Systematic variations in the abundances of presolar grains

of Sic and diamond with petrologic type in unequilibrated ordinary chondrites (UOCs) [1,2] probably reflect differences in P-T conditions and/or fluid composition during parent-body metamorphism. It may, therefore, be possible to constrain physical conditions during the metamorphism by determining the conditions under which presolar grains are destroyed

Beckett, J. R.

Grossman, L.

Mendybaev, R. A.

Stolper, E. M.

Caltech Authors - Main

Meteoritics & Planetary Science, Vol. 35, No. 5, Supplement, 2000 A107 
MAGMATIC WATER IN SHERGO'ITY, INFERRED FROM LIGHT- 
LITHOPHILE-ELEMENT PATTERNS AND CRYSTALLIZATION 
EXPERIMENTS. H. Y. McSween Jr.1, R. C. F. Lentzl, T. L. Grovez, and 
J. C. Dannz, lDeparbnent of Geological Sciences, University of Tennessee, 
Knoxville TN 37996, USA (mcsween@utk.edu), 2Department of Earth, 
Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 
Cambridge MA 02139, USA. 
Introduction: Geomorphic evidence for ancient oceans on Mars [ I ]  
contrasts sharply with the dessicated planet inferred from miniscule water 
contents of SNC meteorites [2]. Amphiboles in trapped melt inclusions in 
pyroxene phenocryst cores once fueled speculation that shergottite magmas 
contained 10-2 wt% water at depth [3], but conflicting interpretations of low 
measured amphibole H contents make this evidence unreliable [4,5]. New 
observations on Shergotty, however, may reconcile geologic and geochemi- 
cal constraints on the amount of outgassed water on Mars. 
Secondary Ion Mass Spectrometry Results: We have measured abun- 
dances of Li, Be, and B in cotes and rims of pyroxenes (the earliest liquidus 
phases). Beryllium increases from core to rim, whereas B and Li decrease 
by 5040%. During crystallization, all three elements should behave incom- 
patibly (and thus increase in the magmatically evolved rims). However, the 
presence of a hot aqueous fluid could alter this trend because B, and to a 
lesser extent Li, are soluble in fluids >350"C. Consequently, the depletion of 
B and Li in pyroxene rims suggests loss of a fluid after crystallization of the 
cores, which are thought to have formed at depth [6]. This observation sug- 
gests that water carrying B and Li was lost from the magma by vesiculation 
on ascent or eruption and provides qualitative evidence for originally wet 
shergottite magmas. 
Experimental Results Iron/magnesium ratios of pigeonite and augite 
cores indicate that both pyroxenes cocrystallized [7], yet experiments on 
Shergotty liquids under varying conditions of pressure and f, consistently 
crystallize pigeonite before augite [8,9]. However, new experiments carried 
out at modest water pressures cocrystallize pigeonite and augite with com- 
positions like those in Shergotty cores. m e  onset of pyroxene crystallization 
is at lllO"C, a slightly higher temperature than that derived from pyroxene 
geothermometry but consistent with measured Wo contents. The preeruptive 
water content of -2 w t 8  would correspond to a pressure of 40-50 MPa, 
similar to that inferred from experiments on a Chassigny melt composition 
[lo]. Basaltic magma having this water content would be saturated at a depth 
of -4 km, a minimum depth for formation of the pyroxene cores. Calcula- 
tions indicate that loss of water after pyroxene core crystallization would have 
taken place mostly at depths <2 km [ 1 I] and allowed the rims to evolve along 
compositional trends like the zoned pyroxene rims. 
Conclusion: n o  independent lines of evidence support the hypothesis 
that the Shergotty parent magma contained an appreciable amount of water 
at depth and delivered this water to the planet's surface. 
