1194.pdf
Lunar and Planetary Science XXXII (2001)
Mg, Si, and Fe isotopes in stony cosmic spherules: C. M. O'D. Alexander et al.
1.0
60
aged the results. The resulting ratios of Al, Ca, Ti,
and Mg to Si agree best with CI and CM values; they
50
are lower by a factor of two than corresponding ratios
0.8
Fraction retained
after one second
for Halley dust [10]. The evaporation-compensated
40
δ57Fe
Fe/Si ratios, however, are much lower than those
0.6
δ18O
δ26Mg
found in CI, CM, or ordinary chondrites. We con-
30
δ29Si
clude that evaporation contributes to but does not
MM14
0.4
20
fully explain iron loss from our sCS. Melting, phase
segregation, and loss of iron sulfides could account
0.2
10
for the loss of up to 1/2 (1/4) of the iron in CI (CM)
precursor material. Reduction of Fe(II) to immiscible
0
orimagemgsphfig5a
0.0
1700 1800 1900 2000 2100 2200 2300 2400
Fe(0) by C, as graphite or a kerogen, could then set
Temperature (C)
the stage for more Fe loss provided that atmospheric
oxygen was somehow excluded.
Figure 2. Degrees of isotopic fractionation for CI-like
material evaporating freely for 1 s. Solid diamonds are
The model of Alexander [8] also predicts the
isotopic data for MM14.
isotopic evolution of a CI-like particle with a radius
of 100 m undergoing Rayleigh fractionation for 1
tures (> 1750C) are also indicated by calculated
2 s at various temperatures (Figure 2). The results for
melting temperatures for the sCS [11] (Table 1).
MM14 line up reasonably well for T ~ 2240C, con-
Conclusions: Mass-dependent isotopic fractiona-
sistent with the model's prediction of relative volatil-
tion in Fe-poor, light-colored sCS is endemic for Fe,
ities. Lower temper atures are possible but would
common for Si, and occasional for Mg. Iron loss oc-
require much longer evaporation times. Taken as a
curs by evaporation and probably by separation of
set, the allowed combinations of time and tempera-
immiscible phases as well. Model calculations for
ture fall outside the range previously thought typical
evaporating CI-like material [8] capture the key com-
for micrometeorites with i ncoming velocities up to 15
positional and isotopic trends and point toward tem-
peratures near or above 2000C, which in turn sug-
km/s. Specifically, the model calculations indicate
temperatures in excess of 2000C. High tempera-
gest high entry velocities. CI- or CM- like material
could be progenitors of these special CS.
Table 1. Analyses of micrometeorites MM8, 9, 11, 14, and 15.
References: [1] Brownlee D.E. et al.
(1997) MPS, 32, 157-175; Maurette M. et al.
MM15g
MM8
MM9
MM11
MM14
(1991) Nature, 351, 44-47; Kurat G. et al.
Massa
221
171
5.00.5
3.00.5
4.00.5
(1994) GCA, 58, 3879-3904; Genge M.J. et al.
Densityb
3.70.5
2.40.6
2.60.7
3.91.1
(1996) GCA, 61, 5149-5162. [2] Flynn G.
MgO
51.7
43.2
35.4
45.2
47.0
(1989) Icarus, 77, 287-310. [3] Love S.G. and
Al2O3c
1.13
5.70
1.57
7.04
11.3
Brownlee D.E. (1991) Icarus, 89, 26-41.
SiO2c
41.6
44.5
51.3
38.9
35.7
[4] Herzog G.F. et al. (1999) GCA, 63 , 1443-
CaOc
1.13
4.45
1.33
5.7
3.1
1457. [5] Schnabel C. et al. (1999) Antarctic
TiO2c
0.07
0.23
0.09
0.33
0.51
Meteorites, XXIV, 166-167; Misawa K. et al.
MnOc
0.08
0.37
(1992) Geochem. J., 26, 29-36; Esat T. et al.
c
FeO
1.84
0.25
7.69
0.05
0.04
(1979) Science, 206, 190-197; Davis A.M. et
al. (1991) LPSC, XXII, 281-282. [6] Taylor S.
TMd
1857
1725
1639
1784
1823
et al. (2000) MPS, 35, 651-666. [7] Alexander
δ57Fee
3.51.2
9.82.2
4.50.4
384
C.M.O'D. and Wang J. (2001) MPS in press.
δ
29Sie
-0.10.4
0.90.8
2.10.4
8.40.6
3.90.2
[8] Alexander C.M.O'D. (2001) MPS in press .
δ
30Sie
-0.71.0
1.50.9
4.40.5 15.91.0 6.00.4
[9] Hashimoto A. (1983) Geochim. J., 17, 111-
δ
25Mge
-3.10.8
-1.20.8
7.60.8
145. [10] Jessberger E.K. and Kissel J. (1991)
δ
26Mge
-5.91.4
-3.01.4
14.71.7
Comets in the post-Halley era. Dordrecht,
fFe
0.67
0.33
0.60
0.02
1075-1092; Eberhardt P. (1998) C o m e t a r y
≡1
fSi
0.91
0.77
0.38
Nuclei in Space and Time, Astron. Soc. Pacific
≡1
≡1
≡1
fMg
0.61
Conf. Series. [11] Beattie P. (1993) Min. Pet-
a) g. b) g/cm3. c) mass %. d) C. e) Standard notation relative to
rol., 115, 103-111.
56Fe, 28Si, or 24Mg (‰.) f) fraction retained. g) Mass % re-
normalized by a factor 1.12 to compensate for low mass total.
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