ARTICLE
Zhamanshin astrobleme provides evidence for
carbonaceous chondrite and post-impact exchange
between ejecta and Earth’s atmosphere
Tomáš Magna1, Karel Žák2, Andreas Pack3, Frédéric Moynier4,5, Bérengère Mougel4, Stefan Peters3,
Roman Skála2, Šárka Jonášová2, Jiří Mizera6 & Zdeněk Řanda6
Chemical fingerprints of impacts are usually compromised by extreme conditions in the
impact plume, and the contribution of projectile matter to impactites does not often exceed a
fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and
impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in
irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System
range and suggest a CI-like chondrite impactor. Δ17O values as low as −0.22‰ in irghizites,
however, are incompatible with a CI-like impactor. We suggest that the observed 17O
depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange
with atmospheric oxygen (Δ17O= −0.47‰) following material ejection. In contrast, com-
bined Δ17O–ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial
range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are
indicated.
DOI: 10.1038/s41467-017-00192-5 OPEN
1 Czech Geological Survey, Klárov 3, Prague 1 CZ-118 21, Czech Republic. 2 Institute of Geology of the Czech Academy of Sciences, v.v.i., Rozvojová 269,
Prague 6 CZ-165 00, Czech Republic. 3 Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Universität Göttingen, Goldschmidtstraße 1, Göttingen
D-37077, Germany. 4 Institut de Physique du Globe de Paris, Université Paris Diderot, 1 rue Jussieu, Paris F-75005, France. 5 Insitut Universitaire de France,
Paris F-75005, France. 6 Nuclear Physics Institute of the Czech Academy of Sciences, v.v.i., Husinec-Řež CZ-250 68, Czech Republic. Correspondence and
requests for materials should be addressed to T.M. (email: tomas.magna@geology.cz)
NATURE COMMUNICATIONS |8:  227 |DOI: 10.1038/s41467-017-00192-5 |www.nature.com/naturecommunications 1
Large-scale planetary collisions and smaller impacts areimportant processes that may add to the chemical makeupof the bodies in the Solar System and shape the planetary
surfaces by forming variably sized craters, layers of impact ejecta,
and shock deformation features. Some meteorites, in particular
carbonaceous chondrites, contain high concentrations of organic
matter and volatiles like water, and are considered to potentially
bear on the emergence of oceans and life1. The identification of
projectile types for impacts is thus central for tracing the origin
and aftermath of collisions. In general, chondrites are regarded as
the most frequent projectiles in larger impacts2 (see also ref. 3 for
the recent summary of meteorite types and individual numbers of
specimens). High thermal energy that is released in impacts leads
to almost complete melting and/or evaporation of the impactor as
well as large amount of the target rocks, and may also change the
behavior of the elements and their chemical compounds4, making
the identification of the projectile difficult.
Tektites (distal ejecta) and other impact-related glasses (part of
proximal and fallback ejecta) are natural glassy materials which
are genetically related to hypervelocity impacts of large extra-
terrestrial bodies on the Earth’s surface. They are produced by
intense melting and partial evaporation of the target materials
during the impact event, and may bear traces of the impactor5, 6.
Tektites are found as variably shaped and sized objects; typically,
they are of splash-form shapes with a size of few centimetres.
They usually are interpreted as being formed from the uppermost
layers of the target lithologies, i.e., unconsolidated sediments and
soils, partly vaporized and partly melted and ejected with a high
velocity from the boundary zone between the impactor and the
Earth’s surface by jetting7. They are mostly silica-rich and with
extremely low contents of volatile components (H2O, C, S,
halogens, etc.). In fact, tektites belong to the most volatile-
depleted natural materials on Earth, often with< 100 p.p.m.
H2O8–10 and low contents of other volatiles (<30–40 p.p.m. C)11.
There is not an agreement on the exact formation mechanisms of
tektites and several concepts have been published4, 12–14. A
general model of tektite formation and volatile loss12, 14, 15
developed the original ideas16 considering possible mechanisms
of fragmentation of the overheated tektite melt by separating
vapor phase17, 18. These ideas were further developed for tektites
of the Central European tektite field (moldavites)4. This latter
conceptual model of formation of tektites considers two main
pathways of the behavior of the ejected supercritical fluid during
adiabatic decompression: (i) the supercritical matter approaches
the sub-critical vapor/melt boundary from the vapor phase sta-
bility field and most matter is vaporized, or (ii) the less energetic
part of the ejected matter reaches the vapor/melt boundary from
the melt stability field and a directly formed overheated melt is
fragmented due to separation of the vapor phase (cf. Fig. 1 in refs
17, 18). The melt fragmentation can strongly be amplified by the
escape of water-, carbonate- and/or organic matter-based volatiles
derived from the target, which were effectively lost from the
tektite melts9, 11. Immediate coalescence (accretion) of small melt
droplets into larger tektite bodies, probably underscored by for-
mation of late condensation spherules, appears to be the most
viable process which may explain the general chemical homo-
geneity of tektites, in parallel to their large micro-scale chemical
heterogeneity, and the general absence of condensation and/or
inner diffusion profiles in tektite bodies4.