[ I ]  Head J. W. et al. (1999) Science, 286, 2134-2137. 
[2] Dreibus G. and W&ke H. (1987) Icarus, 71,225-240. [3] McSween H. Y. 
and Harvey R. P. (1993) Science, 259, 1890-1892. [4] Watson L. L. et al. 
(1994) Science, 265, 86-90. [5] King P. L. et al. (1999) GCA, 63, 3635- 
3651. [6] Lentz R. C. F. and McSween H. Y. (2000) MAPS, in press. 
[7] Stolper E. and McSween H. Y. (1979) GCA, 43, 1475-1498. [8] Dann 
J. C. et al. (2000) LPS XXXI,  Abstract #1081. [9] McKay G. et al. (2000) 
LPS XXXI ,  Abstract #2000. [lo] Minitti M. and Rutherford M. J. (2000) GCA, 
in press. [Ill Wilson L. and Head J. W. (1981) JGR, 86, 2971-3001. 
References: 
AN ASTROPHYSICAL MODEL FOR THE FORMATION OF ZONED 
IRON-NICKEL METAL GRAINS IN THE BENCUBBINICH-LIKE 
CHONDRITES QUEEN ALEXANDRA RANGE 94411 AND HAMMA- 
DAH AL HAMRA 237. A. Meibom'. S. J. Deschz, M. 1. Petaevs, A. N. 
b e ,  J. N. Cuzzi2. J. A. W d 3 ,  and K. Keil', Geological and Environmen- 
tal Sciences, Stanford University, Building 320 Lomita Mall, Standford CA 
94305-21 15, USA (meibom@pangea.stanford.edu). 2NASA Ames Research 
Center, Moffett Field CA 94035, USA, 'Harvard-Smithsonian Center for 
Astrophysics, Cambridge MA 02138, USA, 'Hawai'i Institute of Geophysics 
and Planetology, University of Hawai'i, Honolulu HI 96822, USA. 
Introduction: Calcium-aluminum-rich inclusions and chondrules pro- 
vide important physiochemical constraints on thermal processing of solid 
precursor materials in the solar nebula, but offer few constraints on the 
dynamical environment in the disk during high-temperature formation of these 
precursors by gas-solid condensation. Recently, Fe,Ni metal grains display- 
ing smooth chemical zonation, with Ni and Co decreasing and Cr increasing 
from core to rim, were discovered in CH carbonaceous chondrites [l] and 
in the BencubbidCH-like chondrites QUE 94411 (QUE) and Hammadah a1 
Hamra 237 (HH 237) [2]. Some zoned Fe,Ni metal grains in CH chondrites 
formed by disequilibrium, gas-solid condensation from a gas of solar composi- 
tion at a total gas-pressure of 10-4 bar in the temperature interval between 
1370-1270 K [l]. After their formation, they escaped thermal metamorphism 
and other chemical alteration processes (e.g., oxidation or sulfidization) in 
the disk as well as on their parent body. This remarkable survival of pristine 
Fe,Ni metal condensates has opened a new window to high-temperature proc- 
esses in the protostellar disk around the Sun. 
It was suggested [3] that the zoned Fe,Ni metal grains in CH chondrites 
condensed inside of -1 AU in large parcels of gas convectively moving from 
the hot midplane of the protostellar disk (where all infalling solids are va- 
porized) to cooler regions above it. After their formation, some of the zoned 
Fe,Ni metal grains escaped thermal processing and retained their chemical 
zoning because turbulent diffusion transported them outward in the disk 
( 9 . 3  AU) to regions where midplane temperatures were much lower. The 
fraction of Fe,Ni metal grains that would escape thermal processing by this 
mechanism was estimated to about 5 x 10-3, in agreement with the fraction 
of zoned metal grains in CH chondrites (10-2-10-4) [3]. 
In QUE and HH237 the zoned Fe,Ni metal 
grains have characteristics that further tighten the requirements to any model 
for their formation: (1) a very high abundance (-15 vol8) [2,4], (2) system- 
atically low Cr concentrations compared with zoned metal grains in CH chon- 
drites [1,2,4], and (3) zoning patterns in PGEs (e.g., Os, Ir, Pt, Ru, Pd, and 
Re) qualitatively similar to the zoning patterns of Ni and Co [5]. Thermo- 
