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46th Lunar and Planetary Science Conference (2015)
1520.pdf
IDENTIFYING EXTRATERRESTRIAL SIGNATURES IN MAFIC IMPACTITES: AN ASSESSMENT
BASED ON THE LONAR CRATER, INDIA
Christian Koeberl 1,2 and Toni Schulz 2, 1 Natural History Museum, 1010 Vienna, Austria ([email protected]), 2 Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria ([email protected]).
Introduction: During crater formation a small
amount of projectile material, generally less than 1
wt%, can be incorporated into impactites (impact melt
rocks, suevites, impact glasses, and ejecta), resulting in
a measurable geochemical signal. Such extraterrestrial
(ET) components have, so far, been identified for just
over ~40 out of the ~185 currently known terrestrial
impact structures [1]. The vast majority of these structures are excavated in felsic targets (i.e., continental
crust), making attempts for the geochemical identification of ET signatures rather straightforward. It remains,
however, difficult to detect ET signals for the rare examples of impact craters formed in mafic lithologies.
Tools for the identification of ET signatures generally
take advantage of compositional differences between
the impactor and the terrestrial target. For the most
commonly applied tools, highly siderophile element
abundances (e.g., platinum group elements [PGEs]), as
well as 187Os/188Os compositions, the differences between meteorites (i.e., chondrites and iron meteorites)
and the terrestrial target diminish the more mafic the
latter is, making it more difficult to distinguish between
the indigeneous and the meteoritic component.
Fig. 1: Lonar impact crater, India (ASTER image).
The Lonar crater, India (Fig. 1), is one of the rare
terrestrial examples of an impact crater excavated in a
mafic target, namely the continental flood basalts of the
~65 Myr old Deccan traps. The crater has a diameter of
1830 m and its floor is occupied by a shallow saline
lake. The formation age of Lonar crater was determined at ~0.57 Myr [2]. In the absence of actual meteorite fragments, the impact origin was mainly support-
ed via the presence of shocked plagioclase (e.g., [3]),
as well as meteoritic components of up to 17% postulated based on Cr, Co, and Ni abundances within aerodynamically shaped impact spherules found in the
Lonar area [4]. In an attempt to search for a meteoritic
signature within the impact melt and breccia rocks, we
here present an assessment of the first PGE and Os
isotope data for impact melts, breccias and target rocks
from the Lonar crater, India.
Methods: The samples analyzed for this study
were described by [5] and [6]. Target rocks lack any
obvious evidence of impact-generated shock features
and were collected from basalt flows exposed in the
upper 50 m of the crater wall. In contrast, impact melt
rocks and breccia samples were either collected outside
the crater rim within the ejecta blanket or inside the
crater rim and around the Lonar Lake. About 1 to 3 g
of powdered sample material was spiked with a mixed
tracer composed of 99Ru, 105Pd, 185Re, 190Os, 191Ir, and
194
Pt isotopes and digested in a HNO3/HCl (5+2) acid
mixture at 250°C and 125 bar pressure in an AntonPaar
high pressure asher for 12 hours. Osmium was purified
using carbon tetrachloride (CCl4) based solvent extraction techniques as described in [7], followed by
microdistillation purification [8]. The Os total processing blank was 0.5 ± 2 pg (n = 5). Rhenium, Ir, Ru,
Pt, and Pd were separated and purified from the residual acid by conventional anion exchange techniques
using AG1x8 resin. Total blanks (n = 5) averaged at 515 pg for Re, 1-3 pg for Ir, 20-70 pg for Ru, 10-30 pg
for Pt, and 20-50 for Pd. All Os isotope analyses were
performed using a Thermo Triton Thermal Ionization
Mass Spectrometer, equipped with an SEM detector,
used in negative mode at University of Vienna, Austria,
whereas the contents of rhenium and the PGEs were
measured using a Thermo Element ICP-MS in single
collector mode at University Bonn, Germany.
Results: Platinum-group element abundances and
chondrite-normalized PGE patterns for all samples are
broadly similar to that of the average upper continental
crust. However, several impact melt rocks exhibit a
factor of 2 to 20 higher Ir, Os, and Ru concentrations
(up to ~113, ~202 ppt and ~806 ppt, respectively). The
elevated PGE contents in several impact breccia rocks
are also significantly higher compared to basalts from
other areas within the Deccan traps, averaging at 25.1
± 7.2 ppt for Ir [9] and 6.5 ± 2.8 ppt for Os [10] and
~200 ppt for Ru [11]. Rhenium concentrations are sig-
46th Lunar and Planetary Science Conference (2015)
10
Target basalts
Impact breccia rocks
Mixing curves
UCC Upper Continental Crust
187Os/188Os
nificantly lower in impact melt rocks and breccias
(mostly <200 ppt) compared to target basalts (mostly
>200 ppt). Notably, both lithologies exhibit an apparent dichotomy in Re/Os ratios, which are significantly
lower in impact melt rocks and breccias (0.7 to 10.9)
compared to the target basalts (4.9 to 53.7). The
impactites with the highest Ir, Os, and Ru concentrations and the lowest Re/Os ratios consistently show PG
interelement ratios that are up to an order of magnitude
closer to the chondritic value compared to all other
analyzed rocks.
