(2022) 9:28
Lin ﻿Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-022-00483-6

Progress in Earth andPlanetary Science

Open Access

REVIEW

Enstatite chondrites: condensationand metamorphism under extremely reducing
conditions and contributions to the Earth
Yangting Lin*   

Abstract
Enstatite chondrites are a small clan of meteorites, only ~ 1% out of all meteorite collection. However, they are themost reduced meteorites and have almost identical isotopic compositions to those of the Earth, suggestive of significant contributions to the latter and other terrestrial planets. Enstatite chondrites contain a unique mineral inventoryof sulfides of typical lithophile elements, Si-bearing metal, silicide and phosphide, which record the nebular processes
and the thermal metamorphism in asteroidal bodies under extremely reducing environments. EH group is mainly
characteristic of the higher Si content of metallic Fe–Ni and the higher MnS contents of sulfides than EL group, indicative of a more reducing condition than the latter. However, the fugacity ­pH2S could be the same in both EH and EL
regions, because it was buffered by kamacite and troilite. The majority of sulfides condensed from the nebula, partiallyenclosing schreibersite micron-spherules formed probably by early melting. Another part of troilite, sphalerite anddjerfisherite, intergrown with perryite, were produced via sulfidation of metallic Fe–Ni. Minor exotic components werealso found in enstatite chondrites, including Ca-, Al-rich inclusions and FeO-rich silicate clasts. The Ca-, Al-rich inclusions are identical to those in carbonaceous chondrites except for the alteration under reducing environments, andthe FeO-rich silicate clasts show reduction reactions, both suggestive of migration of dust in the protoplanetary disk.
The highly reducing conditions (as C/O ratios) might be established via repeating evaporation and condensation ofwater ice and organic matter across the snow line along the protoplanetary disk, but need to find evidence. Anotherissue is the preservation of submicron-to-micron-sized presolar grains during high-temperature condensation of the
major constituent minerals. After accretion, the parent bodies of EH and EL chondrites probably experienced distinct
thermal histories, indicated by their distinct petrologic-type distributions and different correlations with the closure
temperatures determined by the FeS contents of sulfides in contact with troilite.
Keywords: Enstatite chondrite, Condensation of the solar nebula, Reducing conditions, Inner solar system, Building
blocks of the Earth
1 Classification
1.1 Chemical groups and petrographic types

Chemical groups: Enstatite chondrites are a small clan ofchondrites (~ 1%) (Meteoritical Bulletin Database, 2021),
characteristic of a unique mineral inventory, including FeO-poor enstatite, Si-bearing metallic Fe–Ni and*Correspondence: LinYT@mail.iggcas.ac.cn
Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing 100029, China

various sulfides of typical lithophile elements (e.g., K, Na,
Mg, Ca, Mn and Cr). This unique mineral inventory indicates highly reducing conditions. Enstatite chondrites
were classified as types I, intermediate and II, with type
I referred to unequilibrated and type II to highly recrystallized (Keil 1968). Later, types I and intermediate werere-designated as EH (high-Fe) and Type II as EL (low-Fe)groups, following the classification scheme of H and L
groups of ordinary chondrites (Sears et al., 1982). Besides
EH and EL chondrites, a few of enstatite chondrites were

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(2022) 9:28

anomalous, with a part of them classified as an intermediate grouplet based on their mineral compositions
plotted between EH and EL (Lin and Kimura 1998). It isnoted that most of the anomalous enstatite chondrites
are highly recrystallized due to impact melting or thermal metamorphism.
Petrologic types: Before the classification of MAC
88136 as the first EL3 (Lin et al. 1991), all EL chondrites
were highly recrystallized and classified as EL6, whereas
EH chondrites ranged from EH3 to EH5. Although Reckling Peak A80259 was reported as the first EL5 (Sears
et al. 1984), it contains niningerite and high Si content
of metallic Fe–Ni (Sears et al. 1984), typical of EH group.
This meteorite was later assigned as EH5 (Meteoritical
Bulletin Database). Both EHs and ELs consisted of anapparent continuum of petrographic types from EH3-5
to EL6, implying origination from a same parent body.
However, the discovery of the EL3 MAC 88136 supplies
with robust evidence for separate thermal metamorphism sequences of EH and EL groups (Lin et al. 1991).
With more collection of enstatite chondrites (a totalof ~ 400), EH and EL groups display distinct patterns ofpetrographic types, with EHs (184) dominated by type
3 (from type 3 to 6: 75.4%, 15.8%, 3.9%, 4.9%), while ELs(177) by type 6 (from type 3 to 6: 14.1%, 10.7%, 5.1%,
70.1%) (Meteoritical Bulletin Database, 2021). In addition, melt rocks (probably impact-induced) are common
in both EHs (8%) and ELs (11.5%). The distinct distribution patterns of petrographic types between EHs and ELs
suggest different thermal histories and internal structuresof their parent bodies.
1.2 Taxonomic parameters

Following the Van Schmus–Wood two-dimensionalclassification scheme (Van Schmus and Wood 1967),
EHs were designated to high-Fe enstatite chondrites
with higher abundances of metallic Fe–Ni and total Fe,
whereas ELs to those with low-Fe (Keil 1968; Sears et al.
1982). However, studies of more enstatite chondrites
demonstrated a wide range of metallic Fe–Ni abundances, which overlap between EH3s (4.1–13.4 vol%) andEL3s (9.4–10.7 vol%) (Weisberg and Kimura 2012). The
wide range of modal abundance of metallic Fe–Ni could
be attributed to heterogeneous sampling; hence, it is nota key parameter to distinguish EHs from ELs. Instead,mineral chemistry preserves the different forming conditions of EH and EL parent bodies.
Si contents of metallic Fe–Ni: The Si contents of metallic Fe–Ni are distinguishably higher in EHs (> 1.9 wt%
Si) than in ELs (< 1.8 wt% Si) (Fig. 1). EH and EL groups
show significant overlapping of the Ni contents of metallic Fe–Ni, but both EH3s and EL3s intend to contain Nipoor metallic Fe–Ni than the equilibrated and/or impact

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4

EH4-5/IM

3

Si wt%Lin P
﻿ rogress in Earth and Planetary Science

SM

EH3

EH6/IM

2

EL6
1

0

EL3
0

2

4

EL4-5
6

8

10

Ni wt%
Fig. 1 The Si and Ni contents of metallic Fe–Ni from enstatitechondrites. Abbreviation: SM-St. Marks, IM—impact melt. Data from
(Lin and Kimura 1998; Patzer et al. 2001, 2004; Lin and El Goresy 2002;
Kimura et al. 2005; Rubin and Wasson 2011; Weisberg and Kimura
2012; Horstmann et al. 2014; Weyrauch et al. 2018)melts, respectively (Fig. 1). It is noticed that the metallic Fe–Ni in the anomalous enstatite chondrites Y793225
and Itqiy show a wide range of Si contents covering bothEHs and ELs (Fig. 1), and Y793225 was designated as theintermediate grouplet (Lin and Kimura 1998). Consistent with the higher Si content of metallic Fe–Ni in EHs,
perryite ((Ni, Fe)5(Si, P)2), a common minor silicide in
enstatite chondrites, is more abundant in EH3s (average
0.8 vol%, range 0.4–1.3 vol%) than in EL3s (average 0.1
vol%, range < 0.1–0.4 vol%) (Weisberg and Kimura 2012).
Furthermore, perryite also contains higher Si in EHs(11.9–13.4 wt% Si) than that in ELs (10.4–11.2 wt% Si)(Lin and El Goresy 2002).
Presence of niningerite or alabandite: Another criticaldifference between EHs and ELs is the composition of(Mg, Fe, Mn)S solid solution, which is referred to as niningerite (Mg-predominant, Mg > Mn) in EHs, whereas asalabandite (Mn-predominant, Mn > Mg) in ELs (Fig. 2).
The FeS contents of both niningerite and alabandite
were mainly determined by equilibrium with troilite,
increasing with temperature (Skinner and Luce 1971).
The presence of alabandite in ELs cannot be a result of
thermal metamorphism of niningerite, which has been
demonstrated by the presence of alabandite in MAC
88136 (Lin et al. 1991) and other EL3 chondrites (Weisberg and Kimura 2012). As shown in Fig. 2, niningerite
within individual EHs plots parallel to the MgS–FeS jointline with the constant MnS contents. The homogeneous
MnS contents of niningerite within the individual meteorites have also been demonstrated by the zoning profiles
conducted on the Qingzhen EH3 chondrite, which show