Oxygen isotope (16O, 17O, 18O) ratios have been used to
classify asteroidal and planetary materials in the Solar System19
and O isotopes could thus be used to help identify the impactor.
Large mass-independent fractionation (MIF) of oxygen isotopes
has been recognized for different Solar System bodies most likely
inherited from MIF processes during early Solar System forma-
tion, whereas common terrestrial and other planetary materials
(e.g., Mars, Moon, Vesta) have 18O/16O and 17O/16O ratios which
are solely fractionated by mass-dependent processes20, 21. The
only isotopically anomalous components of Earth are found in
the atmosphere. Molecular oxygen has a distinctly negative Δ17O
anomaly22 (see Methods for definition of Δ17O) that is mainly
caused by biological proceses (i.e., photosynthesis, respiration),
and by minor effects of photochemical reactions in the upper
atmosphere23. Due to high temperatures of impact ejecta at the
time of formation and during atmospheric re-entry, O isotope
anomalies in tektites and related glasses could also be caused by
partial post-impact exchange with the Earth’s atmosphere24
although this process has not yet been reported for truly terres-
trial samples. In fact, small negative Δ17O offsets (as low as
−0.18‰) were reported for some tektites20 but it remains unclear
whether these low values could provide evidence for such a
process or, alternatively, for the presence of impactor material
with low Δ17O values.
The utility of chromium isotope (50Cr, 52Cr, 53Cr, 54Cr) sys-
tematics to characterize the impactor type was proven in several
cases, including the Chicxulub impact at the
Cretaceous–Paleogene boundary2. Distinct planetary bodies and
meteorite groups show resolved differences in the 53Cr/52Cr and
54Cr/52Cr ratios25, 26, underscored by particularly high Cr
abundance in chondrites, compared to the terrestrial continental
crust. Because individual chondrite groups have distinctive 17O–
54Cr isotope fingerprints3, combined O–Cr isotope measurements
may allow identification of impactor material and distinction
between impactor- or atmospheric exchange-related variations in
Δ17O.
To test the viability of O–Cr isotopes to constrain the impactor
type for tektites and impact-related glasses, we measured triple-
oxygen isotope compositions for a set of 14 impact-related tek-
tite-like glasses from the Zhamanshin astrobleme (basic splash
forms, and acid splash forms—irghizites, two chemically dis-
parate groups of impact-related glasses), and 14 Central European
tektites (moldavites), related to the Ries impact event in Ger-
many. Chromium isotope compositions were obtained for two
–0.5
–0.4
–0.3
–0.2
–0.1
0.0
0 5 10 15 20 25
Δ1
7 O
 (‰
)
δ18O (‰)
Fig. 1 Oxygen isotope systematics in tektites and impact glasses from this
study. Mixing lines between siliciclastic and mafic materials (δ18O= 10.0‰
and 7.0‰, respectively) and air O2 are indicated, bullets represent 10%
increments. Circles—moldavites; diamonds—irghizites; squares—basic splash
forms; reversed triangle—composite splash-form; brown field—the range of
common crustal rocks54 with revised data for San Carlos olivine76; green
field—the recalculated oxygen isotope compositions of other tektites20
(Australasian tektites, North American tektites). The oxygen isotope
compositions of the Earth’s mantle (star)76 and air (hexagon)22 are plotted.
Carbonaceous chondrite groups other than CI have more negative Δ17O
values (dashed arrow). The maximum 2 s.d. error bars for δ18O and Δ17O
are plotted
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impact-related glasses from Zhamanshin, one moldavite, and
terrestrial BHVO-2 reference basalt. The detailed analytical pro-
cedures for O and Cr isotope measurements and notations for O
and Cr isotope compositions are detailed in the Methods section.
All samples were previously characterized for major and trace
element systematics4, 27, 28.
Zhamanshin and Ries differ in the size of impactor (estimated
diameter of ~0.6 km vs. ~1.2–1.5 km)27, 29, paleogeography (dry
vs. wet area) and indications of the presence vs. absence of
extraterrestrial addition in some types of impactites. The Zha-
manshin impactor is interpreted to have been about one order of
magnitude lower in mass than that of the Ries impact event and it
probably was below the size limit necessary for production of
distal ejecta, i.e., true tektites. In addition, taking into account the
distribution of proximal ejecta, trajectories of both projectiles
differed; in case of Zhamanshin the impact must have been much
steeper27 than in case of a relatively shallow trajectory of the Ries
projectile12, 30. The target surface conditions of Zhamanshin and
Ries were also different. While the target of the Ries impact event
in the Miocene was a wet area near the limit of the Miocene
Upper Freshwater Molasse (‘Obere Süsswassermolasse’—OSM)
sedimentary basin31, probably densely overgrown by forests, dry
and possibly cold desert conditions are to be expected during the
Quaternary in central Asia27. Therefore, the quantity of volatiles
(derived from H2O, organic carbon, etc.) produced during the
Zhamanshin impact can be estimated to be more than one order of
magnitude lower than in the case of Ries. The compressed and
ejected material of shallow layers of Zhamanshin target thus did not
travel to a great distance. Instead, the glass droplets which formed
from disintegrated near-surface melts were falling back through the
explosion plume and collected small particles both of the
mechanically disintegrated impactor matter and condensed parti-
cles from the portion of the matter which was evaporated. These
glass droplets then coalesced together to form irghizites27, 28. In a
number of cases, the surface layers of primary droplets within
irghizites are strongly enriched in Fe, Mg, Cr, Co, Ni and P28, 32.
