Jiddat al Harasis 556: A howardite impact melt breccia with an H

Meteoritics & Planetary Science 47, Nr 10, 1558–1574 (2012)
doi: 10.1111/j.1945-5100.2012.01419.x
Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite
component
E. JANOTS1,2*, E. GNOS3, B. A. HOFMANN4, R. C. GREENWOOD5, I. A. FRANCHI5,
K. BERMINGHAM2, and V. NETWING2
1
Univ. Grenoble 1, ISTerre, 38041 Grenoble, France
Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, 48149 Münster, Germany
3
Natural History Museum Geneva, 1 Route de Malagnou, 1211 Genève, Switzerland
4
Natural History Museum Bern, Bernastrasse 15, 3005 Bern, Switzerland
5
Planetary and Space Sciences, The Open University, Milton Keynes MK7 6AA, UK
*
Corresponding author. E-mail: emilie.janots@ujf-grenoble.fr
2
(Received 05 October 2011; revision accepted 06 August 2012)
Abstract–A petrographic and geochemical study was undertaken to characterize Jiddat al
Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict
impact melt breccia containing highly shocked clasts, including mosaicized olivine and
recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–
10 lm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with
orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate
that it has an anomalous bulk-rock composition as howardite, resulting from a mixture
between HED material and at least one second reservoir characterized by a higher D17O. The
bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile
elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second
reservoir. This is independently confirmed by the occurrence of chondrule relics composed of
olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen
isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and
En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H
chondrite material. This percentage is high compared with that observed petrographically,
likely because chondritic material dissolved in the impact melt. This conclusion is supported
by the observed reaction of orthopyroxene to olivine, which is consistent with a reequilibration in a Si-undersaturated melt. JaH 556’s unique composition enlarges the
spectrum of howardite-analogs to be expected on the surface of 4 Vesta. Our data
demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and
impactor materials and must be interpreted with extreme caution within the HED group.
INTRODUCTION
The achondrite Jiddat al Harasis (JaH) 556 was
discovered during a systematic search for meteorites in
the Sultanate of Oman in 2008 (in the Meteoritical
Society database). Petrological observations suggest that
this meteorite belongs to the howardite, eucrite, and
diogenite (HED) group. HEDs represent between 2 and
3% of all meteorites so far collected on a worldwide
basis and, with the exception of lunar rocks, are the
largest suite of extraterrestrial crustal igneous rocks
The Meteoritical Society, 2012.
(Mittlefehldt et al. 1998; Barrat et al. 2003). JaH 556 is a
heterogeneous polymict breccia containing both eucritic
and diogenitic clasts, suggesting it is a howardite.
Amongst the HED group, howardites correspond to
fragmental or regolith breccias composed of mineral
and ⁄ or lithic clasts set in a fine-grained matrix, primarily
consisting of a mixture of eucritic and diogenitic material
(Wahl 1952; Mason 1983). These polymict breccias
are thought to form via surface processes, such as
impacts, that combined eucrite, diogenite, and additional
nonindigenous clasts (Wilkening 1973; Bunch 1975;
1558
JaH 556: A howardite mixed with an H chondrite
Chou et al. 1976; Labotka and Papike 1980; Fuhrman
and Papike 1981; Laul and Gosselin 1990; Zolensky
et al. 1996; Pun et al. 1998; Lorenz et al. 2007).
Chondritic components in howardites were first
identified in the early 1970s (Wilkening 1973) and have
since been found in up to 20 different HED achondrites
with compositions including CM2 and CR2 (Pun et al.
1998; Gounelle et al. 2003; Buchanan et al. 2009).
Although chondritic contribution is commonly at levels
of only a few percent, three Antarctic finds have high
proportions of chondrite clasts (Herrin et al. 2011),
reaching up to 60% in the howardite PRA 04401. The
chondritic clasts led some scientists to conclude that
these inclusions may in fact sample the impactor(s)
which produced the howardites. On the other hand,
exotic clasts found in howardites are also commonly
viewed as having a similar mode of formation to
micrometeorites presently falling on the surface of the
Earth (Lorenz et al. 2007).
While eucrites and diogenites provide information
about the differentiation and subsequent magmatic
processes on the HED parent body, howardites inform
us about the surface of the HED parent body (Delaney
et al. 1983). Howardites, eucrites, and diogenites are
generally thought to originate from 4 Vesta, which is one
of the largest differentiated asteroids identified to date
(Binzel and Xu 1993). This genetic link is based on
spectral comparisons between 4 Vesta and HED
meteorites (McCord et al. 1970; Pieters et al. 2005), and
also the identification of an efficient delivery mechanism
from 4 Vesta to Earth (Binzel and Xu 1993).
Documenting new howardite samples and understanding
their petrogenesis is crucial to the accurate interpretation
of spectroscopic analyses of 4 Vesta’s surface and the
calibration of instruments during the Dawn mission
(McSween et al. 2010).
Recently, some eucrites were reclassified as
ungrouped basaltic achondrites because their oxygen
isotopic compositions do not match the HED group
(Scott et al. 2009). It is inferred that the anomalous
eucrites are probably derived from distinct Vesta-like
parent bodies. Bland et al. (2009) proposed that the
orbital properties and the oxygen isotopic composition
of the Bunburra Rockhole fall belong to a V-type
asteroid other than Vesta. Preliminary oxygen isotopic
data for JaH 556 indicated that it had an anomalous
isotopic oxygen composition. In this study, the petrology
and geochemistry of JaH 556 are investigated to
elucidate its origin and to discuss why its bulk-rock
oxygen isotopic composition deviates from reported
HED values. We will show that JaH 556 documents a
unique type of howardite, in which the isotopic
composition is affected by a significant contribution of
an ordinary chondrite impactor.
1559
ANALYTICAL METHODS
Petrographic and quantitative mineral analysis and
clast classification were completed on a polished thin
section of JaH 556 using a JEOL JXA 8900 EMP
(Institut für Mineralogie, Universität Münster). Beam
conditions were fixed at 15 kV, 20 nA, and 1 lm spot
size. A u(qz) matrix correction procedure was applied
(Pouchou and Pichoir 1984). Clasts with a diameter of
‡30 lm were arbitrarily selected for quantitative
analysis. Smaller clasts are considered as part of the
matrix. Point counting using optical microscopy
techniques was hampered by the small clast size and
presence of shock features. Even in backscattered mode
(BSE), it was difficult to distinguish clinopyroxene
clasts from the matrix. Thus, clast abundances were
estimated from X-ray maps of the studied thin section.
Elemental imaging was obtained with an Eagle III
micro-XRF (ISTerre, University of Grenoble) using a
30 lm-sized beam, at 20 kV and 250 lA. The entire
thin section was mapped in two successive acquisitions
using dwell times of 200 and 400 ms. Step size was
adjusted to fit the beam diameter (approximately
30 lm).
