Mineralogy of a Mudstone at Yellowknife Bay, Gale Crater, Mars
Authors
: D.T. Vaniman
1*
, D.L. Bish
2
, D.W. Ming
3
, T.F. Bristow
4
, R.V. Morris
3
, D. F. Blake
4
,
S. J. Chipera
5
, S.M. Morrison
6
, A.H. Treiman
7
, E.B. Rampe
3
, M. Rice
8
, C.N. Achilles
9
†
, J.
Grotzinger
8
, S.M. McLennan
10
, J. Williams
11
, J. Bell III
12
, H. Newsom
11
, R.T. Downs
6
, S.
Maurice
13
, P. Sarrazin
14
, A.S. Yen
15
, J.M. Morookian
15
, J.D. Farmer
12
,
K. Stack
8
, R.E.
Milliken
16
, B. Ehlmann
8
,15
, D.Y. Sumner
17
,
G. Berger
13
, J.A. Crisp
15
, J.A.
Hurowitz
10
, R.
Anderson
15
, D. DesMarais
4
, E.M. Stolper
8
, K.S. Edgett
18
, S. Gupta
19
, and N. Spanovich
15
, MSL
Science Team
‡
Institutions
:
1
Planetary Science Institute, Tucson, AZ, 85719, USA
2
Department of Geological Sciences, Indiana University, Bloomington
, IN, 47405, USA
3
NASA Johnson Space Center, Houston, TX, 77058, USA
4
NASA Ames Research Center, Moffett Field, CA, 94035, USA
5
Chesapeake Energy, Oklahoma City, OK, 73154, USA
6
Department of Geosciences, University of Arizona, Tucson, AZ, 85721, USA
7
Luna
r and Planetary Institute, Houston, TX, 77058, USA
8
Division of Geologic and Planetary Sciences, California Institute of Technology, Pasadena, CA,
91125, USA
9
ESCG/UTC Aerospace Systems, Houston, TX, 77058, USA
10
Department of Geosciences, SUNY Stony Brook
, Stony Brook, NY, 11794, USA
11
Institute of Meteoritics, University of New Mexico, Albuquerque, NM, 87131, USA
12
School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA
13
Institut de Recherche en Astrophysique et Planétologi
e (IRAP), Universite de Toulouse/CNRS,
Toulouse, 31400, France
14
SETI Institute, Mountain View, CA, 94043, USA
15
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
16
Department of Geological Sciences, Brown University,
Providence, RI, 02912, USA
17
Department of Earth and Planetary Sciences, University of California, Davis, CA, 95616, USA
18
Malin Space Science Systems, San Diego, CA, 92121, USA
19
Department of Earth Science and Engineering, Imperial College London, SW7 2
AZ, UK
*Correspondence to:
dvaniman@psi.edu
†
Current address:
Department of Geological Sciences, Indiana University, Bloomington, IN,
47405, USA
‡
MSL Science Team authors and affiliations are listed in the suppleme
ntary materials.
Abstract
Sediment
ary rocks
at Yellowknife Bay (Gale Crater)
on Mars
include
mudstone
sampled by t
he
Curiosity
rover. The samples, John Klein and Cumberland,
contain
detrital
basaltic minerals
,
Ca
-
sulfates,
Fe oxide/hydroxides,
Fe
-
sulfides, amorphous material, and
trioctahedral
smectite
s
.
The
John Klein
smectite
has
basal spacing of ~10 Å
indicating
little interlayer hydration.
T
he
Cumberland
smectite
has basal spacing at
~
13.2
Å
as well as ~10 Å. T
he ~13.2 Å spac
ing
suggests
a
partially chloritized
interlayer
or interlayer
Mg
or Ca
facilitating H
2
O
retention
.
Basaltic minerals in the mudstone are
similar to those in nearby
eolian deposits
.
However,
th
e
muds
tone has
far less
Fe
-
forsterite
, possibly lost with formation of
smectite
plus magnetite
.