We explore several types of glasses from the Zhamanshin
impact structure (48°24’N/60°58’E), all belonging to the fallback
and proximal ejecta. The major focus of the study is on tektite-
like splash-form glassy objects sized usually up to several centi-
metres. Three major chemical types are distinguished: (i) SiO2-
rich (usually 69–76 wt.% SiO2) irghizites33, 34 which have com-
monly been formed by coalescence of< 1 mm glass droplets and
have elevated concentrations of Ni, Co and Cr27, 28, (ii) mor-
phologically rather uniform drops and their fragments composed
of more basic glass, termed ‘basic splash forms’ (53–56 wt.% SiO2)
27, 28, 32, 35–38, and (iii) composite splash forms, defined as
inhomogeneous acidic splash forms with abundant mineral
inclusions28. Impact glasses occurring in larger irregularly shaped
fragments and large blocks called ‘zhamanshinites’33 were not the
subject of this study because earlier studies excluded any presence
of the impactor matter in these objects28. We refer to the Sup-
plementary Note 1 for further details.
Tektites of the Central European strewn field (moldavites) are
genetically related to the Ries Impact Structure, Germany (48°
53’N/10°33’E), a complex type astrobleme with a diameter of ~26
km39. Unconsolidated sandy, silty and clayey sediments of the
OSM, probably covering most of the target area at the time of the
impact, are generally considered to be the dominant source
material of moldavites4, 13, 29, 30, 39–46. The origin of moldavites in
relation to the Ries Impact Structure has also been confirmed by
generally consistent ages of moldavites and impact-related glasses
from the Ries Impact Structure itself at 14.75± 0.20 Ma31, 47. The
nature of the impactor remains elusive48, 49 due to difficulties in
finding unequivocal extraterrestrial chemical signatures both in
moldavites and impact-related glasses50.
Results
Oxygen isotopes. The Δ17O values of moldavites (from −0.11 to
−0.07‰; Table 1) overlap with crustal rocks (Fig. 1) and the
range of δ18O values from 9.4 to 12.6‰ exceeds that reported
previously (from 10.7 to 11.9‰)41, 51–53. The larger variability in
δ18O likely is a consequence of analyzing a chemically more
variable suite of moldavites4, 29. The new data set confirms the
~3–5‰ offset in δ18O between moldavites and unconsolidated
OSM quartz-rich sands (Fig. 2), where the likely major moldavite
source material analogs such as bulk Miocene sediments from
southern surroundings of the Ries Impact Structure showed
distinctly higher and more variable δ18OSMOW values from 14.1
to 23.3‰ (ref. 41). The OSM sands, which are are assumed to be
the dominant source for moldavites4, 41, showed δ18O values
from 14.1 to 16.5‰ with an average of 15.3‰ (ref. 41) and the
predominance of sands in moldavites is also reflected in low MgO
and CaO contents and moderate δ18O (Fig. 3), excluding large
Table 1 Oxygen isotope composition of impact-related
materials from this study
δ18O (‰)a δ17O (‰) Δ17O (‰)a n
Tektites
SBM-11 11.18 5.83 −0.088 1
SBM-11 replicateb 9.62 5.02 −0.090 1
SBM-23 10.98 5.72 −0.080 1
SBM-35 12.46 6.50 −0.084 2
SBM-44 10.95 5.71 −0.086 2
SBM-88 12.57 6.55 −0.096 3
SBM-192 11.79 6.15 −0.085 1
MM-60 10.85 5.65 −0.092 2
MM-67 10.84 5.66 −0.070 1
CHBM-5 11.81 6.15 −0.086 2
CHBM-6 9.44 4.92 −0.078 1
CHBM-7b 10.13 5.30 −0.086 2
MCB-2 12.14 6.33 −0.091 2
LM-1 11.10 5.79 −0.085 2
MOLD-SBb 11.08 5.30 −0.105 1
MOLD-SB replicateb 10.45 5.79 −0.092 1
MOLD-SB replicateb 11.12 5.47 −0.095 1
Irghizites
IRG-IZ1 12.76 6.58 −0.170 2
IRG-IZ3 11.85 6.12 −0.145 2
IRG-IZ3 replicate 13.36 6.92 −0.141 1
IRG-IZ3 replicate 13.05 6.76 −0.135 1
IRG-IZ3 replicateb 12.98 6.76 −0.147 1
IRG-IZ9 14.42 7.42 −0.198 2
IRG-IZ9 replicateb 13.59 7.03 −0.201 1
IRG-IZ9 replicateb 13.49 7.01 −0.173 1
IR-2 14.24 7.33 −0.200 1
IR-4 12.30 6.33 −0.175 1
IR-8 12.49 6.44 −0.166 1
IR-9 12.09 6.18 −0.219 1
IR-11 12.90 6.63 −0.188 1
IR-12 12.10 6.22 −0.178 1