Lin ﻿Progress in Earth and Planetary Science(2022) 9:28

Page 3 of 16

FeS
90

80

80

Keilite70
EH6/IMEET 90102

60

Ab

50

MS (EL3-6)

40

20
10

LEW87223

Y189

KK

EL5-6

Itqiy

SM

Y169

QZ
SO
Y743701020IlafeghNiningerite
30

40

Y79322550

60

MS-17 (EL3)QUE94204

In

30

70

PCA91020 (EL3)

RKPA

Y691

EL4

50

MAC88136(EL3)

Alabandite
60

70

80

M AC88136

EL3

Grein 002
(EL4)

Y82189Y8218990Fig. 2 MgS–MnS–FeS diagram of (Mg, Mn, Fe)S solid solution fromenstatite chondrites. Dashed lines are impact melt (IM). Abbreviation:
KK—Kota-Kota (EH3), QZ—Qingzhen (EH3), Y169 (EH3), In—Indarch(EH4), SO—South Oman (EH4), Y74370 (EH4), SM—St. Marks (EH5),
Y189—Y82189 (EH IM), RKPA—RKPA 80259 (IM), Ab—Abee (EH IM);
EH6/IM including Y8404, Y980211, Y980223, LEW 88180; Grein 002(EL4), LEW 87223 (EL IM), Ilafegh–Ilafegh 009 (EL IM); LEW 87119 (EL6,IM); MS (EL3-6) including MS-7 (EL5/6), 52 (EL6), 159 (EL6), 196 (EL5),200 (EL3/4), MS-D (EL6) and MS-MU003 (EL3/4); Data from (Zhang
et al. 1995; Fogel 1997; Lin and Kimura 1998; Patzer et al. 2001, 2004;
Lin and El Goresy 2002; Keil 2007; Rubin and Wasson 2011; Weisberg
and Kimura 2012; Weyrauch et al. 2018)

the constant MnS contents regardless of variation in the
FeS and MgS contents (Lin and El Goresy 2002). For alabandite in ELs, it shows constant or small variation in the
MgS contents. Hence, the presence of niningerite can bereferred to as the diagnostic parameter for EHs, and alabandite for ELs. A part of the (Mg, Fe, Mn)S solid solution contain higher FeS than MgS and MnS, which wasnamed as keilite (Keil 2007). The keilite-bearing enstatitechondrites are usually highly recrystallized or melt rocks.
In addition, a few of enstatite chondrites have an intermediate Mg/Mn ratio of the (Mg, Fe, Mn)S solid solution and plot between niningerite and alabandite (Fig. 2).
They were referred to as the intermediate grouplet (Linand Kimura 1998), including LEW 87119, QUE 94204,
Itqiy and Y-793225. It is also noticed that a number of
enstatite chondrite clasts of Almahata Sitta classified as
EL3-6 (Weyrauch et al. 2018) also have the intermediate
compositions of the (Mg, Mn, Fe)S solid solution (Fig. 2),
which is consistent with their higher Mn contents of troilite than those of typical EHs (Weyrauch et al. 2018).
Mn contents of sulfides: The Mn/Mg ratios of othersulfides also show systematic differences between
EHs and ELs, consistent with the presence of niningerite in EHs and alabandite in ELs. Figure 3 plots

ZnS(EH IM)

10

20

30

40

MnS

Fig. 3 ZnS–FeS–MnS content ranges of sphalerite from enstatitechondrites. Abbreviation: QZ—Qingzhen, IM—impact melt. Data
from (El Goresy et al. 1988; Nagel et al. 1989; Lin and Kimura 1998; Lin
and El Goresy 2002; Patzer et al. 2004)

the compositions of sphalerite, with the MnS content
significantly higher in ELs than those in EHs. Furthermore, sphalerite grains within and among EH
chondrites show variation parallel to the FeS–ZnS
joint line, suggestive of relatively constant MnS contents. The relatively constant MnS content of sphalerite is remarkable in Qingzhen, with the sphalerite
inclusions in kamacite varying in a wide range of the
FeS content. In addition, the FeS contents tend to correlate with the petrographic type, with an exception of
Y-82189 impact melt overlapping EH3s (Fig. 3). Thesevariations are similar to those of niningerite describedabove. Daubréelites contain higher Zn and lower Mn
in EHs (up to 10 wt% Zn, < 1.4 wt% Mn) than those inELs (< 4 wt% Zn, 0.8–4.0 wt% Mn) (Keil 1968; Lin and
El Goresy 2002), although partly overlapping between
both groups. These systematic differences in the Mn/
Mg ratios of sulfides between EHs and ELs could not
be attributed solely to buffering of niningerite or alabandite. Oldhamite contains minor Mg, Mn and Fe. It
probably predated niningerite in EHs and alabandite
in ELs, based on its higher REE concentrations (Lodders and Fegley 1993; Crozaz and Lundberg 1995) andhigher condensation temperature than the latter (Lattimer and Grossman 1978; Fegley 1982; Lodders andFegley 1993). The Mn/Mg ratio of oldhamite is alsosignificantly higher in the EL3 Almahata Sitta (~ 1.3)
(El Goresy et al. 2017) than that of the EH3 Qingzhen(~ 0.14) (Lin and El Goresy 2002). Hence, the higher
MnS contents of sulfides in ELs than EHs were established from the beginning of condensation.

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

2 Initial status of the innermost protoplanetarydisk
2.1 Low temperature for preserving presolar grains

Noble gas analysis of the acid residues from primitiveEHs indicated the presence of presolar diamond and SiC(Huss and Lewis 1995). The presolar diamond is a few
nm in size and separated from the acid residue as colloid (Lewis et al. 1987; Lin et al. 2002a), and the majorityof presolar SiC is submicron to micron in size (Lin et al.
2002a). Hence, the presence of presolar grains is very
sensitive to thermal metamorphism. In fact, the abundance of presolar grains drops fast with the petrographic
types (Huss and Lewis 1995). Separation and analysis of
the presolar grains from the Qingzhen EH3 reveal various types of presolar SiC and S­ i3N4 grains, including X
grains, type AB and Main Stream, with the relative abundance ratios and the size distribution pattern similar to
those separated from carbonaceous chondrites (Lin et al.
2002a, 2010). Presolar graphite was also identified in theacid residue from Qingzhen, with similar morphologies and isotopic compositions of those from the CM2
Murchison (Xu et al. 2016, 2018). Furthermore, presolar
silicate grains were found in EH3s by in situ isotope mapping (Zhao et al. 2010).
The presence and high abundance of presolar grains
in EH3s indicate that the temperature at enstatite chondrite-accreting regions has never been entirely heated
up to a maximum temperature of 440 ℃, otherwise thenano-sized diamond and submicron-sized SiC would
have been largely destroyed (Davidson et al. 2014). On
the other hand, the majority of the constituent minerals
condensed from the nebular gas at high temperature. For
instant, oldhamite condensed from the reducing nebulaat > 1000 K (Larimer and Bartholomay 1979), and at least
a part of troilite and sphalerite condensed at a temperature ~ 700 K (Lin et al. 2002a), both are too high to preserve the presolar grains. This discrepancy requires thatenstatite chondrites accreted with materials condensed
in hot zones and the presolar dusts preserved separately
in cold regions. The presolar grains were probably transported from the outer cold zones or dropped from thecold periphery of the protoplanetary disk to the midplane, where the unique constituent minerals condensed
under highly reducing conditions and enstatite chondrite
parent bodies finally accreted. However, the scenario that
presolar grains delivered from the outer solar system isinconsistent with the high abundances of presolar SiC
and diamond in Qingzhen (Lin et al. 2002a), which arecomparable with those in primitive carbonaceous chondrites. Boyet et al. (2013) analyzed Nd isotopes in primitive enstatite chondrites. They found the largest 142Nd
excess in ALHA77295 even than the CM2 Murchison,
which is attributed to presolar SiC as the major carrier.

