Krot et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00437-4(2021) 8:61

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Infiltration metasomatism of the Allendecoarse-grained calcium-aluminum-richinclusions
Alexander N. Krot1* , Michail I. Petaev2 and Kazuhide Nagashima1

Abstract
We report on the mineralogy, petrography, and O and Al-Mg isotopic systematics of secondary mineralization in
the metasomatically altered igneous Ca,Al-rich inclusions (CAIs) [compact type A (CTA), B1, B2, forsterite-bearing B(FoB), and C] from the CV3 carbonaceous chondrite Allende. This alteration affected mainly melilite, and to a lesser
degree anorthite, and resulted in the formation of a variety of secondary minerals, including adrianite, Al-diopside,
andradite, anorthite, calcite, celsian, clintonite, corundum, dmisteinbergite, ferroan olivine, ferroan monticellite,
ferroan Al-diopside, forsterite, grossular, heazlewoodite, hedenbergite, hutcheonite, kushiroite, margarite,
monticellite, Na-melilite, nepheline, pentlandite, pyrrhotite, sodalite, spinel, tilleyite, wadalite, and wollastonite. The
secondary mineral assemblages are mainly defined by chemical compositions of the primary melilite replaced andelements introduced by an aqueous fluid. Gehlenitic melilite (Åk<35) in CTAs and mantles of B1s is mainly replacedby anorthite + grossular; clintonite, corundum, spinel, and Al-diopside are minor. Åkermanitic melilite (Åk35-90) in
type B2s, FoBs, and cores of B1s is replaced by the grossular + monticellite + wollastonite, grossular + monticellite,
and grossular + Al-diopside assemblages; forsterite, spinel, clintonite, and Na-melilite are minor. In type Cs, lacy
melilite (åkermanitic melilite with rounded inclusions of anorthite) is pseudomorphically replaced by the grossular +forsterite + monticellite and grossular + Al-diopside assemblages; Na-melilite is minor. Primary and secondaryanorthites in the peripheral portions of CAIs are replaced by nepheline, sodalite, and ferromagnesian olivine. Some
CAIs contain voids and cracks filled by andradite, hedenbergite, wollastonite, ±sodalite, ±grossular, ±monticellite, ±tilleyite, and ±calcite. All CAIs studied are surrounded by Wark-Lovering rims, fine-grained matrix-like rims
composed of lath-shaped ferroan olivine and abundant nepheline grains, and a layer of salite-hedenbergite
pyroxenes + andradite + wollastonite. Grossular associating with monticellite, Al-diopside, and forsterite andreplacing åkermanitic melilite (27Al/24Mg ~ 2) has high 27Al/24Mg ratios (30−100) and shows no resolvable excess ofradiogenic 26Mg (26Mg*). The 27Al/24Mg ratios (7−10) and 26Mg* (2−3‰) in the nearly monomineralic grossularveins crosscutting gehlenitic melilite are similar to those of the host melilite and plot along a regression line with26
Al/27Al ratio of ~5×10−5. Oxygen isotopic compositions of secondary minerals in the most Type Bs measured insitu with the UH Cameca ims-1280 and matrix-matched standards plot along mass-dependent fractionation linewith Δ17O of ~ −3±2‰ with δ18O ranging from ~0 to ~10‰. Primary melilite and anorthite in the host CAIs are
similarly 16O-depleted, whereas spinel, forsterite, and most Al,Ti-diopside grains have 16O-rich compositions (Δ17O ~* Correspondence: sasha@higp.hawaii.edu
Dedicated to Ahmed El Goresy March 22, 2021, revised May 4, 2021
Submitted to a special issue of Progress in Earth and Planetary Science
1
Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth
Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822,
USA
Full list of author information is available at the end of the article© The Author(s). 2021 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 the original author(s) and the source, provide a link to the Creative Commons licence, and indicate ifchanges were made. The images or other third party material in this article are included in the article's Creative Commonslicence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commonslicence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 2 of 37−25±2‰). Secondary grossular and forsterite in type Cs and type B1 CAI TS-34 show a range of Δ17O, from ~ −15
to ~ −1‰; the 16O-enriched compositions of grossular and forsterite plot along the carbonaceous chondrite
anhydrous mineral line. The similar ranges of Δ17O and positions on the three-isotope oxygen diagram areobserved for primary anorthite; melilite is generally 16O-depleted compared to anorthite (Δ17O ~ −5 to −1±2‰);
spinel and fassaite are 16O-rich (except very Ti-rich fassaite in TS-34 and CTA CAIs). We conclude that Allende CAIs
experienced an open-system in situ metasomatic alteration at relatively high temperatures (200-250 °C) in thepresence of CO2- and H2O-bearing fluid with Δ17O of ~ −3±2‰ followed by thermal metamorphism at ~ 500 °C
on the CV chondrite parent asteroid. During the alteration, most elements were mobile: Si, Na, Cl, K, Fe, S, and Ni
were introduced; Al, Ti, Mg, and Ba were locally mobilized; Ca and some Mg and Al were lost from the hostinclusions. The alteration occurred after nearly complete decay of 26Al, >3 Ma after crystallization of CAIs with thecanonical (26Al/27Al)0 of (5.25±0.02)×10-5; 26Mg* in grossular was inherited from the primary melilite and provide no
chronological significance. Oxygen isotopic heterogeneity of primary minerals in the Allende CAIs at least partly is
due to isotopic exchange with an aqueous fluid that largely affected melilite, anorthite, perovskite, Zr- and Sc-richoxides and silicates, and possibly very Ti-rich fassaite.
Graphical Abstract

1 Introduction
CV (Vigarano type) carbonaceous chondrites experienced metasomatic alteration (fluid-assisted thermal
metamorphism) that affected their all major components—refractory inclusions [Ca,Al-rich inclusions(CAIs) and amoeboid olivine aggregates (AOAs)], chondrules, matrix, and chondritic lithic clasts (dark inclusions) (e.g., Kimura and Ikeda 1995; Kojima andTomeoka 1996; Krot et al. 1995, 1998a, 1998b; Brearley
and Krot 2013 and references therein). It is generally accepted that the alteration of the CV chondrules, matrices,and dark inclusions occurred in an asteroidal setting in
the presence of aqueous solutions > 3 Ma after
crystallization of CAIs having the canonical initial
26
Al/27Al ratio [(26Al/27Al)0] of (5.25±0.02)×10-5 (e.g., Larsen et al. 2011; Brearley and Krot 2013 and references
therein; Doyle et al. 2015; MacPherson et al. 2017). However, there is no agreement on the timing (early vs. late)and location (nebula vs. asteroid) of alteration of the CV
refractory inclusions (e.g., Wark 1981; Hashimoto and
Grossman 1987; McGuire and Hashimoto 1989; Fagan
et al. 2005, 2006, 2007, 2015; Ushikubo et al. 2007; Krot

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

et al. 2007a, 2007b, 2008; Ford and Brearley 2008), theoldest solids formed in the solar nebula (Bouvier andWadhwa 2010; Connelly et al. 2012).
Refractory inclusions are thought to have formed near
the protoSun in a highly reduced gas of approximately
solar composition (H2O/H2 ~ 6.6×10−4; Lodders 2021,
which is consistent with an oxidation state of titanium(Ti3+/Ti4+ ratio) in CAI fassaites (Simon et al. 2007) andthe observations that vast majority of CAIs in unmetamorphosed (petrologic type 2-3.0) chondrites haveclose-to-solar Δ17O (deviation from the terrestrial fractionation line, = δ17O − 0.52×δ18O), ~ -24±2‰ (e.g.,
Yurimoto et al. 2008; Makide et al. 2009; McKeegan
et al. 2011; Bodénan et al. 2014; Kööp et al. 2016; Ushikubo et al. 2017; Krot 2019 and references therein). In
contrast, refractory inclusions in the metasomatically altered and metamorphosed CV chondrites of petrologic
type ≥ 3.1 have heterogeneous Δ17O: spinel, hibonite,
forsterite, and most Al,Ti-diopside grains have solar-like
Δ17O (~ -24±2‰), whereas melilite, anorthite, and, occasionally, Ti-rich pyroxenes are 16O-depleted to various
degrees (Δ17O range from ~ -24 to ~ -1‰) (Clayton
et al. 1977; MacPherson et al. 2017; Kawasaki et al.
2018). Bulk oxygen-isotope compositions of CAIs andtheir mineral separates from the CV3.6 chondrite
Allende (Bonal et al. 2006) follow a mass-independent
fractionation line with a slope of 0.94, named the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line
(Clayton et al. 1977). The nature of this oxygen isotopic
heterogeneity and the nature of the CCAM line remain
controversial. They may reflect gas-melt O-isotope exchange during incomplete melting of coarse-grained igneous CAI precursors in the nebular gas of variableoxygen isotopic composition (e.g., Yurimoto et al. 1998;
Kawasaki et al. 2018). Alternatively or in addition tothese nebular processes, the CV CAIs could have experienced mineralogically controlled O-isotope exchange
during aqueous fluid-rock interaction on the CV chondrite parent asteroid that largely affected their anorthite,melilite, perovskite, grossite, and Zr- and Sc-rich oxides
and silicates (e.g., Wakaki et al. 2013; Krot et al. 2008,2019a, 2019b, 2019c). We note that chondrules and refractory inclusions from the least altered carbonaceouschondrite Acfer 094 (C3.0 ungrouped) have uniform
oxygen isotopic compositions and on a three-isotope
oxygen diagram plot along the line having a slope of
0.987±0.013, named the Primitive Chondrule Mineral(PCM) line (Ushikubo et al. 2012, 2017), which is different from the CCAM line.
In order to understand the nature (place, time, andmechanism) of metasomatic alteration of the CV CAIs
and its possible effect on their O-isotope heterogeneity,
we report here on the mineralogy, petrography, and insitu measured O-isotope compositions of secondary

Page 3 of 37

minerals in all major types of the Allende coarse-grained
igneous CAIs (CTA, B1, B2, FoB, and C; Table 1) using
the UH Cameca ims-1280 and matrix-matched standards. For classification of the CV coarse-grained CAIs,
see MacPherson (2014) and references therein.
Aluminum-magnesium isotope systematics was measured in grossular of several type B1 CAIs.
Preliminary results of this study have been previously
reported by Brearley and Krot (2013) and Krot et al.
(2010, 2020a).

2 Previous studies
The Al-Mg and O isotopic compositions of secondary
minerals in Allende CAIs have been previously reported
in several papers and abstracts. In this section, we briefly
summarize the major results obtained and interpretations proposed in these publications. Secondary minerals, their chemical formulae, and abbreviations used in
the paper are listed in Table 2.
Wark (1981) described pockets of secondary alteration
minerals enriched in Na, K, Rb, Cl, I, Fe, and Zn,
Table 1 A list of coarse-grained igneous CAIs from Allendestudied here and compositional ranges of their melilites
CAI

Type

Åk, mol%

Na2O, wt%

Ref.

ALH-2

FTA/CTA

1−24

n.d.

[1]

818-G

CTA

12−27

n.d.

[2]

CG-13/15

CTA

3−32

n.d.

[2]

TS-2

CTA

9−65

Up to 0.15

[3]

TS-68

CTA

8−33

Up to 0.18

[2,4]

AJEF

B1

12−61

Up to 0.19

Big-All

B1

n.a.

Egg-3

B1

n.a.

TS-23

B1

10−72

Up to 0.30

TS-34

B1

10−65

Up to 0.30

A-39

B2

n.a.

2

B2

9−55

Up to 0.17

[2,7]

B2

n.a.

[2]

3529Z

[2]
[2]
[2]
[2,6]
[2,6]
[2]

CG-6

B2

13−75

Up to 0.20

[2]

CG-10

B2

10−74

Up to 0.25

[2]

CG-11

B2

n.a.

n.a.

[2]

TS-21

B2

10−75

Up to 0.30

[2,6]

TS-31

B2

76−87

0.10−0.20

[2]

All-2

FoB

79−87

0.12−0.23[2]

All-5-2

FoB

66−90

100

C

58−74

0.15−0.25

[8,9]

160

C

44−72

Up to 0.36

[8,9]

[2,5]

References: [1] Simon et al. (2001); [2] this study; [3] Simon et al. (1998); [4]Simon et al. (1999); [5] Bullock et al. (2012); [6] Simon and Grossman (2006);
Kawasaki et al. (2018); [7] Ushikubo et al. (2007); [8] Wark et al. (1987); [9] Krotet al. (2007b)

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Table 2 A partial list of secondary minerals in the Allendecoarse-grained igneous CAIs, their formulae, and abbreviations
used in the text
Mineral/abbreviation

Chemical formula

Adrianite

Ca12(Al4Mg3Si7)O32Cl6

Anorthite/an

CaAl2Si2O8 (tricl.)

Andradite/andr

Ca3Fe2Si3O12

Awaruite

Ni3Fe

Beckettite

Ca2(V,Al,Ti,Mg)6Al6O20

Calcite/cal

CaCO3

Celsian/cls

BaAl2Si2O8

Clintonite/cln

Ca(Mg,Al)3(Al3Si)O10(OH)2

Corundum/cor

Al2O3

Coulsonite(Fe,Mg)(V,Al)2O4

Diopside-Ca-Tschermakitess

CaMgSiO6-CaAl2SiO6

Dmisteinbergite/dms

CaAl2Si2O8 (hex.)