Basic splash forms
IRG-M4 8.01 4.17 −0.068 2
IRG-M4 replicate 9.18 4.76 −0.104 1
IR-10 8.47 4.42 −0.066 1
IR-13 8.26 4.30 −0.075 1
IR-14 8.56 4.45 −0.084 1
Composite splash forms
IR-7 7.03 3.63 −0.094 1
aEstimated measurement uncertainties are <0.3‰ and <0.008‰ for δ18O and Δ17O, respectively,
based on the typical reproducibility (1 s.d.) of results for the standard reference material NBS 28
throughout the analytical sessions, except for samples denoted with superscript ‘b’
bSamples analyzed with an improved extraction line (see text for details). Estimated measurement
uncertainties are <0.15‰ and <0.007‰ for δ18O and Δ17O, respectively, based on the typical
reproducibility (1 s.d.) of measurement results for San Carlos olivine
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volumes of carbonates in moldavite melts. The basic splash forms
from Zhamanshin show Δ17O values (−0.10 to −0.07‰) that also
overlap with crustal rocks. The Δ17O values measured for irghi-
zites (−0.22⩽Δ17O⩽ −0.14‰) are significantly lower than those
measured for basic splash forms, for the moldavites, and also
those reported for common crustal rocks (ref. 54 and discussion
therein; Fig. 1). This difference in underscored by distinct δ18O
values for basic splash forms and irghizites (8.0–9.2‰ vs.
11.9–14.4‰). The Δ17O= −0.09‰ for composite splash-form
IR-7 is at the low end of values for a given δ18O= 7.0‰ (Fig. 1).
Chromium isotopes. Irghizite IR-8 and composite splash-form
IR-7 display a significant positive anomaly in 54Cr (ε54Cr up to
1.54) which is clearly outside the terrestrial range (Fig. 4) and
indicates extraterrestrial origin of a large part of Cr27, 28 in IR-8
and IR-7. This is different from the results for moldavite SBM-88
and basalt BHVO-2 (Table 2), which are indistinguishable from
typical terrestrial values26, 55, 56.
Discussion
Moldavites have uniform terrestrial Δ17O values (mean Δ17O=
−0.09± 0.02‰, 2 s.d.) that are consistent with common sedi-
mentary precursors and weathered residues4, 41 (Fig. 1). This
indicates insignificant contamination by isotopically anomalous
extraterrestrial matter. Neither is any exchange with air O2
obvious. The absence of any apparent evidence for impactor
material is underscored by Cr isotope data for the slightly Cr-
enriched57 moldavite SBM-88 with ε54Cr= 0.09± 0.07 (2 s.e.),
which is identical to the values reported for a range of pristine
terrestrial materials56 (Table 2). Because Cr is a sensitive tracer of
chondritic contamination in impact-related materials2, 58, the
new coupled 17O–54Cr isotope data support the lack of extra-
terrestrial addition for moldavites outside the currently achievable
analytical precision for these elements.
The consistent negative δ18O offset of ~3–5‰ found for
moldavites relative to the Ries area target rocks is extended for a
larger chemical range of moldavites, analyzed in this study (Fig. 2;
this study and refs. 41, 51), and requires mass-dependent 18O
depletion (Fig. 1). This offset can generally be interpreted by
either addition of a low-δ18O component, or additional mass-
dependent O isotope fractionation during tektite formation41, 59.
To explain this several per mil offset, incorporation and isotope
homogenization of ~22 vol.% of a low-δ18O water (−10‰) partly
filling ~40–45 vol.% of pore spaces of the OSM sands, followed by
conversion of the matter to plasma and its condensation back to a
silicate glass was advocated13, 41. However, to shift the δ18OSMOW
of the moldavite parent mixture from the OSM sand average
value of 15.3‰ to the moldavite average value of 11.5‰ would
require addition (and full isotope homogenization) of a much
higher proportion of meteoric water. The mid-Miocene climate of
this part of Europe was significantly warmer than modern cli-
mate60, 61 and the average δ18O values of Miocene meteoric water
were also higher, most probably in the range −5.6± 0.7‰ (ref. 62).
