Microtextures in the Chelyabinsk impact breccia reveal the history of Phosphorus‐Olivine‐Assemblages in chondrites

Microtextures in the Chelyabinsk impact breccia reveal the history of

Phosphorus-Olivine-Assemblages in chondrites

Craig R. WALTON

, Ioannis BAZIOTIS

, Ana

CERNOK

, Ludovic FERRI

ERE

Paul D. ASIMOW

, Oliver SHORTTLE

1,6

, and Mahesh ANAND

3,7

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Athens,

IeraOdos 75, 11855 Athens, Greece

Department of Physical Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Natural History Museum, Burgring 7, A-1010 Vienna, Austria

Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena,

California 91125, USA

Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK

Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK

Corresponding author. E-mail: crw59@cam.ac.uk

(Received 21 July 2020; revision accepted 28 February 2021)

Abstract–

The geochemistry and textures of phosphate minerals can provide insights into the

geological histories of parental asteroids, but the processes governing their formation and

deformation remain poorly constrained. We assessed phosphorus-bearing minerals in the three

lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning

electron microscope, electron microprobe, cathodoluminescence, and electron backscatter

diffraction techniques. The majority of studied phosphate grains appear intergrown with

olivine. However, microtextures of phosphates (apatite [Ca

(PO

)

(OH,Cl,F)] and merrillite

[Ca

NaMg(PO

)

]) are extremely variable within and between the differently shocked

lithologies investigated. We observe continuously strained as well as recrystallized strain-free

merrillite populations. Grains with strain-free subdomains are present only in the more

intensely shocked dark lithology, indicating that phosphate growth predates the development of

primary shock-metamorphic features. Complete melting of portions of the meteorite is recorded

by the shock-melt lithology, which contains a population of phosphorus-rich olivine grains. The

response of phosphorus-bearing minerals to shock is therefore hugely variable throughout this

monomict impact breccia. We propose a paragenetic history for P-bearing phases in

Chelyabinsk involving initial phosphate growth via P-rich olivine replacement, followed by

phosphate deformation during an early impact event. This event was also responsible for the

local development of shock melt that lacks phosphate grains and instead contains P-enriched

olivine. We generalize our findings to propose a new classification scheme for Phosphorus-

Olivine-Assemblages (Type I

–

III POAs). We highlight how POAs can be used to trace

radiogenic metamorphism and shock metamorphic events that together span the entire

geological history of chondritic asteroids.

INTRODUCTION

Phosphorus in Meteorites: Knowns and Unknowns

Meteorites provide direct samples of some of the

most primitive solid materials found in the solar system,

yielding insights into early disk processes including

chemical partitioning in the protoplanetary disk, the

assembly of dust into planets, and subsequent

dynamical evolution (e.g., Scott 2007). We now have a

broad understanding that the parent asteroids of

chondritic meteorites experienced heating (recorded by

Meteoritics & Planetary Science

56, Nr 4, 742–766 (2021)

doi: 10.1111/maps.13648

742

2021 The Authors.

Meteoritics & Planetary Science

published by Wiley Periodicals LLC on behalf of The Meteoritical Society (MET).

This is an open access article under the terms of the Creative Commons Attribution License, which permits use,

distribution and reproduction in any medium, provided the original work is properly cited.

thermal metamorphism) predominantly by the decay of

short-lived radionuclides for the first 60 million years

(Myr) of solar system history (e.g., Bouvier et al. 2007).

A large body of evidence shows that impact events

continued to disturb these objects beyond the end of

thermal metamorphism, resetting a number of mineral

geochronometers in the process (e.g., Wittmann et al.

2010). Both radiogenic and impact-induced heating are

thought to mobilize fluids within chondrites (Lewis and

Jones 2016; Zhang et al. 2016), resulting in

redistribution of volatile components and growth of

secondary phases. Thus, the elemental and isotopic

compositions of volatile-bearing phases in meteorites

and other planetary samples have the potential to

provide important insights into both planetary volatile

reservoirs and processes that operated on their parent

bodies (e.g., Jones et al. 2014; Stephant et al. 2019;

Cernok et al. 2020). Volatile-bearing phases that retain

such information include halides (Jones et al. 2016),

sulfides (Visser et al. 2019), and phosphates (Jones et al.

2016).

In this context, studies of phosphorus (P)-bearing

minerals have been especially informative. In the most

primitive unequilibrated chondrites (e.g., carbonaceous

chondrites), P is found dissolved in Fe-Ni metal as a

neutral minor element (Zanda et al. 1994), indicating

siderophile behavior during early condensation. Indeed,

thermodynamic models predict that P initially condenses

from the solar nebula as schreibersite ([FeNi]

P).

Schreibersite is often observed to be non-stoichiometric,

with P

1 atom per formula unit (Zanda et al. 1994;

Pasek 2019). Conversely, in thermally metamorphosed

and aqueously altered chondrites, P is found in its

oxidized form (P

) within the phosphate minerals

merrillite (Ca

NaMg[PO

]

) and apatite (Ca

[PO

]

[OH,

Cl,F]), implying that P is scavenged from metal and

phosphide grains by fluids and redistributed within the

parent body (Jones et al. 2014).

The direct link to volatile components arises

because apatite requires structural volatile components

(OH, Cl, F) in order to crystallize. Apatite and

merrillite also incorporate U into their crystal lattices,

making both phases viable for U

–

Pb dating (e.g., Merle

et al. 2014; Yin et al. 2014; McGregor et al. 2019) and

hence for time-resolved studies of volatile component

evolution. Phosphate U

–

Pb dating in chondrites has

helped to map out radiogenic heating on million-year

timescales, as well as near instantaneous resetting

during shock-metamorphic events (e.g., Yin et al. 2014).

It is therefore possible to tie the history of asteroidal

phosphates to the compositional evolution of

metasomatic fluids during both early and late-stage

heating events on asteroidal parent bodies. Additionally,

a third mineral reservoir of P has recently been

identified in chondrites: P-enriched olivine grains of

enigmatic origin (e.g., McCanta et al. 2016; Li et al.

2017).

It is clear that P-bearing minerals have a role to

play in elucidating the complex histories of asteroidal

parent bodies. However, there are numerous unresolved

issues both in the study of P-bearing phases and

asteroidal evolution in general

—

many of which would

appear to be linked. In particular, it remains unclear (1)

how oxidized P-bearing minerals form in chondrites

from initially reduced Fe-bound P (Jones et al. 2014,

2016), and (2) how these P-bearing phases respond to

impact processing (Krzesi

nska 2017; McGregor et al.

2018; Cox et al. 2020). Given widespread evidence for

shock metamorphism in meteorites, unravelling which

aspects of the phosphate record pertain to radiogenic

versus impact-induced metamorphism is critical for a

full understanding of the physicochemical evolution of

asteroids.

Here, we assess both the phase associations and

microtextures of phosphate minerals and P-bearing

olivine in a highly shocked chondritic meteorite, that is,

Chelyabinsk. We use a combined imaging approach in

order to access records of heterogeneous shock

processing throughout the meteorite. By comparing our

results with published literature, we are able to establish

new paragenetic constraints on P-hosting phases

—

from

their initial formation, through subsequent impact-

induced metamorphism and melting.

