C.

Analysis under oxidizing or reducing conditions would liberate a different complement of

volatiles. For example, graphite is stable (non-volatile) at 1200 °C in N2, but would be

oxidized to CC>2 (i.e., considered a volatile) in an oxidizing atmosphere. Furthermore,

there are strong indications that the degree of impact-induced devolatilization depends

on the ambient atmospheric conditions during both the shock experiments [7, 14, 17] and

planetary accretion. In the experiments described here, the sample was exposed to air

during the shock. Details of these atmospheric dependences are subjects for further

investigations.

Figure 2 also shows impact-induced H2O loss for crystal-density serpentine [7] and

for 20% porous serpentine [9], which approximate the range of densities of many carbonaceous chondrites [18]. These results bracket the data for Murchison, consistent with

the observation that septechlorites are the major H2O-bearing phase in Murchison. This

also indicates that the results of devolatilization experiments on Mg end-member
- 6-

serpentine may be extrapolated to carbonaceous chondrites.

Laboratory shock pressures can be related to planetesimal impact velocities by

assuming uniform, homogeneous accretion. For symmetric impact, the impact velocity

required to generate a given shock pressure is twice the particle velocity of the shocked

state. Minimum impact velocities onto a planetary surface are equal to the escape velocity of the planet. Thus, assuming constant density, the radius of the planet is a linear

function of impact velocity

r = Yp (87T Gp/3)-1/

2

 = 2 up (8* G^/3)-1/

2

 (1)

where r is the planetary radius, Vp is impact velocity, G is the gravitational constant, ~p

is the average density of the planet, and up is the particle velocity. Figure 3 is a plot of

shock-induced volatile loss versus fractional radius for Earth, Venus, and Mars using the

parameterized devolatilization versus initial shock pressure results for Murchison

presented here (solid line in Figure 2). For the Earth and Venus, impact-induced devolatilization of incident planetesimals would have begun by the time the planetary radii

were about 12% of their present-day values, and complete devolatilization would have

occurred when the planets reached about 27% of their present day radii. For Mars, incipient and complete devolatilization of incident planetesimals would occur at about 26

and 58%, respectively, of its present day radius.

These results indicate that the initial volatile distribution will be extremely nonuniform in an accreting planet. In the absence of thermal effects, the net result of accretion

as outlined above is a core with the volatile content and elemental composition of the

incident material; a volatile depleted mantle will probably form in association with a

metallic core, the core will sink into the center of the planet, and the volatiles depleted

from the mantle will contribute to a volatile-rich veneer at the planetary surface. The

distribution of volatiles between the interior and surface regions depends on planetary

radius and mean density. Integrating with respect to mass along the curves in Figure 3

indicates that for the Earth, Venus, Mars, and a Moon-like planet, 99.4, 99.2, 93.2, and
- 7-

42.2 mass %, respectively, of the incident volatiles end up at the planetary surface at the

end of accretion. The possible existence of a planetary magma ocean in the later stages

of accretion, whether caused by a runaway greenhouse effect due to the accumulating

protoatmosphere [4] or by retention of heat deposited during impact [20], does not alter

the conclusion that a volatile-containing core surrounded by volatile-depleted region

exists early in planetary accretion. The existence of heat generated by core formation

and radioactive decay, of course, profoundly affects planetary differentiation.

Conclusions

Impact-induced H2O loss and total volatile loss from a carbonaceous chondrite have

the same dependence on shock pressure. Thus, there is no enrichment or depletion of

H2O relative to other volatile species in gases released from a carbonaceous chondrite

upon impact. The actual impact pressure dependence is bracketed by results for crystal

density serpentine and 20% porous serpentine. For the Earth, incipient devolatilization

of accreting planetesimals occurred when the planet reached 12% of its present day

radius, and complete devolatilization occurred at 27% of its present day radius. Therefore, impact-induced devolatilization of accreting planetesimals played a major role in

altering the distribution of volatiles during accretion. Prior to metallic core formation

and internal differentiation, 99.4 mass % of the incident volatiles would be found in the

near surface environment (including a possible surficial magma ocean), the remainder

residing in the core region, with a volatile depleted 'mantle' between them.

Acknowledgments. Supported under NASA Grant NGL-05-002-105. We are grateful to

Professor Carleton Moore and the Center for Meteorite Studies, Arizona State University

for samples. We appreciate the use of laboratory facilities operated by George Rossman

and E. M. Stolper as well as helpful comments on the manuscript preferred by William

Anderson. Division of Geological and Planetary Sciences Contribution no. 4293.
References

[I] G. Arrhenius, B.R. De and H. Alfven, Origin of the ocean, in: The Sea, E. D.

Goldberg, ed., pp. 839-861, Wiley, New York, 1974.

[2] A. Benlow and A.J. Meadows, The formation of the atmospheres of the terrestrial

planets by impact, Astrophys. Space Sci. 46, 293-300, 1977.

[3] M.A. Lange and T.J. Ahrens, The evolution of an impact-generated atmosphere,

Icarus 51, 96-120, 1982.

[4] Y. Abe and T. Matsui, The formation of an impact-generated H2O atmosphere and

its implications for the early thermal history of the Earth, Proc. Lunar Sci. Conf.

15th, Part 2, J. Geophys. Res., 90, C545-559, 1985.

[5] M.A. Lange and T.J. Ahrens, Impact induced dehydration of serpentine and the

evolution of planetary atmospheres, Proc. Lunar Planet. Sci. Conf. 13th, Part I, J.