This higher δ18O value would further increase the necessary
proportion of fully homogenized meteoric water to unrealistic
levels at ~30–40 vol.% of the mixture.
The Fig. 2 clearly shows that the carbonate-rich OSM sediment
types could not have been an important component in the mol-
davite parent mixture. The OSM sands and clayey sands, with
only a minor carbonate component, are much more suitable with
respect to both major element chemistry and O isotope compo-
sition. Correlations between the major element contents and δ18O
are generally poor (Fig. 3). The dolomitic carbonate, present as a
minor clastic component in some types of OSM sands4, was
clearly present but only at quantities of a few per cent, at
4
8
12
16
20
24
28
30 50 70 90
δ1
8 O
 (
‰
)
SiO2 (wt.%)
Moldavites (new)
Moldavites41,51
Marly sands41
Clays41
Sands41
Fig. 2 New and published δ18O values vs. SiO2 contents in moldavites and
sediments from the Ries area. The wider range in δ18O values of moldavites
from this study is due to analyzing chemically more variable specimens29
compared to those analyzed earlier41, 51. Silica contents in some earlier
analyzed moldavites51 were reported elsewhere83. Available combined
SiO2–δ18O data for chemically diverse Ries area sediments41 are plotted.
Clays and marly sands can represent only a small proportion of parental
material to moldavites while SiO2-rich sands dominate the budget4, 13, 41.
The consistent δ18O offset of ~3–5‰ between moldavites and possible
parental sediments from Ries area appears to be a combination of several
aspects, explored in detail in the main text. The maximum 2 s.d. error bars
for δ18O (±0.4‰) and SiO2 (±2%) are plotted
8.0
9.0
10.0
11.0
12.0
13.0
14.0a
b
0.0 1.0 2.0 3.0 4.0
δ1
8 O
 (
‰
)
MgO (wt.%)
8.0
9.0
10.0
11.0
12.0
13.0
14.0
δ1
8 O
 (
‰
)
0.0 1.0 2.0 3.0 4.0
CaO (wt.%)
5.0
Fig. 3 New and published δ18O values vs. MgO a and CaO b in moldavites.
The wider range in δ18O values from this study (yellow circles) is not
reflected in distinctive major element compositions4, compared with other
published data51, 83. The 2 s.d. error bars are plotted
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maximum63. Such a small admixture of high-δ18O carbonate was
thus only one of several factors controlling the δ18O range of
moldavites. The carbonate as an important source component is
also ruled out from high 87Sr/86Sr ratios in moldavites42, 64,
which indicate that a major part of Sr in moldavites is derived
from silicates rather than from clastic carbonates. Similarly, the
documented mass-independent effects65 during high-temperature
decomposition of carbonates would have only a limited effect on
Δ17O of moldavites as evidenced by the measured values which
are not shifted from the common crustal range.
Therefore, it can be assumed that the observed δ18O values of
moldavites resulted from a combination of several factors: (i)
mixing of quartz, clay, carbonate and some other minor compo-
nents in varying proportions; (ii) addition of ~10–30 vol.% of water
into the parent mixture; and (iii) isotope fractionation between the
molten glass and separated volatiles at temperatures above 1200 °C.
These findings are also supported by moderate to significant losses
of highly and moderately volatile elements4, 13, 41, 66–68, while less
volatile elements appear to have been quantitatively transported
into moldavite melts without accompanying isotope fractiona-
tion57, 63, 69. Unfortunately, the O isotope fractionations for the gas
phase-melt systems are not calibrated in the temperature range of
interest (i.e., between the melting temperature of moldavite glass
and that of lechatelierite, generally above 1200 °C). By projection of
the available fractionations70 into this high-temperature region it
can be assumed that escaping volatiles (mainly CO, CO2) should
have their δ18O values several tenths or a few per mil units higher
than the silicate melt, shifting the residual melt δ18O to a lower
values.
In contrast to a resolved difference in δ18O between moldavites
and their inferred parent materials (this study and refs. 41, 51),
overlapping O isotope compositions were found for the Ivory
Coast tektites, and their likely parental metasedimentary rocks
and granitic dykes in the Bosumtwi impact structure, Ghana71.
Therefore, the magnitude of the δ18O shift observed during
tektite formation can perhaps be related to the quantity of
volatile-bearing species (water, carbonates and organic matter)
present in the target area. Such an observation implies that solely
the transformation of target materials into tektites does not
generate mass-independent effects and other pertinent processes
are required to modify Δ17O.
