1
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|
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Unique evidence of fluid alteration
in the Kakowa (L6) ordinary
chondrite
I. P. Baziotis
1
*
, C. Ma
2
, Y. Guan
2
, L. Ferrière
3
, S. Xydous
1
, J. Hu
2
, M. A. Kipp
4
, F. L. H. Tissot
4
&
P. D. Asimow
2
Meteorites preserve evidence of processes on their parent bodies, including alteration,
metamorphism, and shock events. Here we show that the Kakowa (L6) ordinary chondrite (OC)
preserves both shock-melt veins and pockets of detrital grains from a brecciated and altered object,
including corundum, albite, silica, fayalite, forsterite, and margarite in a Pb- and Fe-rich matrix.
Preservation of the observed mineralogy and texture requires a sequence of at least two impacts:
first, a high-velocity collision formed the shock melt veins containing the high-pressure minerals
ringwoodite, wadsleyite, majorite, and albitic jadeite; later, a low-velocity impact formed fractures
and filled them with the detrital material. Oxygen and Pb isotope ratios suggest an OC origin for these
detrital minerals. Although fluid alteration is common in carbonaceous chondrites, the discovery
of margarite with an OC oxygen isotopic signature is novel. Kakowa extends both the impact and
alteration history of L6 ordinary chondrites in general.
Meteorites preserve evidence of the modifications that primitive solar system material experienced due to pro-
cesses such as thermal metamorphism, fluid alteration, and shock damage on their parent bodies. The most
direct evidence for the action of liquid water is the preservation of secondary hydrous minerals, which have so far
mostly been documented in carbonaceous
chondrites
1
. In particular, the oxidized subgroup of CV carbonaceous
chondrites is known to contain margarite, vesuvianite, and
kaolinite
1
,
2
. In ordinary chondrites (OCs), the only
hydrous secondary phase noted by
Brearley
3
is fine-grained Fe-rich smectite in the unequilibrated meteorites
Semarkona (LL3.00) and Bishunpur (LL3.15). The unequilibrated chondrite Tieschitz (H/L3.6) hosts a sodic-
calcic amphibole indicating fluid metasomatism at or close to the peak of thermal
metamorphism
4
. In more
equilibrated OCs, phyllosilicates are even more rare or totally absent, however phases other than phyllosilicates
do indicate alteration in these objects. Metasomatic processes are recorded in OCs from types 3.6 to 3.9 by the
presence of sodalite, scapolite, and nepheline; and from types 4.0 to 6.0 by albite and K-bearing
feldspar
5
.
Many OCs preserve records of impact events due to collision(s) among their parent
asteroids
6
–
10
. Such mete-
oritic impact records help to constrain the shock conditions and hence parameters of impact events such as
encounter velocity and the sizes of impactors and targets. In turn, the co-evolution of planetesimal sizes and their
orbital excitation can distinguish among scenarios for the early evolution of the solar
system
11
. Shock parameters
can be inferred from several lines of evidence, including brecciation, deformation in minerals, and the presence
and textural features of melt veins (MVs) that often contain high-pressure (HP)
minerals
12
–
20
. A notable group
of meteorites known as polymict breccias contain fragments of multiple objects, presumably derived from both
impactor and target of one or more collisions and reassembled as rubble
piles
21
. Although such breccias are not
unusual, they typically represent low-velocity collisions; polymict breccias from impacts fast enough to form HP
minerals are
uncommon
22
,
23
. Although collisions were most common in the early evolution of the solar system,
there is strong evidence that the L chondrite parent body was disrupted by a major collision at 470
Ma
24
,
25
, result-
ing in debris that continues to dominate the current flux of meteorites to the
Earth
26
.
Here, we report new data on the historical fall Kakowa, an L6 ordinary chondrite that fell in Romania on May
19th 1858 and was collected within minutes while, according to historical records, still
hot
27
. Kakowa is consid-
ered to be of shock stage S4–S5 (Fig.
1
). We studied its texture, mineralogy, and mineral composition by optical
and electron microscopy, electron probe microanalysis (EPMA), micro-Raman spectroscopy, and electron back-
scatter diffraction (EBSD). In addition, we also acquired in situ oxygen isotope ratios of some mineral phases
OPEN
1
Agricultural University of Athens, Iera Odos 75, 11755
Athens, Greece.
2
Division of Geological and Planetary
Sciences, California Institute of Technology, Pasadena, CA
91125, USA.
3
Natural History Museum Vienna, Burgring
7, 1010 Vienna, Austria.
4
The Isotoparium, Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, CA 91125, USA.
*
email: ibaziotis@aua.gr
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by secondary ion mass spectrometry (nanoSIMS) and Pb isotope ratios by multi-collector inductively-coupled
plasma mass spectrometry (MC-ICP-MS). Our studies document, first, that Kakowa (like many L6 meteorites)
contains HP phases, concentrated in and adjacent to melt veins, that require a strong shock to form. Second, we
document pockets containing a series of novel minerals, including hydrous phases, that appear to be exogenous
to the L6 host rock and were probably emplaced in fractures during a subsequent low-velocity collision. We use
the term “exogenous” to indicate material that appears to have been added to the rock late in its history.
We argue that the low-velocity collision must have occurred after the high-velocity collision (though we have
no constraint on the interval between the two events), as the hydrous phases in the exogenous material would
not have survived a strong shock event. The assemblage of HP minerals and the sizes of their host MVs yield
constraints on the pressure–temperature–time conditions of the strong shock experienced by Kakowa and con-
tribute to the shock record of the L chondrites in general. It is likely, based on literature data from L chondrites,
that the strong shock recorded by Kakowa was due to the large collision event that disrupted the L chondrite
parent body at
~ 470
Ma
24
,
25
, in which case the low-velocity impact would represent the continuing collisional
evolution of the resulting asteroid family after this time. Moreover, the hydrous phases in the exogenous material
indicate that altered material was present in the OC-hosting region of the solar system at this late stage.