Late
Noachian/E
arly Hesperian or younger
age
indicates that clay
mineral
formation on M
ars
extended
beyond
Noachian
time
.
Introduction
The recent de
cade of orbiter
-
and
rover
-
based studies of ancient
sedimentary rocks
on Mars
has revealed a diverse mineralogy
that constrains the nature and timing of early environments in
the history of the planet
(
1,
2,
3
)
These studies provide a starting point for considering the
habitability of Mars, based on a
n
understanding of the
aqueous geochemistry and mineralogy of
rocks place
d
within a geologic framework
(
4,
5
)
.
Such an approach has been adopted by the
Mars Science Laboratory (MSL)
mission
, where
the
science payload and advanced capabilities
of the
Curiosity
rover were designed for assessment of past habitability
(
6
)
.
M
ission goals f
or MSL
place
d
high priority on aqueous
-
system mineralogy, particularly clay
minerals and sulfate salts
(
6
)
.
The mission concept for
the landing site
in Gale Crater was to
leave the landing spot
quickly
and
drive
to
Aeolis Mons, a central mound informally known as
Mount
Sharp. Interpretation of Mars
Reconnaissance
Orbiter
CRISM
(Compact Reconnaissance
Imaging Spectrometer for Mars)
visible
-
near infrared
spectroscopy suggests the presence of
hydrated minerals in sedimentary layers at the base of the mound
(
7
)
. However, soon after
landing
a contact between three different
geologic
units, one with relatively high thermal
inertia,
was
recognized
within
~450
m
of the landing spot, just beyond the alluvial lobe of the Peace
Vallis fan
(
8
)
.
The decision to
drive away from Mount
Sharp toward this
location
has
provided
early
samples of
a
mudstone
th
at contains both clay minerals
and sulfat
e salts.
The John Klein
and Cumberland drill samples were
collected from
the Sheepbed
mudstone
member
of
the sedimentary
Yell
owknife Bay
formation, which
is interpreted as a shallow
lacustrine deposit
(
8
)
.
John Klein and Cumberland
are the second and third
solid
sample
s,
respectively,
collected by
the MSL rover
Curiosity
. The first sample
,
from
an eolian deposit
named
Rocknest,
is
~6
0
m west of the mudstone
drill locations.
The loose Rocknest deposit
(
9
)
had been
used to commission
Curiosity’s
scoop sampling system
and
t
he
lithified
John Klein
sample wa
s used to commission the drill.
Curiosity’s
sampling system
delivers
both scooped and
drill
ed
powder
s to the same set of sieves
(
10
)
. All scooped or drilled samples were sieved to
<150 μm and
portions
were analyzed by the
Chemistry and Mineralogy (CheMin) X
-
ray
diffraction (XRD) and X
-
ray fl
uorescence (XRF) instrument
(
11
)
and the Sample Analysis at
Mars (SAM
) quadrupole mass spectrometer/
gas
chromatograph/
tunable laser spectr
ometer suite
of instruments
(
12,
13
)
.
CheMin XRD
data are the focus of this paper.
Altho
ugh the CheMin XRD
instrument is the
prime mineralogy tool carried by
Curiosity
, constraints
on
the mudstone
mineralogy
are provided
by
the temperatures at which volatiles are released in
SAM evolved gas analyses
,
particularly
H
2
O release profiles
(
13
)
. Other instruments on
Curi
osity
provide
additional
insight into
mineralogy.
Mastcam
(
14,
15
)
multispectral images are capable
of sand
-
size resolution and
potential identification of
certain hydrated minerals
us
ing
short
-
wave
length
near
-
IR filters
(
16
)
.
ChemCam
h
as a narrow laser beam
that can target veins and nodules for remote chemical
analysis by laser
-
induced breakdown spectroscopy (LIBS)
,
with sensitivity to many elements
including hydroge
n
(
17,
18
)
; this capability can aid in constraining
mineral compositions
where
individual
minerals are
~0.5 mm
or larger and is particularly sensitive to alkali and alkaline
-
earth
elements
(
19
)
.
