Progress in Earth andPlanetary Science

Hammouda et al.
Progress in Earth and Planetary Science(2022) 9:13
https://doi.org/10.1186/s40645-022-00471-w

Open Access

RESEARCH ARTICLE

The message of oldhamites from enstatitechondrites
Tahar Hammouda1* , Maud Boyet1, Paul Frossard1 and Camille Cartier2

Abstract
We have determined rare-earth element (REE) abundances in oldhamites (CaS) from 13 unequilibrated and equilibrated enstatite chondrites (5 EH and 8 EL) and in a few enstatites by in situ, laser ablation ICP-MS. In EH chondrites,
oldhamite REE patterns vary from the most primitive petrographic types (EH3) to the most metamorphosed types(EH5). In EH3, CI-normalized REE patterns are convex downward with strong positive Eu and Yb anomalies, whereas
EH5 display flat patterns with enrichments reaching about 80 times CI abundances. The positive anomalies of Eu
and Yb found in oldhamites of primitive EH chondrites indicate that they represent the condensation of a residual
gas fraction, in a manner similar to fine-grained CAIs of carbonaceous chondrites. The early condensate may havebeen preserved in the matrix of unequilibrated EH. Equilibrated EH oldhamite patterns may result from metamorphic
evolution and REE redistribution on the EH parent body. On the contrary, all the oldhamites from EL chondrites (EL3to EL6) display a single kind of patterns, which is convex upward and is about 100 times enriched relative to CI, with anegative Eu anomaly. In addition, the EL pattern is similar to that of oldhamites from aubrites (enstatite achondrites).
The latter observation suggests that oldhamites of all EL metamorphic types (including primitive ones) bear the signature of a magmatic event accompanied by FeS loss as vapor, prior to the assembly of the EL parent body. Given the
difficulty of obtaining precise ages on enstatite chondrites, it is not possible to discuss the chronology of the events
recorded by the oldhamite REE patterns.
Keywords: Enstatite chondrite, Oldhamite, Rare-earth-elements, Redox conditions, Condensation, Vaporization
1 Introduction
Enstatite chondrites (EC) represent the most reduced
type of chondrites. Although rare (about 0.6% of total
known meteorites, Krot et al. 2014; Weisberg and Kimura2012), EC are of great interest because they share many
isotopic characteristics with terrestrial samples [oxygen(Clayton 1993), nitrogen (Javoy et al. 1986; Cartigny et al.
1997), molybdenum (Dauphas et al. 2002; Burkhardt et al.
2011), chromium (Trinquier et al. 2007); titanium (Trinquier et al. 2009); Xe and Kr (Miura et al. 2007); nickel(Regelous et al. 2008)] and it has been suggested that they
could represent the major building blocks of our planet*Correspondence: tahar.hammouda@uca.fr
1
CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Université Clermont
Auvergne, 63000 Clermont‑Ferrand, France
Full list of author information is available at the end of the article(Javoy 1995; Javoy et al. 2010). Enstatite chondrites are
divided into two groups, EH and EL, which differ by theirbulk iron and sulfur contents (H = high; L = low, Wasson
and Kallemeyn 1988; Krot et al. 2014). The two groups
also differ in their sulfide mineralogy. In addition to troilite (FeS), daubréelite ­(FeCr2S4) and oldhamite (CaS) arecommon to both groups, whereas niningerite ((Mg,Fe)S)is the diagnostic sulfide of EH chondrites and alabandite((Mn,Fe)S) is specific to EL (Lin and El Goresy 2002). The
presence of unusual sulfides containing normally lithophile elements (Mg, Ca, Mn, Ti, Na), Si-bearing kamacite,
nitrides, phosphides and carbides reflects highly reducing environment of EC formation. Under these conditions, elements including the rare-earth elements (REE)and the actinides that are lithophile in terrestrial conditions and in other types of chondrites tend to be siderophile and/or chalcophile (e.g., Barrat et al. 2014; Crozaz© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
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regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 2 of 19

and Lundberg1995; Gannoun et al. 2011; Piani et al. 2016;
Cartier and Wood 2019; Anzures et al. 2020).
Oldhamite has attracted much attention because of
its rare-earth element content. Previous studies haveshown that this phase can carry about half of the EC REE
budget, with abundances up to 100 times the chondritic
values (Crozaz and Lundberg 1995; Gannoun et al. 2011).
Because these elements are highly refractory and large
ion REEs substitute for Ca in condensates (Lodders 2003;
Wood et al. 2019), these observations suggest that oldhamites were formed early by condensation. They can becompared to minerals found in calcium-aluminum richinclusions (e.g., perovskite and hibonite), which are firstto condense from the cooling of a solar gas (Lodders andFegley 1993). Thermodynamical modeling of condensation at more reducing conditions (C/O ratio > 0.5, Lodders and Fegley 1993; Ebel and Alexander 2011) predicts
a sequence in which oldhamite appears during coolingand condensation. Furthermore, at reduced conditions,the models predict that rare-earth elements are incorporated into oldhamite with a final pattern characterized
by negative Eu, Yb, and Sm anomalies, because those elements have the lowest condensation temperature among
the REE (Lodders and Fegley 1993; see Fig. 1). Instead,oldhamites from primitive EH (Crozaz and Lundberg
1995; Gannoun et al.2011) show positive Eu, Yb, and Sm
anomalies. This observation is at odds with a postulatedcondensation origin for the sulfides of primitive EH at
reduced conditions.
Experimental studies on CaS/melt REE partitioning successfully predict positive Eu and Yb anomalies

as observed in primitive EH chondrites (Lodders 1996;
Dickinson and McCoy 1997; Ingrao et al. 2019). However,experimental partition coefficient values are of the orderof 1 (Fig. 1), which makes it difficult to explain the highREE enrichment in oldhamite, unless a complex magmatic history is considered.
Rare-earth element patterns of oldhamites differ whendifferent groups and metamorphic types are compared(Crozaz and Lundberg 1995; Gannoun et al. 2011). These
differences may concern overall REE concentrations, thepresence or absence of anomalies in the REE patterns,
e.g., Eu, Yb, and Sm, as well as the sign of the anomalies.
However, literature data are too scarce to fully understand these chemical signatures. Literature data alsopresent information on enstatite compositions (Hsu andCrozaz 1998; Gannoun et al. 2011), which show different
types of REE patterns. In order to get further insight into
the formation conditions of enstatite chondrites as wellas to their REE budgets, we have analyzed oldhamites anda few enstatites from a collection of enstatite chondrites
that covers most of known types (EH3 to EH5 and EL3
to EL6). We used laser ablation ICP-MS for in situ analysis. With this method, we can relate the textural location
to the mineral composition, which is not possible whenusing the stepwise dissolution techniques (Boyet and
Gannoun 2013; Barrat et al. 2014). Thus, we can also discuss the respective advantages of each method, as well as
their complementary nature.

1

10-2

CaS / melt
partitioning

10-4
condensed
10-6

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 1 Rare-Earth Element pattern (in grey) predicted by modelingcondensation as monosulfides solid solution in oldhamite [CaS,
Lodders and Fegley (1993)]. Conditions are T = 1378.6 K (0.1 K below
oldhamite condensation temperature) and a nebular gas with
C/O = 1.2 (reduced conditions). Also shown are the CaS/melt REE
partition coefficients determined experimentally by Ingrao et al.
(2019)

2 Methods
2.1 Samples

We studied a collection of both groups of EC with metamorphic grades ranging from type 3 (primitive or
unequilibrated) to type 6 (highly equilibrated). With
metamorphic grade, from type 3 to type 6, chondrules
become progressively less visible, the matrix proportion
decreases and minerals become chemically more homogeneous. A detailed list of the studied material is presented in Table 1. We have studied 5 samples from theEH group (2 EH3, 1 EH4, 2 EH5) and 13 from the EL
group (6 EL3, 1 EL4/5, 1 EL5, 5 EL6) including both fall
and find samples. All EH were suitable for CaS analysis.
For EL, 3 EL3 and 2 EL6 yielded no fresh oldhamites andwe obtained data for 3 EL3, 1 EL4/5, 1 EL5 and 3 EL6. For
each sample, we studied only one section. In the discussion, we also considered Sahara 97,072 and ALHA77295
EH3 from Gannoun et al. (2011).
Meteorite fragments or slabs were mounted in epoxy
resin and polished using diamond paste under ethanol. In
case the material was already a resin mount, we simply
polished it, in order to have a clean surface. In all cases,
this preparation allowed for the petrographic observations and subsequent in-situ analyses. For laser ablation,