Unlike the constrained crustal-like Δ17O values reported for
moldavites, oxygen isotope systematics of irghizites and splash
forms from Zhamanshin show several peculiar traits. The most
appealing feature of the data set are the uniformly low Δ17O
values in the irghizites (⩽ −0.14‰) relative to basic splash forms
(⩾ −0.10‰) while, at the same time, irghizites display higher
δ18O values (Fig. 1). The difference in chemical composition
between irghizites and basic splash forms reflects differences in
the composition of target materials in Zhamanshin area28. Basic
splash forms sampled deeper-seated lithologies such as andesitic
volcanic rocks and basement tuffs, while irghizites were formed
from surface sands and clays27, 28, 72, evidenced also by distinct
Sr–Nd isotope systematics64 and cosmogenic 10Be identified
solely in the irghizites, which suggests sampling of surface
material73. This provides unequivocal evidence for the depth
segregation limiting the interaction between impactor and target
rocks for Zhamanshin impact materials but does not explain the
difference in Δ17O between basic splash forms and irghizites
(Fig. 1). The observation is discussed in terms of two different
scenarios: the anomaly is due to admixture of anomalous oxygen
from the extraterrestrial projectile, or the impact melts may have
exchanged with isotopically anomalous air oxygen in the after-
math of the impact. We assume a δ18O≈ 10‰ for the end
member target rock of irghizites, which is compatible with their
sedimentary siliciclastic source74.
The low Δ17O in irghizites could be caused by contamination
with anomalous (i.e., Δ17O< −0.5‰) oxygen from the projectile.
While both chondritic and iron meteorites were proposed as
possible impactors for Zhamanshin27, 28, 36, in the context of an
impactor origin of the O isotope anomaly in irghizites, oridinary
chondrites and iron meteorites would be excluded as impactor.
Addition of ~2–10% oxygen from a carbonaceous chondrite
impactor would explain the O isotope systematics of irghizites but
most classes of carbonaceous chondrites were excluded on the
basis of siderophile element systematics28. The abundances of
highly siderophile (HSE—Os, Ir, Ru, Pt, Pd, Re) and moderately
siderophile (MSE—Ni, Co, Cr) elements as an indicator for
impactor contribution differ greatly between irghizites and basic
splash-forms28. In particular, Ni, Co and Cr are enriched in
irghizites relative to basic splash forms (up to ~2000 p.p.m. Ni in
the former)27, 28, 36, suggesting a resolved addition from a Ni-rich
chondrite. To qualify the impactor more precisely, the unique
54Cr excess of 1.54ε units found for irghizite IR-8 (Table 2 and
Table 2 Chromium isotope systematics of impact-related
materials from this study
Cr (p.p.m.) ε53Cr 2 s.e. ε54Cr 2 s.e. n
Irghizites and splash forms
IR-8 213a 0.31 0.03 1.54 0.08 4
IR-7 122a 0.15 0.05 0.62 0.10 2
Tektites
SBM-88 50b 0.01 0.04 0.09 0.07 4
Terrestrial reference materials
BHVO-2 280c 0.02 0.03 0.07 0.07 2
The letter n represents the number of individual runs where each single run consists of four
successive measurements in static mode with isotopes shifted by one mass unit in the Faraday
detectors
aData from ref. 28
bData from ref. 57
cCertified value from US Geological Survey
–5.0
–3.0
–1.0
1.0
3.0
–1.2 –0.8 –0.4 0 0.4 0.8 1.2 1.6
ε54Cr
Δ1
7 O
Fig. 4 Δ17O vs. ε54Cr in impact glasses and tektites from this study
compared with chondrites, achondrites and large planetary bodies. The plot
is modified from refs. 3, 26, 58 with new data from this study plotted as
yellow symbols (circle—SBM-88; reversed triangle—IR-7; diamond—IR-8),
with 2 s.e. uncertainties plotted. For oxygen, the error bars are smaller than
the corresponding symbol size. Different hues of orange from light to dark
represent different classes of carbonaceous chondrites (CI, Tagish Lake,
CR, CB, CM, CV, CO, CK—red spot), the single point with high Δ17O ~2.5‰
represents Rumuruti chondrites. Different hues of blue from light to dark
represent ureilites (farthest from Earth), mesosiderites–HED
meteorites–pallasites–IIIAB irons group, ordinary chondrites, aubrites,
angrites and enstatite chondrites (near-identical to Earth). Triangle—Mars;
star—Earth; crescent—Moon
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Fig. 4) represents the highest anomaly ever observed in any ter-
restrial sample, including those with resolved meteoritic addi-
tion58, and among the highest anomalies ever reported for any
Solar System material26, 56. This high ε54Cr falls in the field
intermediate between CI-type chondrites and the ungrouped
Tagish Lake chondrite. The observed Cr isotope composition
clearly is inherited from the impactor and disqualifies most car-
bonaceous chondrites as an impactor for the Zhamanshin
astrobleme. A simple model (Fig. 5) demonstrates that 54Cr
systematics in Zhamanshin impact glasses can be reproduced by
mixing with 2–7% of CI chondrite having 2650 p.p.m. Cr and
ε54Cr= 1.7 (refs. 56, 75). It also requires two different target
lithologies with low (IR-8) and intermediate (IR-7) Cr abundance,
fully consistent with recent investigations27, 28 and oxygen isotope
constraints (this study). At present, combined 54Cr–17O isotope
data are only collected for a limited number of individual car-
bonaceous chondrites and the possibility of the existence of
chondrites with yet unaccounted 54Cr–17O isotope systematics is
not eliminated. For example, ~5% admixture of a hypothetical
Ni-rich carbonaceous chondrite with ε54Cr= 1.6 and Δ17O of ~
−2‰, which would fall close to the field of CR carbonaceous
chondrites, would explain the observed composition of irghizites
without a need for isotope exchange with Earth’s atmosphere,
discussed below. However, a meteorite with such an isotope
signature has not yet been reported and 54Cr isotope signature in
particular represents a serious constraint for tracing a suitable
impactor type.