Forsterite/fo

Mg2SiO4

Grossular/grs

Ca3Al2Si3O12

Heazlewoodite/hzw

Ni3S2

Hedenbergite/hed

CaFeSi2O6

Hercynite(Fe,Mg)Al2O4

Hutcheonite/htc

Ca3Ti2Si3O12

Ilmenite

FeTiO3Kaolinite

Al2Si2O5(OH)4Magnetite/mgt

Fe2+Fe3+2O4

Margarite/mrg

CaAl2(Al2Si2)O10(OH)2

Molybdenite

MoS2

Monticellite/mnl

Ca(Mg,Fe)SiO4

Montmorillonite(Na,Ca)0.3(Al,Mg)2Si4O10(OH)•2n(H2O)

Na-melilite/Na-mel(CaNa)2(Al,Mg)[(AlSi)2O7]

Nepheline/nph

NaAlSiO4

Ferromagnesian olivine/ol(Fe,Mg)2SiO4Pentlandite/pnt(Fe,Ni)9S8

Scheelite

Ca(WO4)

Sodalite/sod

Na8Al6Si6O24Cl2

SpheneCaTiSiO5

Spinel/sp(Mg,Fe)Al2O4

Tilleyite/tlt

Ca5Si2O7(CO3)2

Vesuvianite

Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4

Wadalite/wad

Ca12Al10Si4O32Cl6

Wollastonite/wol

CaSiO3including Fe-bearing grossular, plagioclase (An90-100),
nepheline, sodalite, Fe-bearing spinel, andradite, andolivine and pyroxenes of variable compositions in several
Allende CAIs. Wark (1981) concluded that the alteration
affected largely melilite and resulted in formation ofPage 4 of 37

grossular and anorthite followed by crystallization ofnepheline and sodalite, and diffusion of iron into spinel.
Based on the presence of alkali-rich halos around theCAIs, which were interpreted as evidence for outward
diffusion of Na and K, Wark (1981) concluded that alteration occurred in the solar nebula prior to incorporationof CAIs into the Allende meteorite.
Hutcheon and Newton (1981) described secondary
minerals in the Allende Type B1 CAIs TS-23 and TS-34.
They showed that secondary grossular and monticellite
form narrow veins separating primary melilite andanorthite crystals, and occasionally form a corona completely surrounding large melilite grains; bladed wollastonite crystals fill cavities lined with grossular. Hutcheon
and Newton (1981) attributed formation of the grossular−monticellite assemblages to thermal decomposition ofmelilite at ~ 670 °C in the solar nebula:
R1 : 3Ca2 MgSi2 O7ðsÞ þ Ca2 Al2 SiO7ðsÞ þ CaAl2 Si2 O8ðsÞ ¼ 2Ca3 Al2 Si3 O12ðsÞ þ 3CaMgSiO4ðsÞ
a kermanite−gehlenite solid solution

anorthitegrossularmonticellite

ð1Þ
No resolvable excess of radiogenic 26Mg (26Mg*) was
found in grossular and sodalite: the upper limits on the initial
26
Al/27Al ratio [(26Al/27Al)0] are < 4.4×10-6 and < 1×10-6,
respectively, suggesting the alteration occurred > 2.4 and >3.9 Ma after crystallization of the host CAIs characterized
by approximately the canonical (26Al/27Al)0 of ~ 4.5×10-5.
Tomeoka and Buseck (1982) reported intergrown mica
and montmorillonite in a fine-grained CAI from
Allende, but proposed no mechanism of their formation.
Hashimoto and Grossman (1987) and McGuire andHashimoto (1989) described secondary grossular,anorthite, feldspathoids, ilmenite, ferroan olivine, andradite, salite-hedenbergite pyroxenes, and phyllosilicates (amixture of Na-phlogopite and chlorite or Al-rich serpentine) in AOAs and several coarse- and fine-grained CAIs
from Allende. These authors concluded that the Allende
refractory inclusions experienced an open-system alteration in the cooling solar nebula (from ~1000 to ~350
K): Si, Na, K, Fe, Cr, Cl, and H2O were introduced,
whereas Ca was lost, presumably as gaseous Ca(OH)2.
Magnesium may have been lost during the replacementof melilite by anhydrous minerals and subsequently
added during the formation of phyllosilicates.
Keller and Buseck (1991) described secondary micas,
clintonite, and margarite in an Allende CTA CAI. Clintonite crystals, 10-500 nm in size, partially replace grossular in alteration veins crosscutting gehlenitic melilite(Åk10-20), whereas margarite occurs as lamellae intergrown with secondary anorthite replacing melilite. Keller
and Buseck concluded that the micas formed by hydration reactions below 450 K in the solar nebula:

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 5 of 37

R2 : 2CaAl2 Si2 O8ðsÞ þ H2 OðgÞ ¼
anorthite

CaAl2 ðAl2 Si2 ÞO10 ðOHÞ2ðsÞ þ CaOðgÞ þ 2SiO2ðgÞ
margarite

ð2Þ
R3 : 2Ca3 Al2 Si3 O12 þ 2MgAl2 O4ðgÞ þ 2FeðgÞ þ 3H2 OðgÞ ¼
grossularspinelCaðMg2 AlÞðAl3 SiÞO10 ðOHÞ2 þ 2FeAl2 O4ðsÞ þ 5CaOðgÞ þ 5SiO2ðgÞ þ 2H2 ðgÞ
clintonite

hercynite

ð3Þ
Davis et al. (1994) reported on the mineralogy, petrography, trace element abundances, and Al-Mg isotope
systematics of primary and secondary minerals in TS-23
and TS-34. Bulk chemical compositions, trace element
abundances, and textures suggest that alteration is dominated by a reaction of Mg-rich melilite with SiO2; a reaction of melilite and anorthite also occurred, as did a lossof Sr and gain of volatiles. Melilite mantles of TS-23 andTS-34 are only slightly altered and contain mainly grossular veins, whereas larger patches of secondary grossular, monticellite, and wollastonite occur between
partially altered grains of anorthite and åkermanitic
melilite (Åk72-76) in the CAI cores, suggesting that Mgrich melilite is more easily altered than gehlenitic melilite; fassaite and spinel show no evidence for alteration.
No detectable 26Mg* was found in grossular; primaryanorthite in altered zones shows evidence for disturbed
Al-Mg isotope systematics. The rare earth element patterns of altered zones are similar to those of adjacentlate-crystallized melilite. The altered zones are enriched
in B, F, Na, K, Cl, and Pb relative to the adjacent primaryphases; they are also fairly high in Sr, Ba, and Eu. The altered zones appear to have inherited their trace element
abundances from melilite; Sr was lost in varying degrees
relative to Ba and Eu. Davis et al. (1994) concluded thealteration occurred in the solar nebula.
Hiyagon (1998) reported δ17O and δ18O with large uncertainties (2σ for δ17O and δ18O are ~3-4 and 4-9‰,
respectively) of secondary grossular (-12 to -4‰ and -6
to +2‰, respectively; Δ17O ~ -9 to -5‰) replacing melilite and anorthite, and of wollastonite, andradite, andhedenbergite (-3 to + 4‰ and + 2 to + 10‰; Δ17O ~ -4
to -1‰) in cavities and topographic depressions on thesurface of several coarse-grained Allende CAIs. No
matrix-matched standards were used in this study (SanCarlos olivine was used as a standard for all secondary
minerals). As matrix effects could not have been properly corrected, only Δ17O can be trusted.
Ushikubo et al. (2007) reported 36Cl-36S, 26Al-26Mg,
and oxygen isotopic systematics of secondary minerals(San Carlos olivine was used as a standard) in the
Allende Type B2 CAI 2. Two secondary mineral domains were identified: “ragged” anorthite-grossulardomain in the CAI interior and sodalite-anorthitenepheline-ferroan olivine domain in its outer part. Δ17O
of ragged anorthite and grossular (SIMS spots in grossular overlapped with primary fassaite and spinel) range
from -17 to -9±6‰. Sodalite and anorthite in the Narich domain and primary melilite appear to be slightly
16
O-depleted (Δ17O ~ -7±6‰). No resolvable 26Mg*
26[( Al/27Al)0 < 4.4×10-7] is observed in grossular, lathshaped anorthite, and sodalite. In contrast, the inferred(26Al/27Al)0 in ragged anorthite is (1.2±0.2)×10-5. Ushikubo et al. (2007) interpreted these observations as anevidence for multiple alteration events experienced by
the CAI, 1.5 and 5.7 Ma after its crystallization.
Fagan et al. (2007) studied several type B1, B2, CTA,
and fluffy type A (FTA) CAIs from Allende and identified two textural and mineralogic types (domains) of secondary mineralization: a grossular-rich domain
concentrated along melilite grain boundaries in CAI interiors and a feldspathoid-bearing domain confined
mostly to CAI margins, just interior to the WarkLovering rim sequence. In type B1s, most secondary
minerals, and some secondary minerals in other CAI
types, show no resolvable 26Mg*, suggesting formation >3 Ma after primary CAI minerals. All but two analysesof secondary minerals from the FTA CAI yield(26Al/27Al)0 ~ (4.9±2.8)×10-6, suggesting formation ~1.8-3.2 Ma after the primary CAI minerals. One grossular from the Type B2 CAI, and several grossular andfeldspar analyses from a CTA CAI, have 26Mg* consistent with (26A/27Al)0 ~ 4.5×10-5. Fagan et al. (2007) suggested that Allende CAIs experienced a protrackted,
episodic alteration: some alteration occurred during theformation of CAIs; some continued during melting events
associated with chondrule formation in the solar nebula
and/or on the CV parent asteroid.
Ford and Brearley (2008) described secondary anorthite, grossular, nepheline, and sodalite replacing melilite
in a Type A CAI from Allende. The CAI is surroundedby an aureole of Ca,Fe-rich pyroxenes and andradite.
Ford and Brearley (2008) concluded that Na, Cl, and Si
were introduced into the CAI, whereas Ca was lost during metasomatic alteration. Mass-balance calculations
show that amount of Ca lost is sufficient to explain Ca
in the Ca,Fe-rich aureole, suggesting that the alteration
occurred in situ on the CV parent asteroid. Subsequently, Brearley et al. (2014) described dmisteinbergite
associated with disordered biopyriboles replacing melilite in this CAI, indicating the formation of the secondary minerals in the presence of an aqueous fluid.
Che and Brearley (2021) described secondary
mineralization in a Type C CAI from Allende. Primaryanorthite in this inclusion is pseudomorphically replacedby sodalite, nepheline, and ferroan olivine (Fa48); secondary hedenbergite, diopside, and wollastonite occur inKrot et al. Progress in Earth and Planetary Science(2021) 8:61

nodules. Sodalite is more abundant in the core of the
CAI, whereas nepheline is more abundant in its mantle.
According to Che and Brearley (2021), the zoned distribution of Na- and Cl-bearing phases may either reflect a
two-stage alteration process on the CV parent asteroid
or evolution of the fluid composition during thealteration.

3 Analytical methods
3.1 Mineralogy and petrology

Polished sections of the Allende CAIs provided by 1David Wark (Australian National University), Gerald Wasserburg (California Institute of Technology), Ian
Hutcheon (Lawrence Livermore National Laboratory),
and Steven Simon (University of Chicago) were mappedin Mg, Ca, Al, Ti, Na, Cl, Si, S, Ni, and Fe K∝ x-rays
using a 5-10 μm electron beam, 15 kV accelerating voltage, 50 nA beam current, 10-20 ms per pixel acquisitiontime, and resolution of 5-10 μm per pixel withwavelength-dispersive spectrometer detectors with the
University of Hawai’i (UH) field-emission electron
microprobe JEOL JXA-8500F. Elemental maps in (i) Mg,
Ca, and Al, and (ii) Cl, Na, and Al were combined using
an RGB-color scheme (Mg, or Cl—red, Ca or Na—green,and Al—blue). The MgCaAl maps are used to illustratetextures and primary mineralogy of the Allende CAIs
studied. In these maps, spinel is purple; melilite andmost secondary minerals, which typically replace melilite, are bright-green; Al,Ti-diopside is dark-green;
anorthite is dark-blue; and forsterite is red. The ClNaAlmaps are used to illustrate distribution of adrianite/wadalite (red), sodalite (yellow), and nepheline (green)
inside and outside CAIs. All CAIs were subsequently
studied in backscattered electron (BSE) images and analyzed for chemical compositions using the UH JEOL
JXA-8500F. Quantitative wavelength-dispersive analyses
were obtained at 15 kV with counting times of 30 s forpeak and for background measurements for each analysis. Natural minerals were used as standards. Electronprobe data were reduced via the modified ZAF correction procedure PAP (Pouchou and Pichoir 1984).
Representative chemical compositions of secondary
minerals in the Allende igneous CAIs are listed in Supplementary Material (Table EA1). X-ray elemental maps
and backscattered electron (BSE) images of the representative Allende CAIs studied are shown in Figs. 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 11, and 12.
3.2 Oxygen isotopes

Oxygen isotopic compositions were analyzed in situ with
the UH Cameca ims-1280 ion microprobe using the1

David Wark, Gerald Wasserburg, and Ian Hutcheon are deceased

Page 6 of 37

method of Nagashima et al. (2015). Briefly, a primaryCs+ ion beam of ~25 pA focused to ~1–2 μm was used.
The ion microprobe was operated at –10 keV with a 50
eV energy window. Three oxygen isotopes were measured simultaneously: 16O– was measured on a Faradaycup (FC) and 17O– and 18O– were measured on electron
multipliers (EMs). The mass resolving power (m/Δm)for 16O– and 18O– was ~2000, and that for 17O– was~5500, sufficient to separate interfering 16OH–. A
normal-incidence electron flood gun was used for charge
compensation. 16OH− signal was monitored after eachmeasurement. The contribution of 16OH− onto 17O− wascorrected based on a peak/tail ratio. The correction was
typically less than 0.05‰. Instrumental fractionation wascorrected using terrestrial standards including San Carlos olivine (for melilite, Na-melilite, forsterite, andferroan olivine), augite (for Al-diopside and kushiroite),
Burma spinel (for spinel), Miyake-jima anorthite (foranorthite and dmisteinbergite), andradite (for andradite),sodalite (for sodalite), nepheline (for nepheline), calcite(for calcite), wollastonite (for wollastonite), and grossular(for grossular) (Nagashima et al. 2020). To verify the positions of the sputtered regions, the spots analyzed foroxygen isotopes were studied with secondary and BSE
images using the UH JEOL JXA-8500F electron microprobe before and after SIMS measurements. Oxygenisotope compositions are reported as δ17O and δ18O, deviations from Vienna Standard Mean Ocean Water(VSMOW; 17O/16OVSMOW = 0.000380; 18O/16OVSMOW =0.002005; De Laeter et al. 2003) in parts per thousand:δ17,18OSMOW = [(17,18O/16Osample) / (17,18O/16OVSMOW)– 1] × 1000, and as deviation from the terrestrial fractionation (TF) line, Δ17O. Typical uncertainties for δ17O,
δ18O, and Δ17O were ~ 1.5, 2, and 2‰, respectively.
Oxygen isotopic compositions of the primary and secondary minerals in the Allende coarse-grained igneous
CAIs measured here and plotted in Fig. 13a-d are listed
in Supplementary Material (Tables EA2 and EA3,
respectively).

3.3 Aluminum-magnesium isotope systematics

Aluminum and magnesium isotopic compositions ofgrossular were measured with the UH Cameca ims-1280
using the method of Jacobsen et al. (2011). Briefly, a primary 16O− ion beam with a current of ~150 pA focused
to ~5×7 μm2 was used. 24Mg+, 25Mg+, and 26Mg+ were
measured sequentially using a monocollector EM. 27Al+
was measured using a multicollection FC simultaneouslywith 25Mg+. The mass-resolving power was set to ~3800,
sufficient to separate interfering hydrides and doubly
charged 48Ca2+. The 27Al+/24Mg+ ratios were correctedusing a sensitivity factor determined on a terrestrial
grossular standard.