Measured 187Os/188Os isotope ratios of rocks from
the Lonar area range from slightly superchondritic to
highly radiogenic (0.1652 to 2.283), with impact melt
rocks and breccias exhibiting on average lower ratios
(ranging from 0.1652 to 0.6000).
Discussion: Lonar rocks analyzed in this study do
not define an isochronous relationship and exhibit considerable scatter on a Re-Os isochron plot with all
samples plotting above a 65 Myr reference isochron
[10], possibly arguing for post-crystallization disturbance. However, consistently lower Re/Os and near
chondritic 187Os/188Os ratios within most impact melt
rocks and breccias compared to the target basalts point
toward the presence of an extraterrestrial admixture in
the impactites. Modelling the relation between
187
Os/188Os ratios and Os concentrations in impact melt
rocks and breccias in comparison to the target rocks
has proven to be a robust means of quantifying small
meteoritic components and constraining the mass input
from impactors in impact rocks (e.g., [12]). As demonstrated in Fig. 2, our data for impact melt rocks and
breccias show a good fit to a calculated mixing curve
between a chondritic and basaltic end-member (see
Figure caption for details). If interpreted in terms of a
meteoritic component, some of the analyzed impact
melt rocks and breccias have the highest amount of
projectile contamination.
Although mafic targets generally complicate a PGE
and Re-Os isotope based search for an extraterrestrial
contamination in impactites (especially in the Lonar
area, where intense weathering and assimilation of
Archean basement might contribute to element mobility as well; [13]), we conclude that the our observations
can be best explained by the addition of up to ~0.02 %
of a chondritic component to some of the Lonar impact
melt rocks and breccias.
These results are comparable to magnitudes of meteoritic admixtures of impact melt rocks and breccias
from similar sized craters (e.g. [14]), but in contrast to
the enormously high meteoritic components of up to 17
wt.% postulated for (suspected) impact spherules from
the Lonar area [4]. A more detailed discussion based
on additional data is in progress.
1520.pdf
UCC
1
0.1
0
50
100
150
200
250
Os [ppt]
Fig. 2: Plot of 187Os/188Os ratios vs. Os concentrations
for target basalts and impact melt and breccias rocks. The
curves show mixing between two target basalts and a hypothesized impactor material of carbonaceous chondrite
composition [15]. For comparison, the upper continental
crust is also shown (star; data from [16]).
Acknowledgements: We than Ambre Luguet, David van Acken and Wencke Wegner for their contributions to the data acquisition and interpretation.
References: [1] Koeberl (2014) Treatise on Geochemistry; [2] Jourdan et al. (2011) Geology 39, 671674; [3] Fredriksson et al. (1973) Science 180, 862864; [4] Misra et al. (2009) Meteoritics and Planetary
Science 44, 1001-1018; [5] Ghosh and Bhaduri (2003)
Indian Minerals 57, 1-26; [6] Osae et al. (2005)
Meteoritics and Planetary Science 40, 1473-1492; [7]
Cohen and Waters (1996) Analytica Chimica Acta 332,
269-275; [8] Birck et al. (1997) Geostandards Newsletter 20, 19-27; [9] Crocket and Paul (2004) Chemical
Geology 208, 273-291; [10] Allègre et al. (1999) Earth
and Planetary Science Letters 170, 197-204; [11]
Crocket et al. (2013) Journal Earth System Science
122, 1035-1044; [12] Lee et al. (2006) Meteoritics and
Planetary Science 41, 819-833; [13] Chakrabarti and
Basu (2006) Earth and Planetary Science Letters 247,
197-211; [14] Koeberl et al. (2014) Geochimica et
Cosmochimica Acta 58, 1229-1234; [15] Shirey and
Walker (1998) Annual Reviews Earth and Planetary
Science 26, 423-500; [16] Peucker-Ehrenbrinck and
Jahn (2001) Geochemistry, Geophysics, Geosystems 2,
1061, doi:10.1029/2001GC000172.