Page 4 of 16

Another argument against this scenario is that few carbonaceous chondrite components have been identified inprimitive enstatite chondrites. Zhu et al. (2020) reported
two chondrules from the EH3 Sahara 97096 with definitely 54Cr-anomaly (ε up to 1.01± 0.11) within the rangeof carbonaceous group. Alternatively, the majority ofminerals (except for the presolar grains) condensed probably very close to the Sun and then transported outward to the regions where enstatite chondrites accreted
altogether with the presolar grains preserved at low
temperature.
In summary, the presence and abundance of submicron-sized presolar grains in primitive enstatite chondrites provide robust evidence for a low temperature in
enstatite chondrite-accreting regions of the protoplanetary disk. However, this discovery conflicts with the hightemperature condensation of enstatite chondrites, which
took place under extremely reducing conditions in theregions probably closest to the proto-Sun as discussed
below. This discrepancy should be addressed by formation and evolution models of the solar nebula.
2.2 Extremely low fO2

The predominant FeO-poor enstatite and the uniqueopaque minerals in enstatite chondrites demonstrate
highly reducing environments. In EH3 chondrites, there
are some FeO-bearing silicate clasts, which appear as
dusty grains under optic microscope because of thepresence of numerous tiny Ni-free metallic Fe particles(Rambaldi et al. 1984; Lin et al. 2002b). This discovery
indicates that the dusty FeO-bearing silicates were exotic,transported from other oxidizing regions and reduced in
enstatite chondrite-accreting regions.
The Si contents of metallic Fe–Ni are indicative of theoxygen fugacity of enstatite chondrites. The systematically higher Si contents of metal from EH3s than thosefrom EL3s suggest that the minerals condensed at evenlower fO2 in EH region than in EL’s. The more reducing condition of EHs is consistent with the presence ofniningerite and lower Mn/Mg ratios of other sulfides, incomparison with the presence of alabandite and higher
Mn/Mg ratios of sulfides in ELs. Mg is a typical lithophileelement, and Mn behaves more sulfophile. Hence, the
MnS/MgS fugacity ratio is sensitive to the redox conditions, decreasing (mainly due to increasing ­pMgS morethan ­PMnS) in more reducing environments.
The reducing conditions of the solar nebula may beachieved by increasing the C/O ratio from the solar valueof 0.52 up to > 0.8–1.0 (Larimer and Bartholomay 1979).
However, how and when were the C/O ratios of enstatitechondrite-forming regions enhanced from the canonical values to > 0.8–1.0? A scenario is that repeating processes of condensation and evaporation of water ice andLin ﻿Progress in Earth and Planetary Science(2022) 9:28

carbonaceous materials across the snow line (~ 2.8 AU
from the Sun) could play a key role for the evolution ofredox along the protoplanetary disk. The water ice andorganic solid particles radially migrated inward and were
heated and evaporated in the inner solar nebular regions
across the snow line. The organic macromolecular matter retained as solid, whereas water ice and small organic
molecule materials evaporated. The vapor of water andsmall organic molecules then diffused to all directions
and re-condensed beyond the snow line. Repeating theseprocesses, the C/O ratio of the protoplanetary disk could
be enhanced toward the Sun. This scenario is consistent with the redox status of the planets, which appears
more reducing closer to the Sun as indicated by the lower
FeO contents of silicates and the higher average density(related to the metallic Fe–Ni core/silicate volume ratio).
The remote sensing of Mercury revealed the most reducing features of the planets, which are comparable withenstatite chondrites (Vilas 1985; Nittler et al. 2011).
2.3 Buffered fH2S

The modal abundance of sulfides is higher in EH3s thanEL3s (Weisberg and Kimura 2012), indicative of a higher
bulk abundance of sulfur in the former and the latter.
Furthermore, the perryite–sulfide intergrowth is also
more developed in EH3s than EL3s, with metallic nodules commonly surrounded by perryite–sulfide intergrowth rims in EH3s but rather rare in E3Ls as depictedin the next section (Fig. 5). The fugacity of ­H2S might behigher when sulfides started to condense in EH-forming
region than that in EL’s, because of its higher bulk abundance of sulfur. However, the fugacity of ­H2S would be
buffered in both EH- and EL-forming regions, when troilite started to condense. After condensation of troilite, the­H2S fugacity in the nebular gas was buffered by kamacite
and troilite as follows:  
Kamacite + H2 S g = troilite + H2 g
where the activities of Fe in kamacite and troilite andthe fugacity of ­H2 were constant at given temperature.

3 Processes in the reducing nebular regions
3.1 Gas–solid condensation of sulfides

Besides the Si-bearing metal and FeO-poor enstatite,
the majority of various sulfides in enstatite chondrites
formed likely via gas–solid condensation in highly reducing environments. The key lines of evidence for condensation are as follows:
Mineral compositions: Oldhamite and niningerite in EH3s are usually euhedral and coarse-grained inmorphology and commonly present in the centers of
opaque mineral assemblages, indicative of early growth
in a free space. This is consistent with the condensation

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calculations, which predicted oldhamite and niningerite as the first two sulfides deposited from the nebula
under extremely reducing conditions with C/O ratio ≥0.8 (Larimer and Bartholomay 1979). Another criticalline of evidence for condensation is the high enrichmentof REE and other refractory elements in oldhamite andpartly in niningerite. SIMS analyses of oldhamite in both
EH3s and EL3s revealed highly REE-enriched patterns(~ 10–100 × CI) w/o anomalies of Eu and Yb (Crozaz
et al. 2003; El Goresy et al. 2017), which are consistent
with condensation of REE under reducing conditions(Larimer and Ganapathy 1987; Lodders and Fegley 1993).
In addition, the normal FeS-zoning profiles were foundin niningerite grains from Qingzhen, with the FeS content decreasing from 17.5 mol% at the cores to 15.4 mol%at the rims in contact with silicates or oldhamite (Linand El Goresy 2002). These normal zonings of niningerite recorded the growth of the grains as temperature
decreasing, a process of gas–solid condensation but
against sulfidation of silicate grains.
Schreibersite micron-spherules enclosed in sulfides:
Two types of schreibersite were reported in enstatitechondrites (Fig. 4), i.e., (1) large grains (up to hundredsµm) in kamacite, and (2) micron-spherules (< 10 µm in
diameter) entrapped as inclusions in various sulfides. The
coarse-grained schreibersite was enclosed in kamacite
usually without coexisting with other minerals. In contrast, the schreibersite micron-spherules enclosed invarious sulfides share similar shapes, sizes and compositions, regardless the wide compositional range of thehost minerals (e.g., oldhamite, niningerite/alabandite,djerfisherite, sphalerite, minerals A and B, caswellsilverite, daubréelite and troilite). Furthermore, the composition of micron-spherule schreibersite is similar to thoseof the coarse grains enclosed in kamacite (Lin and El
Goresy 2002), indicative of a same origin except that the
micron-spherules experienced melting before entrappedby the sulfides. The presence of schreibersite micronspherules in various sulfides suggests that schreibersite
predated all sulfides and the micron-spherules served as
nuclei for condensation of the latter. The early formationof schreibersite is supported by the occurrence of thecoarse-grained schreibersite in metallic Fe–Ni. Lehneret al. (2010) analyzed trace elements of schreibersite andthe host kamacite in the Sahara 97072 EH3 chondrite,and suggested that schreibersite condensed in a reducing gas followed by incorporation into metallic Fe–Ni.
Hsu (1998) reported schreibersite micron-spherules inoldhamite, but proposed that they formed via melting ofoldhamite with coexisting schreibersite. However, this
scenario cannot explain the same occurrence of schreibersite micron-spherules in all types of sulfides, whereas
no coexistence of non-melted schreibersite with sulfides.