The Earth’s atmospheric molecular oxygen carries a Δ17O
value of −0.47‰ (ref. 22); with a correction of δ17O (see ref. 76 for
discussion). At high temperatures, a limited extent of mass-
dependent isotope fractionation is expected between molecular
O2 and silicate melt so that the composition of the irghizites may
represent a mixture of the target rocks and air O2 (Fig. 1). In such
a scenario, 20–35% of the oxygen in the irghizites would have
equilibrated with air O2 while it would be generally< 10% for
basic splash forms. This scenario would account for the low Δ17O
in irghizites compared to basic splash forms. It also supports the
recent models of formation of impact glasses from Zhamanshin27, 28,
in which small particle-sized irghizites represent the last phase of
fallback deposition, thus allowing for the extended period of time
in contact with a progressively cooling and collapsing vapor
cloud, in comparison with larger particles such as basic splash-
forms77. A detailed petrographic and microchemical study con-
firmed that mm- to cm-sized irghizite bodies frequently formed
by coalescence of ~1 mm large glass droplets, a feature not
observed for basic splash forms28. A surface layer 0.1 mm thick
represents ~50% of the volume of such a sphere. Iron in interior
parts of these glass spheres is in Fe2+ form whereas more oxidized
iron is found in surface layers. This is indicated by the observa-
tion that rims of the spheres display lower analytical totals and
higher total Fe contents than their interiors. In particular, the
analytical totals for the rims vary between 97 and 99 wt.% and
FeOtot ranges from 8.6 to 10.4 wt.%, while the inner parts of the
spheres show analytical totals of ~100 wt.% with FeOtot ranging
from 5.6 to 7.0 wt.%. The balancing of the analytical totals in rims
requires some 30–50% of total iron in rims to be present as Fe3+.
Apparently, lower analytical totals associated with Fe enrichment
in the rims suggest the origin of rims in a more oxidizing
environment than for interiors of spherules, indicating exchange
with the atmosphere24. We note that the measured Δ17O data are
from bulk homogenized irghizite samples and that in-situ Δ17O
values would likely vary strongly within individual irghizite
bodies. The model of slow gravitational settling77 and chemical
and isotope exchange with a slowly collapsing vapor plume
cannot be applied to moldavites (distal ejecta) due to their
instantaneous dislocation from the area of origin.
Combining O and Cr isotope data allow both classifying the
impactor type and pinpoiting the process, which gives rise to the
unsually low Δ17O in irghizites. The Δ17O of both CI carbo-
naceous chondrites and Tagish Lake chondrite is close to the
terrestrial fractionation line. The Cr concentration in irghizite IR-
8 is 213 p.p.m., whereas it is presumably low (~20 p.p.m.) in
possible target lithologies28. The 54Cr systematics indicates that
>90% of Cr is derived from the impactor (Fig. 5), and the general
54Cr systematics disqualifies most types of carbonaceous chon-
drites (Fig. 4). Mixing a target material (δ18O= 10‰; Δ17O=
−0.09‰) with a CI-like impactor having δ18O= 16.3‰ and
Δ17O= 0.2± 0.2‰ (ref. 78) (Tagish Lake chondrite has a similar
composition) would not result in the observed low Δ17O of the
irghizites. Therefore, we conclude that exchange with atmo-
spheric O2 in a vapor plume could lead to the observed low Δ17O
in irghizites. Apart from rare sulphates79 and fossil biominerals80,
81, this makes irghizites of the Zhamanshin astrobleme one of the
few terrestrial materials in the geological record that carries traces
of the isotope anomaly of air O2.
