Progress in Earth andPlanetary Science(2021) 8:55
Kimura et al. Prog Earth Planet Sci
https://doi.org/10.1186/s40645-021-00447-2

Open Access

RESEARCH ARTICLE

An Almahata Sitta EL3 fragment:
implications for the complex thermal historyof enstatite chondrites
M. Kimura1,2* , M. K. Weisberg3,4, A. Takaki2, N. Imae1,5 and A. Yamaguchi1,5Abstract
Almahata Sitta is a polymict breccia, consisting of many kinds of clasts. Here we present our mineralogical andpetrological results on an EL3 fragment, MS-177 from Almahata Sitta. This fragment shows a typical type 3 chondritic
texture, consisting of well-defined chondrules, isolated silicate minerals, and opaque nodules. Most chondrules
are enstatite-rich with some having olivine. Although these components are typical of EL3 chondrites, the mineralabundances and compositions are different from the other EL3s. Diopside is unusually abundant in MS-177. On theother hand, perryite and daubreelite were not found. The major pyroxene is orthoenstatite, and the silica phase isquartz. Fe–Ni metal has relatively high P contents. Troilite is enriched in Cr and Mn. Keilite and buseckite are present in
MS-177. From the mineralogy and texture, MS-177 experienced a high-temperature event under subsolidus conditions. Shock-induced heating for a short duration might explain this high-temperature event. We suggest that otherE3 chondrites also experienced heating events under such subsolidus conditions on their parent bodies. On the other
hand, the high abundance of diopside cannot be explained by a secondary thermal event and may have been a
primary feature of MS-177, formed before accretion to the parent body.
Keywords: Enstatite chondrite, Breccia, Thermal history, Opaque minerals
1 Introduction
Almahata Sitta fell in northern Sudan on October 7, 2008
and is a remarkable polymict breccia consisting of various ureilitic and chondritic fragments (e.g., Bischoff et al.
2010a, b; Horstmann and Bischoff 2014). The chondritic
fragments include enstatite (E), ordinary, carbonaceous,and R-like chondrites. E chondritic fragments include
EH3-6, EL3-6, and impact melt rocks. Here, we present
a mineralogical and petrological study of the E chondrite
fragment MS-177, which is classified as an EL3 (Hain,
2012; El Goresy et al. 2012).
Many studies have been conducted on not only EL3
but EH3 chondrites, to understand their complex thermal histories. Lin and El Goresy (2002) and Horstmann*Correspondence: kimura.makoto@nipr.ac.jp
1
National Institute of Polar Research, Tokyo, Japan
Full list of author information is available at the end of the article

et al. (2014) suggested that opaque nodules preserve
evidence of variable thermal histories prior to accretionto the parent body, such as gas–solid condensation andmelting. On the other hand, Zhang and Sears (1996) andWeyrauch et al. (2018) suggested that geothermometers
for E3-6 chondrites do not always reflect their petrologic
type. Rubin et al. (1997) showed that some E3 chondrites
experienced heavy shock effects (S3-5). Kimura et al.
(2017) studied a heavily shocked EH3, Asuka (A) 10164.
A 10164 has a shock vein containing coesite, indicatingthat it experienced shock pressures of 3–10 GPa at temperatures of 1000 °C. The host chondrite was also heated,
between 700 and 1000 °C, during the shock event. All ofthese studies show that the thermal and shock histories
of EL3 chondrites are very complicated.
We selected MS-177 for study because it experienced
an unusual shock-induced thermal history and is important for elucidating the complicated thermal history of E

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Kimura et al. Prog Earth Planet Sci(2021) 8:55

chondrites. El Goresy et al. (2012, 2017) studied opaquenodules in this fragment and in another fragment,
MS-17. They discussed a condensation origin for thephases in the nodules, especially oldhamite and sinoite,
in the early solar nebula. Here we focus on the parent
body thermal history of MS-177. We compare the resultsfrom MS-177 with two other EL3 chondrites, Asuka (A)881314 and the heavily shocked EH3 chondrite, A 10164(Kimura et al. 2017).
It is difficult to evaluate the thermal histories of E chondrites, especially the subsolidus conditions, by studying
their silicate minerals. We do not have the appropriate
silicate geothermometers, except for silica polymorphs(Kimura et al. 2005). Instead, some of the opaque minerals typical of E chondrites have been used to give quantitative constraints on the physical conditions of their
thermal histories (e.g., Zhang and Sears 1996; Lin and El
Goresy, 2002; Horstmann et al. 2014; Kimura et al. 2017).
In addition, Weyrauch et al. (2018) recently proposedthat E chondrites (EH and EL) are divided into “a” and“b” subgroups, based on the compositions of their sulfide
minerals, which are not dependent on petrologic type(i.e., thermal alteration). This classification is significant
in providing hints for understanding the thermal histories of E chondrites. We studied the opaque minerals in
MS-177 to help constrain its thermal history. We expectthat these geothermometers can help constrain the complicated thermal history not only for this fragment but
also other E chondrites.