Geophys. Res. 87, A451-A456, 1982.

[6] M.A. Lange and T.J. Ahrens, Shock-induced CO2-production from carbonates and

a proto-COj-atmosphere on the earth, preprint in preparation, 1985.

[7] M.A. Lange, P. Lambert and T.J. Ahrens, Shock effects on hydrous minerals and

implications for carbonaceous meteorites, Geochim. Cosmochim. Acta, 49, 1715-

1726, 1985.

[8] D.J. Barber, Matrix phyllosilicates and associated minerals in C2M carbonaceous

chondrites, Geochim. Cosmochim. Acta, 45, 945-970, 1981.

[9] J.A. Tyburczy and T.J. Ahrens, in preparation, 1986.

[10] M.H. Rice, R.G. McQueen and J.M. Walsh, Compression of solids by strong shock

waves, Solid State Physics, 6, 1-63, 1985.

[II] S.P. Marsh (ed.), LASL Shock Hugoniot Data, Univ. of Calif. Press, Berkeley, pp.

327, 1980.

[12] D. StbfTler, Deformation and transformation of rock-forming minerals by natural

and experimental shock processes, Fortschr. Miner., 49, 50-113, 1972.

[13] T.J. Ahrens and J.D. O'Keefe, Shock melting and vaporization of lunar rocks and

minerals, Moon, 4, 214-249, 1972.
- 2 -

[14] J.A. Tyburczy and T.J. Ahrens, Dynamic compression and volatile release of carbonates, J. Geophys. Res., in press, 1986.

[15] M.B. Boslough, R.J. Weldon and T.J. Ahrens, Impact-induced water loss from serpentine, nontronite, and kernite, Proc. Lunar Sci. Conf. llth, 2145-2158, 1980.

[16] M.A. Lange and T.J. Ahrens, Shock-induced CO2-production from carbonates and

a proto-CCVatmosphere on the Earth, Lunar Planet. Sci. XIV, 419-420, 1983.

[17] J.A. Tyburczy and T.J. Ahrens, Partial pressure of CO2 and impact-induced devolatilization of carbonates: Implications for planetary accretion, Lunar Planet. Sci.

XVI, 874-875, 1985.

[18] Y. Yomogida and T. Matsui, Physical properties of ordinary chondrites, J. Geophys. Res., 88, 9513-9533, 1983.

[19] L.H. Fuchs, E. Olsen and K.J. Jensen, Mineralogy, crystal chemistry and composition of the Murchison (C2) meteorite, Smithsonian Contrib. Earth Sci., 10, 1-39,

1973.

[20] G.F. Davis, Heat deposition and retention in a solid planet growing by impacts,

Icarus, 63, 45-68, 1985.
TABLE 1

Volatile-containing minerals in Murchison [19]

Wate

Serpentine* - (Mg,Fe)3Si2O6(OH)4

(Septechlorites)

Montmorillonite -

(Na)l/2Ca).33(Al,Mg)2SI,01o(OH)2-nH20

Brucite - Mg(OH)2

Limonite - FejOynl^O

Sepiolite - Mg2Si3O8'2H2O

Mg-Sulfate - MgS04-nH2O

Gypsum - CaSO4-2H20

Bloedite - Na2Mg(SO4)2-2H2O

Whewellite - CaC204-H2O

Carbon

Hydrocarbons*

(H/C ~ 1)

Graphite - C

Calcite - CaCO3

Whewellite - CaC2O4-H2O

Sulfur

Disseminated S*

Troilite - FeS

Pentlandite -

(Fe,Ni),S8

**FeSO phase

Gypsum - CaSO4-2H2O

*Major volatile bearing phase.

**Poorly characterized, unidentified Fe, S, O containing phase.
Table 2. Experimental Results

Shot

number

799

803

785

835

825

781

Sample

mass,

mg

19.6

19.0

19.3

17.6

39.5

19.4

Projectile

Velocity,

km/s

1.08

±0.03

1.21

±0.02

1.40

±0.10

1.67

±0.01

1.84

±0.01

2.06

±0.10

Initial

Shock

Pressure,

GPa

13.3

±0.4

15.1

±0.3

17.3

±1-4

21.5

±0.1

24.0

±0.2

27.3

±1.5

Peak

Shock

Pressure,

GPa

22.9

±0.7

26.2

±0.5

31.1

±2.7

38.4

±0.3

43.2

±0.5

49.7

±3.1

Total

Volatile

Loss,

20° -1200 °C,

%of

initial

-

-

40.0

±5.0

87.4

±2.6

81.3

±3.8

48.0

±10.0

H2O Loss,

20 ' C-1200 ' C,

%of

initial

14.4

±3.5

40.0

±5.0

-

89.5

±6.7

89.0

±5.0
Figure Captions

Figure 1. Representative thermogravimetric analysis (TGA) curves for unshocked and

shocked Murchison. Mass loss increases downward. For the shocked samples, the

impact-induced mass loss is represented by the difference between the final (1200 ° C)

value and 100%. 100% mass loss refers to the mass loss between 200 and 1200 ° C for

unshocked Murchison.

Figure 2. Shock-induced mass loss as a function of initial shock pressure for Murchison.

Squares represent H2O loss; circles, total volatile loss. Solid line is a polynomial fit to

the data. Dashed lines are fits to data on impact-induced dehydration of 20% porous

serpentine [9] and crystal-density serpentine [7].

Figure 3. Impact-induced devolatilization of Murchison as a function of fractional planetary radius and mass for the Earth, Venus, Mars, respectively.
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