Oxygen isotope analysis was carried out at the Open
University using an infrared laser-assisted fluorination
system (Miller et al. 1999). All analyses were obtained on
powdered samples (0.5–2 mg) taken from larger
homogenized aliquots of generally greater than 50 mg.
After fluorination, the O2 released was purified by
passing it through two cryogenic nitrogen traps and over
a bed of heated KBr. O2 was analyzed using a MAT 253
dual inlet mass spectrometer. Analytical precision (1r),
based on replicate analyses of international (NBS-28
quartz, UWG-2 garnet) and internal standards, is
approximately ±0.04& for d17O; ±0.08& for d18O;
±0.024& for D17O (Miller et al. 1999). The precision
(1r) quoted for individual fraction from JaH 556 is
based on replicate analyses (Table 1).
Isotopic analyses were performed on bulk samples of
the meteorite, as well as various groundmass and clast
fractions (Table 1). As initial oxygen isotope analysis
indicated significant levels of terrestrial alteration, all
analyses, apart from a single bulk sample, were
performed on material that had been leached in a
solution of ethanolamine thioglycollate (EATG) prior to
analysis. EATG is known to be efficient at
removing terrestrial alteration products, such as
iron-hydroxides, -carbonates, and -sulfates from a rock
(Cornish and Doyle 1984; Martins et al. 2007).
Oxygen isotopic analyses are reported in standard d
notation, where d18O has been calculated as:
d18O = [(18O ⁄ 16Osample ⁄ 18O ⁄ 16Oref) ) 1] · 1000 (&) and
similarly for d17O using the 17O ⁄ 16O ratio. D17O, which
1560
E. Janots et al.
Table 1. Oxygen isotopic composition of JaH 556 matrix and clast.a
Sample
Bulk samples
JaH 556 bulka
JaH 556 bulk
Matrix samples
JaH 556 matrix
JaH 556 matrix low magnetic
JaH 556 matrix medium magnetic
JaH 556 matrix high magnetic
JaH 556 matrix averageb
Clast samples
JaH 556 large clast CO-1
JaH 556 clast CO-2
JaH 556 clast CO-3
N
d17O&
1r
d18O&
1r
D17O&
1r
2
2
3.15
2.32
0.00
0.00
6.20
4.65
0.01
0.02
)0.09
)0.11
0.00
0.00
3
2
2
2
9
2.19
2.74
2.47
2.53
2.45
0.02
0.11
0.03
0.05
0.23
4.44
5.49
4.97
5.02
4.92
0.06
0.23
0.00
0.08
0.42
)0.14
)0.13
)0.13
)0.10
)0.13
0.01
0.01
0.02
0.01
0.02
7
2
1
1.89
2.10
2.24
0.06
0.07
4.05
4.46
4.72
0.14
0.13
)0.23
)0.24
)0.23
0.01
0.00
a
Untreated; all other samples were leached in EATG.
Average of all matrix samples.
b
represents the deviation from the terrestrial fractionation
line, has been calculated using the linearized format of
Miller (2002):
d17 O ¼ 1000lnð1 þ D17 O=1000Þ
k1000lnð1 þ D17 O=1000Þ;
ð1Þ
where k = 0.5247, which was determined using 47
terrestrial whole-rock and mineral separate samples
(Miller et al. 1999; Miller 2002).
Whole-rock geochemical analyses were obtained
from an aliquot of the powdered sample (1.56 g of
aliquot) by a combination of inductively coupled
plasma–optical emission spectroscopy (ICP-OES) and
inductively coupled plasma–mass spectrometry (ICPMS) at Activation Laboratories Ltd., Ancaster, Ontario,
Canada (code 4 Lithoresearch).
SAMPLE DESCRIPTION AND MINERAL
IDENTIFICATION
JaH 556 is a single, porous, 2–3 cm-sized stone
weighing 36.65 g. It is a polymict breccia with an orangebrown groundmass containing fragments and oblong
vesicles (Fig. 1a). Abundant Fe-oxihydroxides and nearcomplete lack of Fe metal indicate pervasive terrestrial
weathering. Optical microscopy showed that JaH 556
contains abundant mineral and lithic clasts set in a
cryptocrystalline matrix, grayish in transmitted light
(Fig. 1b). The clast size ranges from 30 to 500 lm with
the largest clast (Clast 1) reaching 4 mm (Figs. 1 and 2).
The majority of mineral and lithic clasts are composed
of plagioclase, clinopyroxene, and orthopyroxene.
Olivine clasts are scarce. Hollow vesicles and those filled
with white ⁄ orange material have a maximum length of
5 mm.
Plagioclase, orthopyroxene, clinopyroxene, olivine,
chromite, calcium sulfates, and Fe-oxihydroxide were
identified on X-ray maps (Fig. 2). Plagioclase abundance
is indicated by elevated Al concentrations. Pyroxene
clasts have very low Al contents and they are more
abundant than the plagioclase clasts. Orthopyroxene has
higher Mg and lower Ca contents than clinopyroxene.
The large Clast 1 is mainly composed of orthopyroxene.
In the pyroxene clast fraction <1 mm clinopyroxene is
more abundant than orthopyroxene. Olivine crystals are
rare and difficult to distinguish when using the XRF
imaging. Olivine is best seen at the edge of Clast 1, where
it has higher Fe content than Clast 1. The presence of
chromite is inferred from the high concentrations of Cr
(map not shown). Fe-oxihydroxides are highlighted by a
high concentration of Fe and Co and absence of Si. Ni is
pervasively distributed, but appears to be decoupled
from Fe and Co and does not correlate with any other
element. Sulfur concentrations significantly increase
toward the edge of the sample and correlate well with
Ca. This relationship indicates that the S-rich domains
are Ca-sulfates, apparently formed during the meteorite
alteration in the desert. All phases were independently
confirmed by optical microscopy.
CHEMICAL COMPOSITION OF JAH 556
Oxygen Isotope Analyses
Two bulk fractions of JaH 556 were analyzed for
oxygen isotopes, one untreated and the other leached in
EATG (Table 1; Fig. 3). The untreated fraction
(d17O = 3.15&, d18O = 6.20, D17O = )0.09&) is
strongly shifted to significantly higher d18O values, and
only slightly higher D17O values, compared with the
EATG
leached
bulk
fraction
(d17O = 2.32&,
JaH 556: A howardite mixed with an H chondrite
1561
Fig. 1. a) Slab of JaH 556 with millimeter scale. Clasts occur in an orange-brown matrix containing abundant pores; (b–e) are
transmitted light optical microscope images. b) Mineral clasts and vesicles occurring in a finely crystallized matrix. Plane light and
crossed polarizers. While plagioclase is always limpid in plane light, ferromagnesian silicates are commonly brownish; c) recrystallized
polycrystalline and fibrous plagioclase set in a fine-grained matrix (crossed polarizers); d) orthopyroxene (right) and recrystallized
plagioclase (left) clasts (crossed polarizers). The orthopyroxene shows planar fractures and a rim of clinopyroxene, which is clearly
visible due to its high birefringence; e) diogenite clast with coarse orthopyroxene grains and a protuberance of mosaicized olivine.