2 Sample and experimental methods
We investigated a polished thin section (PTS) of the
MS-177 (EL3) fragment from Almahata Sitta. For comparison, we also studied a PTS of a typical EL3 chondrite,
A-881314, 51-1.
Back-scattered electron (BSE) images were obtainedusing the JEOL JSM-7100F field emission scanning electron microscope (FESEM) at the National Institute ofPolar Research (NIPR). Mineral analyses were conductedusing the JEOL JXA8200 electron-probe microanalyzer at
NIPR with a focused (1–2 µm) beam. The beam currents
used were 4 nA for glass, plagioclase, and silica, and 30
nA for pyroxene, olivine, and opaque minerals at 15 keV.
The analytical and correction methods were the sameas those described by Kimura et al. (2017). The X-ray
overlaps of ­Ka on ­Kb lines of some elements, Fe–Co and
Ni–Cu, were corrected with a deconvolution program.
We identified some phases using a laser micro Raman
spectrometer (JASCO NRS 1000) at the NIPR after the
method of Kimura et al. (2017). The elemental X-ray
maps of the section were obtained using the FESEM. We
determined modal abundances from elemental maps of
the whole areas of the PTS. ImageJ software was used toPage 2 of 12

calculate the modal abundances of minerals based on the
X-ray intensities of each pixel. Imae et al. (2019) appliedan X-ray diffraction (XRD) method to characterize minerals in meteorite sections by using SmartLab, RIGAKU
at the NIPR. We used the same method for the identification of pyroxene in the sample studied here.

3 Results
3.1 Petrography

Figure 1a–d show the section of MS-177 studied here.
MS-177 contains abundant, sharply bounded chondrules of spherical to ellipsoidal shapes, opaque nodules and isolated minerals. The fine-grained matrix is
a minor component and not easily discernible like inother E3 chondrites (e.g., Weisberg and Kimura 2012).
The texture of MS-177 is indicative of a type 3 E chondrite, which is supported by the occurrence of olivine insome chondrules (Weisberg and Kimura 2012; Weyrauch
et al. 2018). The opaque minerals in MS-177 are typicalof E chondrites, such as Si-bearing Fe–Ni metal and a(Mg,Mn,Fe)S phase (Weisberg and Kimura 2012).
Chondrules show well-defined outlines and are
0.69 mm in average diameter, typical of EL3 chondrites(0.6 mm) (e.g., Jones 2012). Most chondrules are porphyritic (pyroxene-dominated) and a few radial pyroxene
types are present. Porphyritic chondrules mainly consistof low-Ca pyroxene phenocrysts and a devitrified mesostasis consisting of fine-grained high-Ca pyroxene andplagioclase, commonly with a silica phase. Olivine, whenpresent, is generally poikilitically enclosed within pyroxene phenocrysts. One of the characteristic features of
MS-177 is a high abundance of high-Ca pyroxene in thechondrules (Figs. 1c and 2a). It is generally not a common mineral in E3 chondrites. In chondrules of MS-177,
high-Ca pyroxene occurs as coarse-grained phenocrysts,
as rims on low-Ca pyroxene phenocrysts, and as finegrained laths associated with plagioclase in chondrulemesostases. Fe–Ni metal and troilite with small amounts
of a (Mg,Mn,Fe)S phase, oldhamite, and an (Fe,Zn)S
phase are present in some chondrules. Isolated silicate
minerals include low-Ca pyroxene, high-Ca pyroxene,
plagioclase, and a silica mineral.
Opaque minerals in nodules and as isolated grains aremainly Fe–Ni metal with troilite and rare schreibersite(bright grains in Fig. 1d). Fe–Ni metal in the opaque nodules includes abundant euhedral laths of pyroxene (bothlow and high-Ca pyroxenes) with plagioclase and theoxynitride sinoite ­(Si2N2O; Fig. 2b). Oldhamite and the(Mg,Mn,Fe)S phase are rare in the nodules.
MS-177 shows some features generally interpreted to
be shock-induced, such as undulose extinction, presenceof irregular fractures in pyroxene and olivine, rare planar
fractures in pyroxene, and rare sulfide veins. We could

Kimura et al. Prog Earth Planet Sci(2021) 8:55

Page 3 of 12

Fig. 1 a Photomicrograph (transmitted plane-polarized light) of a thin section of MS-177 containing abundant sharply bounded chondrules.
The section shows a darkened appearance. b Backscattered electron (BSE) image of the MS-177 section showing abundant spherical chondrules(dark) with well-defined outlines. Opaque minerals (mainly Fe–Ni metal and troilite) (bright) are abundant in MS-177. c Combined elemental map
in Mg (red), Ca (green), and Al (blue) showing a high abundance of diopside (light green) in chondrules and as isolated minerals. d Reflected
light photomicrograph of a part of MS-177 containing chondrules and isolated minerals. Fine-grained Fe–Ni metal and troilite (bright grains) areabundantly distributed in silicate minerals resulting in the darkened appearance