dynamic analysis of compositional profiles across eight zoned Fe,Ni metal 
grains in QUE showed that these grains formed by nonequilibrium, gas-solid 
condensation, at temperatures between -1500-1400 K. from an oxidized 
nebula gas with varying initial dust/gas ratio (-10-40x solar), in a dynami- 
cal environment where earlier formed solids had been isolated from the gas 
to different degrees [4]. The Cr depletion probably resulted from sequester- 
ing of Cr into chondrule silicates (olivinelpyroxene) condensing prior to Fe,Ni 
metal. Each zoned metal grain required distinct condensation parameters (dust/ 
gas-ratio, isolation degree, and Cr-depletion factor); a substantial degree of 
heterogeneity in the formation region was inferred [4]. The apparent diam- 
eter of these metal grains correlates linearly with the inferred condensation 
temperature interval, AT, indicating a constant growth rate [3]. Calculated 
growth timescales range from 10-100 h, and inferred cooling rates of the gas 
during condensation of each grain are consistently low (<-2 K h )  compared 
with chondrule cooling rates (-100-1000 Kih at T - 2100-1500 K [6]). Effec- 
tive removal from the hot condensation region prior to condensation of moder- 
ately volatile siderophile elements (e.g., Ga and Ge [7]) and sulfidization is 
required. We attempt to reconcile all these constraints in an astrophysical 
model akin to that presented in [3], but modified to include radial transport 
of early condensates by the stellar wind. 
[ I ]  Meibom A. et al. (1999) JGR, 104, 22053-22059. 
[2] Weisberg M. K. and Prinz M. (1999) P m .  NIPR Symp. Anrarcr. Mere- 
orifes, 24, 187-189; Weisberg M. K. et al. (2000) MAPS, submitted. 
[3] Meibom A. et al. (2000) Science, 288, 839-841. [4] Petaev M. I. et al. 
(2000) LPS XXXI, Abstract #1606. [5] Campbell et al. (2000) LPS XXXI, 
Abstract #1490. [6] Connolly H., personal communication. [7] Herzog G. F. 
et al., this volume. 
Additional Constraints: 
References: 
SURVIVAL OF PRESOLAR SILICON CARBIDE GRAINS DURING 
POSITION OF METAMORPHIC FLUIDS. R. A. Mendybaevl, J. R. 
Beckett2, L. Grossman1J. and E. Stolperz, Qepartment of Geophysical Sci- 
ences, University of Chicago, Chicago fL 60637, USA, 2Division of Geology 
and Planetary Science, California Institute of Technology, Pasadena CA 91 125, 
USA, 3Enrico Fermi Institute, University of Chicago, Chicago IL 60637, USA. 
Introduction: Systematic variations in the abundances of presolar grains 
of Sic and diamond with petrologic type in unequilibrated ordinary chon- 
drites (UOCs) [ 1.21 probably reflect differences in P-T conditions and/or fluid 
PARENT-BODY METAMORPHISM: CONSTRAINTS ON THE COM- 
A108 Meteoritics & Planetary Science, Vol. 35,  No. 5, Supplement, 2000 
composition during parent-body metamorphism. It may, therefore, be possible 
to constrain physical conditions during the metamorphism by determining the 
conditions under which presolar grains are destroyed. For fluid compositions 
near the H2-C02 binary, lifetimes of micrometer-sized Sic grains are only 
-105-106 yr at 300”-450°C [3], much less than the -107-108 yr required for 
the UOC parent bodies to cool through this temperature range. Based on these 
experiments, presolar Sic  grains would not survive metamorphic processing 
during the evolution of UOCs. Although the composition of fluids in the UOC 
parent body are poorly constrained, they were not restricted to the H,-CO, 
binary. We therefore conducted volatilization experiments on Sic  and SiO, 
in CO-CO,, H,-CO-CO2, N,-H,-CO,, and (He, Ne, Ar)-H,-CO, gas mixtures 
within the same range of f, as in the H,-CO, binary. 
Experiments were conducted within the range of tempera- 