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

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Table 1 List of samples analyzed minerals in this study
MeteoriteType

Fall/findSourceOldhamite

Enstatite

In-situ REE
measurements

Wholerock andleaching

x

[4]

[4]

x

[4]

[4]

ALHA77295

EH3

Find

NASA

Caleta el Cobre 025

EH3

Find

J. Gattacceca, CEREGE

Sahara ­97096a

EH3

Find

L. Labenne

Sahara ­97158a

EH3

Find

L. Labenne

x

Abee

EH4

Fall

L. LabennexOudiyat Sbaa

EH5

Find

A. Irving, University of Washington, Seattlex

St. Mark’s

EH5

Fall

Smithsonian, USNM 3027

MAC ­88136b

EL3

Find

NASA

x

MS ­17c

EL3

Fall

S. Haberer, Haberer Meteoritex

MS ­189c

EL3

Fall

A. Black, ImpatikaxMS ­196c

EL4/5

Fall

A. Black, ImpatikaxAdrar Bous

EL5

Find

L. LabennexxAtlanta

EL6

Find

A. El Goresy, BGI

x

x

x

x

HvittisEL6

Fall

J. Schooler

NWA 974

EL6

Find

Fectayxx[4][4], [6][3], [2][6], [7][3][6], [7][1], [3][6][7][6], [7][6]Previous studies providing data on rare-earth elements (in-situ and whole-rock) on these samples are indicated in bracket [][1] Crozaz and Lundberg (1995): oldhamite, enstatite and niningerite by SIMS
[2] Hsu (1998): albite by SIMS
[3] Hsu and Crozaz (1998): enstatite by SIMS
[4] Gannoun et al. (2011): oldhamite by SIMS and LA-ICPMS, enstatite and niningerite by SIMS, whole-rock measurements[5] Boyet and Gannoun (2013):step-wise dissolution techniques, measurement of leachings and residues[6] Barrat et al. (2014): whole-rock, leachate and residue measurements[7] Boyet et al. (2018): whole-rock measurementsaSahara 97072, 97096 and 97158 are paired meteoritesb

Paired to MAC 88180

c

MS samples are fragments of the polymict breccia of Almahata Sitta fall

the principal limitation is the grain size. This is particularly true for elements with low abundances, because
large spots are needed in order to obtain enough counts
on the detectors. In addition, those minerals that showed
clear evidence of alteration, such as rust or voids, werediscarded.
Except for NWA 974 (EL6), all meteorites studied here
have been subject to previous investigations, either formineralogy and/or whole rock composition. Reports ofwhole rock major and trace elements can be found in Keil
(1968), Kallemeyn and Wasson (1986), Gannoun et al.
(2011), Barrat et al. (2014) and Boyet et al. (2018). Individual mineral major element concentrations obtainedusing electron microprobe have been published (Keil
1968; Buseck and Holdsworth 1972; Rubin 1983; Sears
et al. 1984) with detailed studies on metal and troilite(Keil 1969; Zhang and Sears 1996; Piani et al. 2016, Weyrauch et al. 2018). Trace elements in sulfides have beenreported by Floss et al. (1990), Floss and Crozaz (1993),
Crozaz and Lundberg (1995), Hsu and Crozaz (1998),
Gannoun et al. (2011) and El Goresy et al. (2017).

2.2 Petrographic observations and major and minor
element analysis

Petrographic observations were performed using a JEOL
scanning electron microscope (JSM-5910LV) operating at 15 kV. Major element analyses of minerals wereobtained using an SX100 electron microprobe using a
focused beam, with a 15 kV accelerating voltage, and a
15 nA beam current. For sulfide analyses, we used pyrite­(FeS2) as a standard for Fe and S, metals for Mn, and Ni,
wollastonite for Ca, San Carlos olivine for Mg, C­ r2O3 for
Cr and ­MnTiO3 for Ti. Counting times on peak were 20 s
for Fe, Mg and S, and 10 s for Ca, Ni, Mn, Cr and Ti.
2.3 In‑situ trace element analysis

Trace element concentrations in oldhamites andenstatites were obtained using an Agilent 7500-cs ora Thermo Fisher Element XR ICP-MS, coupled with aResonetics M-50 laser at Laboratoire Magmas et Volcans.
The laser was operated at 193 nm wavelength with energy
varying between 2 and 8 mJ, depending on the nature andsize of the phase of interest. We used 3 Hz frequency andHammouda et al. Progress in Earth and Planetary Science(2022) 9:13

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nominal laser spot size of 20 µm for oldhamite and up to
100 µm for enstatite. In order to improve detection limits
and counting statistics, we used block counting times of
60 ms. For major elements, we used 2, 10 or 20 ms. Total
ablation duration was usually about 80 s and was donein pure He atmosphere. The analyte was carried to the
ICP torch by a gas mixture wherein N­ 2 + Ar were added
to He. We used the following gas flows: He, 750 ml/
min; ­N2, 7 ml/min; Ar nebulizer gas, 0.88 l/min; Ar coolgas, 15 l/min; Ar auxiliary gas, 1 l/min. Plasma RF was
1300 W. NIST 610 glass standard (values from Gagnon
et al. 2008) was used for calibration of relative sensitivities. Analyses were normalized using average Ca and Mg
concentrations determined by electron microprobe foroldhamite and enstatite, respectively. NIST 612 and BCR
glasses were used as secondary standards. Trace-element
concentration reductions were done with the GLITTER
software (van Achterberg et al. 2001). Accuracy on external standards from the instruments has been discussed
recently (Hammouda et al. 2010; Cartier et al. 2014). Allanalyzed elements are accurate within 10% except for Nb(less than 15%) versus Gagnon et al. (2008) and Ti (lessthan 20%) versus Pearce et al. (1997).

are also present. In Hvittis, a chondrule fragment showing a radial intergrowth of oldhamite and enstatite (similar to that found in MS 189) is present.

3 Results
3.1 Description of oldhamite occurrences

The petrographic texture of some EH chondrite sections
studied is illustrated in Fig. 2. In unequilibrated EH3(such as Sahara 97158), oldhamite occurs in metal andsulfide aggregates as well as isolated grains in the matrix.
Grain size ranges between less than 10 µm to more than
100 µm (Fig. 2a, b). Gannoun et al (2011) reported similar descriptions and images for oldhamites of the paired
meteorite Sahara 97072. In Abee (EH4, impact-melt
breccia), oldhamite grains are disseminated in the matrix(Fig. 2c). They can also be found as intergrowths withenstatite (Fig. 2d). Grain sizes range between a few to
100 µm. In St. Mark’s (EH5), oldhamite grains are present in the matrix (Fig. 2e) but chondrules consisting of
an intergrowth of oldhamite, troilite and enstatite are frequent (Fig. 2f ). In both cases, oldhamite grains can reach
100 µm.
Oldhamites of some of the EL chondrites studied areillustrated in Fig. 3. In unequilibrated EL3 (MS 189, MAC
88136), oldhamite grains are disseminated in the matrix(Fig. 3b, c). One chondrule in MS 189 consists of a radial
intergrowth of oldhamite, troilite, and enstatite (Fig. 3a).
In EL5 (Adrar Bous), oldhamite grains are disseminatedin the matrix (Fig. 3d, e). Their size is usually small (lessthan 10 µm) but some larger grains of a few tens of µm
are present (Fig. 3e). Equilibrated EL6 chondrites (suchas Hvittis, Fig. 3f ) contain oldhamite grains that can belarger than 100 µm. Some smaller grains (few tens of µm)

3.2 Oldhamite major element concentrations

Oldhamites of EH are close to CaS end-member composition, with other components (Fe, Mn, Mg) amounting
to less than 1 wt. % each. St. Mark’s (EH5) has slightly
more Mn and Mg, and slightly less Fe than the EH3 oldhamites. In EH3, Fe is the most abundant cation, afterCa, whereas in EH5, it is Mg. In all cases, Mn concentration is intermediate between Fe and Mg. Finally, Fe is theelement with the highest variability.
Oldhamites of EL have less Ca than those of EH.
Within each meteorite, oldhamite composition is rather
homogeneous, except in EL3, where Fe-poor (0.7 wt.%)
and Fe-rich (up to 4 wt.%) grains were found. In Adrar
Bous (EL5) Fe is about 1.1 wt.%, whereas in EL6, Fe islower (0.3 wt.%). It cannot be excluded that high Fe values found in EL3 oldhamite are due to contamination.
In EL3, Mn amounts to 0.8 wt.%, and in the other types,
Mn concentration ranges between 1.2 and 1.4 wt.%. Mg
ranges between 0.18 and 0.24 in EL3 and between 0.4 and0.6 wt.% in EL 5 and EL6.
Major element concentrations analyzed in oldhamites
are provided in the Additional file 1, together with thedetection limits.
3.3 Rare‑earth element concentrations in oldhamites