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Fig. 1 Combined x-ray elemental maps in (a, c, g, k, m, o) Mg (red),Ca (green), and Al (blue), (b, d, h, l, n) Cl (red), Na (green), and Al(blue) and elemental maps in (e, j) Ca, (f) Si, (j) Mg, and (p) Na of
representative samples of coarse-grained CAIs from the CV3
carbonaceous chondrites Allende studied: ALH2 (FTA/CTA), TS2 (CTA),
TS23 (B1), TS31 (B2), Al-2 (FoB), and 160 (C). Regions outlined in “g−j”
are shown in detail in Fig. X. All CAIs are surrounded by WarkLovering rims (WLR), fine-grained matrix-like rims (FGR), and a layerof Ca,Fe-rich silicates (CaFe-sil). Nepheline and sodalite areconcentrated in the peripheral portions of CAIs. Abundant nepheline
grains occur also in fine-grained matrix-like rims. Wadalite/adrianite
are found exclusively in cores of Type B CAIs (high Cl signal in b
comes from epoxy). fas = Al,Ti-diopside; mel = melilite. Hereafter:
abbreviations of secondary minerals are listed in Table 1

Magnesium isotopic compositions of melilite were
measured simultaneously using 4 FCs. A ~10 nA 16O−
ion beam with ~20 μm size was used. The massresolving power was set to ~3800 using an exit slit widthof 250 μm (multicollection exit slit #2). The count ratesof 24Mg+ ranged from ~1×107 to 5×107 cps. The instrumental mass-fractionation effects and relative sensitivities were corrected using synthetic melilite glass.
Excesses of 26Mg (26Mg*) were determined using an exponential law with an exponent of 0.514. We note differences in 26Mg* calculated using exponents of 0.514 and0.5128 (Davis et al. 2015) are negligibly small (<0.03‰).

4 Results and discussion
4.1 Petrography and primary mineralogy of the Allendecoarse-grained igneous CAIs

Based on the mineralogy, petrography, and bulk chemical compositions, several types of coarse-grained igneous CAIs are recognized in CV chondrites: compact typeA (CTA), Type B (B1, B2), forsterite-bearing type B(FoB), and type C (e.g., Grossman 1980; Wark 1987;
Wark et al. 1987; Simon et al. 1999; Simon and Grossman 2006; Bullock et al. 2012). Bulk chemical compositions of these CAIs projected from spinel onto thegehlenite (Ca2Al2SiO7)−forsterite (Mg2SiO4)−anorthite(CaAl2Si2O8) plane of the CaO−MgO−Al2O3−SiO2(CMAS) system plot in the liquidus fields of melilite(CTA, B1, and B2; B2s are generally more SiO2-rich thanB1s), forsterite (FoB), and anorthite (C).
In this section, we characterize the petrography andprimary mineralogy of representative members of different CAI types studied (Fig. 1). Other Allende CAIs studied, their classification, and variations in chemicalcompositions of their melilite, one of the major silicate
minerals that experienced metasomatic alteration, arelisted in Table 1.
4.1.1 Type A CAIs

Type A Ca,Al-rich inclusions are a major type of coarsegrained refractory inclusions in CV3 chondrites. Type AKrot et al. Progress in Earth and Planetary Science(2021) 8:61

Fig. 2 Backscattered electron images of a fine-grained matrix-likerim and a Ca,Fe-rich silicate layer around Type B2 CAI TS31. The
matrix-like rim consists predominantly of lath-shaped ferroan olivine,
nepheline, and salite-hedenbergite pyroxene nodules. The Ca,Fe-richsilicate layer is composed of salite-hedenbergite pyroxenes,
andradite, and wollastonite

Page 8 of 37

CAIs, originally defined by Grossman (1975) as extremely melilite-rich inclusions with minor spinel andAl,Ti-diopside, were subsequently subdivided into irregularly shaped, fluffy ones (FTA), and round, compact(CTA) ones (Grossman 1980). Fluffy Type A CAIs arethought to have formed by gas-solid condensation (MacPherson and Grossman 1984), whereas CTA CAIs crystallized from melts (Simon et al. 1999). However, spineland hibonite grains in FTA CAIs often have rounded or
corroded outlines, and melilite grains often show inverse
compositional zoning, probably reflecting incomplete
melting and evaporation (Allen et al. 1978; Simon et al.
2001). As a result, there is a textural and mineralogicalcontinuum between CTA and FTA CAIs. One of suchintermediate FTA/CTA CAIs studied, ALH-2, is described below.
Fluffy/compact type A CAI ALH-2 (Fig. 1a, b) consists
mostly of gehlenitic melilite (Åk1-24), hibonite, spinel,
perovskite, and traces of Al,Ti-diopside (MacPherson
and Grossman 1984; Simon et al. 2001). Melilite ranges
from small, prismatic crystals 40−70 μm across to anhedral grains ~ 200 μm across. The melilite grains show
inverse chemical zoning with Åk-poor rims and relatively Åk-rich cores. The CAI is surrounded by a multilayered Wark-Lovering (WL) rim (from inside outward)of hibonite + spinel + perovskite, nearly pure gehlenite,
and Al,Ti-diopside, a coarse-grained forsteritic olivine
accretionary rim (Krot et al. 2001a), a fine-grained
matrix-like rim, and a layer of Ca,Fe-rich silicates (salitehedenbergite pyroxenes and andradite).
Compact type A CAIs, including those studied here
(818-G, CG-13/15, TS-68, and TS-2) consist of melilite,
spinel, Al,Ti-diopside (16−20 wt% TiO2, 18−21 wt%
Al2O3), perovskite, ±hibonite, and ±platinum groupelement-bearing (PGE) metal nuggets. Melilite in most
CTA CAIs is typically highly gehlenitic (Åk<33); åkermanite content increases towards the CAI cores. Individual
melilite grains typically show normal igneous zoning.
Nearly pure gehlenite (Åk3) occurs in and near WL rims.
The anomalous CTA CAI TS-2 (Fig. 1c−f), characterized
in detail by Simon et al. (1998), shows a much larger
compositional range of melilite (Åk9−65). Many melilite
crystals consist of patches of åkermanitic melilite(Åk32-62, median Åk52) set in or partially enclosed by,
and optically continuous with, relatively gehlenitic melilite (Åk25-53, median Åk38). The gehlenitic melilites havedendritic shapes and enclose numerous, fine inclusionsof spinel and minor perovskite and Al,Ti-diopside.
Coarse-grained spinel, ~50−150 μm across, occurs inclumps and chains enclosed in relatively åkermanitic
melilite. The sample also contains a spinel-free palisadebody composed almost entirely of Åk-rich (45−65 mol%)Na-bearing (up to 0.15 wt% Na2O) melilite. Melilite near
WL rim is highly gehlenitic (Åk9).

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Fig. 3 Backscattered electron images of secondary minerals replacing gehlenitic melilite in CTA CAIs and mantles of Type B1 CAIs. In the CAI
peripheries, melilite is replaced by anorthite and grossular. The abundance of anorthite decreases, whereas the abundance of grossular increases
towards the CAI interiors. Other phases coexisting with grossular in the CAI interiors are micron-sized spinel, clintonite, FeO-enriched monticellite,
and FeO-enriched grossular. The ferroan monticellite and grossular overgrow FeO-poor grossular. Clintonite is typically enclosed by grossular.
Spinel grains growing into void space in veins have euhedral outlines (indicated by arrow in f). Near the CAI edges, secondary anorthite is
replaced by nepheline, sodalite, and ferromagnesian olivine

4.1.2 Type B CAIs

Type B CAIs consist of melilite, Al,Ti-diopside, spinel,
anorthite, and minor perovskite, hibonite, and ±PGE
metal nuggets. A small subset of Type B inclusions

contain igneous forsterite; these are classified as
forsterite-bearing Type B CAIs (FoB, Bullock et al.
2012). Type Bs are subdivided into two textural subtypes: the B1s, which have a melilite-rich outer mantle

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Fig. 4 Backscattered electron images of secondary mineral assemblages replacing åkermanitic melilite in Type B1, B2, FoB, and CTA CAIs: agrossular-monticellite-wollastonite, b−d grossular-monticellite, and e, f grossular-Al-diopside. All these mineral assemblages may contain minor
clintonite, forsterite, spinel, Na-melilite, and wadalite. Na-melilite is commonly observed at the boundary between altered melilite and igneousanorthite (see a). Arrows in i indicate a boundary between grossulars with different MgO contents (dark, 3.7 and light, 2.3 wt%)

enclosing a pyroxene-rich core; and the B2s, which havea rather homogeneous distribution of minerals (Warkand Lovering 1982b; Simon and Grossman 2006). Thereis a textural continuum between B1s and B2s. Melilite
compositions of Type B1 and B2 inclusions are generally
similar (Åk5−75). The composition of melilite in FoBs
spans even a larger range (Åk6−90). There is a positive
correlation between Åk and Na2O contents in Type Bs.
Melilite in the outermost parts of type B1 mantles isgenerally more gehlenitic than that in B2s, FoBs, andcores of type B1s. Both normally and inversely zoned regions are observed within individual melilite grains intype Bs (MacPherson et al. 1984; Simon and Grossman2006). Fassaite grains are igneously zoned with decreasing TiO2, Sc2O3, and V2O3 contents from cores to edgesof the grains. Anorthite and spinel are compositionallypure.

Type B1 CAI TS-23 (Fig. 1g−j) consists of melilite,
Al,Ti-diopside (3−8 wt% TiO2; 15−20 wt% Al2O3),
spinel, and anorthite (Grossman 1975; Hutcheon et al.
1978; MacPherson et al. 1984; Simon and Grossman2006). It has a gehlenitic melilite mantle ~0.5-1 mm
thick that encloses sparse spinel and fassaite grains andsurrounds a relatively fassaite-rich and melilite-poorcore. Melilite crystals (Åk27-72) in the core show complex(normal and reverse) compositional zoning (MacPherson
et al. 1984). The CAI is surrounded by a spinel−Al-diopside WL rim, a fine-grained matrix-like rim composed offerroan olivine and nepheline, and a layer of Ca,Fe-richsilicates composed of salite-hedenbergite pyroxenes, andradite, and wollastonite.
The anomalous Type B2 CAI TS-31 (Fig. 1k, l) consists of highly åkermanitic melilite (Åk76-87; 0.1-0.2 wt%
Na2O), sector-zoned Al,Ti-diopsides (TiO2, 4-11 wt%;

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Fig. 5 a−d Backscattered electron images, e combined x-ray elemental map in Mg (red), Ca (green), and Al (blue), and f elemental map in Mg ofa vein crosscutting gehlenitic melilite mantle around Type B1 CAI TS-23. Regions outlined in a are shown in detail in b, c, and d−f, respectively.
Towards the CAI interior, the melilite becomes more åkermanitic. The mineralogy of the vein changes towards the CAI interior from anorthite-rich
to grossular-rich; minor phases include Al-diopside, forstetrite, and spinel. Åkermanitic melilite in the core is replaced by coarse-grained grossular,monticellite, wollastonite, and minor wadalite

Al2O3, 12-22 wt%), and compositionally pure anorthiteand spinel. The peripheral portion of the CAI is extensively altered and contains small anhedral Ti-rich fassaites (TiO2, 17-20 wt%; Al2O3, 18-20 wt%). WarkLovering rim layers cannot be clearly defined due to extensive alteration. The CAI is surrounded by a finegrained matrix-like rim composed of lath-shaped ferroan
olivine and nepheline grains, and by a discontinuous
layer of salite-hedenbergite pyroxenes + andradite +wollastonite.

Forsterite-bearing Type B CAI Al-2 (Fig. 1m, n) consists mainly of Al,Ti-diopside (TiO2, 0.6-5.5 wt%; Al2O3,
4.7-21.7 wt%), highly åkermanitic Na-bearing melilite(Åk79-87, 0.12-0.23 wt% Na2O), and compositionally pureanorthite; all phases poikilitically enclose Ca-rich (1.01.3 wt% CaO) forsterite and spinel grains. Distributionof forsterite is very heterogeneous. Anorthite is preferentially concentrated in the peripheral part of the inclusion. The CAI is surrounded by a spinel-rich layer
containing rare grains of hibonite (TiO2, 0.97 wt%;

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Fig. 6 Backscattered electron images primary anorthite (a, b) and secondary anorthite (c, d) in the peripheries of the Allende coarse-grained
igneous CAIs. Both types of anorthite are replaced by nepheline, sodalite, and ferromagnesian olivine

MgO, 8.5 wt%), a fine-grained matrix-like rim composedof lath-shaped ferroan olivine and nepheline, and a layerof salite-hedenbergite pyroxenes + andradite + wollastonite.

4.2 Alteration of primary and secondary minerals in the
Allende CAIs
4.2.1 Alteration of gehlenitic melilite in compact type A CAIs
and type B1 CAIs

4.1.3 Type C CAIs

Gehlenitic melilite is common in CTAs and mantles ofType B1s. Nearly pure gehlenite occurs near WL in most
CAIs studied. Highly gehlenitic melilite (Åk<10) in theoutermost regions of all CAIs studied is replaced by nearlymonomineralic anorthite/dmisteinbergite (Fig. 3a−c; hereafter anorthite; Raman or EBSD studies are required to
distinguish between anorthite and dmisteinbergite; Fintor
et al. 2014; Park et al. 2013). The anorthite/grossular ratio
decreases towards the CAI cores (Fig. 3a−c), which correlates with the increase of åkermanite content in melilite(Åk5−20). Minor clintonite, spinel, and corundum occur ingrossular-rich veins (Fig. 3d−g). Secondary spinel and corundum grains are generally less than 3 μm in size, andoften form subhedral-to-euhedral grains (Fig. 3d−f, i, j).
Occasionally, large concentrations of μm-sized spinel
grains occupy central zones of some grossular-rich veins(Fig. 3e,f). Subhedral-to-anhedral clintonite grains generally occur in interstitial regions between grossular grains(Fig. 3g). In the clintonite-bearing regions, the compositionally distinct grossular grains are observed—FeOpoor/free and FeO-bearing; the latter typically overgrow
the former and may associate with ferroan monticellite(monticellite−kirschsteinite solid solution) (Fig. 3g−j).