Phosphorus in the Chelyabinsk Impact Breccia: An Ideal

Case Study?

Chelyabinsk is an LL5 (low metal, low iron,

petrologic type 5

—

i.e., thermally equilibrated) S4

–

(moderately shocked to shock melted; St

€

offler et al. 1991;

Morlok et al. 2017; Fritz et al. 2019) impact-brecciated

meteorite. The Chelyabinsk impact breccia contains three

distinct shock-related lithologies, which are distinguished

at a first-order level by their optical color, including (1) a

light-colored, chondritic-textured lithology with shock

melt veins (SMVs) and high-pressure phases; (2) a dark-

colored lithology with relict shock-darkened chondritic

textures, abundant shock melt pools, and SMVs with

high-pressure phases; and (3) a quenched shock-melt

lithology with some entrained chondritic fragments and

an absence of high-pressure phases (Bischoff et al. 2013;

Koroteev et al. 2013; Popova et al. 2013; Righter et al.

2015; Morlok et al. 2017).

Despite the extensive work already performed on

Chelyabinsk, the textural setting of P-bearing minerals

with surrounding phases, their origin, and internal

microtextures has not yet received specific attention.

Both the light and dark lithologies of Chelyabinsk

Phosphorus-olivine assemblages in chondrites

743

contain apatite and merrillite (e.g., Popova et al. 2013),

yet these phases are conspicuous for their absence in the

shock-melt lithology. The range of lithologies preserved

in Chelyabinsk may contain assemblage-scale textures

and mineral-scale microtextures that pertain both to the

initial growth and to the subsequent impact

metamorphism of chondritic phosphate minerals. The

great diversity of petrological, geochemical, and

geochronologic analyses that have been reported for

each lithology provides a broader context within which

to construct self-consistent models of phosphate

paragenesis.

Previous studies have gained insight into phosphate

genesis and response to impact by evaluating

associations and microtextures of mineral phases that

tend to co-exist together or in proximity to phosphates

(e.g., Jones et al. 2014). Here, we present a thorough

analysis of P distribution in phosphate and olivine

grains within all three of the variously shocked

lithologies found in the Chelyabinsk meteorite. We

consider the implications of observed mineral

microtextures, phase associations, and chemical

compositions for the relative timing of thermal and

shock metamorphic events affecting phosphorus

reservoirs in Chelyabinsk, concluding with a discussion

of newly observed microtextures that demand attention

during future studies of shocked meteorites.

MATERIALS AND METHODS

Samples

Several polished sections of Chelyabinsk from the

collections of The Open University (OU; sections “A”

and “B”) and the Natural History Museum (NHM),

Vienna (NHMV; N9834), were investigated during this

study. Section “A” contains the light lithology of

Chelyabinsk (Pillinger et al. 2013), a moderately

shocked (dominantly S4) chondrite with well-preserved

chondrules and matrix, crosscut by SMVs. Section “B”

contains the dark lithology (Pillinger et al. 2013), more

severely shocked (blackened) material, and numerous

SMVs (dominantly S5). Finally, section N9834 contains

regions of shock-blackened chondrite (S5) as well as

quenched shock melt (S6).

Light Lithology

The light lithology of Chelyabinsk is dominated by

thermally metamorphosed and chemically equilibrated

matrix and chondrule material, in which major and

minor mineral phases include silicates (olivine,

pyroxene, feldspar), troilite (FeS), iron

–

nickel (P-free

Fe-Ni) metal, phosphates (apatite and merrillite), and

chromite (Righter et al. 2015). SMVs provide clear

evidence for the effect of shock metamorphism in the

light lithology, but typically comprise only several

volume percent of a given sample. SMVs have bulk

compositions broadly representative of the whole-rock

composition and preserve evidence of rapid cooling

(e.g., micro-to-cryptocrystalline silicates, metal

–

troilite

globules, and glasses; Righter et al. 2015; Kaeter et al.

2018).

Shock features are also pervasive in the chondrite-

textured portion of the light lithology. For example,

phases with high-melting temperatures, such as olivine

and pyroxene, are generally fully crystalline yet exhibit

planar fractures, shock-induced mosaicism, as well as

evidence of shearing (e.g., displacement across micro-

faults; Righter et al. 2015; Kaeter et al. 2018). Phases

with lower melting temperatures

—

such as plagioclase,

metal, and troilite

—

have widely formed veins through

unmelted silicates (Kaeter et al. 2018).

Phase-specific responses are more complicated on

smaller scales, where local density contrasts between

phases induce locally heterogeneous melting behavior

(Moreau et al. 2018). There are examples of metal and

troilite grains that retain their pre-impact textures, as

well as examples that have clearly experienced

mobilization during the impact (e.g., injection along

grain boundaries and into fractures, forming network

and vein textures; Korotev et al. 2013). Meanwhile, all

plagioclase feldspar would appear to be secondary,

having experienced melting followed by quenching or

recrystallization (Kaeter et al. 2018) and for which a

number of different occurrences have been identified

(e.g., plagioclase-normative glass, maskelynite,

plagioclase

–

chromite symplectites, crystalline plagioclase

feldspar; Kaeter et al. 2018). Where possible, we use the

same terminology as Kaeter et al (2018) to describe

feldspathic phases (i.e., depending on available

observations). Otherwise, the generic term plagioclase is

used.

Dark Lithology

The dark lithology is mineralogically similar to

the light lithology. However, the chondrite-textured

portion is more heavily brecciated. Sulfide, metal,

silicate-rich SMVs, and shock-melt pools constitute a

greater volume percentage of dark samples. Planar

deformation features in olivine are much more

extensive. These features contribute to a pervasive and

diagnostic shock darkening (Rubin 2003a, 2003b;

Krzesi

nska et al. 2015; Righter et al. 2015; Kaeter

et al. 2018). Plagioclase is more widely associated

with chromite in symplectite structures (interpreted as

former melt pools).

744

C. R. Walton et al.

Shock Melt Lithology

The shock-melt lithology is a micro-to-

cryptocrystalline breccia composed mostly of olivine

–

pyroxene glass mesostasis along with fragments of

chondrite-textured material (both light and dark

lithologies) and metal

–

troilite droplets (Righter et al.

2015).

Microtextural Characterization of Phosphates and

Olivine

Both the light and the dark lithology samples of

Chelyabinsk from the OU collection (Pillinger et al.

2013) were characterized using scanning electron

microscopy (SEM), Zeiss Supra 55VP at the OU and

Quanta 650 at the University of Cambridge. Whole-

sample backscattered electron (BSE) images and

elemental maps using energy dispersive X-ray

spectrometry (EDS) were collected for each sample in

order to identify the distribution of key phases and

provide context for higher magnification analyses.