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

Page 6 of 16

Fig. 4 Occurrences of schreibersite (Sch) in EH3s. a A large grain enclosed in kamacite (Ka) and numerous micron-spherules (arrows) enclosed in
djerfisherite (Dj) in Qingzhen; b small spherules in an oldhamite (Old) fragment in Almahata Sitta MS-17; c abundant schreibersite micron-spherulesin troilite and niningerite in Sahara 97159; d the micron-spherules in troilite and sphalerite in the center of metal-rich nodule in Sahara 97159. Notea mantle enriched in fine-grained troilite and silicates (dark)

Another convincing line of evidence for the gas–solid condensation of sulfides entrapping schreibersite
micron-spherules is their systematically different morphology and mineral chemistry in comparison with thoseintergrown with perryite, which is discussed in the next
subsection in detail. Two major occurrences of troilite, sphalerite and djerfisherite were found in EH3s, one
entrapping schreibersite micron-spherules depicted here
and the other intergrown with perryite. Similar to oldhamite, niningerite and various Cr sulfides, the schreibersite micron-spherule-bearing troilite, sphalerite anddjerfisherite are typical of coarse-grained and euhedral,consistent with gas–solid condensation. Furthermore,these troilite and sphalerite grains are highly Ni-depleted(Lin and El Goresy 2002), with Ni/Fe atomic ratios down
to < ­10–5 determined by SIMS (Lin et al. 2011). Thesegrains are the best candidates to detect possible existenceof the short-lived 60Fe (Lin et al. 2011). Such Ni depletionis expected for the schreibersite micron-spherule-bearing

troilite and sphalerite, which condensed at much lower
temperatures (~ 700 K) than metallic Fe–Ni (~ 1350 K)
and almost all Ni had deposited into metallic Fe–Ni.
Oldhamite and niningerite in EHs were mainly foundas coarse-grained mineral fragments scattered in silicate matrix. In opaque assemblages, both of them usually occur in the centers, suggestive of predating othersulfides. The large fragments of troilite, sphalerite anddjerfisherite usually have euhedral shapes. As mentioned
above, these troilite, sphalerite and djerfisherite commonly contain schreibersite micron-spherules. Chromium sulfides are daubréelite, caswellsilverite, mineral
A and B (El Goresy et al. 1988), and a new aqueous Fe–
Cr sulfide (Lin et al. 1990). Except for the new aqueous
Fe–Cr sulfide exclusively enclosed in metallic Fe–Ni in
Qingzhen (Lin et al. 1990), all other Cr sulfides coexist
often with troilite scattering in silicate materials or on the
peripheries of metal–sulfide nodules, suggestive of latercondensation probably after troilite.

Lin ﻿Progress in Earth and Planetary Science(2022) 9:28

Page 7 of 16

Fig. 5 Intergrowth of kamacite (Ka), perryite (Per) and sulfide in EH3s. a A opaque nodule consisting mainly of the intergrown troilite (yellow
brown) and perryite (light yellow) in Qingzhen under reflection light; b fine-grained troilite (Tr) intergrown with perryite (arrows) in the margin of anodule from Sahara 97159; c the fine-grained troilite intergrown with perryite, note a coarse-grained troilite in the center of the nodule from Sahara
97159; d the absence of sulfide and perryite in a metal nodule from Almahata Sitta EL3. Euhedral grains of sinoite (Sin) are enclosed in metal, andenstatite (En) crystals intergrown with metal along the periphery3.2 Sulfidation of metal

Perryite is a minor phase in enstatite chondrites andshows a common textural occurrence as elongated irregular grains along the boundaries between metal andsulfides (Rambaldi et al. 1986; Lin and El Goresy 2002). In
case of the absence of metal, there are only perryite andsulfides intergrown with each other (Lin and El Goresy2002). It is noticed that the sulfide–perryite intergrowthoccurs along the periphery of opaque assemblages anddevelops inward (Fig. 5). Only troilite, sphalerite anddjerfisherite were found to intergrow with perryite (Linand El Goresy 2002). These perryite-intergrown sulfides
are typical of small and irregular, distinguished fromthose with schreibersite micron-spherules describedabove. On the other hand, the schreibersite micronspherule-bearing sulfides usually do not intergrow withperryite, even enclosed in metal. Except for troilite,
sphalerite and djerfisherite, all other sulfides including
oldhamite, niningerite (in EHs) or alabandite (in ELs), Cr

sulfides (daubréelite, caswellsilverite, minerals A and B,
and the new aqueous Fe–Cr–S), do not coexist with perryite, and they could contact directly with metal without
perryite rims along the interfaces with sulfides. Theseobservations indicate that the perryite-intergrown troilite, sphalerite and djerfisherite were produced by sulfidation of metallic Fe–Ni. The sulfidation reaction took
place from the peripheries of metal grains and developed
inward. The reaction temperature was about ~ 700 K,
based on the thermometer of sphalerite (Lin and El
Goresy 2002). With sulfidation, the concentrations of
Ni and Si in the metal increased, with perryite nucleating along the boundaries in contact with the sulfides.
This scenario is also convincingly supported by the Nibearing compositions of the perryite-intergrown troiliteand sphalerite (Lin and El Goresy 2002), distinguishingfrom the highly Ni-depleted grains enclosing schreibersite inclusions discussed above. The distinctly higher Nicontents of the sulfides can be attributed to the enhanced

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

Ni concentrations in the metal due to sulfidation of Fe
preferred to Ni. Although the compositional difference
between both occurrences of djerfisherite (schreibersite
micron-spherule-bearing versus perryite-intergrown) is
less significant than those of troilite and sphalerite, theperryite-intergrown grains contain slightly higher Cu
than the schreibersite spherule-bearing ones (Lin and El
Goresy 2002).
The perryite–sulfide intergrowth was found common in EH3s, but significantly underdeveloped in EL3s.
Metallic Fe–Ni nodules in the Almahata Sitta EL3 areusually free of perryite–sulfide intergrowth (Fig. 5d). Thisdifference is consistent with the lower bulk abundanceof sulfur (i.e., sulfides) in EL3s than EH3s (Weisberg andKimura 2012).
3.3 Sulfidation of silicates

In contrast to the gas–solid condensation, another scenario, e.g., sulfidation of silicates, was also proposed to
produce oldhamite from diopside and niningerite fromMg-rich olivine and enstatite (Rubin 1983; Fleet and
MacRae 1987; Lehner et al. 2013), or destabilizationof the gaseous S­2, SiO and Na components dissolvedin chondrule melt (Piani et al. 2016). The evidence forsulfidation of silicates includes coexistence of niningerite with corroded silicate clasts (Lehner et al. 2013) andexperimental study (Fleet and MacRae 1987). It was suggested that silicate phases may have been sulfurized in
an O-poor and S-rich environment, and metal–sulfide
nodules in EH3s could have originated and expelled out
from the silicate chondrules (Jacquet et al. 2015, 2018).
Although sulfides in silicate chondrules could form via
sulfidation of silicates and/or crystallization from S-bearing melts, the majority of sulfides, especially metal–
sulfide assemblages and the coarse-grained fragments inmatrix, are inconsistent with such scenarios, according to
the following observations: (1) oldhamite grains with various occurrences have high and similar REE patterns (ElGoresy et al. 1988). As depicted above, the REE patternsof oldhamite are consistent with condensation of REEfrom the nebula under reducing conditions. (2) The REE
and other refractory element abundances and patterns
are inconsistent with the partitioning coefficients withsilicate melt (Ingrao et al. 2019). (3) The normal zoningof niningerite indicates condensation with temperature
decreasing, against a solid–gas/melt reaction. (4) The
identical features of the schreibersite micron-spherules
in oldhamite, niningerite and other sulfides, a robust line
evidence for gas–solid condensation as discussed above.
(5) The reported evidence for sulfidation of silicates was
mainly confined to silicate chondrules, but the majorityof sulfides in opaque mineral assemblages and the large

Page 8 of 16

sulfide fragments show no reaction textures coexistingwith relict silicates.
3.4 Precursor materials of enstatite chondrites

