A82
72nd Meeting of the Meteoritical Society: Abstracts
5364
PRELIMINARY NANOSIMS ANALYSIS OF CARBON ISOTOPE OF
CARBONATES IN CALCIUM-
ALUMINUM-RICH INCLUSIONS
Y. Guan J. M. Paque D. S. Burnett, a
nd J. M. Eiler. Division of Geological
and Planetary Sciences, California In
stitute of Technology, Pasadena, CA
91125, USA.
Introduction:
Carbonate minerals observed
in primitive meteorites are
products of either terrestrial weathe
ring or aqueous alteration in the early
solar system. Most of the carbonat
e minerals in carbonaceous chondrites
occur primarily as isolated grains in
matrix, as crosscutting veins, or as
replacement minerals in chondrules [e
.g., 1, 2]. A few calcium-aluminum-
rich inclusions (CAIs) have been repo
rted containing carbonate minerals as
well [2, 3]. The C and O isotopes of
carbonates in carbonaceous chondrites,
mostly measured by stepwise extrac
tion of bulk samples with phosphoric
acid [4–7], are largely distinctive
from those of terrestrial carbonates,
whereas textural and petrographic evid
ence indicates that some carbonates in
primitive meteorites are terre
strial in origin [2]. This study attempts to
investigate from the aspect of C isotop
e the origin of rare carbonate minerals
in some CAIs. If of extraterrestrial
origin, carbonates in CAIs can provide
important information and constraints
on the ubiquitous aqueous alteration
process in the early solar system.
Samples and Analytical Techniques:
We have selected carbonate-
bearing CAIs from Murchison (CM) and
Leoville (CV) for this study. One of
the CAIs from Murchison
is the extensively studied
Blue Angel—an altered
hibonite inclusion containing
large amount calcite (10–70
μ
m) [3]. The other
Murchison CAI is a 100
×
50
μ
m calcite fragment enclosing small grains of
spinel, melilite, fassaite and perovskite
. The calcite in the type B Leoville
3537-2 CAI occurs as narrow veins (less than ~10
μ
m) at the center of the
inclusion. Carbon isotope
was measured with the Ca
ltech NanoSIMS 50L ion
microprobe. A rastering (3x3
μ
m) primary beam of ~10 pA was used to
sputter the sample and gene
rate secondary ions. Both
12
C and
13
C were
simultaneously collected with EMs. Ca
rbonate standards (calcite, dolomite,
magnesium, and siderite) were used to
check possible matrix effect and
instrumental mass fractionation (IMF).
Typical analytical errors under such
conditions are ~1–2‰ (1ó) for
δ
13
C.
Results and Discussion:
The results from standard measurements
indicate that there are significant matrix effects in C isotope analysis of
carbonates with a NanoSIMS. The IMF
increases by ~30‰ from calcite to
dolomite, magnesium, and side
rite. To date, three calcite grains from the Blue
Angel CAI were analyzed fo
r their C isotope, yielding
δ
13
C values from
+
16
to
+
23‰. Three spots on a calcite vein
in Leoville 3537-2, on the other hand,
show lower
δ
13
C values from—2 to—8‰. Thou
gh these CAI carbonate data
fall within the ranges obtained from
bulk measurements of CM or CV
meteorites [4], the
δ
13
C values are not high enough
to exclude a terrestrial
origin for the Leoville sample. Oxygen
isotope data are needed to further
constrain the nature of th
e CAI carbonates. Nonetheles
s, the data suggest that
there is no presolar C component in CM CAI carbonates that could be
responsible for the peculiar high
δ
13
C values observed in some bulk
carbonates.
References:
[1] Benedix G. et al. 2003.
Geochimica et Cosmochimica
Acta
52:1577–1588. [2] Abreu N.
M. and Brearley A. 2005.
Meteoritics &
Planetary Science
40:609–625. [3] Armstrong
J. T. et al. 1982.
Geochimica
et Cosmochimica Acta
46
:
575–595. [4] Grady M. M. et al. 1988.
Geochimica
et Cosmochimica Acta
52:2855–2866. [5] Zito et al. 1998.
Meteoritics &
Planetary Science
33:A171–A172. [6] Brearley
A. J. et al. 1999. LPSC
XXX, Abstract #1301. [7] Guo W. a
nd Eiler J. M. 2006. LPSC XXXVII,
Abstract #2288.
5166
A PRESOLAR SPINEL GRAIN OF PROBABLE NOVA ORIGIN
F. Gyngard
and E. Zinner. Laboratory for Sp
ace Sciences and Department of
Physics, Washington University
, St. Louis, MO 63130. Email:
fmgyngar@wustl.edu.