not observe undulose extinction of plagioclase becauseof its small grain size. However, the plagioclase is notmaskelynite, as discussed below, and mosaicism of pyroxene was not observed. All these features indicate that theshock stage of MS-177 is S2-3, after Rubin et al. (1997).
We found neither melt pockets nor melt veins in
MS-177. The latter was reported in the heavily shocked
A 10164 EH3 chondrite (Kimura et al. 2017). Euhedral
laths of enstatite typical of melt rocks (Rubin and Scott
1997; Lin and Kimura 1998; Kimura and Lin 1999)
were not found in MS-177, except in the opaque nodules, which is a characteristic of most EL3 chondrites

regardless of shock stage. Figure 2c shows the texture of metal and troilite as isolated minerals. They
do not show a eutectic intergrowth texture, which iscommonly present in a shock melt veins in A 10164(Kimura et al. 2017). Fagan et al. (2000) reported shockinduced features in an EL chondrite, RKP 80249, suchas enstatite prisms and opaque minerals in feldspathic
glass. These features were not encountered in MS-177.
On the other hand, MS-177 shows darkening underplane polarized transmitted light (Fig. 1a) and reflected
light (Fig. 1d) in the optical microscope. Many silicate
phases contain abundant fine-grained opaque minerals
resulting in the darkened appearance.

Kimura et al. Prog Earth Planet Sci(2021) 8:55

Page 4 of 12

Fig. 2 BSE images of chondrules, an opaque nodule and metal from MS-177: a a chondrule consisting of abundant diopside (Di) with enstatite(En) and devitrified glass (DGl), in close association with abundant Fe–Ni metal (white). Black prismatic void spaces are encountered. b An opaquenodule consisting of Fe–Ni metal (Met) including laths of enstatite, diopside, and sinoite (Sin). c Isolated Fe–Ni metal, troilite (Tro), and schreibersite(Sch). d A chondrule mainly consisting of enstatite with oldhamite (Old) and buseckite (Bus)3.2 Modal compositions

Table 1 shows the modal composition of the constituent
minerals in MS-177 and average EL3 chondrite (Weisberg and Kimura 2012). The most abundant phase is
low-Ca pyroxene (mostly enstatite), as is typical of EL
chondrites. In comparison to the average EL3 (Weisberg
and Kimura 2012), high-Ca pyroxene is highly abundantin MS-177, whereas olivine is not so abundant. Troilite isslightly more abundant than the average, but within therange of other EL3s. Schreibersite and the (Mg,Mn,Fe)
S phase are slightly less abundant, and perryite anddaubreelite were not found although they are typicalminerals in EL chondrites.
3.3 Silicate mineral chemistry

Table 2 shows representative analyses of silicate minerals.
Low-Ca pyroxene contains ­Fs<1.3 and ­Wo<1.4 components,

whereas high-Ca pyroxene is ­Fs<1.4 and ­Wo43–48. Hereafter we call them enstatite and diopside, respectively.
FeO-rich low- and high-Ca pyroxene were not found in
MS-177 or A-881314. Enstatite contains < 0.8 wt% ­Al2O3,
and ­TiO2; ­Cr2O3, and MnO are below detection limits (Table 3). Diopside contains < 0.2 wt% ­TiO2, 0.2–1.1­Al2O3, < 0.13 ­Cr2O3, and < 0.12 MnO. In comparison,
enstatite in the A-881314 EL3 contains ­Fs<1.7 and ­Wo<0.8,
which is consistent with that in the MS-177 fragment.
On the other hand, diopside was not found in A-881314.
Olivines, both in MS-177 and A-881314, are F­ a<1. They
contain < 0.15 wt% MnO, whereas the other minor elements are below detection limits. FeO-rich olivine was
not encountered in MS-177 or A-881314.
Plagioclases, both in MS-177 and A-881314, areenriched in the albite component, A­ b61–74 and A­ b63–91,
respectively. Devitrified glasses in chondrules of MS-177

Kimura et al. Prog Earth Planet Sci(2021) 8:55

Page 5 of 12

Table 1 Modal composition (vol%) of the MS-177 and averageEL3 chondrite
MineralMS-177EL3 (average)
65.2

Enstatite

62.2

Diopside

3.4

0.3

Olivine

0.4

2.4
10.4

components. Some devitrified mesostasis glass contains
up to 2.4 wt% Cl. El Goresy et al. (1988) also reported
Cl in the mesostasis in chondrules in the Qingzhen EH3chondrite. Chlorine-bearing phases were not identified in
the chondrules from MS-177, such as djerfisherite, lawrencite, or hibbingite. Therefore, most chlorine is contained in the glassy mesostasis of MS-177 chondrules.
The silica mineral in MS-177 is nearly pure ­SiO2.