Merrillite and apatite were distinguished on the basis of

EDS measurements in point mode, with 10 s spectral

acquisitions, based on the abundance of Na, Mg, and

Cl. Additional SEM imaging of the shock-melt lithology

was performed at the California Institute of Technology

(Caltech) Geological and Planetary Sciences Division

Analytical Facility using a Zeiss 1550VP field-emission

SEM equipped with an angle-sensitive backscattered

electron detector, 80 mm

active area Oxford X-Max Si

drift detector EDS, and an HKL EBSD system. The

SEM imaging and EDS analyses used a 15 kV

accelerating potential and a 120

m field aperture in

high-current mode (

4 nA probe current), yielding

imaging resolution better than 2 nm and an activation

volume for EDS analysis

–

on silicates.

SEM-cathodoluminescence (CL) images were

collected on selected phosphate and plagioclase grains

previously imaged in BSE, in order to obtain an initial

view of crystal structure integrity across the sample. The

CL images were acquired on the OU SEM with a

Deben Centaurus CL panchromatic detector with

Hamamatsu Photo Multiplier Tube (model R316), with

spectral response to wavelengths from 400 to 1200 nm.

Imaging conditions were 7

–

10 kV accelerating voltage,

–

13 mm working distance, and high vacuum mode.

We quantify phosphate phase associations in

Chelyabinsk by counting the phases that share a grain

boundary with a phosphate mineral. Phases associated

with phosphate in this study include olivine, pyroxene,

plagioclase, metal, and sulfides (see Fig. 3).

Lattice orientation, internal microtexture, and

structural disorder of selected phosphate minerals were

studied by EBSD. For the light and dark lithologies,

after SEM analyses, isopropanol and, if needed,

0.25

m diamond paste were used for removing the

carbon coat from the samples. Subsequently, the thin

sections were lightly polished for approximately 10 min

using either 50 nm alumina or colloidal silica in water

suspension with a LabPol-5 system with a LabForce-1

head and an automated doser at OU, to ensure removal

of surface defects. This process minimizes scatter of the

electron beam due to surface interaction and is a critical

step for accurate analysis by EBSD.

We constructed images from rasters of EBSD

solutions using an Oxford Instruments Nordlys EBSD

detector mounted on the OU SEM. Diffracted electrons

were collected at a tilt angle of 70

°

. Raster images were

constructed with 400

–

550 nm step sizes and collection

times

120 ms per step. Binning on the EBSD area

detector was set to 4

9

m (e.g., White et al. 2017;

Cernok et al. 2019). Voltage was set to 20 kV with a

largest aperture of 120

m and high current mode. Such

conditions typically generate a beam current of 9.1 nA,

measured in a Faraday cup at the same conditions. The

interaction volume for diffracted electrons using these

parameters is estimated to be a few tens of nm in width

and depth (Darling et al. 2016). Raw patterns are

automatically background corrected during acquisition,

removing an averaged 64 frames of “noise” from each

pattern. Wild spike reduction and minimal zero solution

correction were the only raw data corrections applied.

Collected diffraction patterns were indexed either as

apatite, with the hexagonal unit cell parameters

9.4555,

6.8836,

°

°

and

120

°

(Wilson et al. 1999) or as merrillite with

the trigonal unit cell parameters

10.3444,

37.0182,

°

°

, and

120

°

(Xie et al. 2015).

For the shock-melt lithology, single crystal EBSD

analyses of olivine at a submicrometer scale were

performed at 20 kV and 6 nA in focused beam mode

with a 70

tilted stage on uncoated specimens in

“variable pressure” mode (25 Pa of N

gas in the

chamber to reduce specimen charging). Imaging,

mapping, semiquantitative EDS analysis, and EBSD

were conducted using the SmartSEM, AZtec, and

Channel 5 software packages, respectively.

Crystallographic orientation of individual grains

and their internal microtexture obtained by electron

diffraction can be visualized in different ways. Band

contrast (BC) is a measure of the pattern quality of a

material, based on the contrast between observed

Kikuchi bands and the background. This measure is

affected by phase crystallinity, lattice orientation, defect

density, surface polishing conditions, and SEM/EBSD

setup parameters (Kang et al. 2013). Texture component

Phosphorus-olivine assemblages in chondrites

745

(TC) maps reveal orientation of individual grains and

subdomains within grains, and reveal distortion of the

crystal lattice from a reference point. Inverse pole figure

(IPF) maps show absolute orientation of the normal to

the polished surface in the crystal lattice coordinate

system and help visualize the orientation of the lattice

of the entire grain. In particular, TC and IPF maps help

distinguish strained from unstrained grains and

subdomains.

Phase Association Quantification

We quantified phosphate phase associations by

manually counting the grain boundary contacts shared

by each phosphate grain studied with neighboring

phases. This form of quantification results in an overall

percentage of grains that share a contact with, for

example, olivine.

Electron Microprobe

We studied the composition and zoning of phases

in point and mapping modes using a JEOL JXA-

8530F field emission electron microprobe (EPMA) at

the NHMV and a Cameca SX100 EPMA at the OU.

All analyses were performed with an accelerating

voltage of 15 kV. For minerals, a 20 nA focused

beam current, 20 s counting time on peak position,

and 10 s for each background were used. Selected

sites were reanalyzed for P at 50 nA beam current

with 20 s peak and 10 s background counting time.

Detection limit (1

) for P was

76 ppm. For glass

analyses, a slightly defocused (5

m diameter) beam

and 10 s counting time were used. Natural mineral

standards used were albite (Na, Si, Al), wollastonite

(Ca), olivine (Mg), almandine (Fe), spessartine (Mn),

orthoclase (K), rutile (Ti), chromite (Cr), Ni-oxide

(Ni), and apatite (P).

RESULTS

Whole-section BSE image mosaics are shown in

Fig. 1. Areas studied at higher resolution in each

section (and which are discussed in the main text) are

called out with subframe rectangles or grain numbers.

Individual phosphates are named according to their

image number, phase identity, and section number; for

example, 001-AP-A would be an apatite from image 001

of section A (our sample of the light lithology), whereas

001-MERR-B corresponds to a merrillite from image

001 of section B (our sample of the dark lithology).

Silicate grains from the shock-melt lithology are named

according to the sequence and analytical session in

which they were studied.

Phosphorus Phase Associations

Light and Dark Lithologies

Phosphates in both the light and dark lithologies of

Chelyabinsk occur mostly as individual grains, as either

apatite or merrillite. Only a few examples were found of

close association between the two phases (occurrences

as replacement or intergrowth). Phosphate grain

boundaries truncate against other phases sharply except

in the case of olivine, where contacts are often irregular

and complex/embayed (Fig. 2A). Both apatite and

merrillite, across all textural settings and associations,

were observed to contain small (

m) rounded

inclusions of olivine (Fig. 2A, label 2), as well as, more

rarely, plagioclase, chromite, sulfide, and pyroxene.

Phosphate inclusions are often observed to occur in

olivine (Fig. 2A). Some small (

m) phosphate and

phosphate

–

olivine inclusions also occur in pyroxene

(Fig. S3 in supporting information).