Rare-earth element concentrations of oldhamites(averages and 1 sigma standard deviations) are given
in Table 2. CI-normalized REE patterns are illustrated
in Fig. 4. Individual patterns for all laser shots in eachsample are displayed in the Additional file 1. In EH
chondrites, CI-normalized REE patterns vary according
to metamorphic type. In primitive EH3 (Sahara 97158,
Caleta el Cobre 025), oldhamites are characterized bystrong positive anomalies for Eu (4 < Eu/Eu* < 5), Yb(2.5 < Yb/Yb* < 3), and Sm to a lesser extent, whereas thepatterns are overall convex downward. Values of [­Eu]Nand ­[ Yb]N are about 60 and 40, respectively. All other
CI-normalized REE abundance decrease from La to Lu,
with values ranging between 46 and 8. These patterns
are similar to the C-D type patterns previously reported
in the literature for unequilibrated EH3 chondrites(ALHA77295 and Sahara 97072, Gannoun et al. 2011;
Qingzhen, Crozaz and Lundberg 1995). Variations
between grains in Eu concentration are limited, whereas
other REE concentrations (including Yb) spread overabout one order of magnitude. Oldhamites from Abee
(EH4), St. Mark’s and Oudiyat Sbaa (EH5) are homogeneous, within each meteorite. In Abee (EH4–IMB),
all analyzed grains display the same CI-normalizedHammouda et al. Progress in Earth and Planetary Science(2022) 9:13(a) EH3 - Sahara 97158

Page 5 of 19(b) EH3 - Sahara 97158

200 µm(c) EH4 - Abee

300 µm(d) EH4 - Abee

100 µm(e) EH5 - St-Mark’sm

100 µmm(f ) EH5 - St-Mark’s

tr

m
m
tr
m
100 µmm

100 µmFig. 2 Scanning electron images of some of the EH samples studied, illustrating the textural position of the oldhamite grains, indicated by the reds
arrows. Also indicated are the metal (m) and troilite (tr) grains

REE patterns, slightly convex upward, with ­[REE]Nranging between 60 and 80, and a negative Eu anomaly(Eu/Eu* = 0.66). In EH5 (St. Mark’s and Oudiyat Sbaa),
oldhamite CI-normalized REE patterns are flat and no
anomaly is observed. ­[REE]N range between 60 and
70. Therefore, a major difference between metamorphic types is the presence of strong positive Eu and Yb(and Sm) anomalies in unequilibrated EH3. No positive

Eu and Yb anomalies were found in Abee, St. Mark’s,
and Oudiyat Sbaa. In Indarch (EH4), Crozaz and Lundberg (1995) found slightly fractionated REE patterns­([La]N = 82; ­[Lu]N = 54), with little variability. Most of
the reported REE patterns in Indarch display a positive
Yb anomaly (Yb/Yb* of about 1.5) and a slight negative
Eu anomaly, which is difficult to quantify because of thelight rare-earth element (LREE) and heavy rare-earthHammouda et al. Progress in Earth and Planetary Science(2022) 9:13(a) EL3 - MS-189

Page 6 of 19tr(b) EL3 - MS-189

tr

m
m
m

tr

tr

m
m

100 µm

100 µmm(d) EL5 - Adrar Bous(c) EL3 - MAC 88136

100 µmtr
m

m

500 µm500 µm(e) EL5 - Adrar Bous(f ) EL6 - Hvittis
tr

tr
m

tr
tr

tr

100 µm

tr

tr

tr

tr

tr

tr

tr
tr

tr
tr m

m

tr

tr

m
m

500 µm

tr

tr

m
m

m

m

500 µmFig. 3 Scanning electron images of some of the EL samples studied, illustrating the textural position of the oldhamite grains, indicated by the reds
arrows. Also indicated are the metal (m) and troilite (tr) grains

element (HREE) fractionation that appears to be stepwise around Eu.
In EL samples, all oldhamite types (EL3 to EL6) arecharacterized by upward convex CI-normalized REE
patterns and negative Eu anomalies (Fig. 4b). In addition, oldhamites have constant compositions within eachmeteorite (a feature noted by Floss et al. 1990 in EL6),and REE abundances in all types have close values. CInormalized La values range between 84 for MS-189(EL3) and 143 for Adrar Bous (EL5), whereas [­Lu]N values vary over a tighter range, from 67 in MS-189 and 84
in Adrar Bous. One EL3 (MAC 88136) appears exceptional with REE abundance values in CaS higher thanthe others, all [­REE]N being larger than 150, except for
Eu (Eu/Eu* = 0.57). However, its overall pattern is notvery different from the rest of the EL oldhamites. We do
not exclude contamination by adjacent phases, resulting
in normalization issue in this sample whose oldhamite

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

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Table 2 Rare-earth element concentrations measured in CaS (in ppm) and CI-normalized values
EH group

Concentration

EH3

EH4

EH5
St-Mark’s (8)Oudiyat Sbaa (14)

ppm

ppm

Sahara 97158 (28)Caleta el Cobre (3)Abee (8)

ppm

ppm

ppm

stdev

stdev

stdev

stdev

stdev

La

9.31

2.46

10.97

0.62

18.85

4.11

17.26

2.17

15.22

0.96

Ce

17.05

5.45

20.78

3.68

49.38

7.16

42.33

6.26

39.36

2.91

6.31

1.03

5.83

0.38

38.18

4.44

32.60

3.77

30.40

2.82

Pr

1.88

0.73

2.63

0.73

Nd

7.75

3.10

10.99

3.39

Sm

2.69

1.14

4.20

1.06

12.68

1.73

10.85

1.76

10.09

0.76

Eu

3.48

0.49

3.76

0.48

3.08

0.62

4.11

0.71

3.76

0.44

15.81

2.00

Gd

1.98

0.95

3.42

1.28

Tb

0.33

0.17

0.60

0.22

Dy

2.21

1.14

3.98

1.40

Ho

0.46

0.23

0.86

0.28

Er

1.37

0.68

2.68

0.83

Tm

0.22

0.11

0.40

0.16

17.33

1.47

10.84

1.65

13.95

1.59

12.81

1.42

2.55

0.34

2.46

0.39

17.06

2.01

16.91

1.62

3.81

0.44

3.64

0.26

10.46

1.26

11.04

1.00

1.56

0.19

1.61

0.18

Yb

6.81

2.40

7.39

0.94

10.39

1.04

11.10

1.90

10.88

1.15

Lu

0.21

0.09

0.49

0.14

1.55

0.26

1.67

0.31

1.68

0.21

CI-normalized concentration

stdev

stdev

stdev

stdev

stdev

La

39.62

10.48

46.68

2.64

80.22

17.47

73.46

9.22

64.76

4.07

Ce

28.42

9.08

34.63

6.13

82.30

11.93

70.56

10.43

65.60

4.86

69.29

11.34

64.09

4.14

82.28

9.56

70.26

8.12

65.51

6.09

Pr

20.61

7.99

28.94

7.98

Nd

16.70

6.68

23.69

7.30

Sm

17.56

7.47

27.43

6.93

82.87

11.28

70.88

11.48

65.92

4.97

Eu

59.32

8.33

64.11

8.11

52.50

10.64

70.12

12.07

64.14

7.52

76.75

9.72

Gd

9.61

4.62

16.59

6.20

Tb

8.74

4.59

15.91

5.99

Dy

8.69

4.48

15.67

5.50

Ho

8.08

4.09

15.13

4.90

Er

8.23

4.09

16.16

5.02

Tm

8.33

4.33

15.24

5.95

68.23

5.80

65.31

9.96

67.71

7.73

62.16

6.89

67.91

9.09

65.47

10.47

67.16

7.92

66.59

6.38

67.23

7.73

64.38

4.56

63.01

7.61

66.50

6.03

59.54

7.38

61.26

7.01

Yb

40.54

14.28

43.97

5.59

61.85

6.21

66.09

11.31

64.78

6.82

Lu

8.39

3.84

19.96

5.73

62.81

10.47

68.00

12.57

68.39

8.34

EL group

EL3

Concentration

MAC 88136 (4)MS 17 (12)