Results
Petrography: groundmass-melt veins—fracture.
The gross petrography of Kakowa may be divided
into chondrules, groundmass, melt veins, and fracture fill. In the groundmass, olivine grains show strong mosai-
cism and planar deformation features. A large fine-grained chondrule (6.1 mm in diameter) and a porphyritic
chondrule (3.5 mm in diameter) dominate the studied section (NHMV-N6231); the porphyritic chondrule is
bisected by the thickest MV. The three main sub-parallel MVs and minor MVs with other orientations are pre-
Figure 1.
(
A
) Macrophotograph of Kakowa meteorite (NHMV-A557) with a straight melt vein cross-cutting
the groundmass. (
B
) Back-scattered electron (BSE) image mosaic of Kakowa section (NHMV-N6231) showing
the studied areas within the different melt veins (MVs). A dashed white rectangle shows the area that contains
the pockets of exogenous material. Notable are the two large chondrules (delineated by fine white lines), one of
them clearly cross-cut by MV3.
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sumed to be the result of one shock event (Fig.
1
); they are predominantly in contact with olivine but occasion-
ally also with pyroxene and metal grains. The width of the thickest MV is nearly constant (from
~ 300 to 360 μm)
across the section surface, while the thinner MVs are variable in width. The MVs consist of glass, silicate clasts
(olivine, pyroxene, and plagioclase), sulfides, chromite, and Fe–Ni metal. The thick MVs are zoned from glass-
bearing rims, through metal-rich crystalline layers, to silicate clast-rich cores (Figs.
2
,
3
). The fractures do not
show any systematic structural relation with the host rock (Figs.
3
,
4
), however, one fracture system (about 1 cm
in overall length) crosscuts the melt vein and bifurcates to surround a composite clast of Fe–Ni metal and sulfide
(Fig.
3
A). The fractures are mostly void space, but filling was observed in three pockets surrounding the compos-
ite clast. The fill contains a fine-grained matrix hosting angular phases (see “
Fracture fill
” section), likely formed
as a consolidated aggregate of loose detrital grains that filled open space in the fractures (Fig.
3
B).
Melt veins.
In three MV regions studied in detail, we observed the HP minerals ringwoodite, wadsleyite,
majorite, and albitic jadeite (Fig.
2
). Although the presence of MVs is obvious upon casual examination of the
Kakowa specimen, this is the first report of HP minerals in this meteorite.
MV1a.
The core of this melt vein is mostly a crystallized assemblage of majorite
+ ringwoodite
+ magnesio-
wüstite (Fig.
2
A; magnesiowüstite was identified by EBSD). Aggregates of fine-grained polycrystalline ring-
woodite plus wadsleyite occur locally as clasts in the MV. Along the margins of the MV, host-rock olivine is con-
verted to polycrystalline ringwoodite, followed outwards by olivine containing ringwoodite lamellae and then
by untransformed olivine. The ringwoodite zone in some places extends more than 25 μm into the host rock.
MV1b.
A Raman spectrum obtained from a
~ 2–7 μm long grain in the core of this MV (Fig.
2
B) displays the
characteristic major peak at
~ 927
cm
−1
reported from both synthetic and natural
majorite
15
,
28
,
29
. The EBSD pattern
collected from the same point reveals the garnet structure. EPMA analysis shows two populations of compositions
among the grains with these Raman and EBSD characteristics: (a) calc-aluminous majorite with up to 4.7 wt%
Al
2
O
3
, CaO in the range 1.6–2.4 wt%, and formula
Na
0.05–0.09
Ca
0.12–0.19
Mg
3.22–3.35
Fe
0.45–0.67
Al
0.21–0.38
Si
3.69–3.75
O
12
; and
(b) nearly end-member Fe–Mg majorite with formula
Ca
0.04–0.05
Mg
3.20–3.29
Fe
0.75–0.89
Mn
0.02–0.03
Al
0.01–0.02
Si
3.87–3.92
O
12
.
MV2.
Irregularly shaped felsic domains in this area, up to
~ 20 μm long (Fig.
2
C), mostly consist of feldspathic
glass but commonly contain sub-μm parallel lamellae of a crystalline phase (Fig.
2
D). The EPMA analysis of the
lamellae yields the formula
(Na
0.65
Ca
0.08
K
0.05
□
0.22
)(Al
0.81
Si
0.17
Fe
0.02
)Si
2
O
6
, with Ca# [100 × Ca/(Ca + Na)] of 10.5.
With 22% vacant M2 sites and 17% Si on M1, this is albitic jadeite, which is beam-sensitive, like in most other
published
cases
30
,
31
. The Raman spectrum of Kakowa albitic jadeite is characteristic of clinopyroxene structure,
with a distinct major peak at 698
cm
−1
and minor peaks at 201, 376, 387, 432, 521, 574, 988, and 1035
cm
−1
(Fig.
5
). The two peaks near 1000
cm
−1
, related to vibration of
[Si
2
O
6
]
4−
groups, are resolved but not as distinct
or well-separated as in the ideal jadeite spectrum. The Raman spectrum of near-endmember jadeite has major
peaks at 700, 991, and 1040
cm
−1
and minor peaks at 204, 375, 385, 433, 525, and 575
cm
−1
(RRUFF R050220.2),
which is an exceptionally good match to Kakowa even though our EPMA analysis plainly shows that the Kakowa
material has albitic composition. No EBSD pattern could be obtained from this beam-sensitive material.
