Continued SIMS Trace Element Study of Presolar Graphite Grains

46th Lunar and Planetary Science Conference (2015)
2882.pdf
CONTINUED SIMS TRACE ELEMENT STUDY OF PRESOLAR GRAPHITE GRAINS. M. Jadhav1, K.
Nagashima2, and G. R. Huss2, 1Department of the Geophysical Sciences, The University of Chicago, Chicago, IL
60637, 2Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822. Email: manavi@uchicago.edu
Introduction: A few quantitative trace-element
measurements on bulk SiC fractions [e.g., 1] and individual presolar SiC grains [e.g., 2 – 6] have been carried out to date. Amari et al. [2] compared the observed
trace-element abundance patterns in presolar SiC
grains to results of condensation calculations for circumstellar environments to associate grains with different C-rich stellar environments [7]. However, very
few published studies have attempted to quantify the
abundance and distribution of trace elements in presolar graphite grains. Isotopic and TEM studies of
graphites have indicated that presolar graphite grains
have lower trace-element abundances compared to presolar SiC grains [e.g., 8 – 12]. Low-density (LD)
graphite grains are good candidates for trace-element
measurements compared to high-density grains because
of their turbostratic structure that can accommodate
trace elements better. Elements like Mg, Al, Si, and Ca
appear to have been incorporated into the parent grains
during primary graphite crystallization [10, 13]. Other
trace elements like Ti, Zr, Ru, Mo, Fe, and Ni are
found concentrated within early crystallizing subgrains
[11, 12].
This study is part of the ongoing attempt to measure
trace element abundances in presolar graphite grains by
SIMS. Last year, we presented Mg, Si, Ca, Ti, V, and
Fe data on graphites from Orgueil [14]. Here, we present data for additional trace elements (Sc, Fe, Ni, Rb,
Sr, Y, Zr, and Nb) from the same grains.
Experimental details: Thirteen individual LD
graphite grains from the OR1d (ρ ∼ 1.75−1.92g cm-3)
density fraction were measured with the University of
Hawai‘i’s Cameca ims 1280 ion microprobe. A 30−60
pA O- primary beam focused to ~ 1 µm was rastered
(12×12 µm2) over the graphite grains, and scanning ion
images of 12C+, 24Mg+, 28Si+, 44Ca+, 47Ti+, 51V+, 56Fe+
(Stage 1) and 12C+, 45Sc+, 56Fe+, 60Ni+, 62Ni+, 85Rb+,
86 + 89 + 90 + 93
Sr , Y , Zr , Nb+ (Stage 2) were collected in
magnetic-peak-jumping mode. The sample chamber
was flooded with O2 to enhance the ion yields. In Stage
3, a Cs+ primary beam was used to measure C, N, and
Si isotopic ratios in the same grains. Scanning ion images of the isotopic data were obtained in a combination of both multi-collection and peak-jumping modes.
L’image software was used to extract isotopic ratios
from the regions of interest.
Synthetic SiC grains were used as standards for the
C, N, and Si isotopic measurements. We used USGS24
graphite and NBS610 glass as standards for all other
elements presented here. The required trace-element
concentrations in USGS24 graphite will soon be measured by ICP-MS. Due to the lack of a proper graphite
standard to obtain absolute concentrations, we present
ion counts of the measured ions normalized to 12C
counts and compare relative abundances in this abstract. 60Ni/62Ni is not corrected for instrumental mass
fractionation. The C, N, and Si isotopes and Mg-Fe
elemental data were discussed in [14].
Results: The 45Sc+/12C+ ratios in the LD graphite
grains from this study range from 0.001−0.2, 60Ni+/12C+
from 4×10-4−3, 85Rb+/12C+ from 10-5−0.004, 86Sr+/12C+
from 5×10-5−0.6, 89Y+/12C+ from 7×10-4−0.01,
90 + 12 +
Zr / C from 5×10-4−0.02, and 93Nb+/12C+ from
4×10-5−10-3. Most of these elements are expected to be
concentrated in refractory subgrains within the graphites [11, 12]. Except for large Fe-Ni subgrains, there
was no sign of other subgrains in the depth profiles of
these grains. If the trace element signals are from subgrains much smaller than 1µm (spatial resolution of the
primary beam) then it was possible that they were
missed during data analysis. Thus, the spatial distribution of trace elements within the grains in this study is
unresolved.
No correlations exist between the trace element
content and the C, N, and Si isotopic ratios of the
grains. We show correlations between different trace
element contents in Figs. 1-4. The type II supernova
grains (grains 1, a subgrain within grain 1 (1sub), 4, 6,
9, 12, and 13), identified on the basis of their N and/or
Si isotopic compositions [14], are marked on each plot
to spot possible correlations. We plot raw ion counts
for all the elements. All errors are 1σ. Dashed lines in
Fig. 5 represent solar isotopic values.
A good correlation exists between 85Rb and 90Zr
contents in the grains with Zr being more abundant
(Fig. 1). In Fig. 2, if the grain with the highest 86Sr/12C
ratio (~0.6) is ignored (possibly highly contaminated
based on the 56Fe content) then 86Sr and 90Zr contents
of the grains are fairly well correlated. No such correlation was seen in presolar SiC grains [2]. Zr is more
abundant than Sr in the grains. The average Sr and Rb
abundances in the grains are roughly equal (Fig. 3) and
also fairly well correlated. The relative abundance of Y
in the grains is higher than that of Nb. These elements
are weakly correlated with each other (Fig. 4).
46th Lunar and Planetary Science Conference (2015)
2882.pdf
10-1
Fig.1
1
6
+ 12
1
9
Y/ C
+
6
+
12
1sub
-4
10-3
1sub
13
9
89
+ 12
Rb / C
85
12
13
10-3
10
4
10-2
10-2
4
10-4
10-5
Fig. 4
10-5
10-4
90
10-3
10-2
10-1
93
Fig. 2
Sr+/12C+
86
10-2
1
4
10-3
13
6
1sub
10-4
12
9
10-3
90
10-2
+ 12
10-1
100
+
Zr / C
85
+ 12
Rb / C
+
13
101
102
12
103
13
C/ C
6
10-3
9 12
-4
1sub
1
4
10-5
10-6
10-6
+
Fig. 5
100
100
Fig. 3
10
+ 12
Nb / C
10-2
101
60
10-1
10-2
10-3
sputtered away by the primary beam, contamination
would decrease. Contamination was lower during this
stage of measurements in the grains with already low
56
Fe/12C ratios but the ratio remained the same in
heavily contaminated grains. We also measured 60Ni
and 62Ni in the grains in an attempt to identify good
candidates for future 60Fe measurements. The 60Ni/62Ni
ratios in the grains (including the 6 SN grains) were
solar within errors (Fig. 5) indicating the grains also
have a large amount of solar or terrestrial Ni contamination.
10-1
10-4
10-4
Zr+/12C+
100
10-5
10-5
10-5
10-5
Ni+/62Ni+
10-6
10-6
10-5
10-4
86
10 -3
10 -2
10 -1
Sr+/12C+
A detailed discussion of these and other trace element correlations and their implications for the chemical environments in which these grains condensed will
be presented at the meeting.
Note on Fe-Ni contamination: As observed in [14],
the grains were highly contaminated by terrestrial Fe.
We compared the level of contamination between the
two measurement stages in the hope that as grains are
Acknowledgements: NASA NNX11AG78G (GRH)
and NNX09AG39G (Andrew M. Davis)
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