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46th Lunar and Planetary Science Conference (2015)
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ISOTOPIC CONSTRAINTS ON THE ORIGIN OF IIE IRON METEORITES. M. Fischer-Gödde1, T. S.
Kruijer1, T. Kleine1 and J. T. Wasson2. 1Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany. Correspondence: [email protected], 2Institute of
Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567.
Introduction: Group IIE iron meteorites, together
with groups IAB and IIICD, are classified as nonmagmatic iron meteorites [1]. These are distinguished
from magmatic iron meteorites in having element-Ni
trends that cannot be explained by fractional crystallization [2,3]. They also contain abundant silicate inclusions, which are largely absent in members of the
magmatic groups. The non-magmatic irons, therefore,
are not considered as samples of the metal cores of
planetesimals, but more likely formed on partially differentiated bodies or within impact-generated melt
pools near the surface of undifferentiated asteroids
[e.g. 3]. The trace element systematics of IIE irons are
complex. In log-log plots of Ga, Ge and Ir versus Ni,
the IIE irons display scattered trends that are inconsistent with variations expected from fractional crystallization [3], yet in plots of As and Au versus Ni they
seem to define a fractional crystallization trend [3,4].
Nevertheless, the small variations in W and Ir with Ni
argue against an origin by fractional crystallization.
The IIE iron meteorites, therefore, are interpreted to
have formed in impact-generated melt pools located
near the surface of a chondritic parent body [e.g. 3].
Silicate inclusions in IIE irons have similar O isotope compositions than H chondrites [5], which has led
to the idea that the IIE irons formed by impact melting
on the H chondrite parent body [3]. A genetic link between IIE irons and H chondrites would be consistent
with similar Mo isotope anomalies reported for these
two groups of meteorites [6,7].
Group IIE irons can be subdivided into two distinct
age groups. One group is characterized by formation
ages (mostly based on Ar-Ar) of ~4.5 Ga and cosmic
ray exposure (CRE) ages of 50-600 Ma, whereas the
other group shows evidence for late impact resetting at
~3.7 Ga and is characterized by younger CRE ages of
3-15 Ma [8]. Currently, the most precise constraints on
the formation age of IIE irons come from Hf-W isotope systematics. The IIE irons have significantly more
radiogenic W isotope compositions than magmatic iron
meteorites, indicating late metal-silicate separation or
re-equilibration until up to ~15 Ma after CAI formation [9]. Such young Hf-W ages would be consistent with formation of IIE irons from impactgenerated melt pools.
The objectives of this study are to constrain the
origin and formation history of the IIE iron meteorites
through a combined Ru, Mo, Pt, and W isotope study.
The ultimate goals of our multi-isotopic approach are
(1) to better constrain the origin of IIE irons and in
particular its relation to H chondrites using coupled
Mo and Ru isotope analyses, and (2) to precisely determine the formation time of IIE metal using Hf-W
isotope systematics. Combined Mo-Ru isotope measurements are useful because most iron meteorites and
also ordinary and enstatite chondrites are characterized
by distinct and correlated nucleosynthetic Mo and Ru
isotope anomalies, making coupled Mo-Ru isotope
systematics a powerful tool to assess potential genetic
relationships among meteoritic materials [6,7,10,11].
One caveat when applying high-precision Mo, Ru and
W isotope measurements to constrain the formation
history of iron meteorites is that the isotopic compositions may have been modified by secondary neutron
capture during extended cosmic ray exposure. Such
effects are well documented for W isotopes [12-14]
and we have recently shown them to be also significant
for Ru [11]. Determining the isotope compostion of
iron meteorites to high precision, therefore, also requires us to assess the magnitude of the neutron fluence in each sample, which can be quantified using Pt
isotopes [12-14].
Here we present the first Ru isotope data for five
members of the IIE iron meteorite group (Arlington,
Colomera, Miles, Mont Dieu, and Weekeroo Station).