Type C CAI 160 is a fragmented inclusion partly surrounded by a WL rim of spinel and Al,Ti-diopside (Fig.
1o, p; see also Figs. 10 and 11 in Krot et al., 2007b). It is
composed of Al,Ti-diopside, melilite, and spinel, all embedded into a fine-grained anorthite groundmass.
Anorthite is compositionally pure CaAl2Si2O8. Al,Ti-diopside (TiO2, 4.6-13.9 w%; Al2O3, 18-20 wt%) occurs ascoarse grains with abundant 3-5 μm-sized inclusions ofanorthite (lacy pyroxenes; Wark 1987) and rounded inclusions of spinel, and as rare massive grains poikilitically enclosing euhedral spinel (see Fig. 11 in Krot et al.,2007b). Melilite occurs as coarse subhedral grains with
abundant 3-5 μm-sized inclusions of anorthite (named“lacy melilite” by Wark 1987) and as compact polycrystalline regions with interstitial Al,Ti-diopside and poikilitically enclosing spinel. Melilite in the CAI mantle ismore gehlenitic than in its core (Åk44-56 vs. Åk56-72); in
both occurrences, melilite contains detectable Na2O
(0.10-0.36 wt%) (Fig. 3 in Krot et al., 2007b). Spinel,
nearly pure MgAl2O4, is heterogeneously distributed
within the CAI and occurs as individual grains enclosed
in melilite, anorthite and Al,Ti-diopside, and as framboids.

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Fig. 7 Backscattered electron images of Alteration of primary igneous anorthite in the interior regions of the Type B1, B2, and FoB Allende CAIs.
Regions outlined in a and e are shown in details in b and f. Anorthite along the boundaries with heavily altered regions of åkermanitic melilitecontains irregularly shaped embayments, inclusions along cleavage planes, and veins of Na-melilite, grossular, kushiroite, and wadalite.
Occasionally, wadalite is replaced by sodalite. Na-melilite growing into open space shows euhedral outlines (see e and f). Variations in brightness
of Na-melilite correspond to variations in Na2O content

4.2.2 Alteration of åkermanitic meliliteÅkermanitic melilite is common in B2s, FoBs, Cs, coresof B1s, and CTA CAI TS-2. Several assemblages of secondary minerals replacing åkermanitic melilite are observed in the CAIs studied: (i) grossular + monticellite +wollastonite (Fig. 4a), (ii) grossular + monticellite (Fig.
4b−d), and (iii) grossular + Al-diopside (Fig. 4de, f).
Minor wadalite (Fig. 4a−f), forsterite (Fig. 4b, g), clintonite (Fig. 4g−i), and spinel (Fig. 4i) can be found in all ofthem. In the clintonite-bearing regions, two compositionally distinct grossulars are observed with sharpboundaries between them—FeO-poor/free and FeO-

bearing (Fig. 4i). In contrast to secondary minerals replacing gehlenitic melilite, secondary anorthite replacingåkermanitic melilite is virtually absent.
Changes in secondary mineral assemblages replacing
melilite of different chemical compositions are illustrated
in Fig. 5 showing a vein crosscutting a gehlenitic melilite
mantle around Type B1 CAI TS-23. Near the WL rim,
the vein consists predominantly of anorthite. Towards
the CAI core, the abundance of anorthite decreases,
whereas the abundance of grossular increases. In theanorthite-free regions of the grossular vein, minor Aldiopside, monticellite, and forsterite are observed. The

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Fig. 8 a, c Combined x-ray elemental maps in Mg (red), Ca (green), and Al (blue), and (b, d−f) backscattered electron images of the interior
regions of Type C CAIs (a, b) 160 and (c−f) 100. Regions outlined in c and e are shown in detail in d–f, respectively. Lacy melilite composed ofåkermanitic melilite with numerous rounded inclusions of anorthite is pseudomorphically replaced to various degrees by grossular-forsteritemonticellite and grossular-Al-diopside. Anorthite groundmass around melilite pseudomorphs shows little evidence for alteration: small amount ofNa-melilite occurs along edges of melilite pseudormorphs

abundance of Al-diopside is highest in a region wherethe vein enters the CAI core containing åkermanitic
melilite (Fig. 5e). The åkermanitic melilite in the core is
replaced by a coarse-grained mineral assemblage of grossular, monticellite, wollastonite, and wadalite (Fig. 5d).
4.2.3 Alteration of primary and secondary anorthite

Both primary and secondary anorthites in the peripheral
portions of CAIs are replaced by nepheline, sodalite, andminor ferromagnesian olivine (Figs. 1 and 6). There are
no clear textural relationships between nepheline andsodalite. Sodalite tends to be located closer to the CAI
interiors than nepheline (Fig. 1d, l); however, this is notalways the case (Fig. 1b, n). Ferromagnesian olivine
ranges in composition from Fa~10 to Fa~40; fayalite content typically increases towards CAI peripheries.
In contrast to åkermanitic melilite, coarse-grained primary anorthite in type B and FoB CAI cores is generally
only weakly altered. In contact with altered regions ofåkermanitic melilite, the edges of anorthite grains appear
to be corroded by Na-rich melilite and nearly pure grossular (Fig. 7a, b). In some cases, Na-rich melilite andgrossular occur along cleavage planes of the anorthitegrains (Fig. 7c). In others, subhedral grains of Namelilite grow in open spaces within anorthite crystals(Fig. 7d, e). These subhedral grains of Na-melilite areKrot et al. Progress in Earth and Planetary Science(2021) 8:61

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Fig. 9 Backscattered electron images of multiple generation of secondary mineral assemblages in the Allende Type B CAIs. Region outlined in e
is shown in detail in f. a−d Replacement of grossular-monticellite-wollastonite assemblages to various degrees by Al-diopside. e, f Grossularmonticellite-wollastonite-Al-diopside assemblage is crosscut by a wollastonite vein. Wollastonite also fills veins and irregularly shaped regionsin fassaite

often chemically zoned: sodium content increases towards grain edges. Occasionally, anorthite grains arecrosscut by veins composed of Na-melilite, grossular, ±kushiroite, and ± wadalite (Fig. 7e, f).
4.2.4 Alteration of lacy melilite in Type C CAIs

In type C CAIs 160 and 100, melilite and fassaite grains
have “lacy” textures characterized by the presence ofabundant rounded inclusions of anorthite ~ 5−10 μm in
size. The lacy melilite and fasssaite are embedded infine-grained (~ 5 μm in size) anorthite groundmass(Figs. 1 and 8o). Anorthite inclusions in lacy fassaite andmelilite, and anorthite groundmass in the CAI coresshow little evidence for alteration. At the same time, lacy
melilite grains are pseudomorphed to varying degrees bygrossular + forsterite + monticellite (Fig. 8a−d) and grossular + Al-diopside assemblages (Fig. d−f). Although theoutlines of the original lacy melilite grains are generally
well-preserved, the replaced portions are highly porous.
The secondary grossular, forsterite, and Al-diopside
growing into the pore spaces often show euhedral outlines(Fig. 8d, f). X-ray elemental mapping revealed enrichment
in sodium along the peripheries of the pseudomorphedlacy melilites (see Fig. 2EA in Krot et al., 2007b). This enrichment is due to the presence of micron-sized anhedralgrains of Na-rich melilite (Fig. 8b, d).

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Fig. 10 Backscattered electron images of chemically zoned secondary minerals in the Allende coarse-grained CAIs. The oscillatory zoned
grossular and Al-diopside are commonly observed in grossular-Al-diopside mineral assemblages and often contain FeO-rich secondary minerals:
andradite, hedenbergite, ferroan monticellite, and ferroan grossular

4.2.5 Secondary minerals replacing, overgrowing, andcrosscutting earlier formed secondary phases

Some secondary minerals and/or mineral assemblages
can be replaced, overgrown, and crosscut by newly
formed ones, as illustrated in Figs. 9, 10, and 11. For example, (i) grossular + monticellite + wollastonite mineral
assemblages replacing highly åkermanitic melilite in the
FoB CAIs All-2 and All-5-2 are in turn replaced to avarious degree by Al-diopside (Fig. 9a−d). (ii) In theType B2 CAI TS-31, grossular + monticellite + wollastonite mineral assemblages replacing åkermanitic melilite are crosscut by wollastonite veins (Fig. 9e, f). (iii) In
the Type B2 CAI CG-10, a wollastonite vein crosscuts
an Al-diopside-rich region replacing an earlier formed
grossular + monticellite−wollastonite assemblage (Fig.
10a); grossular in the Al-diopside-rich region shows oscillatory zoning (Fig. 10b). The coexisting oscillatory

zoned grossular and Al-diopside are also found in the
CAIs AJEF (B1, Fig. 10c), CG-11 (B2, Fig. 10d, f), TS-2
(CTA, Fig. 10e), and CG-6 (B1, Fig. 10h). (iv) Occasionally, the earlier formed FeO-poor secondary minerals(Al-diopside, grossular, and monticellite) are either
overgrown or crosscut by veins composed of ferroan
grossular, ferroan monticellite, ferroan Al-diopside,
hedenbergite, wollastonite, andradite, pentlandite, andheazlewoodite (Figs. 10 and 11g−i).

4.2.6 Secondary minerals in voids inside CAIs

Some coarse-grained igneous CAIs from Allende containvoids filled by anhedral-to-subhedral secondary grossular, monticellite, ferroan monticellite, wollastonite, tilleyite, calcite, andradite, hedenbergite, and Al-diopside(Fig. 12).

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4.2.7 Secondary minerals around CAIs

All Allende CAIs studied are surrounded by the multilayered WL rims, fine-grained matrix-like rims, and alayer of Ca,Fe-rich silicates (Fig. 1). The peripheral regions of the CAIs contain abundant nepheline and sodalite (green and yellow grains, respectively, in Cl:Na:Al
combined elemental maps) replacing primary and secondary anorthite. Sodalite tends to occur closer to the
CAI interiors than nepheline. Sodium-free Cl-rich minerals, wadalite and/or adrianite (red grains in Cl:Na:Al
maps; EBSD studies are required to distinguish between
these minerals) occur almost exclusively in CAIs containing åkermanite-rich melilite—B2s, FoBs, cores ofB1s, and the anomalous CTA CAI TS-2.
The fine-grained matrix-like rims consist of lathshaped ferroan olivine, Fe,Ni-sulfides, and abundantnepheline grains typically enclosing lath-shaped ferroan
olivine (Fig. 2). Occasionally, fine-grained rims containsalite-hedenbergite pyroxene nodules (Fig. 2b).
The calcium- and iron-rich silicate layers separate thefine-grained rims from the Allende matrix and consist ofsalite-hedenbergite pyroxenes, wollastonite, and andradite (Fig. 2c).
4.3 Oxygen isotopic compositions

Fig. 11 Backscattered electron images of different generations ofveins in the Allende coarse-grained CAIs. Regions outlined in a andg are shown in detail in b−d and h−j, respectively. Central regionsof some grossular-rich veins are occupied by pentlandite,heazlewoodite, or andradite + wollastonite + hedenbergite. g−l
Occasionally, grossular-rich veins are crosscut by veins composed offerroan grossular and ferroan-Al-diopside

In Fig. 13a−d, we plotted oxygen isotopic compositionsof primary and secondary minerals in the coarse-grained
Allende CAIs measured with the matrix-matched mineral standards. In Fig. 13g, h, we plotted the previously
reported data for secondary andradite, grossular, magnetite, salite-hedenbergite pyroxenes, sodalite, and wollastonite measured without matrix-matched standards(magnetite is the only exception) in the Allende matrix,
chondrules, and dark inclusions. Like in the previously
reported dataset, most secondary minerals in the Allende
CAIs plot along mass-dependent fractionation line with
Δ17O of ~ −3±2‰, but show a significantly smaller
range of δ18O (~ 0 to + 10‰ vs. ~ −15 to ~ + 20‰).
Larger ranges of Δ17O were found in grossular and forsterite in the Type B1 CAI TS-34 (~ −10 to ~ −1‰) andType C CAIs 100 and 160 (~ −15 to ~ −1‰); these minerals plot along the CCAM line. The similar ranges of
Δ17O are observed for primary anorthite in these CAIs;
melilite is generally more 16O-depleted (Δ17O ~ −5 to ~−1‰). Spinel and fassaite are 16O-rich (Δ17O ~ −25 to ~−20‰); the only exception is very Ti-rich pyroxenes in
TS-34, which are 16O-depleted to various degrees.
4.4 Aluminum-magnesium isotope systematics

In Fig. 14, we plotted excess of radiogenic 26Mg (26Mg*)
vs. 27Al/24Mg ratio in gehlenitic melilite in the mantleand åkermanitic melilite in the core of the Type B CAI
TS-34, and of secondary grossular replacing the melilitein this and several other Allende Type B CAIs. All

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 18 of 37

Fig. 12 Secondary minerals growing in voids inside coarse-grained Allende CAIs. Region outlined in a is shown in detail in b

melilite analyses in TS-34 show resolvable 26Mg* andplot along correlation line corresponding to the initial
26
Al/27Al ratio of (5.4±0.3)×10−5, which is indistinguishable from the canonical value of (5.25±0.02)×10−5 (Larsen et al. 2011). Grossular grains in veins crosscutting
the gehlenitic melilite mantle in TS-34 show resolvable26
Mg* and 27Al/24Mg ratio (in most cases, ~ 7−10) similar to those of melilite and plot along the Al-Mg isochron defined by the melilite. The coarse grossulargrains replacing åkermanitic melilite and associatingwith monticellite, wollastonite, Al-diopside, and forsterite have significantly higher 27Al/24Mg ratios, ~30-100(50−62 in TS-34), and show no resolvable 26Mg* within
2σ uncertainty of our measurements, ~±2‰.
4.5 Secondary mineral assemblages in the Allende CAIs
and inferred metasomatic reactions
4.5.1 Alteration of gehlenitic melilite

Highly gehlenitic melilite (Åk<10) in the outermost regions of the Allende CAIs is largely replaced and crosscut by veins of nearly monomineralic anorthite;grossular and corundum are minor. The abundance ofanorthite in veins decreases and the abundance of grossular increases towards the CAI interiors containing
more åkermanitic melilite (Åk10−30). Other secondary
minerals observed in the grossular-rich veins include
clintonite, spinel, and Al-diopside. The presence ofclintonite provides clear evidence for the involvement of
H2O in the alteration reactions, which can be expressedin mass-balanced form as the following:
R4 : Ca2 Al2 SiO7ðsÞ þ SiO2 → CaAl2 Si2 O8ðsÞ þ CaO
gehleniteanorthite

ð4Þ
R5 : 2Ca2 Al2 SiO7ðsÞ þ 3SiO2 → Ca3 Al2 Si3 O12ðsÞ þ CaAl2 Si2 O8ðsÞ
gehlenitegrossularanorthite