ppm

ppm

stdev

stdev

EL4/5

EL5

MS 189 (27)MS 196 (18)Adrar Bous (11)

ppm

ppm

ppm

stdev

stdev

stdev

La

34.06

8.59

20.96

1.49

19.78

2.24

21.96

0.78

33.67

3.09

Ce

97.36

14.04

54.95

3.49

50.23

4.77

57.88

3.20

82.57

8.65

Pr

15.50

3.03

8.00

0.48

7.59

1.42

8.90

0.88

12.19

1.39

Nd

80.20

12.96

42.46

2.38

38.94

9.79

44.27

2.27

62.73

6.33

Sm

26.08

3.19

14.59

0.68

12.83

1.96

13.53

1.66

19.70

2.09

Eu

5.57

1.43

2.60

0.17

2.37

0.40

2.33

0.43

2.79

0.25

Gd

33.57

3.35

19.91

1.07

15.74

1.98

19.99

2.27

25.78

1.99

Tb

6.38

0.92

3.56

0.18

2.98

0.43

3.35

0.25

4.57

0.35

Dy

43.58

4.83

25.14

1.47

19.59

2.40

23.76

1.14

29.69

2.25

Ho

8.94

1.46

5.33

0.26

4.44

0.77

5.12

0.43

6.30

0.45

Er

28.01

3.02

15.73

0.90

12.63

1.61

15.02

1.21

16.93

1.34

Tm

4.17

0.63

2.24

0.12

1.75

0.23

2.05

0.28

2.35

0.21

Yb

29.52

1.86

15.37

0.73

13.39

2.24

12.84

1.84

15.78

1.24

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 8 of 19

Table 2 (continued)
EL group

EL3

EL4/5

EL5

MS 189 (27)MS 196 (18)Adrar Bous (11)

ppm

stdev

ppm

0.41

2.05

MAC 88136 (4)MS 17 (12)Concentration

ppm

stdev

ppm

stdev

ppm

stdev

Lu

3.77

0.66

2.08

0.11

1.65

0.21

CI-normalized concentration

stdev

stdev

1.99

stdev

stdev

stdev
0.15
stdev

La

144.93

36.54

89.20

6.36

84.19

9.53

93.56

3.32

143.28

13.14

Ce

162.26

23.40

91.58

5.82

83.72

7.95

95.95

5.33

137.62

14.42

Pr

170.36

33.27

87.94

5.26

83.42

15.58

99.93

9.67

133.9715.24

Nd

172.85

27.93

91.51

5.13

83.92

21.10

97.86

4.89

135.20

13.63
13.68

Sm

170.42

20.83

95.35

4.47

83.83

12.82

91.96

10.87

128.73

Eu

95.09

24.33

44.31

2.93

40.48

6.77

41.62

7.35

47.68

4.24

Gd

162.94

16.27

96.63

5.21

76.40

9.63

101.6811.02

125.17

9.66

Tb

170.00

24.50

94.93

4.80

79.35

11.34

92.21

6.59

121.87

9.27

Dy

171.56

19.02

98.97

5.80

77.14

9.46

97.91

4.49

116.898.86

Ho

157.99

25.84

94.15

4.65

78.47

13.66

92.10

7.52

111.257.98

Er

168.72

18.22

94.78

5.40

76.08

9.73

94.51

7.31

101.97

8.10

Tm

159.26

23.99

85.57

4.77

66.81

8.64

84.91

10.53

89.72

7.88

Yb

175.71

11.10

91.47

4.34

79.67

13.31

79.00

10.96

93.92

7.40

Lu

153.15

26.68

84.64

4.41

67.19

8.43

81.80

16.77

83.41

6.25

EL group

EL6
NWA 974 (14)Atlanta (15)Hvittis (8)Concentration

ppm

stdev

ppm

stdev

ppm

stdev

La

26.70

1.93

22.84

1.07

22.56

1.05

Ce

69.97

4.94

59.06

2.62

59.45

1.62

Pr

10.46

0.67

8.99

0.42

9.31

0.44

Nd

57.55

4.27

48.79

2.56

52.41

2.74

Sm

18.59

1.11

15.76

0.78

17.02

0.88

Eu

1.98

0.19

2.10

0.19

1.68

0.12

Gd

25.39

2.74

21.90

0.98

23.98

1.36

Tb

4.42

0.27

3.87

0.16

4.23

0.24

Dy

29.51

2.52

25.46

1.27

28.08

2.12

Ho

6.22

0.50

5.55

0.34

6.04

0.47

Er

16.53

1.52

14.86

0.80

16.00

0.92

Tm

2.26

0.22

2.06

0.14

2.18

0.14

Yb

14.56

1.26

13.04

0.90

14.10

0.94

Lu

1.95

0.28

1.82

0.11

1.90

0.14

96.02

4.45

CI-normalized concentration
La

stdev
113.60

8.23

stdev
97.18

4.53

stdev

Ce

116.62

8.23

98.43

4.36

99.08

2.69

Pr

114.91

7.32

98.77

4.64

102.31

4.87

Nd124.039.21

105.155.52

112.945.91

Sm

121.537.28

103.005.10

111.21

5.76

Eu

33.78

3.22

35.76

3.26

28.67

1.98

Gd

123.2313.31

106.294.78

116.396.61

Tb

117.787.21

103.094.32

112.706.37

Dy

116.189.90

100.225.00

110.568.34

Ho

109.83

8.83

98.02

6.07

106.74

8.24

Er

99.56

9.15

89.51

4.83

96.39

5.53

Tm

86.12

8.27

78.60

5.25

83.29

5.33

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 9 of 19

Table 2 (continued)
EL group

EL6
NWA 974 (14)Atlanta (15)Hvittis (8)

Yb

86.65

7.49

77.60

5.37

83.92

5.58

Lu

79.08

11.21

73.95

4.57

77.06

5.88

CI values were taken from Barrat et al. (2012)(a)oldhamite / CI

100

10

1

EH3 - Sahara 97158
EH3 - Caleta el Cobre
EH4 - Abee
EH5 - Oudiyat Sbaa
EH5 - St-Mark's

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

(b)

oldhamite / CI

100

10

EL3 - MAC 88136
EL3 - MS-17
EL3 - MS-189
EL4/5 - MS-196
EL5 - Adrar Bous
EL6 - Atlanta
EL6 - Hvittis
EL6 - NWA 974

La

Ce

Pr

Fig. 4 CI-normalized REE abundances in oldhamites from EH chondrites (a) and EL chondrites (b). CI values are from Barrat et al. (2012)Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 10 of 19

grains are small and often altered. Concerning the Eu
anomaly, it seems that Eu/Eu* decreases from 0.57 in EL3(MAC 88136) to 0.25 in EL6 (Hvittis). Our REE results
are compatible with literature data on EL3 MAC 88136
and EL6 Jajh Deh Kot Lalu and Khairpur (Crozaz andLundberg 1995; Floss and Crozaz 1993).

Overall, enstatites in MAC 88136 have REE patterns thatare close to those found in Sahara 97096 (EH3). The patterns in MAC 88136 are slightly fractionated ([La/Lu]Nin MAC 88136 less than1 in most cases) and a negative
Yb anomaly is present. Some patterns also have a negative Eu anomaly, which is accompanied by a negative
Sm anomaly in most cases. When shifting to EL chondrites of higher metamorphic grades, REE element patterns in enstatite display two changes: (1) a clusteringof the concentration values, which is particularly clear
for the HREE, with ­[Lu]N of about 1.1; (2) a HREE/LREE
fractionation, reaching a value of [Lu/La]N of 348 forthe most extreme case (in NWA 974). This evolution isaccompanied by enstatite grain coarsening from Adrar
Bous (EL5) to NWA 974 (EL6). The latter case illustrates
the potential analytical biases. For HREE, the standard
deviation is very small, reflecting the smaller potential forcontamination by material adjacent to enstatite grains.
Toward MREE and LREE, the effect of contamination
increases as the concentrations decrease, resulting in alarger dispersion of the measured concentrations, and isparticularly critical for Eu. If we consider that the mostfractionated pattern represents the ‘best’ values, we candocument a negative Eu anomaly (Eu/Eu* = 0.067). We
also note that the contaminant phase is enriched in LREE
and has a positive Eu anomaly, suggesting that plagioclase could be a good candidate. We did not attempt toidentify nearby plagioclase, because we focused on oldhamite features.
Our results on enstatite REE patterns compare wellwith those of Hsu and Crozaz (1998) and Gannoun
et al. (2011). In unequilibrated enstatite chondrites, weobserve high variability for all REE concentrations, which
may reflect contaminations or non-equilibrium processes. Highly fractionated patterns (accompanied by anegative Eu anomaly) found in EL6 enstatite may reflect
equilibration with melt, as suggested by Hsu and Crozaz(1998).