Fracture fill.
The fractures are filled, in three pockets that we have identified, with exogenous material, com-
posed of angular grains of corundum + fayalite + forsterite + albite + margarite + silica + FeS (troilite) + Fe–Ni-
metal embedded in an Fe- and Pb-rich matrix (Figs.
3
,
4
). A series of energy-dispersive X-ray (EDS) analyses of
the fracture-fill matrix show it to be heterogeneous in composition. Idiomorphic to subidiomorphic, 2
× 3 μm,
bright crystals making up
~ 20 vol% of the fracture fill in some places are recognized as PbO. The adjacent
matrix contains more than 70 wt% FeO and up to
~ 5 wt% MgO. Corundum, albite, fayalite
(Fa
99–100
), and for
-
sterite
(Fa
25–26
) each occur as 10–20 μm anhedral and subhedral grains, many of them angular in shape. Mar
-
garite occurs as prismatic crystals, up to
~ 20 μm long, with a composition by EPMA that is very close to ideal:
Ca
0.97
Na
0.03
Fe
0.06
Al
3.94
Si
2.02
O
10
(OH)
2
(the hydroxyl is inferred here). The Raman spectra of these margarite grains
show distinct peaks at 395, 710, 898, 911, and 919
cm
−1
(Fig.
6
A), matching very well the major peaks of the
margarite reference spectrum at 392, 710, and 918
cm
−1
(RRUFF R060839). The identification of margarite is
further confirmed by EBSD (Fig.
6
B,C).
Pb isotope analysis.
Three spots (each 50–100 μm in diameter) were targeted: first we drilled a spot in the
silicate groundmass (Fig.
3
A) as an assessment of background Pb content, and then two spots were drilled in
the Pb-rich fracture-fill material (Fig.
3
A,E). We obtained two orders of magnitude more Pb from drilling the
exogenous fracture fill material than from the silicate matrix (Table S1). The Pb isotope ratios of the two spots in
fracture fill are the same within error (Table S2):
206
Pb/
204
Pb = 18.385,
207
Pb/
204
Pb = 15.615,
208
Pb/
204
Pb = 38.692
(Fig.
7
). This Pb isotope composition is consistent with either ordinary chondrite (e.g., Richardton (H5) and
Kunashak (L6)
32
) or terrestrial material (e.g., pelagic
clay
33
) but not with carbonaceous
chondrites
34
,
35
. Hence the
Pb isotope data do not help to resolve whether the Pb is a terrestrial contaminant. However, they do help to reject
the hypothesis that the fluid alteration responsible for the margarite happened on a carbonaceous body. Moreo-
ver, the data indicate a Pb isotope evolution for most of solar system history with a μ
=
238
U/
206
Pb ~ 9. Given the
extreme Pb concentration of the sampled material, the data show that the U/Pb fractionation involved in making
the fracture fill did not happen in early solar system history; it is consistent with an age of 470 Ma or less.
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Figure 2.
BSE images of MVs in Kakowa. (
A
) MV1a, showing ringwoodite (Rwt) (#1; Raman spectrum
MV1a-1 in Fig.
5
) in close association with wadsleyite (#2; Raman spectrum MV1a-2 in Fig.
5
). (
B
) MV1b,
with fine intergrowth of majorite-pyrope solid solution (Maj-grt) (#3, Raman spectrum MV1b-3 in Fig.
5
) and
magnesiowüstite (Mg-Wus). (
C
) MV2, hosting albitic jadeite (Jd) (#4; Raman spectrum MV2–4 in Fig.
5
). (
D
)
Glass of feldspathic composition in MV2 showing albitic jadeite lamellae. (
E
) Groundmass orthopyroxene (Opx)
(#5; Raman spectrum MV1b-5 in Fig.
5
) in contact with MV1b. The bands across the Opx are likely mechanical
twin planes (indicated by white arrows) due to
shock
5
. Further, at the contact with MV, Opx is transformed to
majorite (#6; Raman spectrum MV1b-6 in Fig.
5
) and olivine (Ol) is partly transformed to ringwoodite. (
F
)
MV2, Albitic jadeite in contact with majorite and ringwoodite.
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Figure 3.
BSE images of the occurrence of exogenous material in Kakowa. (
A
) Overview showing fracture along the edge of a metal-
sulfide clast, filled in part with exogenous brecciated material. “Spot 1” and “silicate spot” are areas drilled for Pb-isotope analysis. (
B
)
Fracture crossing the groundmass inducing brecciation of the host minerals. (
C
) A fracture that cross-cuts the majorite-rich region
of MV1b contains exogenous material including tabular crystals of margarite (Mar). (
D
) Enlargement of the margarite crystal; the
white squares indicate the location of nano-SIMS O-isotope analysis points. The white circle indicates the location of margarite Raman
spectrum given in Fig.
6
A. The white cross indicates the location of margarite EBSD analysis given in Fig.
6
C. (
E
) Corundum (Crn)
and albite (Ab) in the exogenous material form subhedral to anhedral angular crystals. Spot 2 is a second region of Pb-Fe-rich matrix
drilled for Pb isotope analysis. (
F
) Fayalite (Fa) in the exogenous material is also angular. (
G
) Enlargement of the patch of exogenous
fracture fill shown in panel (
A
). (
H
) Further enlargement of the same patch showing margarite and a silica (Sil) phase (lower right of
the figure) as well as the general texture of angular crystalline grains consolidated in a backscatter-bright matrix.
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Oxygen isotopic analysis.