SMVs are occasionally observed to truncate or

intrude a number of features, including phosphate

grains (Fig. 2B). Phosphates and olivine display

evidence of having interacted with mobile plagioclase,

including thin rims around and linear fills of apparent

planar deformation features in olivine and pyroxene, as

well as zones marked by finely interspersed olivine and

chromite in plagioclase (Fig. 2C). Some phosphate

grains were encountered in close association with Fe-Ni

metal (Fig. 2D) and chromite grains, but the majority

are in contact with silicate phases.

Our quantitative assessment of phosphate phase

associations reveals that in the light lithology, both

apatite and merrillite are preferentially associated with

olivine and plagioclase, with significantly fewer crystals

sharing boundaries with pyroxene, sulfide, or metal

(Fig. 3

—

raw data available in Table S2 in supporting

information). Associations are much less organized and

more complex in the dark lithology, with the majority

of phosphates associated with all five other phases

considered (Fig. 3).

Melt Lithology

We do not observe phosphates in the shock-melt

lithology. The budget of P in the shock-melt lithology is

instead hosted as a minor element in silicate phases. Our

sample of the shock-melt lithology contains around 50

vol% quenched impact-melt, composed of small (

zoned olivine and orthopyroxene crystals, silica-rich

glass, and finely disseminated sulfide grains. Relict

chondrules and individual grains of olivine, pyroxene,

and plagioclase entrained in the melt are highly resorbed

and surrounded by melt. Quench textures include a high

abundance of melt inclusions in silicate phases, along

with concentric-to-interstitial skeletal growths of olivine.

746

C. R. Walton et al.

Compositional gradients are apparent in the melt in

proximity to resorbed mineral phases.

Orthopyroxene occurs as large porphyroclasts

(

150

m in length), as small subhedral to anhedral

crystals (15

–

m in length), and as microlites (

in length) entrained in the melt. The large

porphyroclasts have a composition En

75-78

22-24

; the

small crystals have a broader range of composition

0-5

68-86

14-27

; the microlite crystals show a

homogeneous

Mg-rich

composition

(Table S2B). The P

abundances are low (

≤

0.06 wt%)

in all orthopyroxene grains, with the exception of the

microlite crystals, which exhibit a range from 0.13 to

0.19 wt% (Table S2B). Note that the P enrichment in

microlite orthopyroxene crystals is not an artifact of

contamination from neighboring phases, which have

lower P content than the orthopyroxene itself.

EPMA defocused spot analyses on pools of glass

larger than 10

m in diameter reveal an SiO

-rich

composition (63.3

–

64.9 wt%) with very low P

content

≤

0.05 wt% and Mg# in the range of 24.4

–

40.8

(Table S2C). Unfortunately, the small glass areas

between quenched crystals and the zoned rims around

resorbed phases are too small for clean defocused beam

EPMA analyses, so the compositional range given here

may not be fully representative.

Our sample of the impact melt contains olivine grains

(Fo

64.5-85.8

) with 0.02

–

0.52 wt% P

(Table S2C). Two

populations of olivine in this lithology are distinguished

on the basis of textures, major element chemistry, and P

abundance: Partially resorbed relics and quench olivine,

found as individual crystallites and rim overgrowths

around partially resorbed relics (Fig. 4). Quench olivine

is clearly zoned as visible on BSE-SEM images (Fig. 4A).

Quench olivine is further subdivided into individual

crystals and rim overgrowths that formed around pre-

existing olivine crystals. Figure 4B illustrates a schematic

view of these various olivine populations.

(A)

(B)

(C)

Fig. 1. Backscattered electron maps of Chelyabinsk light (A), dark (B), and shock-melt lithology (lower left-hand side of section

N9834 [C], which is otherwise comprised of the dark lithology). Phosphate/P-bearing olivine grains analyzed with EBSD are

indicated, though many more were studied with SEM. Phosphate grains are absent in the shock-melt lithology. Together, these

three sections span a range of shock stages (S4

–

6), thus preserving in pristine condition the textural outcome of exceptionally

varied pressure

–

temperature

–

time pathways throughout the meteorite. (Color figure can be viewed at wileyonlinelibrary.com.)

Phosphorus-olivine assemblages in chondrites

747

Individual quench and rim overgrowth quench

olivine share geochemical and textural similarities. Both

olivine types are concentrically zoned, with the inner

Mg-rich portion containing a high proportion of silicate

glass inclusions. The inner zones of both individual and

rim overgrowth crystals are also somewhat P-enriched

compared to any preserved core olivines (Fig. 5). The

outer portion is generally inclusion-free, Fe-rich, and

very P-rich (Fig. 5). Phosphoran olivine is defined by

having

1 wt% P

(Boesenberg and Hewins 2010).

Our samples display a maximum of

0.5 wt% P

and

are hence most accurately referred to simply as

variously P-rich olivine.

Figure 6 presents a scatter plot of FeO versus P

contents for the various olivine populations identified in

the different lithologies of Chelyabinsk. Quench olivine

data are divided by backscatter contrast into dark

(typically the inner regions) and light (typically near the

rims) groupings. High backscatter contrast outer parts

of quench olivine in the melt lithology have P

concentrations substantially higher than in resorbed

cores, the inner parts of quench olivine, or olivine in the

light lithology. Phosphorus concentration in high

backscatter contrast outer parts of olivine rims is also

higher than that in any other phase in the quenched

shock-melt lithology, including glass and pyroxene

(Table S2).

We rule out that the P-in-olivine signature merely

represents contamination by silicate mesostases on the

basis of both visual (Fig. 5) and geochemical (Fig. 6)

evidence. All of the high P concentrations (

≥

0.25 wt%

) measured in olivine are in outermost light-colored

quench rims (Fig. 5), which are generally inclusion-free

(see spot overlay; Fig. S6 in supporting information; full

(A)

(C)

(D)

(B)

Fig. 2. Backscattered electron images combined with EDS P element intensity maps (in red) showing key phosphate phase

associations in the light lithology. A) Grain 026-MERR-A, showing close association and complex intergrowth texture of

merrillite with olivine. Specific textures are numbered as follows: (1) curvilinear olivine/phosphate boundaries, (2) olivine

inclusions, (3) a grain of plagioclase, (4) pyroxene, and (5) Fe-Ni metal. B) Apatite 079-AP-A, showing truncation of apatite (1

—

black dashed outline) and barred olivine (2

—

yellow dashed outlines) by shock melt vein (SMV; white dashed outline). C) Apatite

039-AP-A, with textural evidence for plagioclase mobilization. A pool of plagioclase containing large apatite and metal grains (1)

is connected to other plagioclase pools by a variety of mobilization textures (2), including zones marked by finely interspersed

olivine and chromite in plagioclase (i), as well as thin rims around and linear fills of apparent planar deformation features in

olivine and pyroxene (ii). D) Merrillite grain 061-MERR-A (1) closely associated with a metal grain (2) and a void (3). Mineral

abbreviations: Ol

olivine; Pyx

pyroxene. (Color figure can be viewed at wileyonlinelibrary.com.)