3.4 Rare‑earth element concentrations in enstatites

Rare-earth element concentrations obtained in enstatites
are illustrated in Fig. 5, as CI-normalized patterns. The
measured concentrations are given in the Additional
file 1. The dataset presented here is limited given the
difficulty of obtaining reliable data. This limitation isdue to low REE concentrations in most enstatite grains­([REE]N < 1), which are small (< 100 µm) and often contain inclusions. Except in the case of equilibrated EL6,
it is thus not clear whether the observed within-sample
variations are accurate or caused by contaminationfrom neighborhood phases. This also has consequences
for data reduction because it raises uncertainties on theinternal standard concentration used for normalization.
In case the laser beam samples material that is not pureenstatite, the assumed internal standard concentration
used for data reduction may be wrong. Those aspects
have been previously discussed by Hsu and Crozaz (1998)
who distinguished three groups of enstatites according
to their REE patterns. Group III enstatites are strongly
depleted in the middle rare-earth elements (MREE) relative to HREE. LREE could not be measured because theconcentrations are too low. These patterns are compatible with equilibrium with a melt (Cartier et al. 2014).
Enstatites from groups I and II have patterns that are
flatter or slightly depleted in LREE relative to HREE andcould have been influenced by non-equilibrium processes
including melt-inclusion trapping or other processes,
such as discussed by Jacquet et al. (2012, 2015).
In the case of EH3, ALHA77295 enstatites have ­[REE]Nvalues ranging between slightly less than 0.1 and 1 times
CI. Some grains show HREE/LREE fractionation, butmany have flat ­[REE]N patterns. Several grains display anegative Yb anomaly, whereas none is observed for Eu.
In Sahara 97096 (EH3), enstatite CI-normalized REE patterns scatter over a wider range, from less than 0.1 to 4
times CI. The patterns are overall flat and, if not systematic, negative Eu and Yb anomalies appear to be common.
In some cases, HREE/LREE fractionation is present. In
EH of higher metamorphic grades (Abee, St. Mark’s, andOudiyat Sbaa), REE abundances in enstatites were below
detection limits and no data could be obtained.
For EL chondrites, we obtained enstatite REE concentrations for at least one sample in metamorphic grades
3, 5, and 6. In MAC 88136 (EL3), CI-normalized REE
concentration range between less than 0.02 to about 2.6.

4 Discussion
We now discuss possible origins for the 3 major types ofoldhamite REE patterns found in the present study: (1)in primitive EH chondrites; (2) in equilibrated EH chondrites; (3) in EL chondrites, all petrologic types being
similar. Finally, we attempt to reconcile in situ mineral
analysis, leaching, and bulk REE data.
4.1 Oldhamite in EH3 as condensates of residual gas?

In the present investigation, all oldhamites analyzed in
EH3 (28 in Sahara 97158 and 3 in Caleta el Cobre 025)
display the same REE patterns (Fig. 4), that labelled
C-D in Crozaz and Lundberg (1995) and Gannoun et al.
(2011). None of the other pattern types (A, B, E) has beenHammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 11 of 19(a) 10

10

1

enstatite / CI

enstatite / CI

EH3 - ALHA77295

0.1

1

0.1

EH3 - Sahara 97096
0.01

0.01
La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

La

Lu

(b)

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

10

enstatite / CI1

0.1

EL3 - MAC 88136
0.01
10

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

enstatite / CI

enstatite / CI

1

0.1

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

0.1

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

10

EL6 - NWA 974

EL6 - Hvittis
1

enstatite / CI1enstatite / CI

Yb

1

0.01

Lu

10

0.1

0.1

0.01

0.01

0.001

Tm

EL6 - Atlanta

EL5 - Adrar Bous

0.01

Er

10

0.001
La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

La

Ce

Pr

Fig. 5 REE patterns of enstatite in EH (a) and EL (b) chondrite. CI values are from Barrat et al. (2012)

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 12 of 19

found within the 28 oldhamite analyses in Sahara 97158.
Caleta el Cobre 025 contains only a few oldhamite grains
suitable for analysis. Gannoun et al. (2011) found thatpattern type C-D was the most abundant in EH3, representing 56 and 97% of the analyses in ALHA77295 andSahara 97072, respectively. It is possible that we missed
the rare grains displaying other types given their scarcity, taking into account that Sahara 97158 and 97072 arepaired (Grossman 1998).
The salient feature of EH3 oldhamites (this study and
Gannoun et al. 2011) is their marked positive Eu and Yb
anomalies combined with their moderate positive Sm
anomaly and downward convex overall REE patterns.
The positive Eu and Yb anomalies are compatible withexperimental oldhamite/melt REE partition coefficients(Ingrao et al. 2019). The presence of E
­ u2+ and Y­ b2+ inthe reduced environment of enstatite chondrite formation has been confirmed by X-ray Absorption Near Edge
Structure (XANES) measurements of the experimental
products obtained in reduced environments similar tothat of enstatite chondrite parent body formation (Ingrao
et al. 2019). The valence state for Eu has not been measured but results from Ingrao et al. (2019) suggest that
all the Eu is present in its 2 + valence state whereas the
2 + /3 + ratio decreases from Yb to Sm. However, theexperimentally determined partition coefficient values
are between 0.1 and 1, which cannot explain the high
REE concentrations by a single stage of oldhamite/silicate
melt equilibration. In addition, oldhamite/silicate melt
equilibrium is not compatible with the shape (convexity)of the REE patterns found in EH3 oldhamite.
Another possibility is that the positive Eu and Yb
anomalies are inherited from a condensation process.
Gannoun et al. (2011) have dismissed CaS in primitive
EH as early condensates because the positive Eu and Yb
anomalies are not compatible with the REE patterns thatare modeled for condensation sequence under reduced
conditions (Lodders and Fegley 1993). Europium and Yb
are the two REE characterized by the lowest condensation temperatures so they should be depleted in an early
condensate, in a manner identical to that identified in
CAIs (e.g., El Goresy et al. 2002) or in the mesostasisof chondrules of ordinary chondrites (Pack et al. 2004).
Rare-earth element patterns measured in EH3 oldhamites may indicate that the material that is now present inthis phase may originate from a fraction that remained in
the gas after an earlier condensation event, in a manner
described for fine-grained CAI in carbonaceous chondrites (Davis et al. 2018).
We have calculated a first-order model adapted fromthat used by Davis et al. (2018) and by using their spreadsheet with thermodynamic data relevant to low oxygen
fugacity conditions. For our model, we considered thatREE condense in CaS, which appears at high temperaturein the condensation sequence, according to Lodders andFegley (1993). We compiled available thermodynamic
data for REE sulfides (Mills 1974; Pankratz et al. 1987;
Barin 1995). We used the formation enthalpies and theentropy values to parameterize the Gibbs free energy
for the formation of gaseous (REE monoxides) and condensed (REE sesquisulfides, except for Eu and Yb wheremonosulfides were also used) species. Contrary to Lodders and Fegley (1993) we considered REE sesquisulfides
in the condensed phase, by generalizing the experimental
observation using XANES of the presence of ­Ho3+ in CaS
equilibrated at low oxygen fugacity (between 5.8 and 6.6
log unit below the Fe-FeO (IW) equilibrium, Ingrao et al.
2019). Because Ingrao et al. (2019) also identified ­Eu2+and ­Yb2+ in their experiments, we considered these two
species in the calculations. Finally, following Drake andBoynton (1988), we calculated the activity coefficientsfor the REE in oldhamite using the partition coefficientvalues of Ingrao et al. (2019). The obtained activity coefficients are given in Table 3.
Our model predicts a first generation of CaS condensates, with upward convex pattern and negative
Eu and Yb anomalies (Fig. 6). This shape is similar tosome extent to that calculated by Lodders and Fegley(1993). Differences arise because we considered sesquisulfides and activity coefficients for the REE in solution
in oldhamite. The results obtained for the REE fraction remaining in the gas are given in Fig. 6. Calculations have been done using a ∆T of 10 K ­(Tcondensationof oldhamite = 1379 K for P­ tot = ­10–3 atm, after Lodders
and Fegley 1993) so as to reproduce the values of thenegative anomalies measured in Eu and Yb in EH3 oldhamites. It should be noted that the uncertainties arelarge because the thermodynamic database is in largepart extrapolated or obtained by analogy with oxide
energies of formation (see discussion in Mills 1974).