1562
E. Janots et al.
Al
Ca
S
Si
Fe
Mg
Ni
Co
Transmitted light
Fig. 2. XRF element distribution and transmitted light images of the studied thin section. Scale bar represents 5 mm.
JaH 556: A howardite mixed with an H chondrite
a
b
Fig. 3. a) Plot of d18O versus D17O showing average H, L, and
LL ordinary chondrite values (Clayton et al. 1991) in relation
to bulk value for JaH 556. Other data and symbols as in
Fig. 3b. b) Plot of d18O versus D17O showing the oxygen
isotope composition of the JaH 556 bulk-rock, the treated
bulk-rock, the treated groundmass (all black squares) and clast
(black diamonds) in relation to HEDs (gray circles) and
angrites (open squares). HED and angrite data: Greenwood
et al. (2005). TFL: terrestrial fractionation line, AFL: angrite
fractionation line, EFL: eucrite fractionation line.
d18O = 4.65&, D17O = )0.11&). The displacement of
hot desert finds to higher d18O values (and slightly
higher D17O) compared with more pristine samples
(i.e., falls, or in the present case the EATG leached
fraction) has been well documented in other meteorite
groups, including carbonaceous (Greenwood and
Franchi 2004) and ordinary chondrites (Bland et al.
2000). Similar effects were also observed in finds from
the Oman desert (Martins et al. 2007; Janots et al.
2011). The relationship between the untreated and
EATG-treated bulk samples seen in Fig. 3b is therefore
consistent with the petrographic observations indicating
that JaH 556 has experienced significant levels of
terrestrial alteration.
The oxygen isotope composition of the leached bulk
sample falls outside the fields of any of the main
meteorite groups, being intermediate between HED and
angrite values (AFL; Fig. 3b).
The oxygen isotope composition of the JaH 556
matrix (groundmass with clast fraction) was determined
1563
on four fractions, three of which were separated on the
basis of their magnetic properties. All these four matrix
samples were treated with EATG. These different
fractions show no clear systematic differences in their
oxygen isotope compositions (Table 1). The average
matrix value shown in Fig. 3b plots close to the leached
bulk value. However, it is important to note that on a
relatively small, very heterogeneous meteorite such as
JaH 556 it is difficult to obtain a truly representative
bulk sample.
Three clasts from JaH 556 were sampled for oxygen
isotope analysis. A total of seven replicate analyses were
undertaken on the large orthopyroxene-rich clast ‘‘Clast
1’’ (Figs. 1, 2, 8a, and 8b). The average analysis of Clast
1 (labeled CO-1 in Fig. 3a) plots at the high d18O end of
the HED array of Greenwood et al. (2005). With respect to
its D17O value, CO-1 ()0.23 ± 0.01) is indistinguishable
from other HED meteorites (average HED D17O
value = )0.24 ± 0.01 (1r); Greenwood et al. 2005). The
two other clasts sampled for oxygen isotope analysis, CO-2
and CO-3, have somewhat higher d18O values than those
measured by Greenwood et al. (2005), and in the case of
CO-3 it is also slightly beyond the range measured by
Wiechert et al. (2004). However, both samples, CO-2 and
CO-3, have D17O values within error of the average HED
value of )0.24 ± 0.01 (1r; Greenwood et al. 2005). In
view of the heavily altered nature of JaH 556, the relatively
high d18O values measured in CO-1 and CO-2 may indicate
that not all traces of terrestrial weathering were removed
by EATG treatment.
In summary, the oxygen isotope data indicates that
there are at least two isotopic components present in JaH
556, one with a composition given by the HED clasts
and a second with a higher D17O value.
Major and Trace Elements
The JaH 556 whole-rock composition is characteristic
of howardites with major and trace element abundances
falling within the appropriate compositional fields
(Table 2). MgO, CaO, Al2O3, Ti, and Sc abundances are
within the range generally found in howardites (Fig. 4b),
and intermediate between eucrites and diogenites. JaH
556, however, is deficient in SiO2 (43.38 wt%) in
comparison to reported HED material (Fig. 4a). The
FeO and MnO abundances also diverge from HED
values with slightly higher FeO and slightly lower MnO
than reported HED compositions (Fig. 4a). As a
consequence, the Fe ⁄ Mn ratio (approximately 41.8;
Table 2) is higher than those found in HED material
(approximately 33–34; Warren et al. 2009).
The JaH 556 trace element composition shows
enrichment in incompatible elements relative to CI,
including the rare earth elements (REE, Table 3). The
1564
E. Janots et al.
Table 2. Comparison of whole-rock major elements in JaH 556 with averaged values of the HED and H meteorites
taken from Warren et al. (2009) and Wasson and Kallemeyn (1988).
Oxide
wt%
JaH 556
SiO2
Al2O3
FeOa
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOIb
Total
43.4
7.45
19.8
0.467
14.1
6.94
0.400
0.030
0.403
0.070
2.09
97.4
Fe ⁄ Mn
41.8
Detection
limit
Diogenite
(29)
Eucrite
(35)
0.01
0.01
0.01
0.001
0.01
0.01
0.01
0.01
0.001
0.01
52.8
1.13
16.5
0.49
26.5
1.38
0.03
49.6
12.5
18.0
0.54
7.8
9.9
0.39
±
±
±
±
±
±
±
33.2
1.7
0.69
2.2
0.11
2.3
0.66
0.03
±
±
±
±
±
±
±
32.9
Howardite
(40)
0.8
1.6
2.0
0.06
2.0
0.8
0.11
50.3
7.8
17.6
0.51
15.5
6.4
0.26
34.1
±
±
±
±
±
±
±
1.0
1.9
1.0
0.03
2.9
1.4
0.07
H chondrite
36.2
2.14
35.4
0.300
23.2
1.75
0.816
117
a
For simplicity, Fe concentrations assume that all Fe is divalent.
b
LOI = Loss on ignition.
flat chondrite-normalized REE pattern (Fig. 5b), with
values being approximately 6–7*CI, is consistent with
howardite REE compositions: Concentrations of the two
siderophile elements Co (159 ppm) and Ni (3940 ppm)
are the highest ever documented in an HED meteorite
(Fig. 4c). Ni content was independently confirmed by
XRF measurements (about 3460 ppm). In addition, U,
Sr, and Ba concentrations are 10–100*CI (Fig. 5a;
Table 3). The enrichments in U, Sr, and Ba are
comparable to those observed in other <50 kyr old
meteorites found in hot deserts (Al-Kathiri et al. 2005;
Barrat et al. 1999). The enrichment in U, Sr, and Ba was
even identified by comparing samples of the diogenite
Tatahouine collected directly after the fall with samples
collected 63 yr later (Barrat et al. 1999). This study
clearly showed that Sr, Rb, and U may be enriched in
the meteorite by hot desert alteration over a very short
time scale.
PETROLOGY OF JAH 556
Mineral composition has been carefully acquired for
different textural position in the matrix and clasts.