Plagioclase

13.6

Silica0.7

1.1

Fe–Ni metal

9.4

10.1

3.4 Opaque mineral chemistry

Schreibersite

0.2

0.6

Perryiten.i

0.1

Table 3 shows representative analyses of the opaque minerals in MS-177. The Fe–Ni metal contains 0.3–0.6 wt%
Si, which is within the range of A-881214 (Fig. 3a) andother EL chondrites, 0.4–1.0% (Weisberg and Kimura2012). On the other hand, the metal contains 0.3–0.6 wt%
P and 6.4–7.9 Ni, higher than in the metal of A-881314(Fig. 3b). The Ni contents of metal in MS-177 are within
the range of metal in other EL3 chondrites (Weisberg andKimura, 2012), but the P contents are higher than in mostother EL3 chondrites (< ~0.1 wt%, unpublished data;
Weisberg et al. 1995). This is one of the characteristic features of MS-177.
Schreibersite contains < 0.12 wt% Si and 7.0–7.8 wt%
Ni. Although the Si contents are the same as those in

Troilite

9.7

7.6

Oldhamite

0.3

0.6

Keilite–alabandite

trace

0.4

Daubreelite

n.i

0.3

Buseckite

trace

–

Sinoite0.1–

n.i. not identified, Weisberg and Kimura (2012)

contain 12.4–29.2 wt% A­ l2O3, 0.2–15.2 MgO, 0.8–13
CaO, 2.9–9.5 ­Na2O, and 0.2–1.3 ­K2O, indicative thatthey are mainly comprised of plagioclase and diopside
Table 2 Representative analyses of silicate phases (wt%)
Phase

Occurrence

SiO2

TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO

Na2O K2O Cl

Total

Fo/Wo/An En/Ab Fs/Or

Plagioclase Chondrule62.55 b.d

22.83

b.d

0.35 b.d0.04 4.88

8.31

0.42 b.d99.37 23.9

Glass

Chondrule58.45 0.21

23.02

b.d

0.30 b.d0.33 6.66

7.77

0.62 1.73

98.77

73.7

2.4

Glass

Chondrule63.51 b.d

19.69

0.07

b.d

b.d

7.31 4.24

6.15

0.50 0.15 101.64

OlivineChondrule42.63 b.d

b.d

b.d

0.06 0.15

56.60 b.d

b.d

b.d

b.d

99.44 99.9

Diopside

Chondrule56.05 0.05

0.24

0.07

0.39 0.09

19.03 24.51 0.11

b.d

b.d

100.54 47.8

51.6

0.6

Diopside

Isolated mineral 55.63 b.d

0.24

b.d

0.35 b.d

19.18 24.59 0.15

b.d

b.d

100.15 47.7

51.8

0.5

Enstatite

Chondrule58.91 b.d

0.30

b.d

b.d

b.d

39.22 0.49

0.06

b.d

b.d

98.98

0.9

99.1

0.0

Enstatite

Isolated mineral 60.03 b.d

0.05

b.d

0.33 b.d

39.34 0.47

b.d

b.d

b.d

100.22

0.8

98.7

0.5

QuartzChondrule

0.10

b.d

0.13 b.d0.10 0.08

0.13

b.d

b.d

97.22

96.62 0.07

b.d.: below detection limits (3σ), 0.03 for ­SiO2, ­Al2O3, MgO, and CaO, 0.04 for ­TiO2, ­Na2O, and ­K2O, 0.05 for FeO, C­ r2O3, and Cl, and 0.08 for MnO

Table 3 Representative analyses of opaque minerals (wt%)
Phase

Occurrence

Mg

Si

P

S

Ca

Ti

Cr

Mn

Fe

Co

Ni

Cu

Zn

Total

KeiliteChondrule

2.63

0.05

b.d

38.25

2.47

0.15

7.11

18.99

24.73

b.d

b.d

0.10

0.44

94.92

Fe–Ni metal

Chondrule

b.d

0.45

0.42

b.d

b.d

b.d

0.06

b.d

90.13

0.33

7.48

b.d

0.08

98.94

Fe–Ni metal

Isolated mineral

b.d

0.42

0.41

b.d

b.d

b.d

b.d

b.d

90.38

0.29

7.59

0.06

b.d

99.27

Oldhamite

Chondrule

0.61

0.20

0.04

40.90

53.28

b.d

b.d

1.19

0.77

b.d

b.d

b.d

b.d

96.99

Schreibersite

Isolated mineral

b.d

0.05

14.61

b.d

b.d

b.d

0.06

b.d

76.86

0.25

7.46

b.d

b.d

99.30

Buseckite

Chondrule

0.92

0.05

b.d

33.92

b.d

b.d

b.d

5.00

29.09

b.d

b.d

b.d

26.12

95.09

Troilite

Chondrule

0.08

b.d

b.d

36.40

b.d

0.32

2.41

1.33

57.58

b.d

0.20

0.07

b.d

98.40

Troilite

Isolated mineral

b.d

b.d

b.d

36.19

b.d

0.28

2.14

1.28

58.11

b.d

0.24

0.09

b.d

98.34

b.d.: below detection limits (3 s), 0.03 for Mg, Si, P, Ti, and Ca, 0.05 for S, Co, Ni, Cr, Mn, Fe, Zn, and Cu
The data of Fe–Ni metal and schreibersite as isolated mineral in this table are use to calculate geothermometries (see Text)