Table 3 Activity coefficients for REE in oldhamite
La

Ce

Pr

Nd

Sm

1617

970

693

539

441

2+

γ(REE )γ(REE3+)

Gd

Tb

Dy

Ho

Er

Tm

422

441

462

511

571

647

1.49
3+

2+

All coefficients apply to ­REE , except for ­Eu
a

Eu
431a

Yb

Lu

3

2+

4+

and ­Yb , which have also been considered. Given the oxygen fugacity range, ­CeFor ­Eu3+ and ­Yb3+, activity coefficients have been interpolated from other ­REE3+ values

764 a

has not been considered here882

fraction (relative to initial)

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

4.2 Missing early oldhamite condensate in EH3 matrix

1
0.1
0.01
0.001
0.00010.00001
0.000001

fraction (relative to initial)

0.0000001(a)condensed

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1

(b)
0.1

Page 13 of 19

La Ce Pr Nd

remaining in gas
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 6 Results of modeling of rare-earth element patterns duringcondensation in a reduced gas. a REE in the condensed phase. b
REE in the residual. The resulting pattern b is similar to that foundin oldhamite of primitive EH, suggesting an origin similar to that offine-grained type II CAI of carbonaceous chondrites (Davis et al. 2018;see text for discussion). Here, a temperature of 10 K below oldhamite
condensation temperature was considered ­( Tcond = 1379 K, after
Lodders and Fegley 1993)

Despite this limitation, it can be seen that the modeled
REE pattern reflects the higher volatility of Eu and Yb,
and Sm, to a lesser extent. Using the activity coefficient
dataset gives the pattern a significant curvature and theresulting combination of curvature and anomalies is apattern that is qualitatively similar to the observed EH3
oldhamite REE patterns. This mechanism that produces
the oldhamite in primitive EH chondrites is therefore
similar to what has been discussed for fine-grained(group II) CAIs in Allende CV chondrite by Daviset al. (2018), whose model considered the condensation of material remaining in the gas after hibonite
condensation. Recently, Hu et al. (2021) have reconsidered the model of Davis et al. (2018) by taking into
account REE stable isotope compositions. In particular,
Hu et al. (2021) propose that group II CAIs result from
evaporation followed by condensation. Because weare presently lacking REE stable isotope data on CaS,
it is difficult to constrain the process of EH3 oldhamite formation better than proposed here. Whatever thedetailed mechanism involved, it remains clear, however,that EH3 oldhamite cannot result from direct condensation from the nebula and that a two-stage process (atleast) is necessary to explain REE abundance patterns.

The postulated first generation of CaS condensates,
which should be depleted in the most volatile REE andthen characterized by negative Eu, Yb and Sm anomalies, has never been identified so far. One possibility isthat this material is present in the matrix and that it is
too fine grained for sampling with any current methods.
Otherwise, it may be that this material has been lost or
reprocessed during planetary body formation. El Goresy et al. (2017) suggested that small oldhamite grains
associated with sinoite in EL3 MS-17 and MS-177 of
the Almahatta Sitta breccia could be formed by condensation. However, the grains were too small (few µm)
and a comparison with the expected EH signature is
not possible, unfortunately. The possibility for an early
condensate present in the matrix is now addressed.
Table 4 summarizes available REE data on EH3 chondrite phases. Here, we focus on Sahara 97158, which,
together with its paired Sahara 97072 and Sahara 97096(Grossman 1998) represents a well-documented material with REE measured on bulk sample, from leachingexperiments, as well as in-situ measurements on oldhamites, enstatites and niningerites. Whole rock REE
abundances were averaged from Gannoun et al. (2011)
and Barrat et al. (2014). In situ analyses for oldhamiteand enstatite are from this study and niningerite data
are from Gannoun et al. (2011). Using the mineral proportions defined by Gannoun et al. (2011, 60% enstatite,
1.1% oldhamite, 1% niningerite), we have calculated a
REE budget of Sahara 97158. The results show a deficit for most REE, which has been labelled ‘missing’ inTable 4. The missing component can represent as muchas half the element budget for Ho and Er. For Eu andYb, the deficit is lower, with values of 5.9 and 18.1%,
respectively. Therefore, the missing component has
negative Eu and Yb anomalies (Fig. 7). This component
bears similarities with some chondrule mesostasis analyzed in the EH3 Sahara 97096 by Jacquet et al. (2015).
Uncertainties in the mass balance exist due to uncertainties in mineral proportions and REE concentrations in individual minerals. Nevertheless, there islittle doubt that a significant part of the REE budget ismissed when reconstructing the whole rock from individual mineral analyses. We suggest that the missing
component is present in the matrix. The REE patternof the missing component is shown in Fig. 7, together
with those of the whole rock, the individual minerals,
and the budget based on mineral analyses. In Fig. 7, weconsider that the missing component is carried by 1%of the whole rock, as the matrix may represent between
5 and 12% of the bulk EC (Rubin et al. 2009b). Changing the proportion of the missing component would

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 14 of 19

Table 4 Mass balance and EH3 reconstruction (1.1% oldhamite; 1% niningerite; 60% enstatite)ppm

Sahara 97158

Oldh

Enst

Nining.cFractiona

WRb

0.011

0.60

0.01

BudgetMissingd

Missing
%e

La

0.227

9.31

0.177

0.075

0.21

0.017

7.6

Ce

0.571

17.05

0.520

0.087

0.50

0.070

12.3

Pr

0.088

1.88

0.065

0.021

0.06

0.028

32.1

Nd

0.434

7.75

0.301

0.070

0.27

0.168

38.6

Sm

0.136

2.69

0.086

0.049

0.08

0.054

40.0

Eu

0.054

3.48

0.020

0.022

0.05

0.003

5.9

Gd

0.189

1.98

0.136

0.016

0.10

0.086

45.3

Tb

0.034

0.33

0.025

0.001

0.02

0.016

45.8

Dy

0.238

2.21

0.167

0.026

0.12

0.113

47.5

Ho

0.054

0.46

0.036

0.012

0.03

0.027

50.2
51.2

Er

0.159

1.37

0.103

0.046

0.08

0.081

Tm

0.024f

0.22

0.018

0.012f

0.01

0.011

44.5

Yb

0.154

6.81

0.080

0.340

0.13

0.028

18.1

Lu

0.025

0.21

0.021

0.020

0.02

0.010

CI-norm

Sahara 97158

Oldh

Enst

NiningBudgetMissing

La

0.964

39.62

0.753

0.320

0.89

7.317

Ce

0.951

28.42

0.866

0.145

0.83

11.728

Pr

0.966

20.61

0.711

0.235

0.66

31.031

Nd

0.936

16.70

0.649

0.152

0.57

36.129

Sm

0.889

17.56

0.561

0.323

0.53

35.595

Eu

0.915

59.32

0.342

0.377

0.86

5.359

Gd

0.918

9.61

0.660

0.077

0.50

41.578

Tb

0.914

8.74

0.665

0.039

0.50

41.833

Dy

0.937

8.69

0.658

0.103

0.49

44.524

Ho

0.946

8.08

0.634

0.209

0.47

47.43248.933

Er

0.956

8.23

0.622

0.276

0.47

Tm

0.932f

8.33

0.701

0.457f0.52

41.508

Yb

0.917

40.54

0.475

2.022

0.75

16.557

Lu

1.013

8.39

0.866

0.817

0.62

39.315a38.8
dMineral fractions were adapted from Gannoun et al. (2011)b

WR is the average of whole rock analysis given by Gannoun et al. (2011) and Barrat et al. (2014)cNiningerite data from Gannoun et al. (2011)d

The missing component was estimated by subtracting the contributions of the phases from the whole rock (WR) REE contente% represents the REE in the missing component relative to the WR compositionf

Tm in WR was not measured in Barrat et al. (2014), and Tm in niningerite was not analyzed by Gannoun et al. (2011). For mass balance purposes, Tm was interpolatedusing reported Er and Lu values

only shift its REE pattern, without altering its main features, that is, the negative Eu and Yb anomalies.
Stepwise dissolution and leaching, as performed byBoyet and Gannoun (2013) and Barrat et al. (2014), canbe a way to access the information carried by the EH
matrix that cannot be documented by in-situ analysis.
Usually, the leachates of EH3 obtained with the weakest acid solutions are interpreted as representing thesulfide (CaS, MgS) contribution whereas the residual
fraction should carry the silicate (enstatite) signature. It

is striking, however, that the leachates of EH3 obtainedwith the weakest acid solutions yield CI-normalized patterns with a slight positive Yb anomaly and only a slightnegative Eu anomaly, which is different from those foundin single CaS grains (by in situ analysis) that have strong
positive anomalies for both Eu and Yb. These results confirm that the missing component is a sulfide phase present in the fine-grained matrix because it is dissolved in
the first leaching using 0.5 M acetic acid (Boyet and Gannoun 2013).