Selected representative EMP analyses are given for the
different mineral populations in Tables 4–6.
Matrix
The finely crystallized matrix has an igneous texture.
It is composed of subeuhedral plagioclase surrounded by
subeuhedral and interstitial clinopyroxene (Fig. 6a).
Matrix plagioclase occurs as approximately 10 lm-sized
crystals and smaller lath-shaped crystals (Fig. 6a).
Plagioclase shows a significant compositional spread
(An76–92; Fig. 7a), with most of the values lying between
An83–92. In BSE images, matrix pyroxene displays zoning
with a darker core progressively evolving toward a
brighter rim enriched in Ca (Fig. 6a). Orthopyroxene
was not found in the matrix. All pyroxene compositions
fall within the pigeonite range (En48–62Wo6–15; Fig. 7b).
The matrix also contains abundant Fe-oxihydroxide, and
minor olivine and spinel (chromite). Mineral assemblage
and compositions are consistent with eucritic material,
with the exception of the olivine occurrence. Olivines are
especially numerous in the area surrounding the
orthopyroxene clasts (Figs. 6b and 8b), where they occur
as homogeneous and subeuhedral grains (Fo50–53,
Fig. 7c), 5 to 25 lm in size. Their morphology suggests
that they formed coevally with the matrix crystallization.
Eucrite Clasts
Eucritic material is represented by clinopyroxene and
plagioclase clasts (Fig. 2). Plagioclase clast size is up to
500 lm. Most of the plagioclase clasts are polycrystalline
consisting of fine-grained and oriented needles ⁄ plates
(Figs. 1c and 1d), and can display flow-deformed shapes
(Figs. 6c and 6d). They commonly contain Fe-phases
and occasionally pyroxene and olivine (Fo57). Ironphases are commonly aggregated and aligned within the
polycrystalline
plagioclase
aggregates
(Fig. 6c).
Occasionally, the plagioclase contains melt inclusions.
Recrystallized and deformed plagioclase has a
composition (An76–92) overlapping that of the matrix
(Fig. 5a). A second, less common group of plagioclase
clasts is characteristically undeformed and corresponds
to single homogeneous grains with no shock features.
However, this second plagioclase group has a similar
JaH 556: A howardite mixed with an H chondrite
45
a
clinopyroxene clasts (>100 lm) are clearly observable in
BSE. Most clinopyroxene grain boundaries are sharp.
However, in some instances, the clast boundaries are
unclear and embayed by the matrix (Fig. 6e). In terms of
composition, clinopyroxene clasts are homogeneous, but
frequently contain abundant inclusions that are probably
responsible for the brownish clinopyroxene clouding.
Howardites
Regolithic
Howardites
40
JaH556
FeO/MnO
1565
35
30
Diogenite Clasts
25
40
35
45 SiO2 (wt %)
50
55
b
Abundance
30
Diogenites
Eucrites
All howardites
25
20
Regolithic
howardites
JaH556
15
10
5
0
MgO
(%)
180
Al2O3
(%)
CaO
(%)
Sc
ppm
Ti
ppm
c
160
140
Co (ppm)
120
100
80
60
Howardites
40
JaH556
20
Regolithic Howardites
0
0
1000
2000 Ni (ppm) 3000
4000
5000
Fig. 4. Whole-rock major and trace elements in JaH 556
compared with other meteorites from the HED group (data
from Warren et al. 2009). a) Graphs showing the SiO2 and
FeO ⁄ MnO variations. b) Comparison of averages of oxide and
trace element. Graph clearly shows the similarity in
composition between our sample and the average values for
howardite meteorites except for Ti. c) Plot of Ni versus Co
demonstrates uniquely high Co and Ni values in JaH 556.
composition (An83–92) as the deformed plagioclase clasts.
This compositional range is typical of eucritic material.
Clinopyroxene clasts (clinopyroxene) are clearly
visible in optical microscopy (Fig. 1b) with high
birefringence in crossed polarizers. In plain light, they
are commonly masked by an orange-brownish clouding,
similar to those described in the Kapoeta howardite (Pun
et al. 1998). Regarding the shock state, clasts present
mosaic texture indicating high shock levels. In BSE,
these clinopyroxene clasts (En46–57Wo5–15, Fig. 7b) are
easily confused with the intergranular clinopyroxene
matrix having a similar composition. Only the largest
Clast 1 is the largest fragment in the sample (about
4 mm). It is a polycrystalline clast with a ‘‘subeuhedral
shape’’ (Figs. 1, 2, and 8), which suggests that it was
originally a single grain that was later brecciated by
impact. It is composed of coarse-grained (up to 1 mm)
orthopyroxene homogeneous in composition (En68–75
Wo2–3, Fig. 7b) and minor chromite (up to 150 lm), Fesulfide (<5 lm, not analyzed), and interstitial olivine. At
the periphery of Clast 1, olivine occurs as a millimetric
protuberance (Fig. 8b) clearly visible on the Fe
X-ray maps (Fig. 2). This olivine lobe is polycrystalline
with homogeneous Fo59–63 composition (Fig. 7c).
Observations in transmitted light with crossed polarizers
reveal a mosaic texture (Fig. 1e). At the contact with the
matrix (Fig. 8c), the olivine lobe is decomposed to finely
crystallized grains (about 1 lm). Within Clast 1, olivine
is also observed interstitially along orthopyroxene grain
boundaries (Fig. 8a). This olivine has a submicrometric
and porous texture and is associated with chromite,
Fe-oxihydroxides, and Fe-sulfide. Quantitative analyses
of interstitial olivine were compromised by the
submicrometric crystal size, but one accurate EMP
analysis yields a composition close to the polycrystalline
olivine aggregate (Fo64; Fig. 7c). At the contact with the
matrix, Clast 1 possesses a lm-sized rim which appears
brighter in BSE mode (Figs. 8a and 8b). The rim consists
mainly of clinopyroxene (En51–64Wo6–11; Fig. 7b), but it
occasionally contains very small olivine inclusions. The
mineralogical composition of Clast 1 unambiguously
indicates that Clast 1 is a diogenitic clast.
Other orthopyroxene clasts dispersed in the matrix
are similar in composition and morphology to the
diogenitic Clast 1, which suggests that all orthopyroxene
clasts represent diogenitic material. Orthopyroxene clasts
are systematically zoned in BSE mode with dark cores and
a brighter, <30 lm wide rim (Fig. 6f). The core
composition corresponds to orthopyroxene (En68–76Wo2–3;
Fig. 7b) and the rim falls within the clinopyroxene range
of pigeonite, with a composition enriched in CaO and
FeO analogous to matrix or clinopyroxene clasts (En52–
60Wo8–10, Fig. 7b). The rim is also visible when using the
optical
microscope
(Fig. 1d):
low-birefringence
orthopyroxene core with decorated planar fractures is
surrounded by a rim of clinopyroxene with high
1566
E. Janots et al.
Table 3. Comparison between selected trace elements in JaH 556 and averaged HED and H chondrite values from
Warren et al. (2009) and Wasson and Kallemeyn (1988), respectively.