ð5Þ
R6 : 3Ca2 Al2 SiO7ðsÞ þ 3SiO2 → 2Ca3 Al2 Si3 O12ðsÞ þ Al2 O3ðsÞ
gehlenitegrossular

corundum

ð6Þ
R7 : 2Ca2 Al2 SiO7 þ Ca2 MgSi2 O7ðsÞ þ 2SiO2 →gehlenite−a kermanite ss
Ca3 Al2 Si3 O12ðsÞ þ CaMgSi2 O6 þ CaAl2 SiO6ðsÞ þ CaO
grossular

diopside−Ca−Tschermakite ss

ð7Þ
R8 : 3Ca2 Al2 SiO7 þ 2Ca2 MgSi2 O7ðsÞ þ H2 O→
gehlenite−a kermanite ss

Ca3 Al2 Si3 O12ðsÞ þ CaMg2 Al4 SiO10 ðOHÞ2ðsÞ þ 6CaO þ 3SiO2
grossular

clintonite

ð8Þ

Krot et al. Progress in Earth and Planetary ScienceFig. 13 (See legend on next page.)(2021) 8:61

Page 19 of 37

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 20 of 37(See figure on previous page.)
Fig. 13 a, b Three-isotope oxygen diagrams and c, d Δ17O values of primary and secondary minerals in the Allende coarse-grained igneous CAIs
TS-34 (B1), TS-31 (B2), AJEF (B1), TS-21 (B2), All-2 (FoB), 100 (C), and 160 (C). Data for primary minerals in TS-34 are from Kawasaki et al. (2018). Spineland most analyses of Al,Ti-diopside have 16O-rich compositions; anorthite and melilite are 16O-depleted to various degrees. Al,Ti-diopside grains in
TS-34 show a large range of Δ17O that correlates with their igneous zoning: very Ti-rich pyroxene cores are 16O-depleted relative to Ti-poor
pyroxene edges. Secondary minerals have generally 16O-poor compositions. The observed range of Δ17O of secondary minerals in individual CAIs
is similar to those of primary melilite and anorthite. e, f Δ17O values of primary minerals in coarse-grained igneous CAIs of different types (CTA, B,
FoB, and Cracked Egg that is rich in krotite (krt, CaAl2O4) and grossite (grs, CaAl4O7)) from CV chondrites. FUN (fractionation and unidentified
nuclear effects) CAIs show large mass-dependent fractionation effects in oxygen isotopes indicating crystallization during melt evaporation in
vacuum (Mendybaev et al. 2017). Melilite and anorthite in CV CAIs of all types are 16O-depleted relative to hibonite, spinel, forsterite, and fassaite.
The observed O-isotope heterogeneity may reflect mineralogically controlled exchange with an aqueous fluid (Δ17O = −3±2‰) on the CV parent
asteroid. Gas-melt isotopic exchange in gaseous reservoirs of variable oxygen isotopic composition cannot be excluded. Data from Bullock et al.
(2012), Connolly et al. (2019c), Krot et al. (2014), and Williams et al. (2017). g Three-isotope oxygen diagram and h Δ17O of secondary minerals in
CAIs, chondrules, and matrices of the oxidized Allende-like CV chondrites. Most secondary minerals were measured without matrix-matchedstandards; magnetite is the only exception. Data from Choi et al. (1997), Krot et al. (2000, 2010), and Cosarinsky et al. (2008)

ð9Þ

R9 : 3Ca2 Al2 SiO7 þ Ca2 MgSi2 O7ðsÞ þ SiO2 →gehlenite−a kermanite ss
2Ca3 Al2 Si3 O12ðsÞ þ MgAl2 O4ðsÞ þ 2CaO
grossularspinel

R10 : 2Ca2 Al2 SiO7 þ Ca2 MgSi2 O7ðsÞ þ 2SiO2 þ Al2 O3 →gehlenite−a kermanite ss

2Ca3 Al2 Si3 O12ðsÞ þ MgAl2 O4ðsÞ
grossularspinelð10Þ

The released calcium (reactions (R) 4, 7, 8, and 9)
could have precipitated as a layer of Ca,Fe-rich silicates(salite-hedenbergite pyroxenes, andradite and wollastonite) outside and inside fine-grained rims surrounding altered CAIs (see below).
4.5.2 Alteration of åkermanitic meliliteÅkermanitic melilite in Type B2s, FoBs, cores of B1s,
and CTA CAI TS-2 is replaced by three major secondary
minerals assemblages: (i) grossular + monticellite +Fig. 14 Aluminum-magnesium isotope diagram of primary melilite in TS-34 (B1) and secondary grossular in TS-34, AJEF (B1), A-39 (B2), and TS-21(B2). All melilite analyses show 26Mg* corresponding to the initial 26Al/27Al ratio, (26Al/27Al)0, of (5.4±0.3)×10−5, which is indistinguishable from thecanonical (26Al/27Al)0 of 5.25×10−5. Grossular grains in nearly monomineralic veins crosscutting gehlenitic melilite mantle in TS-34 show resolvable26
Mg* and plot along Al-Mg isochron defined by melilite; both minerals have similar 27Al/24Mg ratios. Coarse grossular grains replacingåkermanitic melilite and associating with monticellite, ±wollastonite, ±Al-diopside, and ±forsterite show no resolvable 26Mg*; Al-Mg isochron with
(26Al/27Al)0 of 1.7×10−6 is shown for reference. We infer that replacement of melilite by grossular took place after nearly complete decay of 26Al;
26
Mg* in the TS-34 grossular replacing gehlenitic melilite was inherited by the melilite and provide no chronological information

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

wollastonite (R11), (ii) grossular + monticellite (R12),and (iii) grossular + Al-diopside (R7); minor secondary
minerals include clintonite, forsterite, and spinel. Again,
the presence of clintonite and margarite (this study; Keller and Buseck 1991) in the secondary mineral assemblages requires the presence of H2O during alteration
reactions:Page 21 of 37

grossular + Al-diopside assemblages (R17). The pseudomorphs contain neither clintonite nor margarite, and aresurrounded by the virtually unaltered fine-grained
anorthite groundmass, suggesting the alteration may
have occurred under nearly dry conditions:
R16 : Ca2 Al2 SiO7ðsÞ þ 3Ca2 MgSi2 O7 þgehlenite−a kermanite ss

R11 : Ca2 Al2 SiO7ðsÞ þ 3Ca2 MgSi2 O7 þ SiO2 →gehlenite−a kermanite ss

2CaAl2 Si2 O8ðsÞ → 3Ca3 Al2 Si3 O12ðsÞ þ
anorthitegrossularmonticellite

grossular

CaMgSiO4ðsÞ þ Mg2 SiO4ðsÞ

Ca3 Al2 Si3 O12ðsÞ þ 3CaMgSiO4ðsÞ þ 2CaSiO3ðsÞ
wollastonite

monticellite

forsterite

ð16Þ

ð11Þ
R12 : Ca2 Al2 SiO7ðsÞ þ 2Ca2 MgSi2 O7 →gehlenite−a kermanite ss

R17 : Ca2 Al2 SiO7ðsÞ þ 2Ca2 MgSi2 O7 þgehlenite−a kermanite ss

Ca3 Al2 Si3 O12ðsÞ þ 2CaMgSiO4ðsÞ þ CaO
grossular

CaAl2 Si2 O8ðsÞ þ SiO2 → Ca3 Al2 Si3 O12ðsÞ þ

monticellite

anorthite

ð12Þ

grossular

2CaMgSi2 O6 þ CaAl2 SiO6ðsÞ þ CaO
diopside−Ca−Tschermakite ss

R13 : Ca2 Al2 SiO7ðsÞ þ Ca2 MgSi2 O7 þ SiO2 →ð17Þ

gehlenite−a kermanite ss

R18 : Ca2 Al2 SiO7ðsÞ þ Ca2 MgSi2 O7 þ

Ca3 Al2 Si3 O12ðsÞ þ CaMgSiO4ðsÞ
grossularmonticellite

gehlenite−a kermanite ss

ð13Þ
In some cases, grossular + monticellite + wollastonite
assemblages are subsequently replaced by Al-diopside(Fig. 9a-d):

CaAl2 Si2 O8ðsÞ þ SiO2 → Ca3 Al2 Si3 O12ðsÞ þ
anorthitegrossular

2CaMgSi2 O6 þ CaAl2 SiO6ðsÞ
diopside−Ca−Tschermakite ss

ð18Þ

R14 : Ca3 Al2 Si3 O12ðsÞ þ CaMgSiO4ðsÞ þ CaSiO3ðsÞ →grossularmonticellite

wollastonite

These elements may have been also used to form alayer of Ca,Fe-rich silicates (salite-hedenbergite pyroxenes, andradite, and wollastonite) outside fine-grainedrims around CAIs (see below).

The pseudomorphically replaced lacy melilite grains
are surrounded by a thin discontinuous rim of Na-richmelilite (Fig. 8b, d). Because the primary åkermanitic
melilite contains 0.1-0.3 wt% Na2O (Table 1), it is possible that sodium in the Na-rich melilite may have had alocal origin, i.e., it may be a byproduct of alteration of
the primary Na-bearing åkermanitic melilite. The preferential occurrence of Na-melilite along edges of lacy
melilite pseudomorphs may reflect normal igneous zoning of the lacy melilite grains: a positive correlation between Na2O and åkermanite content in melilite wasdescribed in many igneous CV CAIs (e.g., Beckett and
Grossman 1988; Beckett et al. 2000; Simon and Grossman 2006). A similar origin could be inferred for Namelilite in all Allende containing Na-bearing igneousåkermanitic melilite (Fig. 7).

4.5.3 Alteration of lacy melilite

4.5.4 Alteration of primary and secondary anorthite

In contrast to the preferential replacement of coarseåkermanitic melilite grains in Type B and FoB CAIs, inType Cs, lacy melilite grains composed of åkermanitic
melilite and anorthite are pseudomorphically replacedby the grossular + monticellite + forsterite (R16) and by

The occurrences of primary igneous anorthite dependon the CAI types: In type B1 CAIs, it occurs in the CAI
cores; in Type B2 CAIs, it occurs throughout the inclusions (Fig. 1g, k, m). Secondary anorthite occurs almost
exclusively in the peripheral regions of the Allende CAIs

CaMgSi2 O6ðsÞ þ CaAl2 SiO6ðsÞ þ 2SiO2 þ 3CaO
diopside−Ca−Tschermakite ss

ð14Þ
The released calcium and silica may have been used toform a new generation of wollastonite that either crosscuts the earlier formed secondary mineral assemblages
or fills the voids in the CAI interiors (Figs. 9 and 10a):R15 : CaO þ SiO2 → CaSiO3ðsÞ

ð15Þ

wollastonite

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

where it replaces gehlenitic melilite (Fig. 1). Primary andsecondary anorthites in the peripheral portions of the
Allende CAIs are largely replaced by nepheline, sodalite,
and ferromagesian olivine:
R19 : CaAl2 Si2 O8ðsÞ þ Na2 O→ 2NaAlSiO4ðsÞ þ CaO
anorthite

nepheline

ð19Þ
R20 : 3CaAl2 Si2 O8ðsÞ þ 3Na2 O þ 2NaCl→
anorthite

Na8 Al6 Si6 O24 Cl2ðsÞ þ 3CaO
sodalite

ð20Þ
The magnesium content in primary and secondaryanorthite is too low and iron is essentially absent, so thepresence of secondary ferromagnesian olivine associatedwith nepheline and sodalite requires an external sourceof these elements, perhaps matrix. We suggest that Mg,
Fe, and likely SiO2 were supplied to the host inclusions
by an external aqueous solution:
R21 : Mg2þ ðaqÞ þ Fe2þ ðaqÞ þ SiO2ðaqÞ
þ 2H2 OðaqÞ → ðFe; MgÞ2 SiO4ðsÞ þ 4Hþ ðaqÞð21Þfayalite−forsterite ss

Page 22 of 37

The fact that fine-grained rims lack sodalite may indicate an earlier formation of nepheline than sodalite. A
similar conclusion was reached by Che and Brearley(2021) based on the zoned distribution of nepheline andsodalite within Allende CAIs.
Wark (1981) interpreted the presence of alkali-richhalos around altered Allende CAIs as an evidence foroutward diffusion of volatiles and suggested that the alteration occurred in the solar nebula prior to incorporation of CAIs into the Allende meteorite. We conclude
instead that this interpretation is incorrect and cannot
be used as evidence for nebular alteration.
Calcium released during replacement of gehlenitic
melilite by anorthite (R4) and subsequently by nepheline
and sodalite (R19, R20) may have interacted with Fe-,Mg- and Si-enriched fluid that most likely was in equilibrium with the Allende fine-grained matrix (Krot et al.
2001b; Ganino and Libourel 2017). This process resulted
in precipitation of Ca,Fe-rich silicates at the boundary
between fine-grained rims and the Allende matrix (Figs.
1 and 2) and, occasionally, within fine-grained rims (Fig.
2b):
R23 : Ca2þ ðaqÞ þ Fe2þ ðaqÞ þ 2SiO2ðaqÞ
þ 2H2 OðaqÞ → CaFeSi2 O6ðsÞ þ 4Hþ ðaqÞð23Þhedenbergite

4.6 Zoned distribution of Na- and Cl-bearing minerals and
Ca,Fe-rich silicates in and around Allende CAIs: Evidence
for in situ open-system metasomatic alteration of the
Allende CAIs

The observed depletion in calcium and enrichment in silicon in altered gehlenitic melilite regions, the presence of a
Ca,Fe-rich silicate layer outside fine-grained rims aroundaltered CAIs, and the zoned distribution of Na- and Clbearing minerals (nepheline, sodalite, and wadalite) in andaround Allende CAIs (Fig. 1) provide clear evidence for an
open-system behavior of several elements (Si, Na, Cl, Ca,
and Al) during in situ metasomatic alteration of these
CAIs: Si, Na, and Cl were added, whereas Ca and, probably, some Al were removed from the inclusions. The mobility of aluminum during the alteration is consistent withthe presence of grossular, kushiroite, and Na-rich melilite
in veins crosscutting primary anorthite grains inside theinclusions (e.g., Fig. 7f). The outward transport of dissolved Al from CAIs and the influx of Na, K, and Si withan aqueous fluid equilibrated with the Allende matrix
could explain the abundant nepheline grains in finegrained rims around Allende CAIs studied (Figs. 1 and 2):
R22 : Naþ ðaqÞ þ Al3þ ðaqÞ þ SiO2ðaqÞ
þ 2H2 OðaqÞ → NaAlSiO4ðsÞ þ 4Hþ ðaqÞnepheline