In a single session on the nanoSIMS, we analyzed two standard olivines (ter
-
restrial San Carlos olivine and pallasite Eagle Station olivine), four spots on the groundmass minerals of Kakowa
(two olivine and two orthopyroxene points), and four spots on minerals in the exogenous detrital fracture fill
(two spots on corundum, one on albite, and one on margarite). The standard olivines match accepted values in
both δ
17
O and δ
18
O, and the groundmass analyses all plot precisely within the field defined by typical L ordinary
chondrite materials. This confirms that measurements during this session on the Kakowa polished section have
minimal systematic errors, though we cannot quantify possible matrix effects (because all minerals are calibrated
with an olivine standard). What remains in the evaluation of the exogenous phase data is the random error. The
measured O isotope ratios of the detrital phases—corundum, albite, and margarite—cluster around the same
OC-like region of triple oxygen isotope space as the matrix minerals (Fig.
8
). All four points plot above the
terrestrial fractionation line, but the 2σ errors bars on each of the four spots overlap the terrestrial fractiona-
tion line. Hence, we cannot say with confidence that any one of these analyses, in isolation, is chondritic rather
than terrestrial. However, the probability that these four spots are drawn at random from a terrestrial distribu-
tion can be assessed. A Monte Carlo calculation assuming a normal distribution for counting statistical error
on
16
O,
17
O, and
18
O shows that the four analyzed points plot on a mass fractionation line corresponding to
Δ
17
O = + 2.5
± 1.1‰. That is, the null hypothesis that the data are drawn from a terrestrial population is rejected
at the 2.3 sigma level. There is only a 1% probability of these data arising at random from a sample of terrestrial
material. The probability that they are carbonaceous chondrite material is even lower.
Discussion
A first, strong collision.
Like many of the L chondrites, especially the L6 meteorites, Kakowa shows clear
evidence of a strong shock event (Fig.
1
). It is generally classified as shock stage S4–S5 because of the presence of
maskelynite, shock microstructures in olivine (weak to strong mosaicism), and obvious melt
veins
36
,
37
(Fig.
1
B).
Here we document for the first time that this particular meteorite, Kakowa, contains an assemblage of preserved
high-pressure phases, found within the MVs. Their mineralogy and chemistry, alongside the physical width of
their host MVs, yield definite constraints on the parameters of the strong shock experienced by this particular
fragment of the L-chondrite parent body.
The occurrence of wadsleyite suggests pressure (
P
) greater than 13 GPa to at most 22 GPa, whereas ring-
woodite suggests an overlapping but slightly higher
P
range of 18–23 GPa; for both phases the pressure limits on
Figure 4.
Characterization of fine-grained material from exogenous fracture fill. (
A
) BSE image and (
B
) EDS
spectrum of PbO grain adjacent to margarite. (
C
) BSE image and (
D
) EDS spectrum of the Pb–Fe-rich matrix.
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their stability fields depend on temperature (
T
) and Fe content. The measured composition of majoritic garnet
is consistent with
P
in the range 17–20 GPa and
T
between 1800 and 2100 °C
38
. The coexistence of all three HP
phases allows for small-scale spatial or temporal heterogeneity of the
P
field.
The Raman spectrum from a Na-Si-rich glass of feldspathic composition (Figs.
2
D,
3
) suggests a jadeite-like
pyroxene, but EPMA analysis reveals that this material is not true, stoichiometric jadeite. With vacancies on
M2 and excess Si accommodated in the M1 site, this is albitic jadeite. Presently, the implications of formation of
albitic jadeite for shock
P
and
T
are uncalibrated; the well-known experimental stability field of true jadeite may
not be a useful guide. However, the state of the feldspars in Kakowa still yields some constraints on the shock
conditions. Setting aside the preservation question, the absence of lingunite suggests maximum
P
< 21 GPa and
the absence of Ca-ferrite, Ca-perovskite, or Ca-rich garnet suggests, at least locally,
P
≤ 15.5 GPa. The presence of
jadeite-like pyroxene near the center and ringwoodite at the rim of the widest MV again suggests likely temporal
P
gradients (e.g.
39
).
Concerning the time duration of the high-pressure pulse during the strong shock event, it is conventional to
assume that melt veins experience local heating above the liquidus of the host rock, followed by conductive cool
-
ing due to the lower temperature matrix along their walls. Moreover, if pressure is released before cooling below
the liquidus then HP phases will not be observed. In fact, temperature must drop well below the liquidus before
pressure release to ensure preservation of HP phases, which are metastable at ambient pressure. Thermal models
of MV cooling for the width of the widest vein—which hosts ringwoodite, majorite, and wadsleyite—suggest con-
ductive cooling times of 26–37 ms (for details see the “
Modeling strategy
” section). Preservation of ringwoodite
Figure 5.
Selected Raman spectra of HP minerals in Kakowa compared to reference spectra for ringwoodite
(RRUFF R070079), wadsleyite (RRUFF R090004), jadeite (RRUFF R050220.2), pyrope (RRUFF R080060), and
enstatite (RRUFF R040094-3).
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at the center of MV1 suggests temperature dropped below 1000 °C while P remained
> 18
GPa
20
. Wadsleyite can
grow at linear velocities
~ 1 m/s
40
, hence the observed wadsleyite crystal sizes only require the MV to spend a
few μs in the wadsleyite field before quenching. The duration of a high-pressure pulse is set approximately by
the ratio of the diameter of the smaller object involved in a collision to the encounter velocity, or by the two-way
shock travel time across the smaller body, whichever is
shorter
9
. A duration of at least
10
−3
s
41
, given that shocks
strong enough to reach peak
P
> 18 GPa travel through rock at velocity on the order of 5 km
s
−1
, suggests that the
smaller object involved in this collision had a diameter of at least several meters. It is difficult to provide an upper
bound on this diameter; hence this result is consistent with, but does not require, the hypothesis that the strong
shock resulted from the catastrophic disruption event at 470 Ma (which probably involved km-scale
objects
39
).