An issue about the precursors of enstatite chondrites
was whether they were inherited from presolar dust
grains, condensed in situ under reducing conditions, or
reduced from FeO-bearing silicates. The presolar grains
in primitive chondrites are predominant of submicronto micron-sized dust grains. Only a few grains of several
tens microns were reported so far (Gyngard et al. 2009).
Hence, the coarse-grained and euhedral crystals in theprimitive chondrites were condensed from the solar
nebular gas and/or crystallized from melt droplets. Different from the gas–solid condensation under highly
reducing conditions, another scenario proposed thatthe precursors of enstatite chondrites were ordinary ferromagnesian silicates. The unique sulfides of lithophileelements, such as oldhamite and niningerite, were produced by sulfidation reactions of the silicate precursors
under highly reducing conditions (Lehner et al. 2013; Jacquet et al. 2015). However, this scenario is conflict withmost petrological observations and mineral chemistry
discussed above. The presence of dustlike FeO-bearing
silicate clasts in E3 chondrites suggests that they were
exotic with FeO reduced to submicron-to-micron-sized
metallic Fe particles. This is a convincing line of evidence
for the indigenous origin of the predominant FeO-poor
enstatite, whereas exotic origin of the dustlike FeO-bearing silicate clasts. The FeO-free pyroxene in enstatitechondrites likely condensed in situ under highly reducing conditions, instead of being reduced from preexisting
FeO-bearing silicates.
3.5 Heating events
3.5.1 Chondrules

Similar to other chemical groups of chondrites, silicatechondrules in enstatite chondrites were formed by heating events in the nebular regions but under highly reducing environments followed by fast cooling of the molten
droplets. The presence of sulfides in some chondrules
and their igneous textures suggest that the melting took
place after condensation of troilite and other sulfides.
Nine chondrules from the Sahara 97096 EH3 were datedwith high-precision Cr isotopes (Zhu et al. 2020). The
enstatite chondrite-like chondrules plot on a fossil isochron, resulting in 1.6± 0.7 Ma after Ca- and Al-rich inclusions (Zhu et al. 2020). In addition, Al-rich chondrules
from enstatite chondrites have the initial 26Al/27Al ratioof (6.8 ≥ 2.4) × ­10–6 similar to those observed in chondrules from carbonaceous chondrites and unequilibrated
ordinary chondrites (Guan et al. 2006), consistent with
their similar formation time.

Lin ﻿Progress in Earth and Planetary Science(2022) 9:28

3.5.2 Schreibersite micron‑spherules

As discussed above, the wide distribution of micronspherules of schreibersite in various sulfides suggests
a high-temperature heating event in the solar nebula,which melted the preexisting schreibersite. This hightemperature event took place very early, predating thecondensation of oldhamite and all other sulfides. Furthermore, the heating event likely happened in the same
regions where the constituent minerals of enstatite chondrites, including schreibersite and various sulfides, werecondensing from the nebula under a reducing condition.
This is different from the silicate chondrule-forming
events, which postdated most of silicates and probably
troilite and the heating site could be different from thecondensation regions (Hewins 1997; Lauretta et al. 2006).
3.5.3 Opaque nodules

Evidence for melting: In both EH3s and EL3s, it is common that the metal–sulfide assemblages occur as roundshapes, generally referred to as opaque nodules. The
round or oval shapes of opaque assemblages imply thatthey were once molten as silicate chondrules, probably
by the same heating events in the protoplanetary disk. In
addition, there are also kamacite–silicate intergrowths
in EH3s and EL3s, which formed probably via pre-accretionary melting based on the highly siderophile and othertrace element abundance patterns of the metal grains(Horstmann et al. 2014). If the opaque nodules were once
molten, the heating event likely predated the sulfidationof metal, since the intergrowth of sulfides and perryite
usually occurs along the peripheries of the nodules.
Evidence for non-melting: Regardless of the roundor oval shapes, the opaque nodules may not be melted
but accretion of condensates. As discussed above, theschreibersite micron-spherule-bearing sulfides are gas–solid condensates. These schreibersite micron-spherulebearing sulfides are commonly present in the centers ofmetal–sulfide nodules (Fig. 4d) (e.g., Figure 1a by Lin and
El Goresy (2002)), showing identical morphologies andmineral chemistry of those fragments in matrix. If themetal–sulfide nodules were molten droplets, troilite (andalso other sulfides) would have been melted together
with metal, producing en eutectic texture between metal
and troilite and merging schreibersite micron-spherules
together. These observations are against melting of themetal–sulfide nodules. Furthermore, the metal–perryite–sulfide intergrowths occur mainly along the peripheries of the nodules, suggestive of the sulfidation postdating
the nodules. No melting of the nodules took place after
the sulfidation, otherwise the two textural features oftroilite (the irregular grains intergrown with perryite,
the coarse and euhedral grains entrapping schreibersite
micron-spherules) could not have been preserved in the

Page 9 of 16

same nodules (Fig. 4d) (e.g., Figure 1a,b by Lin and El
Goresy (2002)). In addition, as discussed above, the sulfidation took place after the formation of nodules. In contrast, sulfides in silicate chondrules often show igneous
textures (Piani et al. 2016), indicating that the chondruleheating events postdated formation of troilite and othersulfides. These observations are inconsistent with thepossible simultaneous melting of the opaque nodules andsilicate chondrules in enstatite chondrites by the same
heating events.
In the EL3 Almahata Sitta, many metal nodules exhibit
zoning textures, with one and multilayers of abundantfine-grained enstatite and sinoite crystals between metallic Fe–Ni. In addition, under high-contrast BSE images,
the metallic Fe–Ni appears as assemblages of individual
euhedral grains (El Goresy et al. 2017). Prisms of enstatite
are mainly intergrown with metal along the margins
or the rims of the nodules. The zoning texture was also
reported in opaque nodules in many EH3s, including
Qingzhen (Lin and El Goresy 2002), Y-691, Sahara 97159
and the paired ones (Weisberg and Kimura 2012; Jacquet
et al. 2018), and Kaidun (EH3-4) (Ivanov et al. 1996). Lehner et al. (2017) reported porous and S-bearing silica inmetal–sulfide nodules from two EH3 chondrites, which
also show accretionary textures with silicate-rich layers.
Various silicate inclusions in metal nodules from Kaidun (EH3-4) appear to be related to condensation ofmaterial onto the surfaces of metal grains (Ivanov et al.
1996). The nodules appear to have formed by aggregationof separate grains (globules) of metal, with conservationof condensates on the grain surfaces as inclusions. The
inclusions probably condensed in a wide temperature
range from 1400 to 600 K (Ivanov et al. 1996). The aggregation of metal grains and formation of the nodules probably occurred simultaneously with condensation (Ivanov
et al. 1996). All sinoites in the EL3 Almahata Sitta were
found in metallic Fe–Ni, probably condensed before or
together with metal. Although impact origin of sinoite
was proposed (Rubin 1997), these enstatite chondrites
are not primitive type 3 but impact melts. The very highabundance of sinoite in metal, up to 22 vol% in the EL3
Almahata Sitta (El Goresy et al. 2017), argues against
exsolution of sinoite from the metal.
It was also noticed that chromium sulfides, including daubréelite, aqueous F­ eCrS4, minerals A and B, andcaswellsilverite, usually occur in the margins and/or therims of the metal nodules. Lawrencite ­(FeCl2) was also
reported as rims of assemblages in Indarch (Keil 1968)
but more common as inclusions in metallic Fe–Ni in
unequilibrated enstatite chondrites; probably, it can bepreserved better as inclusions in metal since this phaseis extremely sensitive to humidity. Two types of graphite
were found in the Almahata Sitta EL3 (El Goresy et al.