Element (ppm)
Detection
limit (ppm)
JaH 556
Sc
V
Sr
Ba
Cr
Co
Ni
Cu
Zn
Ga
Ge
Rb
Y
Zr
Nb
Sb
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Pb
Th
U
1
5
2
3
20
1
20
10
30
1
0.5
1
0.5
1
0.2
0.2
0.05
0.05
0.01
0.05
0.01
0.005
0.01
0.01
0.01
0.01
0.01
0.005
0.01
0.002
0.1
0.01
5
0.05
0.01
21
72
2907
313
2900
159
3940
20
30
2
2.8
2
9.8
29
1.6
6.8
1.54
3.97
0.67
3.25
0.95
0.365
1.45
0.28
1.87
0.39
1.14
0.163
1.02
0.162
0.8
0.1
10
0.19
0.21
Diogenite
(29)
Eucrite
(35)
Howardite
(40)
H chondrite
14.8 ± 5.6
131 ± 61
1.8 ± 2.4
28.5 ± 3.7
74 ± 16
74 ± 16
22.4 ± 2.8
98 ± 15
44 ± 15
12.5
74
10.0
20.6 ± 7.4
37 ± 36
6.9 ± 2.8
2.6 ± 2.9
23.8 ± 14.2
277 ± 481
810
16,000
0.20 ± 0.10
1.3 ± 0.4
1.0 ± 0.7
0.09 ± 0.12
2.5 ± 1.6
1.68 ± 0.54
0.295
0.07 ± 0.08
0.020 ± 0.034
1.50 ± 0.87
0.60 ± 0.17
1.01 ± 0.30
0.36 ± 0.08
0.185
0.073
0.027 ± 0.023
0.23 ± 0.10
0.16 ± 0.04
0.031
birefringence under crossed polarizers. Locally,
orthopyroxene clasts were also partly replaced by olivine
(Fig. 6h). There, an orthopyroxene core is embayed by
olivine, with a composition (Fo52) similar to that of the
matrix. Olivine textural relations with the matrix suggest
that they crystallized coevally.
Chondritic Clasts
Two lithic clasts (Clasts 2 & 3) are interpreted to be
chondrite relics. Unfortunately, both were too small to be
separated for oxygen isotope analyses. Clast 2 is round,
approximately 200 lm in diameter, zoned, and partly
altered (Fig. 8d). It has a spherical morphology with
elongated crystals in the core and a zoned rim, which is
typical of barred chondrule texture. The elongated
6.0
crystals are olivine (Fo77–80) separated by finely
crystallized orthopyroxene (En79Wo2) and clinopyroxene
(En60Wo7), plagioclase (An62–66), and Fe-oxihydroxide
(Fig. 7). Mineral compositions in Clast 2 are very
distinct from the matrix and the eucritic and diogenetic
materials (Fig. 7), but are consistent with H ordinary
chondrite. Toward the rim, olivine composition evolves
progressively toward more Fe-rich composition (Fo56–74).
Clast 3 is a polycrystalline aggregate, approximately
150 lm in size, for which the limits are defined by a
narrow olivine rim enclosing a fine assemblage of
orthopyroxene and plagioclase (Fig. 8e). As in Clast 2,
plagioclase (An61) is lower in Ca compared with eucritic
material
(Fig. 7a).
Orthopyroxene
compositions
(En68Wo3) plot within the diogenitic field. Olivine
composition (Fo50–57) is similar to that of olivine grains
JaH 556: A howardite mixed with an H chondrite
1000.00
a
zoning in olivine clasts developed coevally with matrix
crystallization.
JaH 556
100.00
Element/CI
1567
Vesicles and Terrestrial Weathering Signatures
Abundant roundish and elongated vesicles reaching
5 mm in length are distributed randomly throughout the
meteorite matrix. Elongation orientation is not clear
(Figs. 1 and 2), but it appears often subparallel to the
largest clasts (e.g., around Clast 1 in Fig. 2). Vesicles are
often partly filled with desert alteration products such as
anhydrite, celestite, Fe-oxihydroxides, and some hydrous
secondary silicates. Amongst them, the sulfates are the
most abundant as seen on the X-ray maps (Fig. 2), which
show that these sulfates are preferentially located at the
border of the sample. Oxidation of sulfides and formation
of secondary sulfates is a common feature in meteorites
found in the hot desert of Oman (Al-Kathiri et al. 2005;
Janots et al. 2011). Among these secondary sulfates,
celestite and barite are common (Al-Kathiri et al. 2005).
10.00
1.00
Al
Sc
Ca
Sr
V
Mg
Cr
Mn
Rb
K
Na
Ba
U
0.10
0.01
100
b
REE/CI
10
CLASSIFICATION AND PETROGENESIS OF
JAH 556—A NEW HOWARDITE
1
JaH 556
diogenite
eucrite
Polymict eucrite
howardite
0.1
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Fig. 5. CI-normalized plots of a) selected
elements and b) REE pattern of JaH 556.
Er
Tm
Yb
Lu
incompatible
in the matrix. Mineral assemblages and compositions of
Clasts 2 and 3 are not typical of HED sources
(Mittlefehldt et al. 1998; Shearer et al. 2010). Based on
the resemblance of their morphology with that of
chondrules and their assemblages with Mg-rich olivine
associated with An60–65, it is proposed that Clasts 2 &
3 correspond to chondritic material. The occurrence of
well-crystallized plagioclase in both clasts indicates that
they derived from an equilibrated chondrite.
A small number of olivine clasts were also
recognized using electron microprobe (Fig. 6g). They are
generally below 50 lm. Most of the olivine clasts are
(sub)euhedral and zoned (Figs. 6g and 7c). Planar
fractures were the only deformation features identified.
In BSE mode, olivine presents a dark core (Ol 1)
surrounded by a brighter, 10 lm-sized rim (Ol 2),
reflecting Mg variations (Fig. 7c): the core has higher
Mg# (Fo73–81) than the rim (Fo56–64). The Mg# in Ol 1 is
analogous to that of Clast 2, and higher than in the
diogenitic Clast 1. Thus, olivine mineral clasts are
attributed to chondritic material. Similarities between the
compositions of Ol 2 and matrix olivine suggest that the
JaH 556 is a heterogeneous polymict breccia
composed of a mixture of HED and chondritic material.
The presence of diogenite clasts is confirmed by mineral
composition and geochemical data. Petrologically, JaH
556 can be classified as howardite (diogenite >10 vol%;
Delaney et al. 1983). As with other howardites (e.g.,
Kapoeta; Pun et al. 1998), it is composed of variably sized
fragments of eucrite, diogenite, and chondrite embedded
in a finely recrystallized cryptocrystalline matrix (grain
size typically <5 lm) composed of subeuhedral
plagioclase and clinopyroxene. Howardite classification is
confirmed by the whole-rock major composition that falls
within the howarditic range of compositions (Table 2;
Figs. 4 and 5).