748

C. R. Walton et al.

silicate chemistry data are provided in Table S2). We

further rule out that the P-in-olivine signature merely

represents contamination by silicate mesostases on the

basis of a positive correlation between FeO and P

the quench olivine population (Fig. 6). These data

support a model in which P is directly incorporated into

the olivine structure, as contamination by silicate

mesostases would yield lower FeO.

The rims of quench olivine in Chelyabinsk exhibit

moderate positive correlations between P

and trivalent

cations (Al

; Fig. 7A) and between P

and a

deficit relative to three cations per four-oxygen formula

unit (

0.64; Fig. 7B). In contrast, P is poorly

correlated with Si

(

0.28; Fig. 7C). Based on these

correlations, we infer that P substitution was

predominantly in exchange with divalent cations,

alongside trivalent Cr and Al, and balanced by M site

vacancies, that is, 4

(Cr, Al)

[]. Core

olivine compositions are identical to the typical

equilibrated olivine found in both the light and dark

lithologies (Fig. 6).

Mineral Microtextures

Microtextural EBSD and CL data provide

information with which we test predictions made by

different models of P-mineral paragenesis. CL imaging

is sensitive to surface and internal mineral textures,

structural defects, trace element distribution, and

elemental zoning in plagioclase and phosphates (G

€

otze

and Kempe 2008). Changes in phase crystallinity and

element mobility during impact metamorphism may

each create visible signatures in CL images, but which

are more reliably interpreted when provided with

context from other techniques, such as EBSD. We

studied the microtextures of P-bearing minerals, as well

as other closely associated phases, in order to determine

a self-consistent paragenetic history.

An important novel finding arising from our

observations is that phosphate microtextures differ

Fig. 3. Counting statistics for association of phosphate grains

with other phases in the light and dark lithologies of

Chelyabinsk (

96 individual phosphate grains), based on an

SEM survey. Mineral abbreviations: Ol

olivine;

Pyx

pyroxene; Plag

plagioclase.

(A)

(B)

Fig. 4. A) Backscattered electron (BSE) image of olivine grain

B6 and surrounding quenched melt. B) Schematic view of the

same olivine grain shown in (A). Two populations of olivine

in this lithology are distinguished: Partially resorbed relics and

quench olivine, found as individual crystallites and rim

overgrowths around partially resorbed relics. Quench olivine is

clearly zoned, and is further subdivided into individual crystals

and rim overgrowths that formed around pre-existing olivine

crystals. Mineral abbreviation: Ol

olivine. (Color figure can

be viewed at wileyonlinelibrary.com.)

Phosphorus-olivine assemblages in chondrites

749

systematically both among lithologies with different

shock stages, and within each lithology owing to the

local phase assemblage. We identify plastically strained

apatite and merrillite grains in the light lithology,

whereas in the dark lithology, we observe strained

apatite grains and recrystallized merrillites. We also

find a novel patchy phosphate population in both the

light and dark lithologies. Figures 8

–

10 present

analyses performed on phosphates. Figure 11 presents

EBSD data for olivine in the light, dark, and melt

lithologies. Figure 12 summarizes in schematic form

the most important phosphate textures observed via

EBSD, CL, and EDS imaging in the course of this

work.

Plastically Strained Versus Recrystallized Phosphates

We identify a dichotomy in phosphate textures

between the differently shocked lithologies of

Chelyabinsk: plastic deformation of phosphates in the

light lithology versus recrystallized phosphates

(exclusively merrillites, given available data) with strain-

free subdomains in the dark lithology. Strained grains

show plastic deformation across the grain that results in

total cumulative misorientations of up to 16

, as visible

in TC and IPF maps and respective pole figures

(Figs. 9A and 9B, and related pole figures). In contrast,

the strain-free subdomains we observe in recrystallized

merrillite show significant misorientations from one

another as demonstrated by TC and IPF maps

(Fig. 9C), but retaining obvious memory of the original

parent crystal orientation, as visible by the dominant

orientation pattern in pole figures (Fig. 9C).

Meanwhile, individual subdomains demonstrate no

internal strain.

Recrystallization textures are visible in both CL

images (Figs. 8 and 9) and EBSD data (Fig. 9), but not

in EDS X-ray maps (Fig. 10). The merrillite subdomain

orientations shown in Fig. 9C appear dominantly

orthogonal to the orientation of the neighboring SMV.

The EBSD maps reveal that individual subdomains are

highly crystalline (returning patterns of sufficient quality

to index at every pixel), while CL images demonstrate

complex patchy areas within each crystal subdomain. In

another recrystallized merrillite, outer subdomain and

grain boundaries are marked by notably bright CL

responses (Fig. 8C). Bright spots also occur in grain

interiors (Figs. 8C and 9C). Recrystallization textures

are universally well developed in dark lithology

merrillites but are apparently absent in dark lithology

apatites (Fig. 8B).

Fig. 5. A) Backscattered electron (BSE) image and (B) P EDS intensity map (scale is in counts) from the rim of the olivine grain

B12

—

shock melt lithology. Highest P intensity is observed at grain margins. (Color figure can be viewed at wileyonlinelibrary.com.)

Fig. 6. Composition of olivine populations in Chelyabinsk.

Early (dark) quench olivines are Fe-poor and variably P-poor

to moderately P-enriched, compared to pre-shock olivines of

both dark lithology resorbed and undisturbed light lithology

type. Late (light) quench olivines are variably P-poor to highly

P-enriched, with Fe-contents that range from slightly lower

than to equivalent to the pre-shock populations. Gray lines

indicate the 1, 2, and 3

detection limit for P

. (Color

figure can be viewed at wileyonlinelibrary.com.)

750

C. R. Walton et al.

In contrast, all phosphates in the light lithology

lack subdomain formation and instead display evidence

of internal plastic deformation (Figs. 8A, 9A, and 9B).

TC images of these phosphates reveal moderate crystal

–

plastic deformation of the grains with up to

internal misorientation from the reference point, with

no evidence of recrystallization (as supported by IPF

maps and relevant pole figures; Fig. 9A).

Patchy Phosphate Cathodoluminescence Signature

In both the light and the dark lithologies, some

apatite grains show complex CL response, displaying

generally thin (

m) patches of lower CL intensity

that may either follow fractures and grain boundaries or

appear to be independent of them (Figs. 8A, 8B, and

9A). This patchy CL response is visible also in some

recrystallized dark lithology merrillite grains (Fig. 8C)

but is apparently absent in light lithology merrillites

(Figs. 8A and 9B). There is no obvious overlap between

the patchy textural features we observe in CL and the

microtextures revealed by EBSD (Fig. 9). The EDS X-

ray maps also do not reveal any visible features in

elemental chemistry that would correspond to the

patchy CL textures that we observe.

Variable Olivine Microtexture

The microtexture of olivine in the shock-melt

lithology (Fig. 11) contrasts markedly with that found

in either the light or dark lithology (Figs. 11A

–

D).