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

2017), as sphere-shaped assemblages of dendritic crystals
enclosed in metallic Fe–Ni in MS-17 and as books in silicate matrix in MS-177.
In summary, the silicate chondrules in enstatite chondrites are similar to those in carbonaceous and ordinarychondrites, produced by transient heating events in thesolar nebula. Although opaque nodules appear round oroval in shapes, implying once melted as silicate chondrules, detailed petrological observations favor aggregation of condensates.
3.5.4 Material transportation

Enstatite chondrites condensed and accreted under
extremely reducing conditions in the protoplanetarydisk, probably closest to the proto-Sun (Wasson 1988;
Rubin 2011). There are also components in enstatitechondrites that formed under higher oxygen fugacities
and other conditions, and hence they are probably exotic.
These candidate exotic components were transported
from other regions before or during accretion of enstatitechondrites. Another scenario is that the FeO-rich components are relicts after the local conditions evolved into
highly reducing conditions.
3.5.5 Ca‑, Al‑rich inclusions

Ca-, Al-rich inclusions (CAIs) were reported as minor
components in enstatite chondrite (Bischoff et al. 1985;
Fagan et al. 2000; Guan et al. 2000b; Lin et al. 2003), andthey share the same initial 26Al/27Al ratio of ~ 5 × ­10–5 asthose typical of carbonaceous chondrites. In addition, theoxygen isotopes of CAIs from enstatite chondrites plot
on the same line with slope of ~ 1.0 in the three-isotope
plot, identical to the isotopic compositions of unalteredminerals in CAIs from carbonaceous chondrites (Guanet al. 2000a; Fagan et al. 2001; Lin et al. 2003).
The EH3 Sahara 97159 contains unusually high abundance of CAIs, with 68 found in two polished sections, or
22 per ­cm2 (0.06 vol%). All of the inclusions are intensely
altered, and the alteration assemblages with the presenceof FeO-poor spinel suggest that the alteration took place
under reducing and S­ iO2-rich conditions, consistent withenstatite chondrite-accreting environments. The prealtered mineral assemblages are type A-like or spinel–
pyroxene-rich ones similar to the predominant types incarbonaceous chondrites, indicative of their same origins
and delivered from the same reservoirs (Lin et al. 2003).
3.5.6 Relict silicates and chondrules

Minor dusty FeO-bearing silicate clasts were found in
enstatite chondrites, which contain abundant tiny Nipoor and Cr-rich metallic Fe (Rambaldi et al. 1984;
Weisberg et al. 1994; Lin et al. 2002b). These clasts
were proposed to originate in the same general nebular

Page 10 of 16

neighborhood of enstatite chondrites and were subsequently transported into the reducing environments(Rambaldi et al. 1984). Weisberg et al (2011) analyzed
oxygen isotopes of the FeO-bearing silicate clasts andfound the 17O values of some olivine and pyroxene fromE3s plotting outside of the enstatite chondrite range, with
one close to R group and several to carbonaceous chondrites. Jacquet et al. (2018) also summarized 17O valuesof several ten chondrules in enstatite chondrites, overlapping with those of LL3. On the other hand, majority of
the FeO-rich pyroxenes have the same oxygen isotopes ofthe enstatite, and they were proposed to have formed at
different times under different redox conditions but fromthe same oxygen reservoir (Weisberg et al. 2011). Alternatively, the similar 17O values of these components
within the range of enstatite chondrites could also be
explained by isotope exchange with the local oxygen reservoir of enstatite chondrites, as indicated by the reduction textures of the dusty FeO-bearing silicate clasts.
Zhu et al (2020) analyzed Cr isotopes of chondrules
from the Sahara 97096 EH3 and found that two out ofnine chondrules have definitely 54Cr-anomaly (ε up to
1.01 ± 0.11) within the range of carbonaceous group. The
discovery of carbonaceous group chondrules in enstatitechondrites suggests early transportation of materials
from the outer solar system to the enstatite chondriteaccreting regions in the inner solar system. This scenario
is supported by numerical simulations of chondruletransportation in the protoplanetary disk (Goldberg et al.
2015).

4 Processes in the parent bodies
4.1 Thermal metamorphism

The FeS content of both niningerite and alabandite incontact with troilite positively correlates with temperature. Two types of normal zoning and one type of reverse
zoning of niningerite were reported in Qingzhen (Linand El Goresy 2002). The normal zoning of niningerite,with the FeS content decreasing from 17.5 mol% in thecenters to 15.4 mol% at the rims in contact with silicate
and/or oldhamite, probably indicates condensation as
temperature decreased, whereas the other normal zoning of niningerite, with the FeS content decreasing from
17.7 mol% to 15.2 mol% toward troilite, suggests equilibrium diffusion of FeS at lower temperature probably
in the parent body. This is consistent with exsolution of
troilite in niningerite in Qingzhen (Lin and El Goresy2002). The same normal zoning and exsolution of troilite
were also found in alabandite in the EL3 MAC 88136, due
to a slow cooling in the parent body. As shown in Fig. 2,
the FeS contents of niningerite in EHs and alabandite in
ELs are generally correlated with the petrographic types.
However, a few of EH and EL chondrites deviates from

Lin ﻿Progress in Earth and Planetary Science(2022) 9:28

this trend, e.g., the EH5 St. Marks and QUE 93372 overlapping the range of EH3s, and Jajh deh Kot Lalu (EL6)within the range of EL3. In addition, the (Mg, Mn, Fe)S solid solution in the EH impact melt rocks are usually FeS-rich, named as keilite. These observations canbe explained by re-accretion of breakup original parent bodies; however, additional evidence is required tomake up a detailed model. Alternatively, Weyrauch et al.
(2018) divided EH and EL into EHa, EHb, and ELa, ELb,
respectively, based mainly on Fe contents in (Fe, Mg, Mn)
S and Cr contents in troilite. They proposed different parent bodies for these subgroups of enstatite chondrites.
However, as discussed above, the systematic variation in
the MnS content of sulfides, especially the presence ofniningerite in EH and alabandite in EL, was establishedduring nebular processes under different conditions. In
contrast, the Fe contents of (Fe, Mg, Mn)S and Cr contents of troilite could be modified by thermal metamorphism in the parent bodies. Hence, it is not necessary tofurther divide EH and EL groups based on these thermal
metamorphism-related parameters.
The FeS content of sphalerite in contact with troilite isalso dependent on temperature and/or pressure (Balabin and Urusov 1995). Sphalerite shows variation in the
FeS contents within and/or among grains with a relatively constant MnS content (Fig. 3). The normal zoning
profiles of sphalerite, with the FeS contents decreasingfrom 50.8 mol% in the cores to 46.0 mol% in contact withtroilite, were reported in Qingzhen (Lin and El Goresy2002). The FeS content of ~ 46 mol% of sphalerite in contact with troilite suggests a final equilibrium temperatureof ~ 500 K according to the thermometer of sphalerite(Balabin and Urusov 1995), assuming a shallow buried
depth.
Troilite contains minor Cr and often shows exsolution of daubréelite, which is another indicator of thermal history. In Qingzhen, it is noticed that troilite incontact with Cr sulfides always shows exsolution of tiny
daubréelite (Lin and El Goresy 2002). This texture can be
explained by two stages of CrS diffusion, first from theadjacent Cr sulfides into troilite during heating and followed by exsolution of tiny daubréelite from the Cr-bearing troilite during cooling. This observation suggests that
the Cr content and exsolution of daubréelite from troilite
are very sensitive to the thermal history. Weyrauch et al.
(2018) divided EH and EL into high and low subgroups
based on the Cr contents of troilite and the FeS contentsof (Mg, Mn, Fe)S solid solution.
The Si contents of metallic Fe–Ni in E3s can be referred
to as indicator of oxygen fugacity of the nebula, and theirvariations may reflect heterogeneous redox conditions
within EH and EL regions. However, the Si contents ofequilibrated enstatite chondrites could be affected byPage 11 of 16

thermal metamorphism, as indicated in Fig. 1. The Si
contents of metal intend to increase with thermal metamorphism (Weyrauch et al. 2018).
4.2 Shock metamorphism