Kimura et al. Prog Earth Planet Sci(2021) 8:55

Page 6 of 12abFig. 4 Compositions of schreibersite in MS-177 and A-881314 on Ni
versus Si plotaFig. 3 Compositions of Fe–Ni metal in MS-177 and A-881314 on a Ni
versus Si plot, and b Ni versus P plot

A-881314 schreibersite, the Ni contents are muchhigher (Fig. 4).
Troilite contains 2.1–2.6 wt% Cr, 0.9–1.6 Mn, and
0.3–0.4 Ti. The Mn and Cr contents are higher thanthose in A-881314 troilite (Fig. 5a), whereas Ti contents
are lower (Fig. 5b).
(Mg,Mn,Fe)S grains are rare in MS-177 (Table 3).
Those found are identified as keilite, based on theircompositions (Fig. 6). This is in contrast to the occurrence of alabandite in A-881314, which is a common
mineral in EL chondrites (Weisberg and Kimura, 2012).
Although the keilite contains 7.1 wt% Cr, daubreelite orother Cr-bearing phases were not observed in MS-177.
Oldhamite is not common in MS-177 and contains
0.5–0.8 wt% Mg, 1.1–1.3 Mn, and 0.7–1.0 Fe.
An (Fe,Zn)S grain was found among enstatite grains
in one chondrule (Fig. 2d). It contains 29.1 wt% Fe and26.1 Zn. From the composition, the chemical formulaof this phase is (­ Fe0.49Zn0.38Mn0.09Mg0.04)1.00S1.

b

Fig. 5 Compositions of troilite in MS-177 and A-881314 on a Cr
versus Mn plot, and b Ti versus Cr plotKimura et al. Prog Earth Planet Sci(2021) 8:55

Page 7 of 12

4 Discussion
4.1 Classification

Fig. 6 Compositions of (Mg,Mn,Fe) S phases in MS-177 and A-881314
on FeS–MgS–MnS plot (atomic ratio)

3.5 Raman spectroscopy

Using laser Raman microscopy, we analyzed 9 feldspar
grains and determined they are all plagioclase, basedon the typical peaks of 506 and 206 ­cm−1 (Fig. 7a).
They are not kumdykolite, a high-temperature polymorph of albitic plagioclase (Németh et al. 2013).
Maskelynite or glass was not identified in any of thefeldspars we analyzed. All of the silica (9 grains) isquartz, from the peak of 465 ­cm−1 (Fig. 7b), regardless of the textural setting. This is different from thatin other EL3s in which the silica mineral is commonly
tridymite or cristobalite (Kimura et al. 2005).
Figure 7c shows the Raman spectrum for the (Fe,Zn)
S phase. The distinct peak of 300 ­cm−1 indicates that
this is buseckite, and not rudashevskyite, a low-temperature polymorph of buseckite. This phase was firstreported in an enstatite-rich achondrite, Zakłodzie, by
Ma et al. (2012). To our knowledge, our finding is thesecond report of this mineral.
Fe–Ni metal contains sinoite grains which wereidentified by the peaks of 984, 734, 495, and 456 ­cm−1,
consistent with the peaks of sinoite in the Y-793225 E
chondrite (Kimura et al. 2005).
3.6 XRD

We identified low-Ca pyroxene by XRD. The diffraction peaks of 321, 511, and 610 indices indicate thatthe dominant pyroxene is orthoenstatite, not clinoenstatite, in MS-177 (Fig. 8). Diopside (Dio) is also abundant, as identified from the XRD data.

MS-177 was classified as an EL3 (Hain 2012; El Goresy
et al. 2012). Our observations support this classification.
The average diameter of the chondrules and Si contents
in Fe–Ni metal and schreibersite are typical of EL chondrites. Although MS-177 experienced shock metamorphism (S2-3), it still preserves abundant well-definedchondrules with devitrified glassy mesostasis. These features are indicative of petrologic type 3. Chondrules also
contain olivine. This is the key evidence for identificationof type 3 E chondrites (e.g., Weisberg and Kimura 2012;
Weyrauch et al. 2018).
Weyrauch et al. (2018) proposed that both EH and EL
chondrites are divided into “a” and “b” subgroups basedon their sulfide compositions, in spite of their petrologic
type, since the compositions of sulfides do not dependon petrologic type. From their criteria, MS-177 is classified as an ELb3, based on the compositions of troiliteenriched in Cr and Mn.
MS-177 has features that distinguish it from typicalEL3 chondrites, including A-881314, such as occurrencesof orthoenstatite, presence of quartz, and keilite, andabsence of daubreelite. The high abundance of diopsideis an especially unusual feature in comparison with anyother E3 chondrite. In the following section, we discuss
the unusual thermal history of MS-177.
4.2 Geothermometry