Individual olivine grains in the light lithology show

minimal internal strain (Fig. 11A). Olivine in the dark

lithology displays local misorientations of a few degrees,

Fig. 7. Olivine compositional data showing correlation between phosphorus and (A) Al

, (B) cation sum deficit, and (C) Si

Representative error bars are shown in each plot. These are calculated as 2 standard deviations from the mean for an olivine grain

where multiple analyses were performed, providing an estimate of external reproducibility.

Phosphorus-olivine assemblages in chondrites

751

which is indicative of weak plastic deformation

(Fig. 11B). Olivine in the melt lithology displays a wider

diversity of textures. We observe

100

m relict olivine

cores that are divided into mosaics of randomly

oriented 5

–

m subdomains (i.e., recrystallization;

Fig. 11C). Also observed are quench growths of olivine

that have large strain-free subdomains, which yield

tightly clustered pole figures (Fig. 11D). Both the relict

recrystallized and quench olivine populations display

good crystallinity and no evidence of internal

misorientation (Figs. 11C and 11D).

DISCUSSION

Genesis of P-Bearing Minerals

Phosphate

Formation:

Metasomatism

Shock

Melting?

The complex microtextures and textural associations

of phosphates in the Chelyabinsk meteorite record key

events in the history of the meteorite and its parent

body. A key observation is that Chelyabinsk phosphates

are predominantly in contact with olivine and

plagioclase (Figs. 2 and 3). Our observations contrast

with Lewis and Jones (2016) and Jones et al. (2014),

who reported that phosphates in unshocked L and LL

chondrites showed no preferred association with any

particular silicate phase. Rather, phosphates in those

lower shock stage samples were nearly equally

associated pyroxene, plagioclase, and olivine, being

most often found in phase assemblages that contain all

of these minerals (Jones et al. 2014; Lewis and Jones

2016).

Moreover, Jones et al. (2014) and Lewis and Jones

(2016) identified (but did not quantify) common

association of phosphates with Fe-Ni metal and sulfide

grains, as well as voids resulting from plucking of Fe-Ni

metal and sulfide grains during sample preparation.

However, light lithology phosphates in Chelyabinsk

almost never occur in such an association (Fig. 3;

association with voids, see Table S1 in supporting

information). Therefore, the distribution of phosphates

in Chelyabinsk is anomalous among ordinary

chondrites. Noting that Chelyabinsk is unremarkable in

(A)

(B)

(C)

Fig. 8. Secondary electron (SE) and cathodoluminescence (CL) images, top and bottom, respectively, showing phosphate

textures in Chelyabinsk. A) 191-MERR/AP-A showing apatite with complex CL response intergrown with a merrillite with

contrastingly smooth CL response. B) 018b-AP-B showing light and dark zones in a patchy texture, as well as micro-fault

displacement textures on the grain scale. C) 003-MERR-B shows variably dark and bright CL response. Bright regions occur on

the whole-grain boundary, some subdomain boundaries, and as internal bright spots. There are also apparently overprinting

patchy regions of dark CL-response. Mineral abbreviations: Ol

olivine; Pyx

pyroxene; Ap

apatite; Merr

Merrilite;

Plag

plagioclase; Chr

chromite. (Color figure can be viewed at wileyonlinelibrary.com.)

752

C. R. Walton et al.

(A)

(B)

(C)

Fig. 9. Secondary electron (SE) annotated phase map, cathodoluminescence (CL) images, texture component (TC) maps, pole

figures, and inverse pole figure (IPF) color maps, for selected phosphates. A red or blue triangle indicates the TC reference point

in respect to which the internal grain misorientation is measured. All pole figures shown are color-coded after the corresponding

IPF map. Chr

chromite; Fe-Ni

iron

–

nickel metal; Ol

olivine; Pl

plagioclase; SMV

shock-melt vein. A) 39-AP-A

showing complex internal structure of apatite with complex CL response. This grain is surrounded by plagioclase. Several apatite

grains showing distinct lattice orientations, according to IPF maps. The pole figure of the selected apatite grain (highlighted in

white) shows moderate crystal

–

plastic deformation. B) Grain 280-MERR-A shows a bright CL response with minor subdomain

heterogeneity as well as plastic deformation with up to 16 degrees misorientation. The pole figure indicates continuous

deformation of the lattice, but no randomly oriented subdomains. C) Recrystallized merrillite from dark lithology displays a

complex internal network of strain-free subdomains (visible in CL; confirmed with EBSD). The pole figure indicates that the

orientation of individual subdomains broadly follows the orientation of the parent grain lattice, but with significant scatter.

(Color figure can be viewed at wileyonlinelibrary.com.)

Phosphorus-olivine assemblages in chondrites

753

terms of its bulk chemical composition and modal

mineralogy among the LL chondrites (Sharygin et al.

2013), we consider several hypotheses to explain our

observations.

One obvious difference is that Chelyabinsk is

heavily shocked, whereas Lewis and Jones (2016) and

Jones et al. (2014) focused on relatively unshocked

ordinary chondrites. However, there remain a number

of aspects of the phosphate distribution in Chelyabinsk

that are similar to textures identified by Jones et al.

(2014). Previous studies have identified phosphate

–

olivine reaction rims suggesting the growth of

phosphate by replacement of olivine during aqueous

alteration (Jones et al. 2014; McCubbin and Jones 2015;

Lewis and Jones 2016). The intergrowth textures present

in Chelyabinsk (e.g., Fig. 2A) could therefore be

interpreted similarly to those in relatively unshocked

ordinary chondrites, that is, as partial replacement of

olivine by phosphate minerals.

Alternatively, the phosphate

–

olivine intergrowths

found in Chelyabinsk may form upon crystallization

from shock melts (Jones et al. 2014; Krzesi

nska 2017).

Phosphate is readily consumed during impact melting of

chondrites (Donaldson 1985; Brunet and Laporte 1998;

Chen and Zhang 2008). In a shock-melt model for

silicate

–

phosphate intergrowths, pre-existing olivine

–

phosphate intergrowths that developed during parent

body thermal metamorphism melt and then regrow as

new olivine

–

phosphate intergrowths. Supporting this

idea requires detailed examination of the textures of the

olivine

–

phosphate intergrowths, for features that

uniquely develop upon growth from melts at high

cooling rates or degrees of undercooling. Distinctive

distributions of P in olivine, for example, can be found

in unaltered igneous rocks and rapidly cooled

crystallization experiments (e.g., Milman-Barris et al.

2008). In the case of Chelyabinsk, this would manifest

as anomalously P-rich compositions for olivine found in

symplectite-like phosphate-olivine assemblages.

The mineral associations and microtextures that we

describe here help to distinguish between these different

possibilities. In summary, we evaluate two possibilities

for the origin of phosphates in Chelyabinsk (1) growth

via metasomatism during thermal metamorphism on its

parent body (an early solar system process), and/or (2)

growth as the crystallization products of impact-induced

shock melts.

Evidence from the Light and Dark Lithologies

The two formation mechanisms we have outlined

are not necessarily mutually exclusive. However, our

results allow us to rule out phosphate formation during

the impact event that formed the Chelyabinsk impact

breccia.