JaH 556, however, shows some differences in
comparison to other howardites in terms of oxygen
isotopic compositions, siderophile abundances, and
mineralogy. These unique characteristics may be mainly
explained by the admixture of an ordinary chondrite
component.
A Significant H Chondrite Component in JaH 556
The occurrence of exotic clasts, especially chondrites,
in howardites has long been recognized. Most of these
clasts are attributed to CM2 (about 80%) and CR2
(about 20%, Zolensky et al. 1996) chondrites. In JaH
556, the presence of chondrules (Clast 2) confirms that it
is partly contaminated by chondritic material, but the
possibility of a significant CM or CR contribution is
1568
E. Janots et al.
Table 4. Selected EMP analyses of the plagioclase from the matrix (Ma), mineral clasts (MCl), clast 2 (Cl2), and
clast 3 (Cl3).
Analysis No.
Position
71
ma
108
ma
225
ma
110
MCl
111
MCl
159
Cl2
160
Cl2
253
Cl3
SiO2
TiO2
Cr2O3
Al2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Ab
An
Or
43.62
0.05
0.02
34.22
0.85
0.02
0.02
0.07
18.30
1.00
0.06
98.23
9.0
90.7
0.4
45.69
0.02
0.02
34.45
0.67
0.04
0.06
0.07
17.77
1.23
0.11
100.12
11.1
88.3
0.7
46.35
0.01
0.03
34.46
0.70
0.00
0.08
0.20
17.50
1.31
0.05
100.69
11.9
87.8
0.3
45.24
0.17
0.01
34.04
0.63
0.00
0.00
0.12
18.16
1.07
0.08
99.52
9.6
89.9
0.5
44.94
0.00
0.00
34.79
0.30
0.03
0.00
0.05
18.52
0.89
0.07
99.59
8.0
91.6
0.4
51.67
0.03
0.09
29.63
0.77
0.02
0.00
0.20
13.44
3.53
0.43
99.82
31.4
66.1
2.5
52.59
0.06
0.07
29.22
0.76
0.03
0.01
0.22
12.90
3.92
0.38
100.16
34.7
63.1
2.2
53.14
0.02
0.04
28.75
0.84
0.02
0.16
0.25
12.27
4.00
0.47
99.96
36.1
61.1
2.8
Table 5. Selected EMP analyses of olivine from the matrix (Ma), from mineral clasts (MCl), clast 1 (Cl1), clast 2
(Cl2), and clast 3 (Cl3).
Analysis No. 92
Position
Ma
SiO2
TiO2
Cr2O3
Al2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Fo
Fa
176
Ma
135
MCl
281
MCl
134
MCl
280
MCl
65
Cl1
66
Cl1
152
Cl2
155
Cl2
163
Cl2
168
Cl2
246
Cl3
254
Cl3
34.86 35.14 36.34 35.77 38.15 39.05 34.93 34.90 39.18 39.37 35.72 35.87 34.92 35.77
0.02
0.03
0.00
0.06 0.01
0.02
0.00
0.00
0.13
0.00
0.03
0.05 0.12
0.05
0.08
0.10
0.17
0.09 0.06
0.10
0.17
0.12
0.49
0.30
0.10
0.20 0.08
0.10
0.02
0.01
0.03
0.02 0.01
0.02
0.01
0.00
0.25
0.10
0.02
0.03 0.06
0.01
40.27 39.82 31.21 36.13 17.97 17.51 35.76 34.65 19.20 18.34 36.74 36.73 37.37 37.03
0.79
0.83
0.65
0.71 0.50
0.49
0.65
0.69
0.37
0.40
0.76
0.69 0.77
0.77
0.05
0.07
0.00
0.00 0.00
0.07
0.06
0.00
0.00
0.03
0.00
0.02 0.15
0.01
25.54 24.56 31.90 27.87 43.05 42.69 29.03 30.52 40.64 42.23 27.37 26.98 25.50 26.76
0.33
0.27
0.22
0.25 0.05
0.14
0.25
0.22
0.24
0.12
0.21
0.24 0.32
0.32
0.01
0.03
0.02
0.03 0.00
0.00
0.00
0.00
0.02
0.01
0.00
0.01 0.00
0.00
0.00
0.03
0.00
0.00 0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.00 0.00
0.00
101.98 100.88 100.53 100.93 99.81 100.08 100.86 101.09 100.51 100.90 100.98 100.81 99.29 100.82
53.1
52.4
64.6
57.9 81.0
81.3
59.1
61.1
79.1
80.4
57.0
56.7 54.9
56.3
46.9
47.6
35.4
42.1 19.0
18.7
40.9
38.9
20.9
19.6
43.0
43.3 45.1
43.7
ruled out by the bulk-rock isotopic oxygen composition
which requires a second reservoir with higher D17O, most
likely ordinary chondrite (Fig. 3b). Based on the d18O
values of the H, L, and LL groups (Clayton et al. 1991),
the best fit to the EATG washed bulk composition of
JaH 556 is obtained by invoking an impactor with a
composition similar to the L group (Fig. 3b). Using the
average HED D17O value of )0.239& (Greenwood et al.
2005) and the average D17O value of +1.06& for L
chondrites (Clayton et al. 1991), this would indicate that
the bulk JaH 556 oxygen isotope value represents a mix
of 10% L chondrite and 90% HED.
However, an H chondrite impactor is compatible
with the mineral composition of the chondrule (Clast 2)
that is composed of elongated olivine intercalated with
finely grained orthopyroxene and plagioclase (barred
olivine chondrule). Inside the chondrule, mineral
composition changes toward the rim as it re-equilibrated
with the matrix. Thus, core compositions are most
probably
representative
of
primary
chondritic
compositions. An H chondrite impactor is consistent
with the core olivine composition in the chondritic clasts
(Fo80 and En79Wo2), which corresponds to that seen in
equilibrated H chondrites (Jones 1998). The high d18O
value of the JaH 556 meteorite may be accounted for by
incomplete removal of the alteration products that
shifted the d18O toward high values (Fig. 3b and oxygen
isotopic compositions of hot desert finds; Bland et al.
JaH 556: A howardite mixed with an H chondrite
1569
Table 6. Selected EMP analyses of pyroxene from the matrix (Ma), from mineral clasts (MCl), clast 1 (Cl1), clast 2
(Cl2), and clast 3 (Cl3).
Analysis No.
