Phosphorus
-
Olivine
-
Assemblages (POAs): a paragenetic model for P
-
bearing phases in primitive meteorites
C. R. Walton
1
, O.
Shorttle
1,2
,
A. Černok
3
,
I. Baziotis
4
, P. Asimow
5
, L. Ferri
è
re
6
,
M. Anand
3
1
Department of Earth Sci-
ences, University of Cambridge, Downing Street, Cambridge CB2 3EQ UK,
2
Institute of Astronomy, University of Cambridge,
Madingley Road, Cambridge, CB3 OHA, UK
.
3
Department
of Physical Sciences, Open University, Walton Hall, Milton Keynes
MK7 6AA, UK
.
4
Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Ath-
ens, Iera
Odos 75, 11855 Athens, Greece.
5
Division of
Geological and Planetary Sciences, California Institute of Technology,
1200 E California Blvd, Pasadena, California 91125, USA.
6
Natural History Museum, Burgring 7, A
-
1010 Vienna, Austria.
Introduction:
Phosphate minerals
(e.g.,
apatite (Ca
5
[PO
4
]
3
[OH,Cl,F])
,
merrillite (Ca
9
NaMg
[
PO
4
]
7
)
)
are emerg-
ing
as
crucial tools
in the investigation of extraterrestrial volatile, thermal, and collisional histories
(
1
–
5
)
. However,
the mechanisms involved in the formation (and deformation/destruction) of phosphate minerals in primitive meteor-
ites remain unclear, hindering our ability to confidently interpret an important record of chemical, isotopic, and
structural informa
tion. Here, we provide further
observations and arguments
in support of a key role for olivine in
the paragenesis of P
-
bearing phases in chondrites
(
1
)
.
Paragenesis of
POAs
:
By
Back
-
Scatter
-
Electron, Electron
-
Back
-
Scatter
-
Diffraction, and Cathodolumins
cence
imaging
investigation of the Chely
abink
LL5 chondrite
, we developed a new model of meteorite phosphate paragen-
esis
.
Figure 1 presents a schematic overview of our proposed Phos
phorus
-
Olivine
-
Assemblage (POA) scheme.
Phosphorus in a reduced oxidation state is initially hosted by metal, following nebular condensation
(
6
)
. Parent body
radiogenic metamorphism results in progressive oxidation of reduced phases, in which siderophile
elements are
known to progressively migrate out of metal hosts
(
7
)
.
Observations of partially equilibrated chondrites reveal P
-
enriched grains of olivine, formed during metamorphic reactions with migrating fluids
(
8
)
–
a type I POA. In more
fully to c
ompletely equilibrated chondrites, P is hosted quantiatively by phosphate minerals
(
1
)
. These phosphate
minerals share a close textural relationship with olivine, in particular sharing grain boundary contacts and large in-
clusions of either phase in the o
ther. Olivine inclusions in a given grain are observed to preserve similar crystallo-
graphic orentiations. These observations support interpretation of olivine
-
phosphate textural association as resulting
from olivine
-
replacement reactions
(
1
)
, presumably initially at the expense of (now
-
consumed) P
-
rich olivine, which
we define as Type II POAs. Finally, highly shocked meteorites contain shock melt portions in which phosphates are
absent; instead, P is found principally
in
the latest generation of
quench olivine crystals
(
1
)
–
Type III POAs.
Conclusions:
Textural constraints
from unequilibrated, equilibrated, and shocked meteorites support a par-
agenesis
for POAs
leading from metal, through P
-
rich olivine and subsequent replacement by phosphate m
inerals, to
P
-
rich silicates in
shock
-
melted lithologies. This model provides a useful paragenetic context for interpreting the
information preserved by each of these phases in a given meteorite. Our POA classification scheme supports the u
se
of metal
/
sili
cate compositions for trac
ing
parent body equilibration
;
phosphates
as tracers of volatile
specie
s during
parent body metamorphism
;
and high
-
T P
-
rich olivine as a sensitive tracer of post
-
impact crystallization processes.
References:
[1]
C. R. Walton
et al
.
Meteorit Planet Sci
(2021), doi
:10.1111/maps.13648.
[2]
L. F.
White, J.
et al
.
Meteorit Planet Sci
.
54
, 1262
–
1282 (2019).
[3]
G. H. Edwards, T. Black-
burn.
Sci Adv
.
6
, eaay8641 (2020).
[4]
J. R.
Darling, L. F.
et al
.
Geochim Cosmochim Ac
.
293
, 422
–
437 (2020).
[5]
A. Stephant
et
al.
Geochim Cosmochim Ac
(2019),
doi:10.1016/j.gca.2019.07.045.
[6]
M. A.
Pasek.
Icarus
.
317
, 59
–
65 (2019).
[
7
]
B. Zan-
da, M.
et al
.
Science
.
265
, 1846
–
1849 (1994)
.
[
8
]
Y. Li
et al
.
Am Mineral
.
102
, 98
–
107
(2017).
Figure 1:
schematic view of POA paragenesis.
Mineral ab-
breviations: Ol = olivine, Pyx = pyroxene, Plag = plagioclase,
SMV = shock
-
melt
-
vein.
6022
.
pdf
84th
Annual
Meeting
of
The
Meteoritical
Society
2021
(
LPI
Contrib
.
No
.
2609
)