(A)

(B)(C)

(D)

(E)

Fig. 10. Energy dispersive X-ray maps

backscattered electron images of selected phosphate grains from light and dark

lithology (sections A and B, respectively). Grains show a homogeneous major element chemical response regardless of their

internal microtextural state in CL or EBSD. A) Smooth merrillite and complex apatite grain (in CL). B) Smooth merrillite grain

(in CL). C) Degraded apatite grain (in CL). D) Patchy apatite and recrystallized merrillite grain. E) Recrystallized merrillite

grain (in CL and EBSD). (Color figure can be viewed at wileyonlinelibrary.com.)

754

C. R. Walton et al.

First, the finely crystalline quench textures of melt

veins appear to post-date (i.e., crosscut) the relatively

coarse intergrowth textures described for most

phosphate

–

olivine textures in the light lithology of

Chelyabinsk (e.g., Fig. 2B). Frictional melting along

planes of displacement (now preserved as SMVs) has

therefore affected some of the phosphates in

Chelyabinsk. At the same time, this observation

requires that the phosphates existed prior to the impact

event

—

rather than having formed during it.

During this event, light lithology phosphates

became deformed and dark lithology merrillites were

recrystallized (e.g., Figs. 7, 8, and 10). The

recrystallization textures of dark lithology merrillites are

pristine (i.e., unstrained subdomains), showing that

there has been no significant metamorphic overprint or

alteration that postdates the shock melting event that

produced the Chelyabinsk breccia. Undeformed igneous

grains show tightly clustered pole figures in EBSD data.

Pole figures of recrystallized merrillites in Chelyabinsk

instead show evidence for memory of parent crystal

orientation in the form of extended arcs: This is

unambiguous evidence that these grains did not grow

from an impact-induced melt (Fig. 9C). Therefore, the

phosphate minerals found in those lithologies must also

predate the shock-melting impact during which they

recrystallized.

Phosphates in the light lithology display significant

crystal

–

plastic deformation (up to 16

internal

misorientation) yet preserve minimally deformed olivine

inclusions with similar crystallographic orientation

(Fig. 8, Panel C2; Fig. S6). Both of these observations

clearly imply that these phosphate

–

olivine intergrowths

must predate the impact that deformed them. Our

observations suggest the growth of coherent phosphate

grains, potentially via olivine replacement, to leave

(A)

(B)

(C)

(D)

Fig. 11. A) Inverse pole figure (IPF) map of olivine surrounding and included within MERR-280-A reveals random orientations

and minimal deformation of individual grains. B) IPF map of olivine inclusions in MERR-003-B reveals that individual grains

are randomly oriented. The pole figures of a large (outlined) olivine inclusion indicate that the entire grain shows moderate

crystal

–

plastic deformation, but no randomly oriented subdomains that would be indicative of recrystallization. C) IPF maps

and pole figures for olivine grain B13 reveal numerous subdomains, indicative of recrystallization. Data are for the orthorhombic

structure of olivine of mmm crystal class. D) IPF maps and pole figures for olivine grain B12 reveals larger strain-free

subdomains, indicative of crystallization from a melt without subsequent deformation. Mineral abbreviations: Ol

olivine;

Pyx

pyroxene; Ap

apatite; Merr

Merrilite; Pl

plagioclase; Chr

chromite. (Color figure can be viewed at wileyonline

library.com.)

Phosphorus-olivine assemblages in chondrites

755

islands of disconnected but similarly oriented primary

olivine.

Dark lithology phosphates also contain large

rounded olivine inclusions. However, in this instance,

SMV-proximal olivine inclusions display complex and

strained microtexture (Fig. 9, Panel C3, and Fig. S5 in

supporting information). We also observed cases where

recrystallized phosphates and their olivine inclusions

have been cut by and displaced by sliding across SMVs

(Fig. 8). Finally, we find that P concentrations are

uniform in olivine throughout the light lithology

(Fig. 6). This finding suggests that olivine intergrown

with phosphate in the light and dark lithologies did not

grow from a P-rich melt. All of these observations

Fig. 12. Generalized phosphate microstructural types observed in Chelyabinsk with (top panel) CL imaging, which consistently

reveals the widest diversity of features, and (bottom panel) EBSD maps, which show strained and unstrained (recrystallized)

phosphates and olivine grains. (Color figure can be viewed at wileyonlinelibrary.com.)

756

C. R. Walton et al.

suggest that metasomatic replacement of olivine by

phosphate occurred before impact metamorphism.

Evidence from the Melt Lithology

The shock-melt lithology lacks any discrete

phosphate phases. This observation suggests the

complete melting of phosphates during impact, followed

by the formation of P-rich olivine during quench

crystallization of the resulting melt. A quench origin is

obvious based on the petrographic context, quench

textures (e.g., high abundance of melt inclusions,

concentric-to-interstitial skeletal growths; Fig. 4) and

the anomalous chemistry of these olivine grains (Fig. 5),

including boundary enrichment of phosphorus.

The EBSD maps of light, dark, and melt lithology

olivine contrast significantly. In the light lithology,

olivine is only very mildly deformed (Fig. 11A). In the

dark lithology, olivine shows weak

–

moderate level of

crystal

–

plastic deformation when in direct contact with

SMVs (Fig. 11B). In the shock melt, relict cores are

recrystallized, and both these and quench olivines are

apparently strain-free (Figs. 11C and 11D). Evidently,

both the quench-crystallized P-rich and relict P-poor

recrystallized olivine, like the recrystallized phosphates,

have not experienced additional deformation since

impact-related (de)formation.

Recrystallized relict olivine cores in the shock melt

served as nuclei for growth of the subhedral to euhedral

branches (Fig. 4; see Erdmann et al. [2014] for similar

textures). Indeed, the texturally distinct core and quench

olivine in the shock-melt lithology also display

contrasting chemical compositions (Fig. 5). Combined

with increasing backscatter brightness toward the rim,

later crystallizing quench olivine contains higher

concentrations of elements that are generally

incompatible in the crystal structures of olivine and

pyroxene (e.g., P), that is, those silicate phases observed

to have freshly crystallized in the shock melt lithology.

This relationship could be simply interpreted as

reflecting growth of later crystallizing silicates from an

increasingly incompatible element-enriched residual

impact melt. This interpretation is consistent with the

presence of P-rich glassy melt inclusions trapped in

olivine quench crystals, visible in EDS maps (Fig. 5B).

Despite an overall trend, the scatter in P content

with olivine Fe-Mg composition across all compositions

suggests that P systematics in the shock melt were

highly heterogeneous throughout the crystallization

sequence (Fig. 6). This heterogeneity is potentially

consistent both with the slow diffusion of P in mafic

silicate melt (Watson et al. 2015) and the seemingly

nonuniform distribution of P-rich olivine throughout

the investigated sample. We infer that the spatial

distribution of P-rich olivine in shock-melt lithology

derives from the melting of pre-existing phosphates,

potentially incomplete homogenization of P through the

melt, and finally local P accumulation as an

incompatible element during progressive quench

crystallization. Future studies may be able to utilize P

systematics in olivine as a crystallization speedometer

for quench-cooled shock melts.