ture from 750” to 1350°C; the range of log f, was between IW-3 and IW2, 
which encompasses the range of estimated f, for UOCs [4,5] . As starting 
materials, we used wafers (typically -6 x 3 x 0.3 mm) of chemically vapor 
deposited Sic  and fused silica glass. Experimental details are as in [3,6]. 
Silicon carbide wafers exposed to our experimental gas mix- 
tures first increased their weight, as was observed in [3], indicating the 
formation of a silica layer. After the silica layer formed, wafers exposed to 
H-bearing gas mixtures lost weight linearly with time within the whole range 
off, in this study in a manner consistent with weight loss of SiO, wafers. 
In H-free systems (CO-C02 gas mixtures), however, no weight loss of Sic  
wafers was observed after formation of SOl. Dilution of H2-C02 by noble 
gases (He, Ne, and Ar) decreases the rate of weight loss of SiO, (and Sic). 
The rate also decreased with increasing molecular weight of the noble gases. 
For example, in H,-CO,, 45% He-H,-CO,, 45% Ne-H,-C02, and 45% Ar- 
H,-CO, at 1206°C and IW-1, the volatilization rates of SiO, are 4 x 10-7, 
2 x 10-7, 1.5 x 10-7, and 1 x 10-7 mg/min/mm2 respectively. In 45% N,- 
H,-CO,, the volatilization rate was -5x lower than in N-free systems. These 
results indicate that diffusion of H-bearing species to the surface of SiUSiO, 
is the rate-limiting step. 
A set of experiments conducted in H,-CO-CO, gas mixtures at 1206°C 
and log fg from IW to IW-I showed that the volatilization rate of SiO, 
strongly depends on concentrations of both H, and CO in the gas mixture. 
In 10% H2-77% CO-13% CO,, for example, the volatilization rate was -2Ox 
lower than in H,-CO, at the same f,. As in H,-CO, gas mixtures, volatil- 
ization rates in H,-CO-CO, gas mixtures are directly proportional to PH,O. 
Our experimental results clearly indicate that survival of 
presolar Sic  grains during parent-body metamorphism is controlled by the 
concentration of water vapor in the metamorphic fluid. In [3], we concluded 
that survival of presolar Sic  grains with diameters of 0.4-2.0 pm in UOCs 
requires that the fluid be water-poor relative to H,-CO, at the same fg. We 
can impose more stringent constraints on fluid composition based on our 
current results. For example, the low activation energies for gases at IW and 
IW-I (-100 kJ/mole), lead to lifetimes of 2-p-sized grains greater than 107 yr 
at 300°C only if PH,O c O.ooO1 atm. For more reducing conditions, the PH,O 
may have been higher (<0.02 atm at IW-2 to IW-3) because of higher ac- 
tivation energy for volatilization (-140 KJ/mol). These PH,O values require 
that the vapor be very H-depleted relative to H,-CO, (e.g., at IW-1, a 10% 
H,-77% CO-13% CO, gas mixture has PH,O = 0.02 atm). We conclude that 
the metamorphic fluid was H-poor in composition. 
[I] Huss G. R. (1990) Narure, 347, 159. [2] Huss G. R. 
(1997) AIP Con$ Proc. 402, 721 pp. [3] Mendybaev R. A. et al. (1998) LPS 
XXIX, Abstract #1871. [4] McSween H. Y. and Labotka T. C. (1993) GCA, 
57, 1105. [5] Brett R. and Sat0 M. (1984) GCA, 48, 111. [6] Beckett J. R 
and Mendybaev R. A. (1997) GCA, 61, 4331. [7] Yang C.-W. et al. (1997) 
GCA. 61, 2943. 
Experiments: 
Results: 
Applications: 
References: 
TRACE ELEMENTS IN AND ON METEORITIC DIAMONDS. S. 
Merchel. B. Spettel, and U. Ott, Max-Planck-Institut fur Chemie, D-55020 
Mainz, Germany (merchel@mpch-mainz.mpg.de). 
Introduction: Although diamond was the first mineral phase identified 
as of presolar origin and moreover is the most abundant mineral in meteor- 
itic matter, its origin has remained enigmatic. Trace-element abundances are 
low, and so far, only Te and the noble gases Kr and Xe show possibly di- 