(2022) 9:13

Page 15 of 19

5100

analyzed phases
mass balance
modeled leachates
analyzed EH3 leachates

410

Eu/Eu*

REE abundance (CI-normalized)

Hammouda et al. Progress in Earth and Planetary Science

1

CaS

3
2

budget
MgS

0.1

0.01

La Ce Pr Nd

oldhamiteenstatiteniningerite

budgetmissing

Sah97158

10

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(a)
1

5

2

Yb/Yb*

3

4

analyzed phases
mass balance
modeled leachates
analyzed EH3 leachates

4

Eu/Eu*We have attempted to reconcile in situ and leachingdata by considering the contribution of a component
present in the matrix, such as the missing component
discussed above. We have modeled REE CI-normalized
pattern of a mixture consisting of the missing component and a fraction of the CaS and the MgS present in
the chondrite. The result is displayed in Fig. 8. Given
the overall uncertainties in phase proportions, REE
abundances in minerals (particularly LREE in enstatite
and MgS), it is worth noting that the resulting combination possesses negative and positive anomalies
for Eu and Yb, respectively, as observed in EH3 leachates. The overall LREE to HREE fractionation of themodeled missing component differs slightly from thatobserved in the leachates, but the LREE budget relies
heavily on enstatite compositions, which bear significant uncertainty. Therefore, we consider that our model
serves to illustrate that in situ and leaching data can bereconciled.
Mixing single grains and matrix contributions can befurther explored by modeling the matrix using a condensation sequence. As mentioned above, CaS of primitive chondrites can be modeled as condensing froma reduced residual gas (Ebel 2006). Given the Eu andYb positive anomalies observed in CaS, the first condensate has negative anomalies for both elements, andtherefore resembles the missing component discussed
above, on the basis of mass balance calculations. We
have mixed this early condensate with fractions of CaS
and MgS, in a manner similar to that used above, forthe missing component. Using appropriate proportionsof both sulfides, it is possible to reproduce the expected
anomalies for Eu and Yb, as illustrated in Fig. 8. Here,

L2

en
miss.
comp. L30

Fig. 7 Summary of CI-normalized REE patterns of EH3 as determinedby whole-rock analysis, together with its constituent minerals and thereconstructed pattern by mixing the minerals in modal proportions.
The mismatch between the whole-rock and the reconstructedcompositions, the missing component, is also illustrated. CI values arefrom Barrat et al. (2012)

L1

WR

5

CaS

3
2

budget
MgS
L1
L2

WR

1

en
L3
early condensate

0
0

1

(b)
2

Yb/Yb*

3

4

5

Fig. 8 Eu/Eu* versus Yb/Yb* of modeled leachates obtained bymixing the matrix component and a fraction of the sulfides present in
EH3 (CaS and MgS). WR = whole rock (averaged from Gannoun et al.
2011; Barrat et al. 2014); budget = reconstruction of the whole rockcomposition using mineral modes and REE abundances in CasS, MgS,
and enstatite; en = enstatite; CaS = oldhamite; MgS = niningerite(from Gannoun et al. 2011). Leachates L1, L2, L3 are modeled byvarying the relative proportions of the matrix component andsulfides, so as to illustrate the possible combinations for the sign of
the Eu and Yb anomalies observed in leachates (Boyet and Gannoun
2013; Barrat et al. 2014). a The matrix component consists of themissing component identified by mass balance. Leachate L1 isobtained by mixing this component with 14% and 75% of the CaS
and the MgS present in the rock, respectively; L2 by using 8% of the
CaS and 100% of the MgS; L3 by using 5% of the CaS and 30% of the
MgS. b The matrix composition is modeled as an early condensate(see Fig. 6). Leachates are simulated by mixing this condensate
with CaS and MgS in the following proportions: L1 = 92.8%
condensate + 1.7% CaS + 5.6% MgS; L2 = 92.5% condensate + 1.3%
CaS + 6.2% MgS; L3 = 94.3% condensate + 0.9% CaS + 4.7% MgS

as elsewhere, the major uncertainties arise from thethermodynamic data used for condensation modeling.
Both models presented have uncertainties and large
degrees of freedom, but it is worth noting that both themissing component and the calculated early condensate
have negative Eu and Yb anomalies. We suggest that thiscontribution that is not detectable by in situ analysis ofHammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 16 of 19

single grains can be documented by step-leaching methods, although its contribution is partly blurred by mixing
with other sulfides.

measured in Abee is more similar to those defined for the
EL groups (Fig. 4). Later on, Rubin et al. (2009a) extended
this observation to EL, which they noticed all presented
evidence for melting, except for one. EL chondrites fromtype 6 are all characterized by depletion in LREE and arealso depleted in Ca relative to EH samples. These features, first described by Rubin et al. (2009a), have been
confirmed by Barrat et al. (2014) who also noted depletions in Nb and Eu and positive Y anomalies in EL6 samples and impact melt samples [e.g., Galim (b), LAP 02225,
Barrat et al. (2014)]. Bulk trace element concentrations of
EL6 clearly suggest that all samples from this subgroup
have been affected by a melting event.
That CaS of all EL chondrite subgroups may have equilibrated with silicate melt is strongly substantiated by the
REE pattern with its upward convex shape. Recently,
Ingrao et al. (2019) reported a complete dataset for REE
partitioning between oldhamite and silicate melt. The
influence of radius size on REE behavior is a strong indication that crystal-melt equilibrium may be at work. A
major difference between REE abundance in CaS of EL
chondrites and partition coefficient values concerns theEu and Yb anomalies. In the experiments, both elements
have a positive anomaly, whereas in the CaS of EL, only
Eu has a strong negative anomaly. One possible explanation for the low concentrations of Eu and Yb is the lossof a Fe and S-dominated vapor phase. Compared to EH,
EL are depleted in iron and sulfur and available partition
coefficients of REE between molten FeS and molten silicate (Lodders 1996; Ingrao et al. 2019) show that both Eu
and Yb are preferentially incorporated in FeS, relative toother REE. In addition, Eu and Yb are the most volatile
REE. It is therefore possible that melting was accompanied by partial vaporization, resulting in Eu and Yb loss
and yielding the presently observed REE patterns in oldhamites of EL chondrites. Although this conclusion is at
odds with the detailed analysis of El Goresy et al. (2017)
on MS-17 and MS-177, we note that El Goresy et al.
(1995) report on the preservation of carbon and nitrogen isotopic heterogeneities in the primitive achondrite
Acapulco, which experienced partial melting. Moreover,
we have no information on the timing because no data isavailable, but the melting event likely occurred very early
before the accretion of the final parent body.
Whole rock data on EL chondrites often show depletion in Eu and/or Yb (Barrat et al. 2014). Recently, on
the basis of metal and sulfide analysis, Horstmann et al.
(2014) and Weyrauch et al. (2018) proposed that a previous parent body of enstatite chondrite affinity underwent
melting and was dismantled, some or all of its fragments
being later assembled to form the EL parent body. Morerecently, Kimura et al. (2021) proposed that MS-177, an
EL3, underwent a high-temperature subsolidus event and4.3 Oldhamite of EH5 as reflecting sulfide/matrix
equilibration?

