Extreme oxygen isotope ratios in the early solar system
Jérôme Aléon 1, François Robert 2, Jean Duprat 3 & Sylvie Derenne 4
Supplementary discussion
Insoluble organic matter (IOM) was recovered from the bulk Murchison meteorite at
LCBOP (Paris) following a well established procedure1 based on HF/HCl dissolution after
extraction of soluble organic compounds. Fluorosilicates formed during the extraction were
removed by hot HCl. The presence of silica in the residue is probably due to the combination
of its resistance to HF (ref. 2) and its protective organic coating. IOM pellets were deposited
and incrusted onto a high purity gold foil, cleaned by HNO3 acid etching followed by heating
at 900°C during several hours. The sample was pyrolyzed in vacuum to remove exchangeable
hydrogen3. After oxygen isotope mapping, several attempts were done to isolate the grains
from IOM. Boiling H2O2 treatment only resulted in the loss of IOM pellets containing grains
M7 to M17, precluding their analysis for Si isotopes. SiO2-rich grains were finally isolated
from their IOM mantle after an unsuccessful attempt to perform Raman microspectroscopy
with a 514 nm Ar laser.
Isotopic measurements were done using the CRPG (Nancy) IMS 1270 ion microprobe.
O isotope images were acquired at high mass resolution (~ 6000) in 4 sessions using a 10 keV
Cs+ beam of 1.5 µm. To ensure the absence of analytical artifacts different settings were
used : peak-switching mode on the axial electron multiplier (EM) with a circular cross-over
(session 1), peak-switching with a rectangular cross-over (session 2), simultaneous detection
of
16
O,
17
O,
18
O on differents EMs (hereafter multicollection) with a circular cross-over
(session 3). Duplicate analyses were done using multicollection in session 4. Typical count
rates for 16O were between 10 and 1000 counts per second per grain (cps/grain). Quantitative
data were extracted by image processing using Visilog 5.1 software. In multicollection mode
a percent level precision is reached in 3 minutes for 3 µm grains. All corrections were
negligible with regard to the observed variations, thus only the instrumental mass
fractionation (IMF) was corrected using a mantle xenolith olivine (313-1) as a standard (18O
= 5.6‰ relative to the Standard Mean Ocean Water). Depending on the sessions, the
instrumental fractionation factors ranged between 0.97 ± 0.06 and 1.01 ± 0.04 (2) for
18
O/16O and between 1.01 ± 0.06 and 1.08 ± 0.04 (2) for 17O/16O.
1
Si isotopes were measured at a mass resolving power sufficient to remove hydride
interferences at mass 29 and 30 (~ 4000) using peak-switching mode, a circular cross-over
and a 10 keV Cs+. To increase precision the beam intensity was increased resulting in a
defocused beam of ~2.5 µm. Counting times were 2 s (background), 5 s (28Si), 30 s (29,30Si)
during 20 cycles. Count rates for 28Si vary between 100 and 5000 cps/grain. Quantitative data
were again extracted by image processing using Visilog 5.1 software. After corrections for
background, dead time of the EM, linear intensity drift due to grain sputtering, IMF and
matrix effects between olivine and silica, the analytical precision is typically < 5‰ on > 5 µm
grains. Background and dead time corrections were negligible. The mantle xenolith olivine
(29,30Si = 0‰) was used as a standard. IMF for olivine was -13.5 ± 1.6‰ (2) for 29Si and 22.1 ± 2.5‰ (2) for 30Si. Matrix effects between olivine and silica were assumed to be 10‰ (30Si) as is typically observed (Engrand pers. comm.).
In the irradiation model, we investigated all reactions leading to
channels involving the short periods
17
16,17,18
O including
F (=64.7 s), 18F (=109.8 m) and 18Ne (=1.67 s) by
bombarding a solar gas target (N, O and Ne in solar proportions) (ref 4) with p,  and 3He
particles in the E = 3-50 MeV energy range. The characteristics of the charged particles
accelerated in Young Stellar Objects being not well constrained, we assumed a differential
power law dN/dE=K E- spectra, with particles similar to typical contemporary gradual solar
flares (GR :
3
3
He/ =5  10-4, /p=0.01, =2,3) and impulsive solar flares (IM :
He/p=/p=0.1, =3,4,5) (refs. 5-7). In such an approach, the nuclear production of oxygen
atoms is dominated by low threshold fusion-evaporation reactions. Experimental crosssections were used when available and otherwise were calculated using the statistical code
EMPIRE II (ref. 8) to infer the total cross-section excitation functions. Irradiation by GR type
particles fails to explain the data by more than an order of magnitude essentially because 3He
particles are necessary to produce 18O/16O above 10-2. With IM type irradiation conditions, we
find
17
O/16O ranging from 0.25-0.5,
18
O/16O from 0.6-1.6, having
18
O/17O from 2.4-3.5,
depending on the chosen . Reactions products are hereafter noted O*. The main reaction
channels contributing to
9,10) and
14
20
16,17,18
O* final production are for
Ne(3He,p)18F()18O* ; and for
N(,p)17O* (ref 12),
16
17
O* :
18
O* : 16O(3He,p)18F()18O* (refs
14
N(,n)17F()17O* (ref. 11) ;
Ne(p,)17F()17O* (ref. 13). The
O(3He,2p)17O* and
20
16
O* is
produced by 14N(3He,p)16O* ; 16O(p,p’)16O* (ref. 14,15) and 16O( ')16O* (ref. 16). For the
two latter reactions, we considered the events in which there is fusion of the incident particle
2
with the target (16O) followed by p or  emission, so that the resulting 16O* will have similar
recoil kinematics (energy and angle) as the others O* production channels.
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4
12
C,
14
N, and
16
O