ð22Þ

R24 : 3Ca2þ ðaqÞ þ 2Fe3þ ðaqÞ þ 3SiO2ðaqÞ
þ 6H2 OðaqÞ → Ca3 Fe2 Si3 O12ðsÞ þ 12Hþ ðaqÞandraditeð24Þ
R25 : Ca2þ ðaqÞ þ SiO2ðaqÞ
þ H2 OðaqÞ → CaSiO3ðsÞ þ 2Hþ ðaqÞð25Þwollastonite

R26 : Ca2þ ðaqÞ þ 0:5Fe2þ ðaqÞ þ 0:5Mg2þ ðaqÞþ 2SiO2ðaqÞ
þ 2H2 OðaqÞ → CaðFe; MgÞSi2 O6ðsÞ þ 4Hþ ðaqÞsalite−hedenbergite ss

ð26Þ
The preferential precipitation of Ca,Fe-rich silicatesoutside fine-grained rims may be due to a relatively
compact nature of these rims compared to the boundaries between rims and matrix, where a geochemical barrier between fluids of different compositions, Ca-richand Fe,Mg,Si-rich, which were in equilibrium with thehost CAIs and surrounding matrix, respectively, could
have been generated.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

4.7 Evidence for multistage metasomatic alteration of the
Allende CAIs and variations in chemical composition of
the metasomatic fluids

Our mineralogical observations indicate that metasomatic alteration of the Allende coarse-grained CAIs
was multistage and resulted in the formation of multiple
generations of secondary minerals. These include (i) replacement of grossular + monticellite + wollastonite
mineral assemblages by Al-diopside (Fig. 9a−d), (ii) wollastonite veins crosscutting grossular + monticellite +wollastonite mineral assemblages (Fig. 9e, f), (iii) crosscutting relationships between grossular-rich veins andveins containing abundant iron-rich minerals (andradite,hedenbergite, ferroan Al-diopside, ferroan monticellite,
pentlandite, and heazlewoodite) (Fig. 11); (iv) overgrowth of monticellite by ferroan monticellite (Fig. 10i);(v) overgrowth of Al-diopside by ferroan grossular (Fig.
10g); (vi) overgrowth of grossular by ferroan grossularand andradite (Figs. 3 and 10g-j and h); (vii) oscillatory
zoning of grossular and Al-diopside grains (Fig. 10b-h);(viii) replacement of secondary anorthite by nepheline,
sodalite, and ferromagnesian olivine (Figs. 1 and 6c, d).
In most cases, the later formed secondary minerals contain higher iron content than the earlier ones. These observations suggest that alteration occurred in thepresence of aqueous solutions of variable chemicalcompositions.
4.8 Physico-chemical analysis of alteration reactions
inferred in Allende CAIs
4.8.1 Transport of elements in aqueous solution and steam
during metasomatic alteration of Allende CAIs

Mineralogical and petrographic observations "Petrography and primary mineralogy of the Allende coarsegrained igneous CAIs", chemical compositions of secondary phases, and the inferred chemical reactions"Secondary mineral assemblages in the Allende CAIs
and inferred metasomatic reactions", "Zoned distribution of Na- and Cl-bearing minerals and Ca,Fe-richsilicates in and around Allende CAIs: Evidence for insitu open-system metasomatic alteration of the
Allende CAIs", "Evidence for multistage metasomatic
alteration of the Allende CAIs and variations inchemical composition of the metasomatic fluids" suggest the involvement of water in metasomatic alteration of CV chondrites and their components (seealso Brearley and Krot 2013; Ganino and Libourel2017). The altered patches and veins in mantles andcores of the Allende coarse-grained CAIs are dominated by Ca, Al, Mg-rich phases which are enrichedin SiO2 and depleted in CaO compared to the primary phases. Such mineralogy requires an efficient
addition of Si to and removal of Ca from CAIs. Most
secondary ferromagnesian silicates in CAI interiors—

Page 23 of 37

forsterite, monticellite, and Al-diopside—are essentially Fe-free compared to the ferroan olivine, kirschsteinite, and salite-hedenbergite in fine-grained rims
and neighboring matrix. It appears that for some reason the Fe, abundant in an aqueous solution circulating through the Allende matrix (Krot et al. 1998a,
1998b; Ganino and Libourel 2017), has not been efficiently delivered to the interiors of the Allendecoarse-grained igneous CAIs. The same may be true
for Na and Cl which are predicted to have high concentrations in an aqueous solution equilibrated withmatrix (Petaev and Moronenko 1997), but in wadalite
replacing åkermanitic melilite in Type B CAIs, theseelements are decoupled from each other.
While the source of water on chondritic parent
bodies remains speculative, the accretion of H2O-richices alongside other nebular components is considered
the most plausible mechanism (Krot et al. 2015).
Upon subsequent heating, either internal or impactinduced, the ices sublimate and/or melt to generate
H2O steam or liquid water or, most likely, both. The
liquid water and steam would react with thesurrounding minerals and amorphous phases andleach out some chemical elements to transform
initially nearly pure water to an aqueous solution withconcentrations of SiO2, Fe2+, Mn2+, Ca2+, Mg2+, Na+,
Cl−, HCO3−, and other ions and neutral aqueous
species gradually increasing with time. Such asolution is capable transporting of Ca, Mn, Fe, Si, Na,
and Cl at low temperatures. The ability of steam totransport at least some of these elements depends
upon P-T conditions. The numerous experimental
and natural data suggest that in terrestrial hydrothermal environments a supercritical fluid (T > 647 K, P
> 22 MPa) can efficiently transport these elements
too, but the estimates of the Allende tensile strength
imply that a gas pressure in excess of ~ 10 MPashould break the rock and release the gas (Grimm
and McSween Jr. 1989). Thus, the presence ofsupercritical fluid on the CV parent body seems unlikely. Still, a low-density steam, either subcritical saturated (PH2O = PH2O, equil, T < 647 K) or dry
superheated (PH2O < PH2O, equil, T > 647 K), canspread out of the aqueous solution source region andcarry chemical species dissolved in it. Such a steam
seems to be of low interest for geochemists, but it isa source of great concern in the steam power industry because it results in the deposition of solids on
turbine blades and corrodes them.
A classic review of experimental data on concentrations of different solutes in subcritical and dry superheated steam (Harvey and Bellows 1997) shows thatsignificant amounts of SiO2, NaCl, NaOH, Na2SO4,
Na3PO4, and Fe oxides orders of magnitude higher

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

than would be expected on their vapor pressures, canbe dissolved in steam in the form of solvated complexes (e.g., SiO2•mH2O). The concentration of a solute (Xs) in the steam depends upon temperature (T),pressure (P), and steam density (ρ) as described byempirical equationsln ðXs Þ ¼ A þ B=T þ ðm1 þ m3 =TÞ ln ρ−lnPð27Þ

ln ðXs Þ ¼ A þ D  Tþ ð m 1 þ m2  ρ  m 3  T Þ ln ρ−lnP

ð28Þ

or

where A, B, D, m1, m2, and m3 are regression coefficients specific for each solute. Because of generally low
distribution coefficients of these solutes between a “parental” aqueous solution and the steam equilibrated withit, subsequent condensation of such a steam would produce a “secondary” aqueous solution containing much
lower concentrations of solutes than its “parental” solution. The “secondary” solution should be essentially freeof solutes which cannot be efficiently transported by thesteam. Because of the low solubility of iron oxides inwater under oxidizing conditions, the equilibrium concentrations of iron oxides in the steam are negligible.
Calcium concentrations in steam are also expected to be
low because of very low distribution coefficients of
CaCl2 and CaSO4 between an aqueous solution andsteam derived from it (e.g., Svoboda 2006). Apparently,
neither Fe nor Ca can be efficiently transported by thesteam.
4.8.2 Physico-chemical analysis of alteration reactions

The inferences about the involvement of an aqueous solution or H2O steam or both in the alteration process
can be made based on a physicochemical analysis of theobserved and/or possible alteration reactions and the
textural relationships and mineral crystallization sequences in altered refractory inclusions. The texturesand mineral chemistry of altered areas leave no doubtthat an aqueous alteration was a non-equilibrium, multistage process, with many observed reactions not going
to completion (see Alteration of primary and secondary
minerals in the Allende CAIs). Given the multitude ofsuch non-equilibrium effects, considering them would
make the problem intractable. Instead, we employ here
an equilibrium approach in evaluating intensive parameters of the observed alteration reactions assuming thatthey have reached a steady state. Then, in order toevaluate the plausibility of these reactions, the calculated
values of fugacities and activities of different gaseous
and aqueous species for each reaction are compared

Page 24 of 37

with each other and with independent estimates of theseparameters. The thermodynamic data for solid, gaseous,and aqueous species are taken from compilations ofRobie and Hemingway (1995) and Oelkers et al. (1995),
except for the gaseous H4SiO4 (Plyasunov 2011) andsodalite (Komada et al. 1995). The gaseous and solid solutions are treated as ideal. For the sake of temperatureinterpolation, the tabulated data were fitted with polynomials, with the deviation between fitted and tabulated
data being typically < 0.1%. For the aqueous species, wehave adopted the lowest pressure (0.004–16.55 MPa)tabulation set for the 25–350 °C temperature range, withthe pressure at any given temperature being equal to thepartial pressure of the saturated H2O vapor; i.e., the gaseous phase involved in alteration reactions is nearly pure
H2O steam equilibrated with an aqueous solution at any
given temperature.
For the same conditions, we calculated maximum
concentrations of solutes in the steam and the aqueous solution formed by its condensation (Fig. 15),
using equations and regression parameters of Harvey
and Bellows (1997). The rather high concentrations of
SiO2, Na, and Cl in both the steam and aqueous solution, especially at higher temperatures and pressures,
imply that these elements potentially can be supplied
to the CAIs from an aqueous solution equilibrated
with matrix without the addition of Fe and Ca insoluble in steam. This may explain the Fe-rich and Fepoor secondary mineralogy in the matrix and refractory inclusions, respectively. However, because both
Na and Cl can be efficiently transported by the steam,they are not expected to decouple unless steam
formed at an early stage of alteration contains only
Cl-bearing species such as HCl and NH4Cl whichmight have been present in accreted ices (e.g., Zolotov
and Mironenko 2007).
Because SiO2, CO2, Na, and Cl can be transported bysteam, the reaction involving these elements do not
require an aqueous solution to be in direct contact withthe solid phases, while the reactions involving Ca, Fe,
and Mg need an aqueous solution for addition and/or
removal of these elements. The inferred alteration reactions are divided into several groups based on chemicalelements added to and/or removed from the primarymineral assemblages of CAIs: (1) SiO2 only, (2) CO2
only, (3) SiO2, CO2, and Ca, (4) Na±Cl and Ca±SiO2, (5)Fe, Mg, Ca, SiO2.
The physico-chemical conditions of reactions describing the formation of Fe, Mg, Ca-bearing silicates during
aqueous alteration have been previously evaluated by us(Krot et al. 1998b, 2001b) and are not considered here.
They should be fully consistent with the current work
because we use essentially the same approach andthermodynamic databases.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 25 of 37

Fig. 15 Maximium solubility of different solutes in the saturated H2O steam (solid lines) and the aqueous solution formed by its condensation(dashed lines)

To evaluate whether an observed alteration reaction
can proceed via the gaseous phase only or requires a direct contact with an aqueous solution, one has to write
appropriate reaction equations, calculate their intensive
parameters, and compare them with each other and independent data if such exist. For example, reaction 7 canbe described by the following equations:
R7g : 2Ca2 Al2 SiO7ðsÞ þ Ca2 MgSi2 O7ðsÞ
þ 2H4 SiO4ðgÞ þ 4CO2ðgÞ
¼ Ca3 Al2 Si3 O12ðsÞ þ CaMgSi2 O6ðsÞþ CaAl2 SiO6ðsÞ þ Ca2þ ðaqÞ
þ 4HCO3 − ðaqÞ þ H2ðgÞ þ H2 OðaqÞ

lgK7g ¼ lg½XDi  þ lg½XCts  þ lg Ca2þ þ 4 lg½HCO3 − hþlgPH2 - 2lgPH4SiO4 - 2 lg½XGel  - lg XA k - 4lgPCO2

R7aq : 2Ca2 Al2 SiO7ðsÞ þ Ca2 MgSi2 O7ðsÞ
þ 2SiO2ðaqÞ þ 4CO2ðgÞ þ 3H2 OðaqÞ
¼ Ca3 Al2 Si3 O12ðsÞ þ CaMgSi2 O6ðsÞþ CaAl2 SiO6ðsÞ þ Ca2þ ðaqÞ
þ 4HCO3 − ðaqÞ þ H2ðgÞ

ð29Þ
ð30Þ

ð31Þ



lgK7aq ¼ lg½XDi  þ lg½XCTs  þ 2 lg Ca2þ
þ4 lg½HCO3 −  - 2 lg½SiO2  - 2 lg½XGel h- 2 lg XA k - 4lgP
CO2

ð32Þ
where subscripts s, g, and aq denote solid, gaseous,and aqueous species, respectively; XDi, XCTs, XGel, andXÅk are mole fractions of diopside, Ca-Tschermakite,gehlenite, and åkermanite, respectively, in Al-diopsideand melilte solid solutions; PH2, PH4SiO4, and PCO2 arepartial pressures of H2, H4SiO4 and CO2 in the gaseous
phase dominated by H2O; and [Ca2+], [HCO3−], and[SiO2] are activities of ions and neutral species in the
aqueous solution. Note, that Si and C in the reaction
R7g are supplied via gaseous phase, but the removal of
Ca requires the presence of an aqueous solution; i.e., theH2O steam must condense to be able to dissolve the released Ca and drain it away from the reaction zone. The
presence of calcite in some Allende CAIs (Fig. 12c) suggests that PCO2 in the gaseous phase was high enough to
produce carbonic acid upon steam condensation, making
the HCO3− main anion of the aqueous solution. Similar
reaction equations can be written for reactions 4−20.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

The results of modeling are shown in Figs. 16, 17, 18,
and 19.
The reactions involving alteration of melilite (R5, R11,
R12) by addition of either gaseous H4SiO4 or aqueous
SiO2 (Fig. 16) can proceed forward at any temperatureand pressure because the required H4SiO4 fugacities insteam and activities of SiO2 in aqueous solution are either much lower than or comparable with the solubilityof SiO2 in the steam or the aqueous solution formed byits condensation.
Of special value here are the reactions describing possible alteration of nepheline to analcime by addition of
SiO2 and ±H2O:
R27aq : NaAlSiO4ðsÞ þ SiO2ðaqÞ þ H2 OðaqÞ
¼ NaAlSi2 O6  H2 OðsÞð33ÞlgK27aq ¼ - lg½SiO2 

ð34Þ

R27g : NaAlSiO4ðsÞ þ H4 SiO4ðgÞ
¼ NaAlSi2 O6  H2 OðsÞ þ H2 OðgÞð35ÞlgK27g ¼ lgPH2O - lgPH4SiO4 ¼ −lgXH4SiO4

ð36Þ

Experimental studies at T−PH2O conditions identicalto considered here (Wilkin and Barnes 1998) showedthat analcime is a stable phase, which can be dissolved
or replaced by other phases if activities of SiO2 and cations in the aqueous solution change. Therefore, the lackof analcime in the CV chondrites effectively places anupper limit on the H4SiO4 fugacity in the steam (Fig. 16,
top panel) and SiO2 activity in aqueous solution (Fig. 16,
bottom panel) implying that during CAIs’ alteration, neither steam nor aqueous solution was SiO2-saturated. Because the upper limit of aqueous SiO2 activity is 3−6
orders of magnitude higher than that in the aqueous solution made by in situ condensation of steam (Fig. 16,
bottom panel), a solution with a higher concentrationof SiO2, tentatively labeled as “external” aqueous solution, can be involved in CAI’s alteration. Such a solution must come to a CAI from outside (matrix?) as aliquid (Krot et al. 2001b; Ganino and Libourel 2017).
In principle, it may be the same aqueous solution,
which feeds the steam. Concentrations and ratios ofsolutes in such two solutions communicating witheach other via steam would differ substantially due todifferent partition coefficients for different solutes.
Another important chemical difference between these
two solutions is that the “condensed steam” solution
is expected to be free of Fe, Ca, and other elements,
which are not efficiently transported by the steam.
The lower limit on the H4SiO4 fugacity and SiO2 activity is set by reactions R11 and R5, respectively,
which are widespread in the Allende coarse-grained
igneous CAIs.