The thermal histories of meteorites have been addressedby mainly using silicate and oxide geothermometers.
However, most of these thermometers cannot be applied
to E chondrites because these minerals do not show solid
solution in E chondrites. Instead, we use texture, mineralpolymorphs, and opaque mineral features to elucidate thethermal history of the MS-177 EL3 fragment (Table 4).
MS-177 does not show evidence for shock-induced
melting such as melt veins or pockets. The texture is different from those of melt rocks, as mentioned above. Thissuggests that the fragment did not experience heating
higher than solidus temperature, < 1000 °C, after McCoy
et al. (1999). The enstatite is identified as orthoenstatite,
indicative of heating to temperatures > 600 °C, after
Smyth (1974), assuming that enstatite in the chondrules
and their fragments was initially clinoenstatite, as in thecase of other type 3 E chondrites (Weisberg and Kimura2012).
The absence of kumdykolite, a high-temperature polymorph of albitic plagioclase, indicates temperatures
below ~ 1000 °C (Németh et al. 2013). The occurrenceof quartz without tridymite, cristobalite or other polymorphs, suggests that equilibration temperatures wereKimura et al. Prog Earth Planet Sci(2021) 8:55

Page 8 of 12

a

b

c

Fig. 7 Raman spectra of a plagioclase in a chondrule, b quartz in a chondrule, and c buseckite in a chondrule. All are from chondrules in MS-177

lower than 900 °C (Swamy et al. 1994). In the case ofother type 3 E chondrites, the silica phases are generally cristobalite, tridymite, and glass, which crystallized
from chondrule melts (Kimura et al. 2005). Therefore,
the original silica phases in MS-177 were likely transformed into quartz during secondary heating below
900 °C.

Fe–Ni metal and troilite never show eutectic intergrowths, although they commonly occur in close association with each other. This observation indicatestemperatures lower than eutectic melting, 970 °C (Ryzhenko and Kennedy 1973). This is consistent with theconclusions of El Goresy et al. (2017) who negatedimpact melting for the formation of metal nodules inKimura et al. Prog Earth Planet Sci(2021) 8:55

Page 9 of 12

The occurrence of keilite, not alabandite, also indicateshigh-temperature conditions. The composition (Table 3;
Fig. 6) indicates ~ 600 °C for the keilite in MS-177, after
Skinner and Luce (1971). Oldhamite contains ~ 2 mol%MgS and MnS components. This also suggests temperatures of ~ 600 °C (Skinner and Luce 1971). The occurrence of buseckite, not rudashevskyite, also supports
high-temperature conditions (Ma et al. 2012), although
the phase relationships between buseckite and rudashevskyite is not yet clear.
4.3 Thermal history

Fig. 8 X-ray diffraction of 2 theta, 29°–33°, showing peaks fororthoenstatite (Oen) and diopside (Di) in MS-177

EL3 chondrites, based on their petrographic and isotopic
features. Schreibersite is stable below 1000 °C (Doan andGoldstein, 1970). Coexisting metal and schreibersite(Table 3) indicates temperatures of ~ 850 °C, based on thepartitioning of Ni between them, after Zhang and Sears(1996). Perryite was not found in MS-177. Although its
stability is not yet known, it seems that the absence ofperryite suggests high-temperature conditions (Kimura
et al. 2005).
Troilite commonly contains relatively high Cr contents(Table 3; Fig. 5). This suggests high-temperature conditions of more than 700 °C (El Goresy and Kullerud, 1969).
Under such conditions, daubreelite is not stable (El Goresy and Kullerud, 1969), and it was not found in MS-177.

From the geothermometers discussed above and frompetrographic observations, we discuss the unusual
thermal history of MS-177. This fragment as well as
A-881314 show typical type 3 E chondritic textures.
However, the geothermometers indicate that MS-177
experienced a high-temperature event under subsolidus
conditions, 600–850 °C. Horstmann et al. (2014) argued
that metal-silicate intergrowths (opaque nodule in thispaper) in EL3 chondrites did not form by impact melting. This is consistent with our temperature estimation.
The temperature variation should reflect different closure temperatures of the different geothermometers.
Nevertheless, we interpret the geothermometry toindicate that MS-177 experienced a high-temperature
event. We suggest the heating event took place on
the parent body because most minerals show similar
high-temperature conditions, reflecting near-equilibrium conditions, as recorded by the geothermometry
assessed in the previous section. This is different fromother E3 chondrites where opaque nodules show different thermal histories in each E3 (Lin and El Goresy2002). After the heating event, MS-177 cooled rapidly
because both olivine and silica survive and primary

Table 4 Geothemometries
Phase

Geothermometry

References

Metal-Schreibersite~ 850 °C

Zhang and Sears (1996)

Schreibersite< 1000 °C

Doan and Goldstein (1970)

Perryite

High-temperature

Kimura et al. (2005)

Keilite~ 600 °C

Skinner and Luce (1971)

Oldhamite~ 600 °C

Skinner and Luce (1971)

Buseckite

High-temperature

Ma et al. (2012)

No daubreelite (Cr-bearing troilite)≥ 700 °C

El Goresy and Kullerud (1969)