Analytical techniques: Iron meteorite samples
(~1 g) were digested in reverse aqua regia in Savillex
beakers on a hot plate. After dissolution, Os was extracted from the aqua regia into CCl4 [15]. An aliquot
corresponding to about 30% was taken from which Ru
was separated from the sample matrix by cation exchange chromatography [16] and further purified by
mirco-distillation [17]. Ruthenium isotopes were
measured on a ThermoScientific Neptune Plus MCICPMS at the University of Münster. The measurements were typically performed with total ion beam
intensities of ~7.5 × 10-11 – ~4.4 × 10-10 A, which was
obtained for a 100 ppb Ru standard solution. Isobaric
interferences of Mo and Pd on Ru masses 96, 98, 100,
102 and 104 were corrected by monitoring 97Mo and
105
Pd. Measured Ru isotope ratios were normalized to
99
Ru/101Ru using the exponential law and are reported
as εiRu units representing the deviation in parts per 104
from the terrestrial Ru isotope composition. Accuracy
and precision of the Ru isotope measurements were
evaluated by replicate digestions and multiple analyses
46th Lunar and Planetary Science Conference (2015)
of the NIST 129c steel doped with ~500 to ~2000 ng
Ru from an Alfa Aesar standard solution. The external
reproducibility (2 s.d.) of the Ru isotope analyses obtained for 49 individual measurements from 9 different
digestions over a period of the last two years is ±0.45
ε96Ru, ±0.51 ε98Ru, ±0.13 ε100Ru, ±0.14 ε102Ru, and
±0.35 ε104Ru.
Results: The IIE irons display negative ε100Ru
anomalies ranging from –0.31 for Arlington to –0.13
for Mont Dieu (Fig. 1). Mont Dieu is the only IIE sample for which the ε100Ru anomaly is not fully resolved
from the terrestrial Ru isotope composition. The
anomalies in ε100Ru are correlated with negative anomalies in ε102Ru, although the latter are in general less
well resolved than those in ε100Ru. Most samples also
display positive ε96Ru and ε98Ru values, but these are
not resolved from the terrestrial standard. The ε104Ru
values of all analyzed meteorite samples overlap with
terrestrial Ru.
100
Fig. 1: ε Ru values of IIE iron meteorites in comparison to ε100Ru values of chondrites and iron meteorites.
Fig. 2: Average Ru isotope composition of IIE iron
meteorites shown as εiRu vs. iRu plot. Error bars represent 95% confidence intervals. The dashed line illustrates a calculated isotopic pattern for a deficit in sprocess isotopes [18].
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Discussion: The average Ru isotope composition
obtained for IIE iron meteorites shows a pattern that is
consistent with a deficit in s-process isotopes (Fig. 2).
Similar patterns have been observed for other iron meteorites [11], but of varying magnitude (Fig. 1). The
Ru isotope composition of the IIE irons overlaps with
that of ordinary chondrites and would thus be consistent with a genetic relationship between these two
groups of meteorites. Note, however, that both also
overlap with the Ru isotope composition of IVA iron
meteorites. Moreover, at the current level of analytical
resolution the Ru isotope composition of the IIE irons
also seems to overlap with enstatite chondrites.
The ε100Ru values of the IIE irons may be slightly
more negative than those measured, because neutron
capture reactions would lower ε100Ru values. The scatter of the ε100Ru values observed for the investigated
IIEs and in particular the less negative ε100Ru value of
Mont Dieu might reflect such neutron capture-induced
isotope shifts. We will quantify these effects by measuring the Pt isotope composition for the same samples.
We will also analyze the W isotope compositions on
aliquots of the same samples used for the Ru isotope
measurements, to determine Hf-W ages for each of the
samples. It will be interesting to see whether all IIE
irons are characterized by a common pre-exposure W
isotope composition (i.e., unaffected by neutron capture) or if there are W isotope differences. The latter
would imply distinct formation times, which might be
expected if the IIEs formed as impact melt pools near
the surface of a chondritic asteroid.
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