Page 26 of 37

The phase equilibria involving loss or gain of CO2:
R28 : 3CaCO3ðsÞ þ2CaSiO3ðsÞ ¼ Ca5 Si2 O7 ðCO3 Þ2ðsÞ þ CO2ðgÞ
calcitewollastonite

tilleyite

ð37Þ
R29 : Ca5 Si2 O7 ðCO3 Þ2ðsÞ ¼ Ca5 Si2 O8 ðCO3 ÞðsÞ þ CO2ðgÞ
tilleyitespurrite

ð38Þ
R30aq : CaCO3ðsÞ þ SiO2ðaqÞ
¼ CaSiO3ðsÞ þ CO2ðgÞ

ð39Þ

R30g : CaCO3ðsÞ þ H4 SiO4ðgÞ
¼ CaSiO3ðsÞ þ CO2ðgÞ þ 2H2 OðgÞð40Þrequire rather low mole fraction of CO2 in the steam(Fig. 17), which seems to be reasonable for a Cbearing system such as carbonaceous chondrites. The
occurrence of wollastonite, tylleyite, and calcite within
the same CAI (All-2; Fig. 12a−c) gives a reasonableestimate of mole fractions (XCO2) and partial pressures (XCO2×PH2O,sat) of CO2 in the steam as well asactivities of the HCO3− ion in aqueous solution. At
such conditions, the observed replacement of meliliteby grossular + monticellite + forsterite (R16) canreadily proceed, providing the released CaO reacts
with CO2 to form calcite, regardless of whether anaqueous solution saturated in calcite is present ornot.
Many alteration reactions require the addition of
CO2±SiO2 accompanied by removal of Ca±SiO2. Whileboth CO2 and SiO2 can be added or removed in either
steam (bottom panels) or aqueous solution (top panels),the very low solubility of Ca in steam requires the presence of an aqueous solution, which would absorb the released Ca as either dissolved Ca bicarbonate, Ca2+ +2HCO3− (Fig. 18) or calcite (Fig. 17). If SiO2 is deliveredin the steam, the latter must condense to form an aqueous solution, initially Ca-free, before alteration reactions
can proceed. In such a case, the amount of aqueous solution is expected to be rather small, with its Ca absorption capacity being limited. The Ca concntration in the
aqueous solution will affect the sequence of alteration
reactions, with the alteration of gelenitic melilite toanorthite being the most sensitive to the Ca buildup. If
such aqueous solution was not effectively drained before
becoming calcite saturated, one would expect to see
widespread calcite associated with altered silicates.
While calcite does present at least in some altered
Allende CAIs, it tends to occur in nodules hinting for effective removal of Ca2+ and HCO3− with drained solution, which can accumulate in cavities and fill them withcalcite and other CO2-bearing minerals upon evaporation of water to form nodules.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

The formation of nepheline and sodalite (Fig. 19)
requires an addition of Na and Cl and removal of Ca(R19, R20) and addition or removal of SiO2 (R31,
R32):

Page 27 of 37

R31aq : 2Ca2 Al2 SiO7ðsÞ þ 3SiO2ðaqÞ þ 2Naþ ðaqÞ
¼ Ca3 Al2 Si3 O12ðsÞ þ 2NaAlSiO4ðsÞ
þþ Ca2 ðaqÞð41ÞFig. 16 Phase boundaries of reactions involving either aqueous (top panel) or gaseous (lower panel) silica. The lack of analcime among alteration
products places an upper limit on XH4SiO4 or XSiO2 in steam and aqueous solution, respectively. In the lower panel, the “condensed steam” field
designates an aqueous solution formed by in situ condensation of steam as opposed to the “external” aqueous solution, which comes to a CAI
from outside (matrix?) as a liquidKrot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 28 of 37

Fig. 17 Phase boundaries of reactions involving CO2 (R16, R28, R29 - black lines) and aqueous SiO2 (R6, R7, R14, R30 - blue lines) at maximal(XSiO2,max defined by R27 - top panel) and minimal (XSiO2,min defined by R5 - bottom panel) values of SiO2 activity

R32aq : 2CaAl2 Si2 O8ðsÞ þ 2Naþ ðaqÞ þ 2CO2ðgÞ
þ H2 OðaqÞ
¼ 2NaAlSiO4ðsÞ þ Al2 O3ðsÞ þ 2Ca2þ ðaqÞ
þ 2HCO3 − ðaqÞ þ 2SiO2ðaqÞð42Þ

Na, Cl, and SiO2 can be transported by either steam
or aqueous solution, but the necessity to remove Ca
requires the presence of an aqueous solution. As discussed above, the latter can be formed either bysteam condensation or arrive from the matrix. The

substitution of anorthite by sodalite at higher temperatures requires a solution with rather low Na (andCl) concentrations that can be formed by the steam
condensation. At lower temperatures, an “external”
aqueous solution with higher Na and Cl concentration is required. Other reactions can proceed only inthe presence of an “external” aqueous solution withhigh concentrations of Na and Cl. It is interesting tonote that the presence of widespread Cl-bearing and
Na-free wadalite in many altered Type B CAIs points
to the decoupling of Na and Cl during at least theKrot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 29 of 37

Fig. 18 Phase boundaries of reactions involving addition of either aqueous SiO2 or gaseous H4SiO4 and removal of Ca in the form of dissolved
Ca bicarbonate, Ca2+ + 2HCO3−, at XCO2 defined by R28 (wollastonite-calcite-tilleyite equilibrium) and different values of SiO2 activity and H4SiO4fugacity defined by reactions R5 and R11 (Fig. 19). In each plot, the labeled curves of different colors separate the “primary” (above the curves)and “secondary” (below the curves) assemblages which correspond to the reactants and products of each reaction, respectively. The gray calcite
saturation line separates two different phase assemblages containing silicates + calcite + aqueous solution above the line and silicates + aqueous
solution below it. The tilleyite and spurrite saturation curves show conditions when a mineral starts to crystallize from an aqueous solution
according to a reaction Ca2+ + HCO3− + SiO2 → xCaO∙ySiO2∙zCO2 + CO2(g) + H2O(g)Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 30 of 37

Fig. 19 Phase boundaries of reactions R19, R20, R31 and R32 involving addition of Na+ and removal of Ca2+ at the XCO2 defined by R28, lowest
XCa2+ defined by R4 (melilite-anorthite equilibrium – top left panel of Fig. 20). The activity of aqueous SiO2 in reactions R31 and R32 is set at theupper limit of XSiO2 defined by R27. The “condensed steam” field designates an aqueous solution formed by in situ condensation of steam as
opposed to the “external” aqueous solution, which comes to a CAI from outside (matrix?) as a liquid

early stages of aqueous alteration. In this case, the Cl
could be supplied by the steam containing HCl and
NH4Cl but neither NaCl nor NaOH.
4.9 Oxygen isotopic composition of metasomatic fluid on
the CV chondrites parent asteroid

Most secondary minerals measured for oxygen isotopic
composition in the Allende Type B and FoB CAIs (TS-34,
TS-31, AJEF, TS-21, and All-2) replace primary melilite andto a lesser degree anorthite. These primary minerals are typically 16O-depleted relative to the minerals largely unaffectedby secondary alteration – spinel, hibonite, forsterite, andmost Al,Ti-diopside grains (very Ti-rich fassaite in TS-34 is
the only exception): Δ17O range from -5 to -1‰ vs. ~ -25 to-20‰, respectively. Larger variations in Δ17O (up to -10‰)
are observed in anorthite of TS-34 (Fig. 13a, b). In TS-31,
AJEF, TS-21, and All-2, secondary minerals plot along massdependent fractionation line with Δ17O of ~ −3±2‰ andshow a much smaller range of δ18O, from ~ 0 to ~ + 10‰,compared to the previously published data, from ~ −15 to +20‰ (Fig. 13b, g) acquired without the use of proper mineral standards. Because metasomatic alteration of Allende
CAIs occurred in the presence of aqueous solution/fluid andsome minerals directly precipitated from it, we infer that
Δ17O of the secondary minerals recorded Δ17O of the fluid.
This conclusion is consistent with similar Δ17O (-1.5±1‰)of aqueously formed fayalite and magnetite in the oxidized
CV3.1 chondrite Kaba (Doyle et al. 2015).
Assuming that oxygen-isotope composition of the fluid
was constant and that secondary minerals were in isotopic equilibrium with it, δ18O of the minerals withknown water-mineral isotope fractionation can be used
to constrain the temperature of alteration (Zheng 1991,
1993). Because O-isotope fractionation between mineral
and water decreases with temperature increase (Fig. 20),
the relatively small range of δ18O of the secondary minerals, ~ 10‰, which is much smaller than those between
aqueously formed magnetite and carbonates in CI, CM,
and CR chondrites (up to ~30‰; see Fig. 16 in Krotet al. 2015), is consistent with a relatively hightemperature of the CV metasomatic alteration, > 200°C,
compared to those of CIs, CRs, and CMs, < 100°C (Krotet al. 2015 and references therein). This is also consistent with a small range of δ18O, ~ 5‰, between
aqueously formed magnetite and pure fayalite in theleast metamorphosed CV chondrite Kaba, which

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 31 of 37

Fig. 20 Calculated oxygen-isotope fractionations for anhydrous silicate-water and magnetite-water as a function of temperature (Zheng 1991, 1993)

apparently experienced lower temperature metasomatic
alteration than Allende (Krot et al. 2018). We note, however, that our petrographic observations indicate that thechemical composition of the Allende fluid evolved withtime and alteration was probably multistage (see above).
Therefore it seems highly unlikely that the O-isotope
composition of the fluid remained constant. As a result,the entire range of δ18O among secondary minerals
measured, ~ 10‰, cannot be used to constrain thetemperature of alteration of Allende CAIs: data for Oisotope compositions of secondary minerals that appear
to have formed contemporaneously within an individual
CAI could be more appropriate. Unfortunately, only alimited number of secondary minerals within individual
CAIs have been measured with matrix-matched standards by now (Fig. 13c), and no special attention has
been to whether the secondary minerals analyzed arecogenetic. To illustrate the latter point, in Fig. 21 a andb, we plotted O-isotope compositions of secondary minerals in two Allende CAIs, TS-31 and All-2. Although
the observed range of δ18O between grossular and wollastonite replacing melilite in these inclusions is rather
limited, 4−6‰, the secondary minerals are apparently
not in isotopic equilibrium (there are some differencesin Δ17O) and may be not cogenetic. Therefore, thesedata cannot be used for estimating a temperature ofmetasomatic alteration as well.
Larger ranges of Δ17O are observed in secondary grossular and forsterite in the Type C CAIs 100 and 160(from ~ −15 to ~ −1‰) and Type B1 CAI TS-34 (from~ −10 to ~ −1‰). In contrast to other CAIs measured,grossular and forsterite in these inclusions plot along

CCAM line (Fig. 21c, d). The similar ranges of Δ17O andpositions on a three-isotope oxygen diagram are observed for primary anorthite in these CAIs. Melilite is
the most 16O-depleted primary phase in these inclusions(Δ17O ~ −5 to ~ −1‰). We suggest that grossular andforsterite in TS-34, 100, and 160 could have largely
inherited oxygen isotopic compositions of the primary
melilite and anorthite which experienced incomplete Oisotope exchange with the fluid phase (see Role of aqueous fluid–rock interaction in O-isotope exchange in primary CAI minerals). In the case of Type C CAIs, thismight be due to alteration under relatively dry conditions (see Alteration of lacy melilite).
4.10 Role of aqueous fluid–rock interaction in O-isotope
exchange in primary CAI minerals

Oxygen isotopic composition of the CV aqueous fluid
inferred from secondary minerals (Δ17O ~ −3±2‰) isvery similar to the most 16O-depleted compositions ofprimary melilite, anorthite, and very Ti-rich fassaite in
the isotopically heterogeneous coarse-grained igneous
CAIs from Allende studied (Fig. 13). Hibonite, spinel,
forsterite, and most fassaite grains in these CAIs have16
O-rich, solar-like compositions. A similar O-isotope
heterogeneity is observed in all types of igneous CAIs
(CTA, B, FoB, and C) from metamorphosed CV chondrites, irrespective of mineral crystallization sequenceand evaporation history (Fig. 13c, e, f). Because the vast
majority of refractory inclusions in unmetamorphosedcarbonaceous chondrites (CM2, CR2, CH3.0, CO3.0, andAcfer 094) have uniform solar-like oxygen isotopic compositions (Δ17O ~ −24‰; Yurimoto et al. 2008; Makide