The presence of discrete veins indicates heterogeneity of the
T
field, likely the result of collapse of spatially
variable porosity during shock compression or slip along localized shear bands. It is likely an ill-defined exercise
to attempt to state a single global peak
P
or
T
condition for the meteorite, much less for the maximum conditions
experienced anywhere on the parent body during the associated impact event. Nevertheless, the conditions are
within the range inferred from studies of HP phases, melt veins, and textures, in other L6
chondrites
16
,
18
,
20
,
39
,
42
–
46
.
Figure 6.
(
A
) Selected Raman spectrum obtained from exogenous margarite in the fracture fill crosscutting
MV1b, compared to reference spectrum for margarite (RRUFF R060839). (
B
) Un-coated BSE image of the
margarite crystal shown in Fig.
3
D, during Electron back-scatter diffraction (EBSD) analysis. (
C
) EBSD pattern
indexed with margarite structure.
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Is the exogenous material in Kakowa of extraterrestrial origin?
We identified pockets of detrital
crystals and Pb-Fe-rich matrix filling fractures that cross-cut the melt veins formed by the strong shock (Figs.
3
,
4
). In principle, this fracture fill could have several sources. It could be derived from: (1) the same parent body as
Kakowa, (2) a different extraterrestrial object that collided with the parent body, or (3) from terrestrial contami-
nation. Given its consolidated detrital texture and its extended history, various parts of the fracture fill could be
Figure 7.
206
Pb/
204
Pb versus
207
Pb/
204
Pb data, plotted together with the geochron (4.55 Gyr), evolution curves
for Pb-rich matrix of Kakowa, and plausible meteoritic and terrestrial Pb reservoirs. The box shows the area
enlarged in the inset, where material plots if it has evolved over solar system history with μ
=
238
U/
206
Pb ~ 8.9.
Inset: a close-up view showing the
206
Pb/
204
Pb versus
207
Pb/
204
Pb data fields for Kakowa and selected terrestrial
Pb reservoirs—mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs), upper continental crust,
lower continental crust, pelagic
sediments
48
, and MVT-type Pb ore
deposits
49
–
52
—as well as a handful of OC
meteorites that plot in this region—Kunashak, Richardton and Forest City. Meteorite data sources: Canyon
Diablo troilite
(CDT
53
), ordinary and carbonaceous
chondrites
32
,
34
,
35
,
53
, and Kakowa (this study).
Figure 8.
Triple oxygen isotope ratio diagram with the reference lines CCAM (Carbonaceous Chondrite
Anhydrous Minerals, slope 1) and TF (Terrestrial Fractionation, slope 0.5)
54
,
55
, data for terrestrial and meteoritic
standards (
SCOL
San Carlos Olivine, terrestrial,
ESOL
Eagle Station Olivine, a pallasite meteorite), and data
from the Kakowa groundmass (olivine and enstatite) and the Kakowa exogenous fracture fill (corundum, albite,
margarite). Data for ordinary chondrites are also
plotted
56
. Error bars are 2σ. Δ
17
O is the vertical distance of a
point from the TF line in this plot.
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from more than one of these sources. Kakowa is a historical fall that was recovered within minutes of
landing
27
,
but to exclude terrestrial contamination (e.g., during sample preparation), we interrogated the origin of this
material by micromilling areas of the fracture-fill matrix for Pb isotope analyses and by nanoSIMS in situ triple
oxygen isotopic analysis of the exogenous phases. The Pb isotope results, as discussed above, are ambiguous and
serve only to exclude ancient Pb enrichment and carbonaceous chondrite sources for the fracture fill matrix. The
oxygen isotope results on the detrital grains are more significant. The groundmass minerals (enstatite, forsteritic
olivine) consistently lie near the OC range on the three-oxygen isotope plot, above the terrestrial fractionation
line (TFL). Neither the groundmass nor the detrital minerals plot on the carbonaceous chondrite anhydrous
minerals (CCAM) line. The Δ
17
O values of all the measured phases from Kakowa are indistinguishable, but as a
population, they are statistically separated from Δ
17
O = 0 (TFL). Therefore, the oxygen isotope results are most
consistent with the detrital minerals, like the Kakowa groundmass minerals, being from an ordinary chondrite
source (Fig.
8
). Although corundum could be associated with CAIs from carbonaceous chondrites, it is also pre-
sent in other meteorite
groups
47
and in the present case its O-isotope signature indicates an ordinary chondrite
origin. We cannot resolve whether the exogenous and native components of Kakowa are from different oxygen
reservoirs, but with some confidence we can exclude the hypothesis that the detrital phases are terrestrial or
carbonaceous chondrite in origin.
Perhaps the most distinctive feature of the exogenous material is the presence of the hydrous calcic mica
margarite. Margarite may form by hydration of anorthite, with or without
corundum
57
. In the absence of corun-
dum, the reaction yields excess
SiO
2
:
whereas in the presence of corundum, margarite can form without producing silica:
In the exogenous material in Kakowa, margarite coexists with both corundum and silica (Fig.