No metal-troilite eutectic intergrowth< 970 °C

Quartz< 900 °C

Ryzhenko and Kennedy (1973)
Presnell (1995)

Plagioclase (not kumdykolite)< 1000 °C

Németh et al. (2013)

Orthoenstatite> 600 °C

Smyth (1974)

No silicate melting< 1000 °C

McCoy et al. (1999)

Kimura et al. Prog Earth Planet Sci(2021) 8:55

chondrule textures remain. None of the geothermometers we used indicate low-temperature conditions, supporting this conclusion. Therefore, the thermal historyof MS-177 is different from equilibrated E chondrites
which were heated for long durations, resulting in lossof well-defined chondrule boundaries and disappearance of olivine (e.g., Kimura et al. 2005).
The lower abundance of schreibersite in MS-177 thanin A-881314 is consistent with the high Ni and P contents of the Fe–Ni metal, reflecting a high-temperature
event. The low abundance of keilite is explained by thehigh concentration of Mn in troilite. The absence of
daubreelite is consistent with our interpretation of thethermal history. Cr was mostly concentrated into thetroilite under high-temperature conditions.
MS-177 contains a lower abundance of olivine thanin other EL3 chondrites (Table 1). One possibility isthat the olivine reacted with silica to form secondary
enstatite under high-temperature conditions. However,
we did not observe evidence of secondary enstatite.
Poikilitic occurrence of olivine in enstatite phenocrysts
indicates that some olivine survived without reaction
with silica. Olivine is not very abundant in MS-177
and the abundance of silica is not different from that ofother EL3s. Therefore, the low abundance of olivine is a
primary feature of MS-177, although the possibility ofolivine depletion through secondary reactions cannot
be totally excluded.
In MS-177, diopside is highly abundant whereas
oldhamite is rarely encountered. A possible explanation is that oldhamite reacted with silicate minerals toform diopside during a thermal event on the parent
body. Fogel (1997) reported such metamorphic diopside in equilibrated E chondrites. However, diopsidein MS-177 does not show the textures expected if it
formed from secondary reactions in the parent body.
Instead, diopside shows euhedral to subhedral morphologies in chondrules and opaque nodules, suggesting crystallization from the chondrule melts. Ikeda(1989) also reported that diopside crystallized in chondrules in Y-691, EH3 chondrite. A diopside-rich chondrule was reported in the MAC 88136 EL3 (Weisberg
et al. 2012). Therefore, we conclude that the abundant
diopside in MS-177 is a primary feature, present inchondrules and opaque nodules before their accretion
onto the MS-177 parent body. This may be consistent
with the pre-accretionary origin of opaque nodules in
E chondrites, and not formation by an impact on theparent body (Hortsmann et al. 2014; Rindlisbacher
et al. 2021). Horstmann et al. (2014) excluded a nebular
origin for such nodules, from their trace element abundances. To further evaluate this possibility, we would
need trace element data for the nodules in MS-177.

Page 10 of 12

4.4 Formation process

In summary, the MS-177 EL3 fragment experienced secondary heating under subsolidus conditions on the parent body. The potential heat sources could be (1) internal
heating by short-lived isotopes, such as 26Al, as suggested
for equilibrated O chondrites (e.g., Huss et al. 2006),(2) external heating such as solar-radiation and electromagnetic induction (Huss et al. 2006), and (3) shockinduced heating for short duration which is proposed
for heated chondrites, such as in CM and CR chondrites(e.g., Kimura and Ikeda 1992; Nakato et al. 2008; Mahan
et al. 2018). This may be related to the observation thatmany E3 chondrites experienced shock effect (mostly
S3) (Rubin et al. 1997). As argued above, metamorphism
for long duration by internal heating can be excluded
for MS-177. The high temperatures induced by internal
heating is long lasting, ­106–108 years (Miyamoto et al.
1981). In this case, the texture of MS-177 should havebecome similar to those of E4-6 chondrites, in whichrecrystallization degrades chondrule boundaries and olivine disappears. Nevertheless, we cannot totally exclude
the possibility that the parent body was disrupted intofragments during this high-temperature stage.
It is difficult to assess solar-radiation heating and electromagnetic induction as heat sources. We would needdata for implanted solar-wind noble gases. MS-177
would have acquired implanted noble gases by the radiation on the parent body surface in the case of such heating. Additionally, it has been suggested that these heating
mechanisms cannot supply enough energy to efficiently
heat parent bodies (e.g., Huss et al. 2006).
On the other hand, MS-177 was shocked as suggestedby the shock-induced features described above (and isS2-3). A 10164 also experienced a shock event (S4) andformed a melt vein. Not only the vein, but the host of A
10164 was heated during the shock event (Kimura et al.
2017). From the observation of A 10164 and the shockfeatures of MS-177, shock-induced heating seems to be
the most plausible heat source but not to the same degree
as A 10164. The absence of high-pressure minerals suggests that the pressure conditions for MS-177 were low
in comparison with A 10164, which experienced shockinduced partial melting under conditions of 3–10 GPa
and 1000 °C (Kimura et al. 2017).
However, the post-shock temperature is not high in thecase of shock stage S2-3. The post-shock temperature isestimated to be lower than 150 °C in ordinary chondrites(Fritz et al. 2017). If such temperatures can be applied to
E chondrites, the estimated geothermometries outlined
above give much higher temperatures than expected. If
MS-177 experienced S5 stage, the temperature may be
600–900 °C (Fritz et al. 2017). However, mosaicism ofpyroxene and melt pockets (or their relicts), which areKimura et al. Prog Earth Planet Sci(2021) 8:55