Oldhamites from the two EH5 studied (St. Mark’s andOudiyat Sbaa) have flat REE patterns. Oldhamites from
Abee (EH4/IMB) are also flat but, in addition, have aslight negative Eu anomaly. Here, we only discuss theEH5 data, and ignore Abee, which is an impact melt breccia that has experienced some degree of melting. Abee isstrongly heterogeneous as observed in the REE patternobtained from bulk rock measurements. The Abee rareearth element pattern obtained by Boyet et al (2018) has
a negative Eu anomaly (Eu/Eu* = 0.83) whereas the pattern is flat in the piece analyzed by Barrat et al (2014).
Therefore, Abee appears to have been affected by secondary processes (i.e., impact metamorphism and/or melting) and is not a good candidate to discuss parent-bodyprocesses.
The flat character of REE patterns in EH5 oldhamites can be interpreted as representing unfractionatedREE condensation. It can also be interpreted as reflecting inter-grain redistribution during textural maturation
associated with metamorphic evolution incomplete in
EH3 and nearly complete in EH5 on their parent body.
Unfractionated condensation could be caused by disequilibrium processes during condensation, but detailed
mechanisms of this process are difficult to decipher (e.g.,
Jacquet and Marrocchi 2017). However, if we assume agenetic link between EH3 and EH5 oldhamites, it is necessary to consider that EH5 oldhamites have gained a significant fraction of all the REE but the most volatile ones
(Eu and Yb), which are at the same level of concentrationin EH3 and EH5. In the present state of our knowledge,
the only mechanism possible would involve the contribution of a complementary reservoir present in the matrixof EH3, such as the missing part discussed above.
4.4 Oldhamite of EL chondrites as a record of a melting
event?

The main characteristics of the REE patterns of oldhamite from EL chondrites are (1) that they bear no resemblance to those found in all EH types, (2) that they are
all similar, with minor differences between types, and(3) that they are similar to those found in aubrites, thatis enstatite achondrites (Floss and Crozaz 1993; Lodders
et al. 1993; Wheelock et al. 1994).
From their observation of euhedral enstatite crystals
growing on chondrule relicts and metal globules, Rubin
and Scott (1997) argued for melting in Abee (EH4), which
is an impact melt breccia. The oldhamite REE patternHammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 17 of 19

suggested that other E3 also underwent heating events.
Moreover, Rindlisbacher et al. (2021) suggested thatthe metal-rich nodules found in a thin section of NWA
8785, which is classified as an EL3, recorded melting andevolved separately before being accreted on their parent body. Here, we suggest that all EL chondrites went
through a high-temperature stage that involved somedegree of melting and vaporization, whose effects are
now recorded in EL oldhamite REE patterns.
There is agreement between leaching experiments
on EL3 (Barrat et al. 2014) and REE patterns obtained
from in situ single CaS grain analysis. They both show
upward convex patterns with a negative Eu anomaly. Thisis markedly different from findings on EH3, as discussed
above. The similarity between leaching and grain analyses
in EL3 indicates that the contribution of the EL matrix to
the REE budget (if any) is not different from that of thesingle grains. The high temperature (melting) stage discussed above to explain similar oldhamite patterns for all
EL types may have contributed to matrix/grain equilibration as well.

and chemical equilibration between the large oldhamite
grains and the matrix. This evolution is accompanied by
no bulk loss of material, even for the most volatile REE,
such as Eu and Yb.
The third type of oldhamite is found only in EL chondrites, whatever their metamorphic type. It is similar
to pattern measured in oldhamites from aubrites. The
EL REE patterns bear some characteristics indicative ofoldhamite/melt equilibrium, although some expectedfeatures are lacking, such as the positive Eu and Yb
anomalies documented in the experimental petrology
literature. This paradox can be solved by consideringthat melting was accompanied by partial volatilization,
wherein a fraction Eu and Yb were lost to a Fe and S-richgaseous phase. Given the high temperatures required for
melting, it is not excluded that EL oldhamites could also
carry the signature of evaporation followed by condensation, in a manner discussed by Hu et al. (2021). Perhaps
data on REE stable isotope compositions, when available,
will further contribute to our understanding of the origin
of EL oldhamites. It remains that interaction with melt isalso attested by enstatite REE patterns, but this is visible
only in EL5 and EL6, although it may also be hinted at inEL3.
A striking finding from our survey is that oldhamites
from all EL chondrites, even the least equilibrated ones
(EL3) carry a signature that can be attributed to hightemperature modification, melting and volatilization.
Here, one must admit that overall chondritic elemental
abundances were not lost during the process. One possibility is that melting and partial evaporation took placeon the EL parent body itself and that either the melting
degree or the gravity field was low enough to prevent
large-scale differentiation of the EL parent body. The present data do not allow to determine at which stage of theEL history the high-temperature event took place. It may
be on the parent body or prior to parent-body formation.
It cannot be excluded that several heating events happened. It seems clear, however, that all EL, including themost unequilibrated ones have been processed at somestage and that they do not represent primary nebularmaterial.

5 Conclusions
The detailed analysis of REE in oldhamite from most
known types of enstatite chondrites by laser ablation
ICP-MS documents a striking contrast between EH and
EL. Three major types of oldhamite REE patterns weredistinguished. The first type, with marked positive Eu
and Yb anomalies, is characteristic of unequilibrated EH
chondrites (EH3). It may correspond to the condensationof a residual gas, following a two-stage process similar tothat invoked by Davis et al. (2018) for fine-grained, type
II CAIs. Because the whole rock EH3 REE budget cannot be reproduced by mass balance using individual graincompositions, a complementary reservoir must be present, perhaps in the fine-grained matrix. This component
cannot be characterized by in situ methods, but its composition may be approached by mass balance. Stepwise
dissolution of unequilibrated EH can sample this reservoir but published data seem to indicate that its signal ismixed with that of sulfide grains. The matrix contribution
may consist of an early-condensed (but not macroscopic)
sulfide (CaS). If, alternatively, EH oldhamite resulted
from evaporation followed by condensation following themodel recently proposed by Hu et al. (2021) for group II
CAIs, the evaporated fraction must have remained on
EH3 parent body to satisfy chondritic abundances for
EH3 bulk rock REE. The lack of data on REE stable isotope composition of CaS does not allow to conclude further at the present stage.
A second type of oldhamite is characterized by flat REE
patterns. It is found in equilibrated EH5 chondrites only.
We suggest that it is the result of textural maturation

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00471-w.
Additional file 1. Major element concentrations analyzed in oldhamites
by electron microprobe, together with detection limits.
Acknowledgements
This work would not have been possible without Ahmed El Goresy who
introduced us to the world of exotic sulfides that form in reduced conditions.
We started to study and measure REE in sulfides from unequilibrated EH only.

Hammouda et al. Progress in Earth and Planetary Science(2022) 9:13

Page 18 of 19

The first results obtained on EL samples led to many discussions and stormy
debates. We would have liked Ahmed El Goresy to give us his opinion on thismanuscript. We thank him warmly for having transmitted us his passion. This
work benefitted from discussion with Jean-Alix Barrat and Bertrand Moine.
Comments and suggestions by Alan Rubin and Denton Ebel, as well as byeditor Makoto Kimura are gratefully acknowledged. Assistance by J.-L. Devidal
on electron microprobe and laser-ablation ICP-MS analyses and by J.-M. Hénotand E. Voyer with the scanning electron microscope is acknowledged. We
thank I. Chargros and B. Wasilewski for their help at an early stage of this project. We thank INSU-CNRS (Programme National de Planétologie) for financial
support. This research was partly supported by the French Government Laboratory of Excellence initiative ANR-10-LABX-0006, the Région Auvergne, andthe European Regional Development Fund. This project has received funding
from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 682778—
ISOREE). This is Laboratory of Excellence ClerVolc contribution № 511.

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Authors’ contributions
The project was designed by TH and MB. All authors contributed to the dataacquisition. TH wrote the manuscript with discussions with MB, PF, and CC. All
authors read and approved the final manuscript.
Funding
This work was supported by Programme National de Planétologie of CNRSINSU, by the French Government Laboratory of Excellence initiative ANR-10LABX-0006, the Région Auvergne, and the European Regional Development
Fund. This project has received funding from the European Research Council(ERC) under the European Union’s Horizon 2020 research and innovation
program (Grant Agreement No. 682778—ISOREE).
Availability of data and materials
All data discussed in this paper are provided in the main text and in the supplementary information files.
Declaration
Competing interests
The authors declare that they have no competing interest.
Author details
1
CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Université Clermont
Auvergne, 63000 Clermont‑Ferrand, France. 2 CRPG, CNRS, Université de Lorraine, UMR 7358, 54501 Vandœuvre‑les‑Nancy, France.
Received: 26 August 2021 Accepted: 31 January 2022

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