3
). Albite is
present but anorthite is not. Given the detrital mode of occurrence of these phases, we do not know whether
margarite formed in the presence of the phases with which it now coexists. It is very likely that a Ca-bearing
feldspar was the precursor, since calcic feldspars are both found as primary phases (Semarkona LL3.00
58
) and
in equilibrated ordinary chondrites. Studies show that anorthite is present at degrees of thermal metamorphism
up to L4 but only albite is found at L5 or
higher
59
,
60
. Finding margarite in an L6 is therefore puzzling, except
that we find it in an exogenous detrital fracture fill. The simplest explanation is that the fracture fill is derived
from ordinary chondrite material that experienced thermal metamorphism of stage 4 or lower as well as fluid
alteration. Moreover, the EBSD analyses of the exogenous material indicate well-crystallized minerals. This
indicates that margarite is not simply a product of low-temperature aqueous alteration, which would be expected
to yield poorly crystallized and fine-grained phases. Rather, the margarite indicates a two-stage process of low-
temperature hydration followed by thermal metamorphism and recrystallization. Such thermal processing may
have destroyed other phases that would be expected to develop during the aqueous alteration process (or these
phases may remain but be too small or poorly crystalline to characterize). Yet the olivine in the exogenous
material is heterogenous (forsterite and fayalite are present) and so is probably from a type 3 object that did
not experience such thermal metamorphism. Hence the exogenous material itself is a detrital juxtaposition of
ordinary chondrite-derived material with distinct histories, and not an equilibrated assemblage. The low-speed
impactor may itself have therefore been a polymict breccia.
In principle, the fluid alteration that formed the margarite might have occurred either before or after injection
of the exogenous detritus into fractures in Kakowa. However, there is no evidence (at the scale of one section)
for fluid infiltration into the Kakowa groundmass, chondrules, or melt veins. There is sufficient porosity that
fluids percolating through the exogenous material would likely have altered other parts of the sample as well,
if alteration followed injection. Hence, we prefer a scenario in which fluid alteration and subsequent thermal
maturation formed the margarite before its injection into fractures in Kakowa.
The fayalite and silica in the fracture fill are consistent with the sequence of events that we infer from the
large crystalline margarite grains. Silica in achondrites has been inferred to be deposited from water during
fluid
alteration
61
. Thermodynamic calculations then show that the assemblage of fayalite and silica in ordinary
chondrites reflects an initial event of low-temperature fluid alteration followed by thermal
maturation
62
.
Pb enrichment in fracture fill.
We do not, at this time, understand the mechanism of Pb enrichment
responsible for forming the PbO crystals and Pb-rich matrix of the fracture fill. Here we consider the plausible
options and constraints provided by our data. The first logical explanation for the source of lead is terrestrial
contamination, either before collection, during museum storage, or during preparation of the section. Our Pb
isotope data do not exclude a common terrestrial source for the Pb. However, we judge that the addition of
enough Pb to constitute several weight percent of the fracture fill during a few
minutes
27
between fall and col-
lection to be highly implausible. Following King et al.
63
, a century of museum storage may lead to oxidation of
minerals that are unstable in the oxidizing and water-bearing terrestrial atmosphere, such as FeS, Fe–Ni-metal,
or Pb sulfates. If Pb were already present in the assemblage, it could have formed Pb oxide during storage, but the
source of Pb would still likely have been extraterrestrial. The only scenario we envision in which the Pb would
be entirely terrestrial in origin would be Pb metal derived from a polishing plate, later oxidized to PbO during
storage of the prepared section.
The second source for the Pb that is consistent with the Pb isotope results is an ordinary chondrite reservoir.
In the rare example of PbO grains at the rim of a chondrule in Chainpur (LL3.4)
64
, U–Pb systematics suggest
(1)
2CaAl
2
Si
2
O
8
+
H
+
=
CaAl
4
Si
2
O
10
(
OH
)
2
+
2SiO
2aq
+
Ca
2
+
(2)
CaAl
2
Si
2
O
8
+
Al
2
O
3
+
H
2
O
=
CaAl
4
Si
2
O
10
(
OH
)
2
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that heating during the L-chondrite breakup event liberated Pb from Pb-bearing troilite or metal. In Kakowa,
the troilite and metal in the fracture fill contain no observable Pb, which makes it difficult to judge whether all
the Pb was liberated from these phases or was never present.
In any case, the problem of the Pb source, while unsolved at this time, can be considered separately from the
source of the detrital grains hosted in the Pb-rich fracture fill. We rely on the oxygen isotope results from these
to show that the margarite, corundum, and albite are all non-terrestrial and likely from an ordinary chondrite
reservoir that experienced fluid alteration followed by thermal maturation.
A second weak collision brings a hydrated assemblage into Kakowa.
In the studied section of
Kakowa, we find shock melt veins bearing high-pressure minerals that are cross-cut by fractures filled with
exogenous material. We conclude therefore that Kakowa preserves a record of at least two impact events. The
low-velocity impact event must come after the high-velocity impact, but we cannot constrain the time difference
between them. The two events may be unrelated or, on the other hand, it could be that the low-velocity event was
a secondary impact between fragments of debris from the high-velocity
impact
42
,
65
.
Although dating of shock events can be challenging because they may only partially reset some radiometric
systems, it is widely agreed on the basis of numerous studies that many L chondrites preserve a record of a strong
shock at ~ 470
Ma
24
,
25
,
66
–
68
, commonly associated with shock darkening, formation of melt veins, and creation of
HP minerals. For example, the meteorites Peace River, Taiban, Mbale, and Sixiangkou include the above shock-
related
features
12
,
25
,
43
–
46
,
65
,
69
,
70
. This event is so ubiquitous among L-chondrites that it is generally presumed to
represent the age of catastrophic disruption of the L-chondrite parent
body
24
,
25
,
71
.