Position
86
Ma
87
Ma
315
MCl
321
MCl
298
MCl
309
MCl
9
Cl1
23
Cl1
53
Cl1
60
Cl1
151
Cl2
156
Cl2
258
Cl3
260
Cl3
SiO2
TiO2
Cr2O3
Al2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Wo
En
Fs
51.16
0.23
0.64
1.15
20.10
0.63
0.00
21.01
3.87
0.01
0.01
98.81
7.9
59.9
32.2
51.15
0.30
0.65
1.32
20.96
0.71
0.06
20.17
4.21
0.08
0.00
99.61
8.7
57.7
33.6
51.01
0.42
0.46
1.04
22.81
0.66
0.04
19.08
3.95
0.02
0.02
99.51
8.2
55.0
36.9
52.31
0.18
0.52
0.88
21.59
0.80
0.06
19.84
3.87
0.03
0.03
100.11
8.0
57.1
34.9
53.61
0.12
0.49
0.72
17.98
0.67
0.01
24.78
1.39
0.00
0.03
99.80
2.8
69.1
28.1
53.86
0.05
0.46
0.76
15.00
0.52
0.00
27.31
0.99
0.02
0.01
98.98
2.0
75.0
23.1
52.68
0.15
0.42
0.99
18.21
0.60
0.05
24.81
1.40
0.02
0.04
99.38
2.8
68.9
28.3
53.83
0.04
0.41
0.42
15.34
0.59
0.03
27.21
1.07
0.06
0.00
98.99
2.1
74.4
23.5
51.55
0.38
0.45
1.01
21.69
0.77
0.00
20.82
3.43
0.02
0.00
100.12
7.0
58.7
34.3
51.72
0.21
0.54
0.94
20.72
0.65
0.00
20.97
3.29
0.01
0.00
99.04
6.8
60.0
33.2
52.33
0.16
0.84
2.13
19.75
0.67
0.09
21.07
3.40
0.02
0.00
100.44
7.1
60.9
32.0
53.95
0.21
1.09
2.27
12.00
0.47
0.03
28.39
1.16
0.02
0.04
99.62
2.3
79.0
18.7
53.31
0.15
0.42
0.84
19.42
0.64
0.00
22.24
2.06
0.02
0.00
99.09
4.3
64.3
31.5
51.80
0.30
0.85
1.87
20.64
0.66
0.03
21.36
3.23
0.00
0.00
100.74
6.6
60.6
32.8
2000; Greenwood and Franchi 2004; Martins et al. 2007;
Janots et al. 2011).
Additional precision on the nature of the chondritic
component is given by the siderophile element
composition of JaH 556. To date, the siderophile
whole-rock composition of JaH 556 is unique. In JaH
556, Ni and Co concentrations are the highest
documented in any howardite (Fig. 4c), with a Ni ⁄ Co
ratio approximately 25. On average, eucrites and
diogenites contain only a 0.1 to 200 ppm Ni and 5 to
100 ppm Co (Warren et al. 2009), while higher
siderophile concentrations are found in howardites
(average of 503 ± 183 ppm of Ni; Warren et al. 2009).
Although some rare eucritic meteorites can contain
relatively high abundances of Ni and Co (Buchanan
et al. 1993), due to abundant Fe ⁄ Ni metal grains in the
matrix (Duke 1965), high siderophile concentrations are
usually attributed to exogenous contamination. Taking
the Ni content of the L chondrite impactor to be
12,000 ppm (Wasson and Kallemeyn 1988), and the
Howardite target rock to be 270 ppm (Warren et al.
2009), then a 10% L chondrite, 90% HED mix will
give a value of 1550 ppm, too low to account for the
measured value of 3940 ppm Ni in JaH 556.
Considering an H chondrite impactor with an average
D17O = 0.73& (Clayton et al. 1991), JaH 556 would
correspond to a 13% H chondrite, 87% HED mix.
Given an H chondrite Ni content of 16,000 ppm
(Wasson and Kallemeyn 1988) this mix would result in
a Ni content for bulk JaH 556 of 2320 ppm. The lowest
D17O value measured in an H chondrite is that for the
H3 find Willaroy (D17O = 0.40&; Clayton et al. 1991);
a value confirmed by our measurements of this
meteorite. This value would indicate a 20% H
chondrite—80% HED mix for bulk JaH 556. Using the
H chondrite Ni content of 16,000 ppm would give a
predicted Ni content for bulk JaH 556 of 3420 ppm,
which is relatively close to the measured value of
3940 ppm (3460 ppm measured by XRF).
On the one hand, the unique nature of the JaH 556
meteorite suggests that ordinary chondrite admixtures
may be rare at the HED parent body surface.
Alternatively, the scarcity of such howardites could be
explained by a collection bias. The observed meteorite
record indicates that meteorites attributed to 4 Vesta
record an important contribution of carbonaceous
chondrite. Carbonaceous chondrite contribution is
commonly due to micrometeorites in 4 Vesta, as also
observed at the Earth’s surface (Lorenz et al. 2007). If
compared with the terrestrial record of observed
meteorite fall statistics and meteorite finds in desert areas
(e.g., Bevan et al. 1998; Zolensky 1998), exogenic
contribution of H and L chondrite with millimetric or
larger size on HED asteroids should statistically be more
frequent than carbonaceous chondrites.
Impact Melting and Clast Re-Equilibration
Most meteorites from the HED group display
textural features typical of thermal metamorphism and
complex impact events (Takeda 1991), the chronological
sequence of which has been summarized in Metzler et al.
(1995). The first stage involves the primary crystallization
of the magmatic rocks. The second stage is characterized
by recrystallization during slow cooling after a reheating
phase. The last stage corresponds to multiple periods of
impact brecciation and recrystallization due to thermal
metamorphism. Although the question of the heat source
1570
E. Janots et al.
a
Clast1
b
Pl
Pl
Cpx
Ol
Cpx
Ol
Pl
Pl
Px
Cpx
Pl
20 µm
c
50 µm
d
Pl
Pl
Pl
100 µm
e
50 µm
f
Cp
x
Cpx
Opx
100 µm
g
50 µm
h
Ol1
Ol
Ol2
50 µm
Opx
50 µm
Fig. 6. Backscatter electron images of mineral clasts and matrix. a) Matrix composed of plagioclase (dark gray) and oxide grains,
surrounded by zoned clinopyroxene (gray); b) subeuhedral olivine (ol) at vicinity of the diogenite clast (D-clast); c) flow-shaped
polycrystalline plagioclase clast (pl) containing abundant oxide inclusions; d) polycrystalline plagioclase clasts with olivine
inclusion; e) clinopyroxene clast; f) zoned pyroxene grains with an orthopyroxene core (Opx) and a clinopyroxene rim (Cpx). Note
two sets of perpendicularly oriented fractures; g) zoned olivine showing a Mg-rich Ol 1 core (dark gray) and Fe-enriched Ol 2 rim
(bright gray); h) polycrystalline olivine enclosing orthopyroxene (Opx) relics.
is still in debate, impact alone appears insufficient to
supply the heat required for the thermal metamorphism
of the HEDs (Keil 2000).