Enstatite chondrites commonly experienced shock metamorphism, with many meteorites classified as impact
melt rocks. Some enstatite meteorites, previously classified as aubrites, were re-designated as EH impact melts,
based on their undifferentiated modal abundances, presence of niningerite, graphite and high Si content of metal(Udry et al. 2019). Rubin (2015) summarized the shockfeatures of enstatite chondrites and proposed a classification scheme of shock metamorphism. Both EHs
and ELs show correlations between petrologic type andthe degree of shock, which suggest that collisional heating is mainly attributed to the thermal metamorphismof enstatite chondrites. This also explains the relatively
young Ar–Ar ages of enstatite chondrites. Furthermore,most Fe-rich niningerites (i.e., keilite) are present in EH
melt rocks, except for LEW 88180 classified as EH5-6(Zhang et al. 1995) but re-designated as IMB (Keil 2007).
The presence of keilite in EH impact melt consists ofquenching of shock-induced melts. On the other hand,no keilite was reported in EL impact melt, except for EET
90102 (EL6) (Fogel 1997). In addition, keilite was foundin Khor Temiki #2 vitrophyre in the aubrite host (Fogel
2005). This observation supports the distinct post-shock
thermal histories of EH and EL parent bodies.
Oldhamite and other sulfides can crystallize fromenstatite chondrite impact melts, based on the experiments with Indarch as starting material (McCoy et al.
1997a, b; McCoy et al. 1999). The shock-induced silicate
melts can incorporate significant S, and the compositionsof the melts are related to the temperature. Metal–sulfide
melts completely at 1000 °C, whereas silicates start tomelt at 1000 °C and complete at 1500 °C. The post-shock
temperature is based on shock pressure and the porosityof the targets (Moreau et al. 2019), the post-shock temperatures increasing from ~ 540 K at 30 GPa to ~ 1300 K
at 70 GPa, and the post-shock temperature increasesby ~ 800 K and melt fraction by ~ 0.12 with the porosity
increasing up to 15%.
4.3 Qingzhen reaction

Djerfisherite is commonly present in enstatite chondrites
and usually occurs as large fragments in matrix. It showsa breakup texture, partially replaced with fine-grained
troilite and trace sphalerite (Fig. 4a), which was referred
to as “Qingzhen Reaction” by El Goresy (1995). The finegrained sphalerite in the breakdown products of djerfisherite has the lowest FeS content of ~ 42 mol%, indicativeof low temperature of < 450 K (Lin and El Goresy 2002).

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

Isotopic analysis of the breakup products of djerfisheritein Qingzhen shows that the I-Xe chronometer is decoupled from, and more stable than, both Ar–Ar and Rb–Sr
chronometers, which were reset in the 1.4–2.8 Ga period(Ash et al. 1997).
Hopp et al. (2014) conducted Ar–Ar dating of enstatitechondrites and found different Ar–Ar age distribution between EHs and ELs. The Ar–Ar ages of ELs arein a narrow range from 4.50–4.44 Ga, whereas EHs are
more variable from 4.55 to 2.2 Ga. The very young ages
show clear evidence for impact-induced partial resettingat ~ 2.2 Ga, consistent with the I-Xe results of the breakdown products of djerfisherite. The isotope chronologyof EHs and ELs also supports the different metamorphism histories of their parent bodies.

5 Implications to the Earth building components
5.1 Isotopic compositions

The isotopic anomalies, e.g., 17O, ε50Ti and ε54Cr, were
little affected by the solar nebular processes, but preserved the information of the reservoirs. Accordingly,
the available materials of the solar system have beenclassified into the carbonaceous group (representativeof outer solar system) and the non-carbonaceous group(representative of inner solar system) (Warren 2011).
The Earth, the Moon and enstatite meteorites belong to
the non-carbonaceous group and have identical isotopic
compositions. These discoveries supply with strong evidence for the genetic relationship of enstatite meteorites
and the Earth, which were confirmed by analyses of manyother elements.
Lithophile elements: The Ca isotopes imply multiple supernova sources for the neutron-rich isotopes
in the solar system, with ε48/44Ca and δ44/40Ca varying
up to ~ 1‰ in chondrites, whereas enstatite chondrites
and the Earth have identical Ca isotopic compositions(Huang and Jacobsen 2017). Although there is an averagesmall deficit of ~ 10 ppm in 142Nd of enstatite chondrites
relative to modern terrestrial samples, the closest to the
Earth value, the µ142Nd average value of EL3s is -0.8±
7.0(2SD) (Boyet et al. 2018). In addition, the ε96Zr valuesof enstatite chondrites are the same of the bulk silicate
Earth (BSE) (Mezger et al. 2020).
Although Si isotopes of enstatite chondrites appear
lighter by ~ 0.30‰ δ30Si in comparison with the BSE,
which was used to argue against any major contribution of enstatite chondrite-like planetary materials in
Earth accretion (Fitoussi and Bourdon 2012), this discrepancy could be due to fractionation of Si between
silicates and Si-bearing metal during condensation. Sikdar et al. (2020) analyzed metal and silicate separates
from EH3 and reported significantly lighter Si isotopes
in enstatite chondrite metals (δ30Si ≥ − 6.94 ± 0.09‰,

Page 12 of 16

Mg/Si =  ~ 0.001), whereas its silicates are heavier (av. δSi = − 0.33 ± 0.11‰, Mg/Si =  ~ 1.01) and closer to BSE(δ 30Si = − 0.29 ± 0.08‰). First-principles calculations ofequilibrium isotopic fractionation of Si during condensation of the solar nebula under highly reducing conditions
confirmed that the silicates are 30Si-enriched in comparison with Si-bearing metallic Fe–Ni (Javoy et al. 2012).
Refractory siderophile elements: highly siderophile elements in the terrestrial mantle are mainly attributed to
the late veneer, because the original inventory has been
partitioned into the core during the metal–silicate segregation. Hence, the isotopes of highly siderophile elementsof the Earth’s mantle trace mainly the features of the lateveneer. The ε100Ru values of enstatite chondrites are theclosest to the Earth value (Fischer-Gödde and Kleine2017), suggesting that not carbonaceous chondrites but
enstatite chondrites may be the primary late veneer. Theµ92Mo and ε100Mo values (s-process component) show
the same trend as Ru isotopes, with enstatite chondrites
closest to the Earth’s mantle (Render et al. 2017; Mezger
et al. 2020).
Moderate volatile elements: Zhao et al. (2020) analyzed K isotopes of 8 enstatite chondrites and reported
the average δ41 K of 0.47±0.57‰, indistinguishable fromthe BSE value (0.48 ± 0.03‰). δ66/64Zn of enstatite chondrites (0.2–0.4‰) is similar to the Earth (0.3‰), and the δ
122/118
Sn value is only slightly lighter (0.18 ± 0.21‰) thanthe Earth (0.49 ± 0.11‰). The isotopically heavy Sn of thesilicate Earth may be due to loss of light Sn, via partition
into the metallic core or a sulfide matter, or by evaporative loss from Earth. This isotopically light Sn loss is consistent with the correlation between the contents andisotopic compositions (Creech and Moynier 2019).
30

5.2 Bulk chemistry

The isotopic similarity of enstatite chondrites with the
Earth suggests that they could be the major building
blocks of the latter. The most significant chemical difference between enstatite chondrites and BSE is the higher­SiO2 abundance of enstatite chondrites. However, thischemical difference can be explained by differentiation processes in the early Earth (Javoy 1995; Javoy et al.
2010). During core formation, the exchange budget equation ­SiO2 + 2Fe = Si + 2FeO under high pressure andtemperature can deduce the compositions of BSE, thelower mantle and the core from enstatite chondrites. The
building blocks of the Earth were chemically and isotopically similar, but not identical to, enstatite chondrites.
The origins of water and volatile components of the
Earth are another long-standing issue. A model is thatthe building blocks of the Earth are very dry and the
majority of water was delivered by the late veneer of carbonaceous chondrites. Maruyama et al. (2017) proposed