Disruption of the parent body, however, need not be the last impact event experienced by its fragments.
Indeed, the Ghubara L5 chondrite, for instance, contains a cognate xenolith with a slightly younger
40
Ar/
39
Ar
age than the disruption (445 Ma). The L-chondrite parent body broke into numerous asteroid fragments known
as the Gefion
family
71
. In turn, one or more of these fragments may have collided with other asteroids. Polymict
OC meteorites, containing lithologies of various
types
72
, may rarely host xenolith clasts that experienced dif
-
ferent shock histories and preserve different shock stages (e.g., St. Mesmin
meteorite
73
). In the present case, we
document a further collision between a strongly shocked L6 fragment and another object with likely ordinary
chondrite affinity, lower thermal metamorphic type, and a history of fluid alteration. We cannot say, with the
present data, whether this object represents: (1) another fragment of the L-chondrite parent body, excavated
from shallower depth and placed onto a crossing orbit with low encounter velocity, or (2) an unrelated body. The
latter impact involved brecciation in the solid state, without impact heating sufficient to decompose margarite.
The exogenous minerals certainly did not experience shock conditions comparable to the nearby melt veins
(
P
> 18–23 GPa and
T
> 1800–2100 °C). Although impacts among planetesimals were most common in the first
100 million years of solar system history, here we infer that this impact is likely younger than
~ 470 Ma.
The data presented here are compatible with a number of detailed histories of impact and parent body disrup
-
tion. For example, we cannot rule out that Kakowa records a strong shock that failed to disrupt the parent body,
followed by a weak shock that nevertheless represents the disruption event. This scenario seems unlikely, however,
as the weak shock is recorded by the actual presence of exogenous material from the impactor and hence it does
not represent a record of a major collision elsewhere on the body, attenuated by distance. The detailed physical
process (e.g., the role of fluids) by which the fill was emplaced into the fractures is not clear, but it seems likely
that the maximum depth of material injection into narrow fractures during a weak impact is quite limited. Hence,
we may infer that, after thermal and (strong) shock processing (perhaps during the fragmentation of the parent
body), the material that would become the Kakowa meteorite was excavated to the near-surface, where it could
easily be fractured and receive material transferred from a low-velocity impactor. Based on this inference, we sug-
gest that the strong and weak shocks recorded in Kakowa did not both occur on the intact parent body. Another
possibility is that yet a third impact disrupted the parent body without significantly shocking the fragment from
which Kakowa originates. The most likely scenario, in our view, remains the disruption of the L-chondrite parent
body at
~ 470 Ma by a major collision between bodies with high encounter velocity, recorded by the strong shock
assemblage in Kakowa, followed by a low-velocity impact (recorded by the fracture and exogenous fill material)
between fragments of this event, which were placed into related orbits within an asteroid family.
Conclusions
Kakowa hosts a mineral record unique among ordinary chondrites studied to date. One of the melt veins formed
during a strong shock event is cross-cut by a fracture filled by a unique exogenous material containing the
hydrous phase margarite together with corundum, fayalite, forsterite, albite, and silica embedded in an Fe- and
Pb-rich matrix. The mineralogy, oxygen isotope ratios, and Pb isotope ratios of the exogenous material are most
consistent with derivation from an ordinary chondrite that preserves a more intense history of fluid alteration
and a lower degree of thermal metamorphism than the rest of Kakowa, suggesting that alteration of the exog
-
enous material and metamorphism of the bulk of Kakowa both predate their juxtaposition. The injection of the
exogenous phases records a second impact event, with low encounter velocity, that postdates the strong shock
and likely also postdates the disruption of the L-chondrite parent body.
Material–analytical methods–modeling strategy
Material.
A single thick polished section (500 μm in thickness) of the Kakowa meteorite (NHMV-N6231)
was examined for shock indicators with a focus on its melt veins (MVs). Eleven areas located in the three sub-
parallel MVs were analyzed by optical microscopy, scanning electron microscopy, and electron probe microa-
nalysis for texture and mineral chemistry (MV1 to MV11 in Fig.
1
). Two of the regions (MV1 and MV2) were
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further studied with co-located Raman spectroscopy in order to couple structural and compositional characteri-
zation at common spots.
Analytical methods.
We used transmitted and reflected light microscopy to characterize texture, and likely
mineralogy of phases large enough to be resolved optically.
Scanning electron microscopy
(Field-emission SEM; JEOL JSM-IT300LV at NHM Vienna and Zeiss 1550VP
at Caltech) yielded images, preliminary composition by energy-dispersive X-ray spectroscopy, and structure
determination by EBSD. Single crystal EBSD analyses at sub-micrometer scale were performed at 20 kV and
6 nA in focused beam mode with a 70° tilted stage on uncoated sections in “variable pressure” mode (25 Pa of
N
2
gas in the chamber to reduce specimen charging). Imaging, mapping, semi-quantitative EDS analysis, and
EBSD were conducted using the SmartSEM, AZtec, and Channel 5 software packages.
Electron probe microanalyzer
yielded quantitative major element chemistry using a JEOL JXA8530F Field
Emission EPMA instruments (FE-EPMA) equipped with five wavelength-dispersive spectrometers (WDS) and
one energy-dispersive spectrometer (EDS) at the NHM Vienna, Austria. All the analyses were acquired using
15 kV. For minerals, a 15 nA focused beam current, 20 s counting time on peak position, and 10 s for each
background were used. For glass analyses, a slightly defocused (5 μm diameter) beam, 5 nA probe current, and
counting times of 10 s on-peak and 5 s on each background position were used. Natural mineral standards used
were albite (Na, Si, Al), wollastonite (Ca), olivine (Mg), almandine (Fe), spessartine (Mn), orthoclase (K), rutile
(Ti), chromite (Cr), and Ni-oxide (Ni) with ZAF matrix correction.