agnostic isotopic patterns [ 1.21 indicative of a supernova origin. Major 
overabundances are observed for isotopes abundantly produced by r-process 
0 AKL 
0 AMW 
A MKL 
V MMW 
i ‘; 
40 60 80 proton number 0 
Fig. 1. Elemental abundances in diamond separates from Allende and 
.. .. . . . - . . . . . . - -_ . - _- _- . . . . Murchson (nonnahzed to c and solar abundances). AKL and MU = Allende 
and Murchison diamond residues, prepared by variant of “classical” method 
[7]. AMW and MMW were prepared by “faster” chemishy [5]. Arrows denote 
upper limits only. 
nucleosynthesis because they have precursors with magic neutron numbers 
N = 50 and N = 82. Hence measurement of heavy trace elements of mostly 
r-process origin associated with magic N = 126 would help to shed further 
light on the origin of the diamonds; they may also bear on the suggestion 
of different r-process sources [3]. Unfortunately, those are noble metals and 
elements like Os, Pt, and especially Ir to a significant degree survive the 
chemical treatments employed during diamond extraction [4]. 
Recently we started on a program for a 
“faster and better” chemical isolation of presolar grains. First results showed 
our new procedure to be not only “faster” [5], but also “better,” as far as 
contamination of diamonds with Sic  is concerned [5,6]. To check the abun- 
dance of heavy trace elements we have now investigated diamond separates 
prepared by different methods from the Allende and Murchison meteorites 
by instrumental neutron activation analysis (INAA). Preliminary results are 
plotted in Fig. 1. For comparison, data of Lewis et al. [4] for a diamond 
separate from Allende are also shown. For several elements, including Fe, Co, 
Ni, Se, Sb, La, Sm, Hf, and Os, improved data will be available by the time 
of the meeting. 
Discussion: Murchison residues generally have lower trace-element 
concentrations than those from Allende. The abundance of Ir in the classi- 
cally prepared [7] Allende residue (244 ppm) is similar to the one reported 
in [4] (300 ppm) whereas for the three other residues significantly lower values 
were determined (51, 18, and 2.5 ppm). The low Ir concentrations and the 
correspondingly low activities enabled us to determine or give upper limits 
for many more elements than in previous INAA analyses. 
If we take the approximate straight line defined in Fig. 1 by the heavy 
noble gases as a measure for the expected abundance of trace elements 
indigenous to the diamonds, it follows that in the case of Ir the major part 
is still extraneous, a fact consistent with the observation by INAA that its 
isotopic composition is solar within -5%. For some of the other heavy el- 
ements (e.g., Pt), however, we are already at a level that detailed isotopic 
analyses should be feasible. For the rest we are working on further purifi- 
cation. 
We thank A. Besmehn and C. Sudek for the “clas- 
sical” samples (AKL, MKL) and assistance in preparing AMW and MMW. 
[I] Richter S. et al. (1998) Nature, 391, 261-263. [2] Lewis 
R.  S. et al. (1987) Nature, 326, 160-162. [3] Qian Y.-Z. et al. (1998) 
Astrophys. J. ,  494, 285-296. [4] Lewis R. S. et al. (1991) Meteoritics, 26, 
363-364. [5] Merchel S. and Ott U. (1999) MAPS, 34, A81. [6] Besmehn 
Experiments and Results: 
Acknowledgments: 
References: 


Survival of presolar silicon carbide grains during parent-body metamorphism: constraints on the composition of metamorphic fluids

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Survival of presolar silicon carbide grains during parent-body metamorphism: constraints on the composition of metamorphic fluids

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