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Page 32 of 37

Fig. 21 Three-isotope oxygen diagrams on primary melilite and anorthite, and secondary andradite, calcite, grossular, sodalite, and wollastonite in
the Allende CAIs TS-31 (B2), All-2 (FoB), TS-34 (B1), and 160 (C). Grossular and wollastonite (yellow circles) replace melilite and appear to haveformed contemporaneously. Andradite, calcite, and wollastonite in veins (black-and-yellow circles) crosscutting the grossular±wollastonite-bearing
assemblages precipitated directly from aqueous solutions. Primary melilite and anorthite are 16O-depleted relative to spinel, Al,Ti-diopside (except
TS-34), and forsterite; melilite is more 16O-depleted than anorthite. Secondary minerals have similar compositions to those of anorthite andmelilite. In TS-31 and All-2, secondary minerals plot along mass-dependent fractionation line slightly below the TF line (Δ17O ~ −3‰). Insets in “a”
and “b” show O-isotope compositions of grossular and wollastonite that appear to have formed contemporaneously by replacing primary
melilite; there is a small range of δ18O between these secondary minerals, 4 and 6‰ in TS-31 and All-2, respectively. In TS-34 and 160, secondary
grossular together with primary melilite and anorthite plot along CCAM line

et al. 2009; Ushikubo et al. 2017; Krot et al. 2020b), theseobservations may indicate that melilite and anorthite inAllende CAIs, experienced oxygen isotopic exchangewith an aqueous fluid on the CV parent asteroid,
whereas hibonite, spinel, Al,Ti-diopside, and forsterite
retained their original compositions. Since 16O-rich and16
O-depleted gaseous reservoirs appear to have coexisted in the early solar nebula (e.g., Itoh and Yurimoto
2003; Simon et al. 2011, 2016, 2019; Krot et al. 2020b)
and coarse-grained igneous CAIs experienced multiple
melting events (Krot 2019 and references therein), thisinterpretation does not preclude O-isotope exchange
during gas-melt interaction in the solar nebula of variable isotopic composition (e.g., Yurimoto et al. 1998;
Kawasaki et al. 2018). To understand potential role of
the CV aqueous fluid in O-isotope exchange,

experimental data on oxygen self-diffusion in CAI minerals under high temperature and wet conditions are
required.
4.11 Aluminum-magnesium isotope systematics of
secondary minerals in the Allende CAIs

Aluminum-magnesium isotope systematics of secondary
minerals in the Allende CAIs is often used as evidence
for a prolonged duration of the alteration that may havestarted in the solar nebula shortly after the formation of
CAIs, continued during chondrule formation, and on the
CV chondrite parent body (e.g., Ushikubo et al. 2007;
Fagan et al. 2007). This interpretation is based on the occasional presence of resolvable 26Mg* in some secondary
grossular and anorthite grains and its common absence inthe vast majority of secondary minerals measured.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

To test this interpretation, we measured Al-Mg isotope systematics in grossular grains replacing melilite ofdifferent chemical compositions (Fig. 14). Grossular in
the nearly monomineralic veins crosscutting gehlenitic
melilite mantle in the Type B1 CAI TS-34 has a similar27
Al/24Mg ratio to those of the host melilite (6.5-9 vs.
5.8-10; up to 20 in melilite); all grossular grains measured show resolvable 26Mg*. In contrast, grossular associating with monticellite, forsterite, and Al-diopside, andreplacing åkermanitic melilite (27Al/24Mg = 1.3-1.9) in
TS-34 and several other Type Bs has significantly higher
27
Al/24Mg (30-100) ratio and lacks resolvable 26Mg*(0.2±2‰ for TS-34); the latter is indistinguishable froman average 26Mg* in the åkermanitic melilite (0.7±0.4‰).
The high 27Al/24Mg ratio in these grossulars is probably
due to Al-Mg fractionation between the grossular andother cogenetic secondary minerals: most magnesium ispartioned into monticellite, forsterite, and Al-diopside.
The upper limit on timing of this fractionation, andtherefore, the timing of the metasomatic alteration is~ 3.5 Ma. Based on these observations, we conclude
that 26Mg* in grossular replacing both gehlenitic andåkermanitic melilite was largely inherited from melilite after nearly complete decay of 26Al, >3.5 Ma after
crystallization of primary minerals of their host CAIs.
The late-stage formation of grossular with and without resolvable 26Mg* is consistent with their similar
O-isotope compositions. It is also consistent with53
Mn-53Cr ages of the metasomatically formed fayalite
and kirschsteinite in the CV3 chondrites Asuka-881317
þ0:8(4:2þ0:8−0:7 Ma, Doyle et al. 2015) and Vigarano (3:2−0:7 Ma,
MacPherson et al. 2017), respectively.
Ushikubo et al. (2007) reported 26Mg* in coarse-grained
ragged anorthite surrounded by secondary grossular andmonticellite in the Allende Type B2 CAI 2 that corresponds to (26Al/27Al)0 of ~ (1.2±0.2)×10-5. No resolvable
26
Mg* was found in the grossular: (26Al/27Al)0 < 4.4×10-7.
Largely based on the unusual morphology of the anorthitegrains and their close association with grossular, Ushikubo
et al. concluded that the anorthite is secondary, suggestingthat alteration started 1.5 Ma after crystallization of CAIs
with the canonical (26Al/27Al)0 and lasted for several Ma.
Dr. T. Ushikubo kindly provided to us a polished sectionof this CAI. Our preliminary petrographic and mineralogical observations indicate that the ragged anorthite is infact the primary igneous phase. The supporting arguments
include: (i) the tripple junctions between ragged anorthite,
Al,Ti-diopside, and unaltered åkermanitic melilite suggesting co-crystallization of these minerals; (ii) the
presence of unaltered igneous inclusions of melilite andAl,Ti-diopside in some anorthite grains; (iii) the presenceof secondary Na-rich melilite corroding edges of anorthitegrains, as commonly observed in igneous anorthite in the

Page 33 of 37

Allende type Bs and FoBs (Fig. 7). We conclude that (1)secondary grossular, wollastonite, monticellite, forsterite,
and Na-melilite around anorthite grains in the CAI 2
replace åkermanitic melilite, and are not geneticallyrelated to anorthite; and (2) the low initial 26Al/27Al ratio
in ragged anorthite (Ushikubo et al. 2007) reflects thedisturbance of its Al-Mg systematics during thermal metamorphism as often observed in anorthite of many Allendeigneous CAIs (e.g., Kita et al. 2013; MacPherson et al.
2017). As a result, Al-Mg systematics of ragged anorthite
in the Allende CAI 2 cannot be used for dating the CV
metasomatic alteration.

5 Conclusions
1. All major types of coarse-grained igneous CAIs
(CTA, B1, B2, FoB, and C) from the oxidized CV3chondrite Allende experienced metasomatic alteration. This alteration affected mainly melilite and toa lesser degree anorthite, and resulted in the formation of different secondary minerals, including
adrianite, Al-diopside, andradite, anorthite, calcite,
celsian, clintonite, corundum, dmisteinbergite,
ferroan olivine, ferroan monticellite, ferroan Aldiopside, forsterite, grossular, heazlewoodite, hedenbergite, hutcheonite, kushiroite, margarite, monticellite, Na-melilite, nepheline, pentlandite,pyrrhotite, sodalite, spinel, tilleyite, wadalite, andwollastonite. The secondary mineral assemblages
are mainly defined by chemical compositions of the
primary melilite replaced:– Gehlenitic melilite (Åk<35) in CTAs, mantles ofB1s, and the outermost regions of other CAI
types is mainly replaced by anorthite andgrossular; clintonite, corundum, spinel, and Aldiopside are minor;– Åkermanitic melilite (Åk35-90) in Type B2s, FoBs,
cores of B1s, and CTA CAI TS-2 is replaced byseveral mineral assemblages: grossular +monticellite + wollastonite, grossular +monticellite, and grossular + Al-diopside; forsterite, spinel, clintonite, and Na-melilite are minor;– In type Cs, lacy melilite grains arepseudomorphically replaced by the grossular +forsterite + monticellite and grossular + Aldiopside mineral assemblages; Na-melilite is minor;– Primary and secondary anorthites in the
peripheral portions of Allende CAIs are replacedby nepheline, sodalite, and ferromagnesianolivine;– All types of Allende CAIs studied contain voids
and cracks filled by andradite, hedenbergite,
wollastonite, ±sodalite, ±grossular, ±monticellite,±tilleyite, and ±calcite.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

2. Allende CAIs are surrounded by fine-grained
matrix-like rims composed of the secondary lathshaped ferroan olivine and abundant nepheline
grains, and by a layer of salite-hedenbergite pyroxenes + andradite + wollastonite.
3. These observations and physico-chemical analysisof the inferred chemical reactions suggest that
Allende CAIs experienced an open-system metasomatic alteration in situ in the presence of CO2and H2O-bearing fluid on the CV chondrite parent
asteroid at ~150-250 °C. During the alteration, Si,
Na, Cl, K, Fe, S, and Ni were introduced, whereas
Ca, Mg, and some Al were lost from the host inclusions; Ti and Ba were locally mobilized
4. In the Allende Type B CAIs, grossular associatingwith monticellite, forsterite, Al-diopside, and wollastonite, and replacing åkermanitic melilite has high27
Al/24Mg ratios (30−100) and shows no resolvable
26
Mg*. The 27Al/24Mg ratios and 26Mg* in grossularreplacing gehlenitic melilite are similar to those of
the host melilite and plot along a regression line with26
Al/27Al ratio of ~5×10−5. We conclude that metasomatic alteration occurred after nearly complete
decay of 26Al, >3 Ma after crystallization of CAIs withthe canonical (26Al/27Al)0 of (5.25±0.02)×10-5. 26Mg*
in grossular was inherited from primary melilite andprovides no chronological significance.
5. Oxygen isotopic compositions of most secondary
minerals in the Allende Type B CAIs measured insitu with the UH Cameca ims-1280 and matrixmatched standards plot along mass-dependent fractionation line with Δ17O of ~ −3±2‰ with δ18O
ranging from ~ 0 to ~10‰. A smaller range ofδ18O, ~5‰, is observed for cogenetic secondary
minerals within individual CAIs. We conclude that
Δ17O of ~ −3±2‰ corresponds to that of the aqueous fluid on the Allende parent asteroid. A small
range of δ18O among the secondary minerals is
consistent with high-temperature metasomatic alteration of ~ 150-250 °C.
6. Secondary grossular and forsterite in Type Cs andType B1 CAI TS-34 show a range of Δ17O, from ~−15 to ~ −1±2‰; and on a three-isotope oxygen
diagram plot along CCAM line. The similar variations in O-isotope compositions are observed forprimary anorthite in these CAIs; primary melilite is
generally more 16O-depleted (Δ17O range from ~−5 to ~ −1±2‰). Primary spinel, forsterite, andmost Al,Ti-diopside grains have 16O-rich compositions (Δ17O ~ −25±2‰). We suggest that oxygenisotopic compositions of grossular and forsterite inthese CAIs were largely inherited from melilite andanorthite which may have experienced incomplete
O-isotope exchange with a fluid.

Page 34 of 37

6 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-021-00437-4.
Additional file 1: Figure EA1. Backscattered electron images of the
Allende Type B2 CAI #2 characterized by Ushikubo et al. 2007. Primaryand secondary minerals are labeled by white and yellow symbols,
respectively. The CAI consists of the irregularly-shaped (ragged) melilite(mel), anorthite (an), and Al,Ti-diopside (fas), all poikilitically enclosing
spinel (sp) grains. Melilite, anorthite, and Al,Ti-diopside occasionally exhibit triple junctions (see “b”). Most anorthite grains contain exsolutionsof submicron silicates. Some anorthite grains contain melt inclusions
composed of Al,Ti-diopside and unaltered melilite (see “c”). Melilite is replaced to various degrees by grossular (grl), forsterite (fo), and monticellite (mnl). Anorthite is almost unaltered: only minor secondary Na-melilite(Na-mel), grossular, and kushiroite (kush) occur at the boundary withmelilite. We conclude that ragged anorthite in CAI #2 is not a secondary
phase; it is a primary mineral that crystallized from a CAI melt and largely
escaped metasomatic alteration. Therefore, the Al-Mg isotope systematicsof this anorthite cannot be used to date the metasomatic alteration.
Table EA1. Representative electron microprobe analyses of secondary
minerals in the Allende CAIs studied. Table EA2. Oxygen isotopic compositions of primary minerals in the Allende coarse-grained igneous CAIs
measured by SIMS. Table EA3. Oxygen isotopic compositions of secondary minerals in the Allende coarse-grained igneous CAIs measured by
SIMS.
Acknowledgements
We thank Dr. S. Simon (University of New Mexico), Dr. G. J. MacPherson(Smithsonian Institution), Dr. D. Wark (University of Melbourne), and Prof. G. J.
Wasserburg (California Institute of Technology) for the loan of the Allende
CAIs’ thin sections. We thank Drs. G. Libourel, A. M. Davis, G. J. Wasserburg, I.
D. Hutcheon for numerous discussions and useful suggestions. Handling of
the manuscript by Associate Editor Dr. Yangting Lin is appreciated. The
authors thank two anonymous reviewers for their constructive reviews which
help to improve the paper. This work was supported by the National
Aeronautics and Space Administration (NASA) Emerging Worlds grant
NNX17AE22G (ANK, PI). This paper is dedicated to Professor Ahmed El Goresy
who successfully combined mineralogy, petrology, and isotope chemistry instudying extraterrestrial materials.
Authors’ contributions
AK carried out mineralogical and petrographic characterization, and oxygenisotope measurements of refractory inclusions, and drafted the manuscript.
KN established analytical protocols of oxygen and Al-Mg isotope measurements, developed O-isotope standards, and carried out Al-Mg isotope measurements of the samples. MP carried out physico-chemical analysis ofmetasomatic alteration of the Allende refractory inclusions and helped todraft the manuscript. All authors read and approved the final manuscript.
Funding
Funding for this work was provided by the National Aeronautics and Space
Administration (NASA) Emerging Worlds grant NNX17AE22G (AK, Principal
Investigator). MIP was partially supported by the NASA Emerging Worlds(80NSSC20K0346) and DOE-NNSA (DE-NA0003904) grants to Stein B. Jacobsen (Harvard University).
Availability of data and materials
Please contact author (AK) for data requests.

Declaration
Competing interests
The authors declare that they have no competing interests.
Author details
1
Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth
Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI 96822,
USA. 2Department of Earth and Planetary Sciences, Harvard University andHarvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.

Krot et al. Progress in Earth and Planetary Science(2021) 8:61

Received: 31 March 2021 Accepted: 23 June 2021

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