Methods
Oxygen purification and isotope measurements. The procedures for the oxygen
extraction and high-precision triple-oxygen isotope measurements followed the
methodology outlined elsewhere54. In brief, ~2 mg of a powdered sample were
reacted with fluorine gas at a low pressure (~20 mbar) and heated with a CO2-
based laser. Excess fluorine was removed by hot NaCl and chlorine was subse-
quently trapped with liquid nitrogen cold trap. Oxygen liberated from analyzed
samples was collected on 5 Å molecular sieves and cleaned by gas chromatography
for mass spectrometry analysis. For some samples, oxygen was extracted using an
improved protocol that applies BrF5 as the fluorinating agent (details of this
protocol are given elsewhere76). All three oxygen isotopes were measured using a
Finnigan MAT 253 gas source mass spectrometer, housed at the Göttingen Uni-
versity. The results are reported in δ17O–δ18O (‰) notation and Δ17O (‰) values
relative to VSMOW2 reference material. The Δ17O is defined relative to a reference
line with the slope of 0.5305 and zero intercept54 as Δ17O= 1000ln(δ17O/1000 + 1)
− 0.5305 × 1000ln(δ18O/1000 + 1), with San Carlos olivine having a Δ17O=
−0.05‰ (ref. 76). The 1 s.d. reproducibility of the measurements was always better
than 0.3‰ for δ18O and 0.01‰ for Δ17O values, respectively, based on repeated
1
3
10
10
5
Cr (p.p.m.)
ε5
4 C
r
0.0
0.4
0.8
1.2
1.6
2.0
10 100 1,000
Fig. 5 Simple binary mixing model for impact glasses from Zhamanshin
with CI chondrite impactor. CI chondrite with 2650 p.p.m. Cr and ε54Cr=
1.7 (yellow circle with a red rim; see main text for data sources) was mixed
with different target materials, a Cr-poor (20 p.p.m.) composition for
irghizite IR-8 (diamond) and intermediate-Cr (70 p.p.m.) composition for
composite splash-form IR-7 (reversed triangle), reflecting their different
sources28. The numbers along the calculated mixing lines indicate a
proportion of the impactor matter in the mixture. The 54Cr results are
identical to 53Cr results (not shown), supporting the robust nature of the
mixing process. The nil effects for moldavite SBM-88 (yellow circle with a
green rim) and terrestrial standard BHVO-2 (star) are documented. The
errors for ε54Cr values are 2 s.e. (see Table 2), the errors for Cr
concentrations (1–3% RSD)28 are smaller than the corresponding symbol
size
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00192-5
6 NATURE COMMUNICATIONS |8:  227 |DOI: 10.1038/s41467-017-00192-5 |www.nature.com/naturecommunications
analyses of reference quartz NBS-28 (National Bureau of Standards, USA) and San
Carlos olivine.
Chromium purification and isotope measurements. The procedures for the Cr
isotope measurements were performed at the Institut de Physique du Globe de
Paris, France. The two irghizite samples (IR-7 and IR-8), one moldavite sample
(SBM-88) as well as the basalt BHVO-2 (Hawaii, USA) were dissolved in a mixture
of concentrated HF and HNO3 in Teflon bombs at 140 °C for several days until
complete dissolution of all phases. Chromium was purified by cation exchange
chromatography following the method described elsewhere55. The 53Cr/52Cr and
54Cr/52Cr isotope ratios were measured using a Triton thermal ionization mass
spectrometer (Fisher Scientific, Bremen, Germany) following the method described
elsewhere26. Briefly, samples were loaded in chloride form on the outgased W
filaments together with an Al-silicagel–H3BO3 emitter. Each run comprised 20
blocks of 20 cycles and each single run was a combination of three successive
multi-collection measurements in static mode. The measured 53Cr/52Cr and 54Cr/
52Cr ratios are reported in ε53Cr and ε54Cr notation (1 in 10,000 difference relative
to NIST SRM 3112a reference material) and were normalized using an exponential
law to 52Cr/50Cr= 19.28323 (ref. 82). Each sample was measured two to four times
(Table 2).
Data availability. All relevant data are available from the authors on request and/
or are included with the manuscript in the form of data tables and data within
figures.
Received: 1 September 2016 Accepted: 8 June 2017
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Acknowledgements
This study was supported by the Czech Science Foundation project 13-22351S.
F.M. thanks the ERC under the European Community’s H2020 framework program/ERC
grant agreement #637503 (Pristine), the ANR for a chaire d’Excellence Sorbonne Paris
Cité (IDEX13C445) and the UnivEarthS Labex program (ANR-10-LABX-0023 and
ANR-11-IDEX-0005-02). Part of this work was supported by IPGP multidisciplinary
program PARI, and by Region île-de-France SESAME Grant no. 12015908.
Author contributions
T.M. and K.Ž. conceived the study. R.S., J.M. and Z.Ř. provided the samples. A.P., S.P., F.
M. and B.M. collected the data. Š.J. and R.S. characterized the microchemistry of key
samples. All authors contributed to data interpretation. T.M., K.Ž., A.P. and F.M. took
lead in writing.
Additional information
Supplementary Information accompanies this paper at doi:10.1038/s41467-017-00192-5.
Competing interests: The authors declare no competing financial interests.
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© The Author(s) 2017
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00192-5
8 NATURE COMMUNICATIONS |8:  227 |DOI: 10.1038/s41467-017-00192-5 |www.nature.com/naturecommunications