Lin ﻿Progress in Earth and Planetary Science(2022) 9:28

formation of the Earth as two steps. At 4.56 Ga, the Earthwas born from enstatite chondrite-like dry materials
without atmosphere and ocean components, and peaked
at 4.37–4.2 Ga the subsequent secondary accretion of
bio-elements, H, C, O and N probably from carbonaceous chondrites. This model was referred to as ABEL
(the advent of bio-elements model). However, the isotopic compositions of highly siderophile elements favor
for addition of the s-process deficit carbonaceous chondrites. On the other hand, the primitive enstatite chondrites may not mean dry or volatile-poor, because of thepresence of the aqueous Cr sulfides (minerals A and B,
aqueous Fe–Cr sulfide), sinoite and graphite as depictedabove. These aqueous Cr sulfides are not terrestrial
weathering origins, since the Qingzhen and several otherenstatite meteorites are witnessed falls. In fact, the bulk
compositions of enstatite chondrites (especially EH3s) do
not show significant depletion in moderate volatile elements. Furthermore, the analysis of water contents andH isotopes of enstatite chondrites reveals that the watercontents ranging from 0.08 to 0.54 wt% (negatively correlated with type) with bulk δ D of -100 ~ − 150‰, andenstatite chondrites attributed at least three times themass of water in the ocean (Piani et al. 2020).
Of the meteorite collection, enstatite chondrites show
the most similar isotopic compositions of the Earth, suggestive of important contribution to the building materials of the latter. However, there are large differencesin bulk compositions between the Earth’s mantle andenstatite chondrites (e.g., higher FeO content of the former, whereas more SiO2 of the latter), which are required
to be addressed in detail. Nevertheless, the highly
reduced properties, high abundance of graphite, and thepresence of N-bearing phases and aqueous Cr sulfides of
enstatite chondrites could have critical constraints on theearly evolution of the Earth and the compositions of terrestrial mantle and core.

6 Issues remained
6.1 Relative abundances of presolar grains

Presence of Si3N4: SiC is not expected to condense under
the conditions of carbonaceous chondrites or ordinarychondrites; hence, its presolar origins are understandable. In contrast, SiC can condense under highly reducing conditions (Larimer and Bartholomay 1979), but all
SiC grains from the primitive enstatite chondrites arepresolar origins. This discovery indicates that in fact no
SiC condensed under the highly reducing conditions inthe solar nebula. On the other hand, few AGB grains of­Si3N4 have been found in meteorites (Zinner et al. 2007),
although ­Si3N4 grains were detected in the circumstellar shells of a number of extreme carbon stars (Clément
et al. 2005) and the predominant S­ i3N4 grains in enstatite

Page 13 of 16

chondrites are solar origin (Lin et al. 2002a). A part of­Si3N4 grains were likely exsolution from metallic Fe–Ni,
usually with a needle-like shape (Alexander et al. 1994).
Other ­Si3N4 grains found in metal, schreibersite, sulfides
and silicates probably condensed in the solar nebulawith δ15 N of − 60 ± 1‰ (Leitner et al. 2018). All of thepresolar ­Si3N4 grains are supernova origin (Nittler et al.
1995; Lin et al. 2002a). The relative abundance of X-Si3N4
is comparable with that of X-SiC, suggestive of condensation of both phases from the supernovae ejecta. In
contrast, SiC but no ­Si3N4 condensed from AGB ejecta,
whereas both were reverse in the reduced solar nebula.
Rareness of presolar O-rich grains: The matrix-normalized abundances of presolar SiC and diamond in Qingzhen are comparable to those in Murchison and otherprimitive chondrites. However, the presolar O-richgrains are extremely depleted in Qingzhen (Lin et al.
2002a). This discrepancy is remarkable, regarding thatthe relative abundances of various types of presolar SiC
and ­Si3N4 grains are similar between enstatite chondrites
and carbonaceous chondrites (Lin et al. 2002a). A scenario is that the presolar O-rich grains have been largely
destroyed under the highly reducing environments. This
scenario may be attributed to the absence of presolar
silicates in enstatite chondrites. However, corundum andspinel are very stable and resistant to strong acid, andthey were separated via being concentrated in the acidresidue of chondrites.
6.2 Coexistence of high‑T condensates and low‑T
preservation of presolar grains

The submicron-sized presolar grains are very sensitive
to temperature. The abundance of presolar grains in E3
chondrites is comparable to those of ordinary chondrites
and carbonaceous chondrites. This discovery supplies
with robust evidence that the temperature at chondriteaccreting regions was low and there was no significant
increase in temperature from carbonaceous chondrites
in the outer solar system to enstatite chondrites in the
inner solar system. On the other hand, the majority of
the constituent minerals in enstatite chondrites condensed at high temperature under highly reducing conditions. The coexistence of high-T condensates and low-T
preservation of presolar grains suggests that both components formed and preserved in separated areas andfinally transported into the enstatite chondrite-accreting
regions. However, it is unknown whether the presolar
grains were added from the low-T zones of the outersolar system and/or from the periphery of the protoplanetary disk to the midplane, or the main constituents condensed at high-T zones of the protoplanetary disk andtransported outward into enstatite chondrite-accreting
regions.

Lin P﻿ rogress in Earth and Planetary Science(2022) 9:28

The systematically different mineral chemistry between
EHs and ELs favors local condensation and consequentin situ accretion without significant transportation. This
scenario is supported by the relatively constant mineralchemistry within the same chondrites, e.g., the Mn contents of sulfides, which indicate similar forming conditions of the minerals in the same chondrites but show
spatial variation. Graphite from Almahata Sitta MS-17
and MS-177 EL3 has distinct morphologies and C isotopic compositions (MS-177: books in silicate matrix
with δ13C = − 18 to -2‰; MS-17: feathery spherules inmetal with δ13C = − 26 to -6‰) (El Goresy et al. 2017).
Not only the differences between EH and EL regions,but also within ELs and EHs, the chemical and isotopic
variations have been well preserved during accreting process. Preservation of the delicate differences establishedduring condensation supplies with convincing evidence
for local accretion of enstatite chondrites. This discovery favors a scenario that the presolar grains were addedfrom low-T zones.
6.3 Origins of the highly reducing conditions

The solar nebula has a bulk composition of CI chondrites with the C/O ratio of ~ 0.54. The enstatite chondrite-forming regions were highly reducing with C/Oratios > 0.8, different from the canonical value of the solar
nebula. Before condensation from high temperature, theenstatite chondrite-forming regions should have been
evolved into highly reduced conditions from the homogeneous solar nebula with a CI composition. As discussed above, repeating evaporation and condensationof water ice and organic matter cross the snow line along
the protoplanetary disk could produce an increasing C/Oratio toward the Sun. Isotopic compositions of N and C
of insoluble organic matter (IOM) from Sahara 97096(EH3) are within the range of values observed for carbonaceous chondrites, suggestive of a common component
widespread in the solar nebula (Piani et al. 2012).
Furthermore, enstatite chondrite-forming regions
could be rather heterogeneous with a large gradient inoxygen fugacity from EH to EL regions, as indicated by
the Si contents of metal, the MnS contents of sulfides andC isotopes of graphite. However, few records of the evolutional processes have been found.
Pignatale et al. (2016) computed a 2D condensation
model for the inner solar nebula. They found that an
enstatite-rich environment could be achieved via vertical settling and radial migration of dust in the protoplanetary disk, and enstatite chondrites could form in the
inner surface layer of the disk within 0.4 AU.

Page 14 of 16

Acknowledgements
The author is grateful to Prof. Ahmed El Goresy for his supervision when theauthor conducted his PhD thesis research in the Max Planck Institute fürKernphysik, Heidelberg, more than 30 years ago. The manuscript has beensignificantly improved by the constructive reviews by Dr. M. Kimura and A. E.
Rubin and the guest editor T. Sharp. This work was supported by the Strategic
Priority Research Program on Space Science, Chinese Academy of Sciences(XDA15020300), the Key Research Program of Frontier Sciences, CAS (QYZDJSSW-DQC001), and the Natural Science Foundation of China (41673069).
Author contributions
YL contributed to the review and writing of the paper. The author read andapproved the final manuscript.
Funding
Funding for this work was provided by the Strategic Priority Research Programon Space Science, Chinese Academy of Sciences (XDA15020300), the Key
Research Program of Frontier Sciences, CAS (QYZDJ-SSW-DQC001), and the
Natural Science Foundation of China (41673069).
Availability of data and materials
Please contact the author for data requests.

Declarations
Competing interests
The author declares that he has no competing interests.
Received: 8 November 2021 Accepted: 6 May 2022

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