Overall, there is no evidence that any of the

phosphates in Chelyabinsk are the products of impact

mobilization. Melting did affect phosphates in the

shock-melt lithology; however, here, P has been trapped

as a minor element in silicate phases (principally olivine)

upon cooling and crystallization. The textural evidence

we present requires that all preserved phosphate grains

existed prior to impact but responded differently to

metamorphism depending on lithology-scale conditions

and the propagation of shock waves through local

phase assemblages. Building on this conclusion, we now

highlight textural evidence for the important role

played by plagioclase mobilization during impact

metamorphism.

Shock Effects on P-Bearing Minerals

Modification of Phosphate Phase Associations

We explain the anomalous phase associations of

Chelyabinsk phosphates using a novel mechanical shock

modification model, which relies upon selective

plagioclase melting and its subsequent mobilization

toward olivine, pyroxene, and phosphate. We propose

that phosphates formed initially during thermal

metamorphism with no preferential phase association

(as described by Lewis and Jones 2016). Phosphates in

Chelyabinsk then had these initial growth-related phase

associations modified during the impact melting and

mobilization of easily shock-melted phases such as

plagioclase and metals/sulfides (Rubin 2003a, 2003b).

We further propose that, owing to their intergrown

nature, phosphate

–

olivine grain boundaries were

mechanically more resilient to plagioclase-melt

propagation during impact. The common association of

light lithology phosphates with olivine and plagioclase,

but infrequent association with metal and sulfides

(Fig. 3), provides support for this model.

We observe smooth-textured plagioclase proximal

to many light lithology phosphates, as linear trails

interstitial to olivine and pyroxene (Fig. 2C), and as

thin borders around some silicate grains (Fig. 2C). In

places, plagioclase has locally entrained numerous very

small (

m) inclusions of silicate, metal, and

chromite. Correlating the CL images and EBSD maps

further indicates that at least some of this plagioclase is

poorly crystalline (Figs. 7 and 8). It is also highly

unlikely that surface condition is responsible for the

Phosphorus-olivine assemblages in chondrites

757

observed poor EBSD response, as other phases respond

extremely well. In accordance with previous authors

(e.g., Kaeter et al. 2018), we interpret these features as

indicative of plagioclase being melted upon initial

shock, mobilized in the turbulent flow regime behind

the shock front, and quenched by rapid cooling to

amorphous material (i.e., maskelynite; Ferri

ere and

Brandst

€

atter 2015). As such, plagioclase-normative

melts appear to have preferentially flowed into pre-

existing grain boundaries and fractures opened during

passage of the initial shock wave (e.g., Fig. 2C).

A “preferential-melting model” for increasing the

probability of finding plagioclase in association with

phosphate in the light lithology only operates over a

limited range of shock states. For instance, a much

greater percentage of dark lithology phosphates display

contacts with metal and sulfide phases than in the light

lithology (Fig. 3). This is consistent with the fact that

metal and sulfide vein networks are more extensively

developed in the dark lithology, as these phases also

became mobilized during the higher peak and post-

shock conditions experienced (Morlok et al. 2017;

Moreau et al. 2018). As such, preferential mobilization

of plagioclase resulted in general phosphate separation

from metal and sulfide in the light lithology (i.e.,

destroying growth assemblages), but additional

mobilization of Fe-Ni metal and sulfide created a more

randomly mixed set of phase associations in the dark

lithology. In both cases, associations with olivine were

widely retained (Fig. 3).

Our observations suggest that plagioclase in

Chelyabinsk was extensively mobilized under S4

conditions, in the light lithology, but that this

metal/sulfide mobilization was increasingly extensive

under the S5 conditions of the dark lithology. An even

stronger shock deformation would cause wholescale

melting of both low-melting point and high-melting

point phases and, in particular, would consume pre-

existing phosphates. In general, the post-shock phase

associations should vary depending on the phases

mobilized. Figure 13 schematically illustrates our

preferred model for the sequence of phosphate phase

associations in the light and dark lithologies of

Chelyabinsk, in the context of the overall evolution of

phosphorus-bearing mineralogy in ordinary chondrites.

Modification of Phosphate Microtexture

There is clear evidence for variable phosphate

microtexture as a function of different degrees of shock

metamorphism, represented by the distinct lithologies in

Chelyabinsk. In particular, recrystallization textures in

CL and in EBSD are only observed in the dark

lithology; although there are sparse SMVs in the light

lithology, phosphates proximal to these veins are not

recrystallized. The presence of SMVs in the light

lithology indicates that SMV-proximal peak

temperatures reached

1950 K for

70 ms (Ozawa et al.

2014). However, their sparse distribution implies that

SMVs would have cooled more quickly in contact with

less shocked and less heated matrix in the light

compared to the dark lithology, thus providing

insufficient heat for recrystallization of phosphates.

Moreau et al (2018) correlated mesoscale modeling

of shock processes to observe textural features in

ordinary chondrite samples, finding that the S5

–

transition associated with shock darkening corresponds

1200 K post-shock temperatures in troilite and

600 K in olivine. Connecting the shock stage

evaluation of Morlok et al (2017) with the results of

Moreau et al (2018), peak post-shock temperatures in

the dark lithology should have been greater than in the

light lithology by a minimum of 200 K, prior to

thermal equilibration at much lower temperatures.

Given that subdomain formation in a deformed matrix

is more efficient at elevated temperatures (Urai et al.

1986), this difference in post-shock heating intensity is a

plausible explanation for the contrasting microtextural

state of light versus dark lithology phosphates in

Chelyabinsk.

Other recent studies have also highlighted the role

of shock metamorphism for inducing recrystallization

textures in phosphate minerals observed in a wide range

of terrestrial (e.g., McGregor et al. 2019; Cox et al.

2020; Kenny et al. 2020) and lunar (

Cernok et al. 2019)

samples. Our findings for the heterogeneously shocked

Chelyabinsk breccia suggest that recrystallization

textures develop principally in response to prolonged

lithology-scale heating during impact-induced strain at

the grain scale. Therefore, we mark out phosphate

recrystallization as a sensitive indicator of micro-to-

mesoscale

–

–

pathways in the postimpact

environment.

It is notable that recrystallization textures are

visible in CL (Fig. 9C). Together, the bright internal

spots as well as conspicuous bright subgrain boundaries

suggest that the diffusive migration and/or grain

boundary enrichment of trace elements during shock

processing may be responsible for the specific response

displayed (e.g., Gros et al. 2016; McGregor et al. 2019).

The EDS images (Fig. 10) do not reveal any such

chemical zonation in major elements, but EDS does not

resolve variation in the trace elements that typically

modulate CL response.

In the scope of this work, we can robustly state that

CL imaging provides a way to rapidly assess phosphate

microtextures prior to EBSD work. As such, our

combined CL-EBSD approach allows us to interpret

only limited number of high-resolution EBSD maps as

758

C. R. Walton et al.