The textural observations in JaH 556 indicate that
melting took place (Figs. 1 and 6) during at least one major
impact event (Stöffler et al. 1991). JaH 556 contains highly
JaH 556: A howardite mixed with an H chondrite
0.04
a
a
Clast1
matrix
mineral clast
0.03
Ol
Opx
Cp
clast2
x
clast3
K (afu)
1571
rim
0.02
50 µm
Matrix
0.01
b
Clast1
Ol
Opx
0
0
0.40
20
An (%)
60
80
100
b
Ol
matrix
Cpx clast
zoned clast core : Opx
zoned clast rim : Cpx
clast1 : Opx
clast1rim : Cpx
clast2
clast3
0.30
Ca (afu)
40
0.20
Matrix
200 µm
c
Ol
Ol
Cpxrim
0.10
0
0
0.1
0.2
c
XFe
0.3
0.4
matrix and homogeneous clasts
zoned clast core: Ol1
zoned clast rim : Ol2
clast1
clast1 intergranular
clast2
clast2 rim
clast3
Mn (afu)
0.02
Matrix
0.5
d
0.01
20 µm
Matrix
Clast2
100 µm
0.00
0
0.1
0.2
0.3
XFe
0.4
0.5
0.6
e
Fig. 7. EMP analyses of a) anorthite contents versus K (atoms
per formula unit—afu) in plagioclase; b) XFe versus Ca (afu) in
pyroxene; c) Fe (afu) versus Mn (afu) in olivine. The different
symbols correspond to distinct textural positions mentioned in
the text.
shocked clasts (mosaicized olivine and orthopyroxene
displaying sets of decorated planar fratures) in a fine and
vesicular matrix, which is interpreted to result from
recrystallization of an impact melt. Other original shock
features include the flow-deformed shape and
polycrystalline structure of plagioclase containing
abundant inclusions. These features are reminiscent of
plagioclase described in the Martian shergottites GRV
99027 (Wang and Chen 2006) where the plagioclase texture
is interpreted to be the result of shock-induced melting,
Clast3
Matrix
100 µm
Fig. 8. Electron backscattered images of the lithic clasts. a)
Clast 1 with a brighter rim composed of clinopyroxene; b) Clast
1 with olivine protuberance; in (a) and (b) Clast 1 limit is
marked by a white line; c) zoom out of the rectangle in Fig. 8b.
It shows that the contact between clast1 and matrix consists of
clinopyroxene and microcrystalline olivine; d) spherical Clast 2
with barred olivine intercalated with orthopyroxene, and
plagioclase. Clast 2 rim is enriched in Fe in comparison to the
core; e) Clast 3 displaying olivine rim enclosing an assemblage
of plagioclase and orthopyroxene.
1572
E. Janots et al.
recrystallization, and exsolution. Based on the plagioclase
texture, Wang and Chen (2006) proposed that GRV 99027
experienced a slower cooling than samples with
maskelynite. Similar slow cooling can be envisaged in JaH
556, where the silicates are completely recrystallized with no
remaining relics of impact melt pockets.
In JaH 556, olivine and orthopyroxene clasts are
commonly zoned with decreasing Mg# toward the matrix.
Locally, orthopyroxene reacts to form clinopyroxene and
olivine. The low SiO2 bulk composition may explain why
orthopyroxene is absent in the microcrystalline matrix and
olivine is found instead. It is noteworthy that
orthopyroxene replacement by olivine is common in
terrestrial peridotite xenoliths and ophiolites (e.g., Braun
and Kelemen 2002) and is caused by reaction with a SiO2undersaturated
melt.
This
was
demonstrated
experimentally by Shaw et al. (1998), who were able to
reproduce similar replacements when orthopyroxene was
placed in contact with an SiO2-undersaturated melt.
In JaH 556, the thermal increase due to impact and
thermal annealing provoked plagioclase melting and
possibly caused partial dissolution of the eucritic,
diogenitic, and chondritic material. It is highly probable
that SiO2-undersaturated melt was produced by partial
dissolution of chondritic material in the impact melt and
that it was this dissolution reaction that generated
numerous olivine grains in the matrix. This also explains
why so little chondritic material was observed
petrographically. This mechanism precludes the idea that
chondrite clasts were added to the HED after the impact
melting. However, it is unclear whether chondritic
material was assembled before, or during the impact that
caused the melting. In the case of other howardite
samples, it is thought that the carbonaceous chondrite
material is incorporated in the regolith before its
lithification and communition (Pun et al. 1998). In JaH
556, the chondritic contribution is much higher than that
seen in other howardites, which suggests that the H
chondrite material is derived from a major impactor.
From our sample alone, it is impossible to estimate the
size of the original H chondrite impactor and so to
conclude whether or not it caused the major shock
features and thermal history observed in JaH 556.
SUMMARY AND PERSPECTIVE
JaH 556 is a unique howardite specimen with a bulkrock oxygen isotopic signature diverging from the HED
field due to a significant contribution (about 20 wt%) of
ordinary chondrite impactor material that may have
caused JaH 556 to melt and recrystallize.
In the available meteorite collection, basaltic
meteorites are mainly attributed to 4 Vesta and there is
only little evidence of differentiated asteroids. Based on
the oxygen isotopic composition, Scott et al. (2009) and
Yamaguchi et al. (2002) proposed that several HED
meteorites with anomalous oxygen isotope characteristics
are derived from other asteroid bodies than 4 Vesta. It is
inferred that anomalous eucrites may be derived from
distinct Vesta-like parent bodies, although the possibility
of contributions from impactors is not ruled out for Ibitira
and NWA 1280. In JaH 556, the oxygen isotopic deviation
is unambiguously attributed to an exogenic source by the
convergence of oxygen isotope data, high siderophile
element concentrations, and petrographic identification of
chondrule relics. Petrographically, chondritic material is
underrepresented (<5% of the clasts). Olivine (re)crystallization suggests that original chondritic material
partly dissolved in the impact melt. This means that the
exogenic component cannot be ruled out although not
visible in other anomalous HEDs. Extreme caution is
therefore recommended before attributing slight isotopic
deviations to an anomalous eucrite with an origin from a
V-type asteroid. Based on the results of this study, it is
probably a good idea to investigate in more detail the
oxygen isotope composition of clasts in other anomalous
HEDs.
The complex JaH 556 meteorite enhances our
understanding of the lithological diversity present on the
surface of the asteroid 4 Vesta, which is currently being
studied by the NASA Dawn spacecraft. The unique
nature of this sample suggests that it may be
unrepresentative of the HED parent body surface.
Alternatively, the high proportion of ordinary chondrite
found on Earth’s surface suggests that rocks similar to
JaH 556 are locally expected at the surface of 4 Vesta.
Acknowledgments—This study was partially financed
through Swiss National Foundation research grants
107681 and 119937. Ing. Salim Omar Al Ibrahim, Dr.
Ali Al Rajhi and Dr. Salim Al Busaidi, Directorate
General of Minerals, Ministry of Commerce and
Industry, Muscat, are thanked for their support during
the project. We appreciated the constructive reviews of
one anonymous reviewer and T. Mikouchi, and of the
editor, M. Zolensky. We also thank J. Berndt-Gerdes for
his help at the electron microprobe.
Editorial Handling—Dr. Michael Zolensky
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