DISCOVERY OF
HIGH
-
PRESSURE POLYMORPH
S
IN
THE RECENT FALL
O
F
VIÑALES (L6
ORDINARY CHONDRITE)
:
IMPLICATIONS FOR COLLISIONS ON ITS PARENT BODY
.
I
. B.
Baziotis
1
,
S. Xydous
1
, A. Papoutsa
1
,
J. Hu
2
,
C. Ma
2
,
L.
Ferrière
3
, S. Klemme
4
, J. Berndt
4
,
and
P.
D.
Asimow
2
,
1
Agricultural University of Athens, Iera Odos 75, 11755, Athens, Greece
(
ibaziotis@aua.gr
)
,
2
California Institute of
Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA
,
3
Natural History Museum,
Burgring 7,
A
-
1010 Vienna, Austria,
4
Westfälische Wilhelms‑Univ. Münster, Correnstrasse 24, 48149 Münster,
Germany
.
Introduction:
Ordinary chondrites
record
impact
-
related features
, including
high
-
pressure (HP) mineral
s
,
usually
occurring
in melt veins (MV
s
)
,
yielding
constrain
ts on
shock conditions and hence parameters
such as the
size of
colliding bodies
,
impact velocity
, etc
. Here, we report the first
observations
of HP mineral polymorphs in the
recent
L6 chondrite fall
of
Viñales
(
Cuba
,
1
st
February 2019
;
proposed
shock stage S
3
[1]
and partly S4
[2]
).
The investigated
thin section
shows
at least nine
major
,
subparallel
,
MV
s
(5
0
-
600
μm wid
e
)
and
a
few thin
ner
MVs
at an
angle
to these
.
It is unclear whether the
y
are of a single or multiple generations
.
W
e have
so far
identified, within a single melt vein,
a
region
hosting the
HP polymorphs
majorite
and
albitic
jadeite
.
T
hese observations
define a definite press
ure range
for this meteorite and contri
b
ute to the shock record of
the L
-
chondrites in general.
Material and
a
nalytical
m
ethods
:
One
polished thin section
(
from a
private collection)
of the Viñales meteorite
was examined for shock indicators
with a focus
on
the MVs.
O
ptical microscopy, scanning electron microscop
y (
Field
-
emission
SEM;
JEOL JSM
-
IT300LV
and
Zeiss 1550VP
)
, electron
micro
probe
(
EPMA;
JEOL JXA 8900 and JEOL
JXA 8530F
)
,
micro
-
Raman spectroscopy
, and electron back
-
scatter diffraction
(EBSD)
were used
.
Fifteen
areas lo-
cated in
nine
MVs were analyzed for
their
texture and mineral chemistry.
Several
regions were studied with co
-
located
Raman spectroscopy
;
one
site, so far, contains
HP polymorphs
.
Results and Discussion
:
Although no HP minerals have
previously
been reported in the
Viñales
meteorite, we
have observed
two
HP minerals
:
majorite
and
albitic
jadeite
.
A Raman spectrum
, obtained from a grain located within
a melt vein
(grain size ~
2
-
3
μm in diameter), displays th
e characteristic major peak at
~
92
5
cm
−1
and
minor peak at
~5
83
cm
−1
,
reported from both synthetic and natural majorite. The EBSD pattern collected
from the same point can
be indexed only with the garnet structure
.
The
EPMA
compositions of grains
in this a
rea
,
with these Raman and EBSD
characteristics
,
show
nearly
end
-
member majorite composition
,
Ca
0.15
Mg
2.96
Fe
0.55
Al
0.41
Si
3.
87
O
12
.
The
albitic
jadeite has
been identified as
crystals
with
irregular shape
(<1
-
2
μm
)
within a Na
-
rich melt pool,
adjacent to
the
majorite
-
bearing
melt vein
.
The
Raman spectrum
of
the
albitic
jadeite
shows
distinct peaks at
~700
,
375, and 381
cm
-
1
.
Unfortunately,
the peaks at ~988 and ~1034 cm
-
1
,
related to vibration of [Si
2
O
6
]
4−
groups
in
jadeite, which are not resolved in albitic
jadeite,
were
not
observed.
EPMA
analysis
yields
the
following
empirical
formula:
(Na
0.52
Ca
0.06
Mg
0.04
K
0.01
□
0.37
)
(Al
0.75
Si
0.26
)Si
2
O
6
, with Ca# [100×Ca/(Ca+Na)] of
10
.
2
; the M2 vacancies and Si on M1
show that
this is albitic jadeite [3]
.
No
suitable
EBSD signal
was obtained
for this beam
-
sensitive material
, but
we
plan
to
confirm the pyroxene
structure by
t
ransmission
e
lectron
m
icroscopy
.
If
a
part of the melt veins reached peak temperatures above the liquidus
of the matrix material and maintained
that
state long enough to reach complete melting, then
the MV
would have evolved according to the liquidus
relations of
a cooling chondritic liquid.
The texture of the melt vein hosting the majoritic garnet suggests
an
evolution from a
liquid,
as
indicated by the equi
g
ranular
crystal
shape
and
12
0
ο
triple junctions.
Although
Raman spectra
from
the
Na
-
rich fine
-
grained region
suggest a jadeite
-
like pyro
xene
, the non
-
stoichiometric composition currently precludes use
of the jadeite stability field as a constraint
.
However,
the
absence
of lingunite suggests maximum
P
< 21 GPa
[4]
and
the absence of Ca
-
ferrite, Ca
-
perovskite, or Ca
-
rich garnet suggests, at
least locally,
P
≤ 15.5
[5]
GPa.
Conclusions
:
Pending further analyses, it is already clear that Viñales hosts a
mineral
record of high
-
P
,
T
. In con-
trast to previously published works, we identifi
ed
two
HP polymorphs.
The
investigated section of
Viñales
shows
features more consistent with shock stage
S5
-
S6 [
6
]
.
T
he presence of discrete
melt
veins indicates heterogeneity of
the
T
field, likely the result of
the
collapse of spatially variable porosity during shock compression
or slip along
localized shear bands
[
7
]
. We cannot presently
define the range of
peak condition
s
or
P
-
T
path
s experienced
by
the
meteorite or rule out
the
recording of multiple shock events on its parent body.
Acknowledgments:
I.B., S.X.,
and
A.P. received support for this research from ESF and the Greek State (call
code EDBM103). I.B. thanks
EU
-
funded SYNTHESYS [AT
-
TAF
-
30] project which provided travel and accommo-
dation funds, and
instrument
time costs while using SEM and EPMA at
the
NHMV.
Ref
erences:
[1]
Gattacceca J.
et al.
(
2020
)
M
APS
55
:
1146
–
1150.
[2]
Yin F.
and
Dai D. (2021)
Science Progress
104(2), 00368504211019859.
[3]
Ma C.
et al.
(
2021
)
Am. Min.
, 10.2138/am
-
2021
-
7925.
[4]
Gasparik T.
(
1989
)
CTMP
102
:
389
–
405.
[
5
]
Bobrov A. V.
et al.
(
2008
)
GCA
72
:
2392
–
2408.
[
6
]
Stöffler D.
et al.
(
2019
)
MAPS
54
:
946
–
949.
[
7
]
Baziotis I.
et al.
(
2018
)
Sci.
Rep.
8(1)
:
1
–
16.
6115
.
pdf
84th
Annual
Meeting
of
The
Meteoritical
Society
2021
(
LPI
Contrib
.
No
.
2609
)