The MSL alpha
-
particle X
-
ray spectrometer (APXS) has proven heritage
from
previous missions and provid
es sensitivity to a wide range
of common rock
-
forming
elements
(
20
)
, although analysis spot resolution is ~1.7 cm diameter, providing a bulk
chemical
analysis
rather than mineral analysis for most samples
.
We use the results from APXS to constrain the
composition of the X
-
ray amorphous com
ponents of the mudstone
.
The Ma
rs Hand Lens Imager
(MAHLI)
provide
s
high
-
resolution images
,
to 14 μm per pixel,
with
Bayer pattern color that can
simulate
hand
-
lens
or close
-
up
sample analysis
(
21
)
.
Figure 1 shows
Mastcam and MAHLI
images of
the boreholes and drill
powders
for John
Klein and
Cumberland
.
Cumberland
(Fig
.
1C
) was targeted
after John Klein, in order to analyze
a part of the mudstone with fewer sulfate
veins
and a greater abu
ndance of res
istant concretions
,
including nodules and hollow nodules that
are regarded as early ceme
ntation
.
The two drill holes
are
~
3 m apart.
Powders
of both
mudstone samples
are notably
gray in color, unlike the red
dish
weathering
and/or dust
evident on the
s
urface of the mudstone.
This reddish surface of the
mudstone
did not contribute to either drill sample, for the auger does not pass powder into the
sampling system until it is ~1.5 cm into the drill target
(
10
)
.
Mineralogical Analysis and Quantitative Mineralogy
The X
-
ray diffraction
patterns for John Klein and Cumb
erland are compared in Fig. 2A.
We
quantified the
c
rystalline
components
,
other than
smectite
s
,
in John Klein and Cumberland
(Table 1)
by
using whole
-
pattern fitting and Rietveld analysis
(Fig
s
.
2B,C)
; smectites and the
amorphous component were quantified using a modified version of the FULLPAT program (
22
).
These methods are described
in
(
23
)
and elaborated
in (
24
).
C
onversion of the
two
-
dimensional
images of Debye di
ffraction rings on the CCD to one
-
dimensional
diffraction patter
n
s
was done
in the same manner for Rocknest, John Klein, and Cumberland.
For all three samples, we
obtained
unit
-
cell parameters (Table 2
)
and phase compositions (Table 3)
for major phases
.
T
he
unit
-
cell parameter an
d phase composition data reported for
Rocknest, John Klein, and
Cumberland were processed in the same manner and are therefore comparable.
An independent
assessment of
the
abunda
nce
of
smectite
and amorphous material can be
obtained from the
fixed or cell
-
parameter
-
constrained
chemical compositions of th
e
phases listed
in Table 1
and
the total sample chemical composition from APXS analysis of the drill tail
ings
.
This approach
(
24
)
uses a mass
-
balance calculation
with XRD con
straints on crystal chemistry
(
25, 26
)
and
, for the present study,
model
smectite
compositions
,
to estimate
the composition of
the
amorphous
component
.
Additional constraints
on
smectite
abundance
come
from
SAM
evolved gas analysis, where the amount of
H
2
O
released at higher temperatures (~
400
-
835
°
C)
can be rela
ted
to dehydroxylation
of
various
clay minerals
(
13
)
.
Within estimated errors these
three methods agree, but in this paper we focus on the XRD
estimates (
Table 1
).
None of the coarse
(
>
m
m width
)
veins that cross the Sheepbed member
(
8
)
were sampled for
CheMin and SAM analysis. However, Mastcam near
-
IR spectral filters are sensitive to
hydration
in certain minerals
(
27
)
, and this method
(
24
)
was used to analyze veins where ChemCam and
APXS data indicate
d
that Ca
-
sulfate p
hases are present
. T
his allows mapping
of
inferred
gypsum
distributions in th
e veins
that were not sampled for CheMin and SAM analysis
.