Raman spectroscopy
analyses were collected from the polished thin section of Kakowa using a dispersive con-
focal Raman microscope, Renishaw inVia Reflex at the National Hellenic Research Foundation. Analyses used
a 514 nm Ar-ion laser and a 100
× objective lens and spectra were collected in the region from 200–1600
cm
−1
.
We acquired the spectra very carefully focusing at the surface of the samples, with a laser power
~ 5 mW to
avoid destruction of the analyzing area. Acquisition time was 30–60 s averaging 5 accumulations. Additional
Raman spectra were collected with a Renishaw InVia Confocal Raman microscope at the Mineral Spectroscopy
Laboratory at Caltech. The 514 nm laser was set to
< 2 mW power to avoid laser damage. Each spectrum was
collected for 5 s with 3000 line/mm diffraction gratings, corresponding to Raman shifts of 200–1100
cm
−1
. Gauss
-
ian–Lorentzian peak fitting (fityk version 0.9.8) was used to remove background and estimate the peak centers
with a precision of
~ ± 0.2
cm
−1
. Raman spectra of high-P phases, and margarite were compared with published
data from RRUFF database.
Multicollector-inductively coupled plasma mass spectrometry—Pb isotope
analysis at the Isotoparium (Caltech).
The mounted section of the NHMV-N6231was micro-drilled using a GEOMILL 326 equipped with a tungsten
carbide drill bit. At each of the three drill spots, three powder aliquots were recovered, by successively drilling
to 10–20 μm, 50–60 μm and 80–100 μm depth. The first powder aliquot was obtained by “dry” drilling the mate-
rial, and then pipetting 4–6 μL of MQ-H
2
O onto the surface to suspend the material, recovering the drop, and
transferring to an acid-cleaned PFA beaker containing 1 mL of 1 M
HNO
3
(twice-distilled from ACS reagent
grade
HNO
3
). The second two spots were “wet” drilled by first pipetting a 4–6 μL drop onto the surface sur
-
rounding the drill bit, then drilling to suspend the released material within the drop, and finally recovering the
drop and transferring to an acid-cleaned PFA beaker containing 1 mL of 1 M
HNO
3
. After the final depth was
drilled, an additional drop of 4–6 μL was pipetted onto and off of the surface to recover any remaining powder.
The beakers containing recovered material in 1 M
HNO
3
were then placed on a hotplate at 140 °C for several
hours to digest the Pb-bearing phases.
Following digestion of the Pb host phases, a 50 μL aliquot (5% of the total digest) was taken and diluted with
0.95 mL of 0.45 M
HNO
3
. The concentration of Pb was checked in these solutions on a Neptune
Plus
MC-ICP-MS
(Thermo Scientific) via one-point calibration with a 200 ppb Pb solution (SPEX). The samples containing
> 10 ng
Pb were then diluted to 15 ng/g or 6.25 ng/g for isotopic analysis. To these solutions, Tl was added to correct
for instrumental mass
bias
74
, such that the final solution had a 4:1 Pb:Tl ratio. Internal standard solutions were
prepared at the same Pb and Tl concentrations (15 ng/g Pb
+ 3.75 ng/g Tl and 6.25 ng/g Pb
+ 1.625 ng/g Tl) using
SPEX certified standards.
The Pb and Tl isotopic compositions of sample and standard solutions were analyzed on the Neptune
Plus
MC-ICP-MS using a glass spray chamber, regular sampler and skimmer cones, and a nominal 50 μL/min PFA
nebulizer, yielding
~ 57 V/ppm of Pb. Each analysis consisted of 50 measurement cycles of 4.914 s in static mode,
with mercury interferences monitored in cup L3 (
202
Hg), and Tl and Pb isotopes measured in L2 to H3 (L2:
203
Tl,
L1:
204
Pb, C:
205
Pb, H1:
206
Pb, H2:
207
Pb, and H3:
208
Pb). All cups were equipped with using
10
11
Ω amplifiers. Raw
data were corrected for instrumental mass bias via external normalization with
Tl
64
. For each analysis, the mass
bias (β in Eq. 10 of Ref.
75
) as calculated using the measured
203
Tl/
205
Tl ratio, a normalization ratio of 0.418922,
and the respective molar masses (M203
= 202.972344 and M205
= 204.974427). The actual Pb isotope ratios
were then calculated using the determined β value, the molar masses of the Pb isotopes (M204
= 203.973043,
M206
= 205.964465, M207
= 206.975897, M208
= 207.976652), and the measured Pb isotope ratio (
20x
Pb/
204
Pb).
Each sample solution was analyzed between five and six times. Final data are reported as the mean and 2σ of
replicate analyses (between
± 0.004 and 0.012 for
206
Pb/
204
Pb). The external reproducibility was assessed using 30
replicate analyses of SPEX Pb
+ Tl solutions, yielding a 2SD of
± 0.026 (2SE of
± 0.011 for n
= 6) for
206
Pb/
204
Pb.
The results (in ng Pb) obtained from the silicate spot, and the other two Pb-rich areas (Spot #1 and Spot #2),
are given in Table S1. It is clearly shown that the first 10–20 μm contain little to no Pb (it seems to be removing
the surface layer of glue/polish covering the sample section). The deeper drills actually sample the Pb-rich mate-
rial. In the silicate spot, 2 orders of magnitude less Pb is recovered, demonstrating that the blank contamination
from the drill itself is not an issue).