indicative of S5 E and O chondrites (Rubin et al. 1997;
Fritz et al. 2017), are not observed in MS-177. We also
expect that such heavy shock would destroy the original
type 3 texture of MS-177, through partial melting andvein formation (Rubin et al. 1997). Therefore, we conclude that MS-177 was not heavily shocked. It may havebeen buried beneath an impact crater or deposited ina hot eject blanket as suggested for annealed ordinarychondrites (Rubin, 2004) and was heated to the subsolidus temperatures mentioned above, for a short duration. Following this heating event, MS-177 experienced
a shock event, resulting in its shock-induced features (ofS2-3).
The duration of high-temperature conditions did notcontinue for a long time, and the primary texture andolivine enclosed in pyroxene survived. Thus, the thermal history of MS-177 is unusual, differing from thehigh temperature and pressure conditions for the heavily
shocked A 10164, and the low-temperature conditions for
A-881314 containing Fe-poor alabandite, Mn-Cr-poor
troilite, and daubreelite. MS-177 is similar to subgroup bof E chondrites after Weyrauch et al. (2018). We suggestthat these Eb chondrites and MS-177 experienced similar high-temperature conditions on their parent bodies,
although the petrographic features, such as the primary
texture, survived during the high-temperature event.
The Almahata Sitta meteorite came from the asteroid
2008 TC3. The formation of this brecciated asteroid is acontroversial issue (e.g., Horstmann and Bischoff 2014).
However, urelilitic and chondritic materials were assembled into the parent body, which was very loosely consolidated (Bischoff et al. 2010a, b). This may suggest thatthe fragments did not react with each other in the parent
body. Therefore, the unusual thermal history of MS-177
originated before final accretion of the Almahata Sitta
parent body and developed from the original E chondritic parent body.

5 Summary
We studied fragment MS-177 from the Almahata Sitta
polymict breccia. MS-177 is classified as an EL3 chondrite based on petrographic observations and mineralchemistry. However, it shows mineralogical features,such as abundant diopside and lack of daubreelite, notobserved in any other E3 chondrite. From the mineralchemistry, MS-177 is also classified as an ELb chondrite,
based on the classification scheme proposed by Weyrauch et al. (2018). Several geothermometers that we usedsuggest that this fragment was subjected to a high-temperature event for a short duration, which is supportedby the preservation of olivine and the primary texture.
This may have resulted from a shock event. Enstatitechondrites, including type 3, experienced complicatedPage 11 of 12

formation processes including shock and thermal events.
Some of them were subjected to high-pressure and temperature conditions like A 10164. Others were heated forshort duration under subsolidus conditions like MS-177,
or not metamorphosed like A-881314, in the parent body.
These differences may reflect wide variation in the shockhistories of the E chondrite parent bodies. E chondrites
may have experienced a greater number of shock events
than other chondrites (Rubin et al. 1997).
Acknowledgements
This paper is dedicated to late Prof. A. El Goresy who accomplished outstanding achievements in meteoritical sciences, especially his work on E meteorites.
The sample of MS-177 was supplied by El Goresy. This work was supported
by a Grant-in-aids of Ministry of Education, Science, Sport, and Culture ofJapanese government, No. 18K03729 to M. K, National Aeronautics and Space
Administration grant #80NSSC18K0589 to MKW. This study was also supported
by National Institute of Polar Research (NIPR) through Project research KP307
and General Collaboration Project no. 26-30. We appreciate thoughtful reviewsby A. Bischoff and T.J. Fagan. We also thank the associate editor T.G. Sharp for
efficient handling of the manuscript.
Authors’ contributions
MK carried out the analyses and designed this study. MKW proposed thisstudy. AT, NI, and AY analyzed the data. All authors read and approved the final
manuscript.
Funding
This work was supported by a Grant-in-aids of Ministry of Education, Science,Sport, and Culture of Japanese government, No. 18K03729 to M. K.
Availability of data and materials
Analytical data is available from the corresponding author upon reasonable
request.

Declarations
Competing interests
The authors declare that they have no competing interest.
Author details
1
National Institute of Polar Research, Tokyo, Japan. 2 Ibaraki University, Mito,
Japan. 3 Kingsborough College and Graduate Center of the City Universityof New York, New York, USA. 4 American Museum of Natural History, New York,USA. 5 SOKENDAI, Tokyo, Japan.
Received: 4 March 2021 Accepted: 10 September 2021

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