Introduction:
Presolar grains from novae
are exceedingly rare. A few
SiC and graphite grains have been identified by large
13
C,
15
N, and
30
Si
excesses [1, 2]. While some purpor
ted nova grains may have actually
condensed in a supernova
[3], some are undoubtedly
nova condensates [4, 5].
Except in rare cases,
the material ejected in nova
explosions is O-rich, and it
remains a puzzle why to
date only carbonaceous ph
ases, with one possible
exception [6], have been identified.
Here we report the discovery of a
presolar spinel grain w
ith a likely nova origin.
Experimental:
The “CG” residue of Murray, prepared by physical and
chemical separation [7] and
consisting of high concentr
ations of spinel grains
(diameter ~0.5
μ
m), was scanned for O-anomalou
s grains with an automated
measurement technique recently develo
ped for the NanoSIMS [8]. Grain C4–
8 was not identified by the particle de
finition software, but was recognized as
an
17
O hotspot in an ion image taken du
ring the automated surveys. High-
resolution SEM images confirmed that
it was surrounded by
multiple grains.
Subsequent removal of
surrounding isotopically
normal material [9] and
manual measurement of
16,17,18
O revealed the grain to be extremely enriched
in
17
O. Following O isotopic analysis,
24,25,26
Mg
+
and
27
Al
+
were measured in
multicollection mode with an O
–
primary beam in a separate measurement
session.
Results:
The O isotopic composition of
C4–8 is characterized by a
huge enrichment in
17
O and a modest depletion in
18
O, with
17
O/
16
O
=
(4.40
± 0.01)
×
10
−
2
and
18
O/
16
O
=
(1.10 ± 0.02)
×
10
−
3
.
This isotopic signature is
similar to that of Group 1 grains
[10]. However, RGB and AGB stars
undergoing 1st and 2nd dredge-up, the
most likely sources of these grains
[11], cannot produce
17
O/
16
O > 4 × 10
−
3
[12]. C4–8 is significantly enriched
in
25,26
Mg (
δ
25
Mg
=
949 ± 9‰ and
δ
26
Mg
=
929 ± 7‰) and, similar to the
situation for O, nucleosynthesis in
the O-rich envelope of AGB stars (
1
3M
)
cannot produce
δ
25
Mg
>
~
40‰ [13]. The most
likely condensation
environment for a grain with
extreme enrichments in both
17
O and
25
Mg is
found in nova ejecta. The best match fo
r the O isotopes in C4–8 is achieved
by models for
CO novae with a 0.8 or 1.15M
white dwarf; however, the Mg
isotopic composition is much
better explained by a 0.6 M
CO model [2].
The only other putative nova oxide iden
tified so far, T54 [14], is also
17
O
rich, but has not been analyzed for Mg
/Al and cannot help refine the model
predictions. Recent model calculations
have indicated nova nucleosynthesis
beyond Ca [5], and future isotopic meas
urements (e.g., Ca, Ti) of C4–8 are
planned to better constrain the
origin of this unique grain.
References:
[1] Amari S. et al. 2001.
The
Astrophysical Journal
551,
1065. [2] José J. et al. 2004.
The
Astrophysical Journal
612, 414. [3] Nittler
L. R. and Hoppe P. 2005.
The
Astrophysical Journal
631, L89. [4] Nittler L.
R. et al. 2006.
Meteoritics & Planetary Science
41, A134. [5] José J. and
Hernanz M. 2007.
Meteoritics & Planetary Science.
42, 1135. [6] Nittler
L.R. 2005.
The
Astrophysical Journal
618, 281. [7] Ta
ng M. et al. 1988.
Geochimica et Cosmochimica Acta
52, 1221. [8] Gyngard F. et al. 2009.
LPSC 40, 1386. [9] Zinne
r E. and Gyngard F. 200
9. LPSC 40, 1046. [10]
Zinner E. 2007.
TOG
vol. 1.02, pp. 1–33. [1
1] Nittler L.R. 1997. in
Astrophys. Impl. Lab.
Study of Presolar Mat.
pp. 59–82. [12]
Boothroyd A.I.
and Sackmann I.-J. 1999.
The
Astrophysical Journal
510, 232. [13] Zinner
E. et al. 2005.
Geochimica et Cosmochimica Acta
69, 4149. [14] Nittler L.R.
et al. 1997.
The
Astrophysical Journal
483, 475.