The
sedimentologic context
of the mudstone is
complex. Observations of the mudstone
(
8
)
lead to interpretation of an
early
-
diagenetic association of
no
dules, hollow nodules
, and
raised
ri
dges
that are crosscut by late
-
diagenetic fractures filled with light
-
toned
Ca
-
sulfates
(S
-
Ca
association in the
se
veins was
identified with ChemCam)
. E
arly
-
diagenetic hollow nodules
are
filled with light
-
toned sulfates only
where intersected by late
-
diagenetic light
-
toned
microfractures
(
8,
19
)
.
MAHLI images show that the John Kle
in drill spot
had a surface footprint
(1.6 cm diameter)
with ~3.9% hollow nodules
, ~2.5% solid nodules, and ~14.5% light
-
toned
sulfate, whereas the Cumberlan
d drill spot had ~8.5% hollow nodules
,
~
2.2% solid nodules, and
no evident light
-
toned sulfate.
Estimates of light
-
toned sulfate abundances from pre
-
drilling
images can be deceptive because their
three
-
dimensional
distribution is dependent on variable
occurrence of thin veins t
hat may or may not be visible on
the
dust
-
mantled surface. A more
accurat
e survey
of the distribution and abundance of light
-
toned sulfates is obtain
ed by analysis
of drill
-
hole
wall images
(
24
), at least to the depth exposed (the lower part of each drill hole
contains some debris)
.
T
he borehole samples John Klein and Cumberl
and provide adequate sampling of the
mudstone matrix with a partial sampling of features that are both early
-
d
iagenetic (nodules and
hollow nodules
) and late
-
di
a
genetic (light
-
toned veins). The drill did not sample any
early
-
diagenet
ic raised ridges. The
raised ridges
were identified in LIBS
and APXS analyse
s as
including an Mg
-
Fe
-
Cl
rich component; in i
m
age
s
they appear to have
an isopachous filling of
several
layers
and may be mineralogically complex
(
19
)
.
Without direct sampling and CheMin
XRD analys
is
,
knowledge of mineralogy i
n the
raised ridges
is speculative
an
d is not addressed
here
.
Si
licates other than
Smectite
s
Several detrital
silicate minerals in the mudstone
bear a strong resemblance to those found in
the Rocknest eolian depo
sit
(
Table
s
2
, 3
)
.
Fe
-
forsterite, p
lagioclase, pigeonite, and augite are
generally
similar between Rocknest, John Klein, and Cumberland, suggesting similar
mafic
sources.
Presence of pigeonite indicates mafic sources that were basaltic.
However, XRD
analyses of the mudston
e
samples reveal presence of orthopyroxene as well as clinopyroxenes,
indicating a source of
some
mafic
minerals
that is either absent from or very minor in the nearby
eolian deposit.
It is notable that Fe
-
forsterite is almost absent in Cumberland and its
abundance in John Klein
is much lower than in Rocknest. Figure 1B shows that the reddish Rocknest sample coats the
walls of the scoop that is filled with the John Klein drill powder. Testbed operations
on Earth
suggest that at least ~4% cross
-
contamination
should be expected
between samples
. By
the time
Cumberland was imaged
(Fig
.
1D), the red Rocknest powder was almos
t gone, so the sampling
system had been largely cleared of this cont
aminant in processing John Klein
. Progressive
dilution and a stronger cle
aning cycle between John Kl
ein and Cumberland left
little if an
y
Rocknest contamination in
Cumberland
–
and conversely, some of the
Fe
-
forsteritic olivine
pres
ent in the John Klein sample might
be contamination from the Rocknest sample.
Phyllosilicates
CheMin
XRD data reveal the presence
of
phyllosilicate
s
in John Klein
and Cumberland
(Fig
.
2
A
).
A
broad 001 diffraction peak
in the John Klein XRD pattern
extends from
12 to 9.4
Å,
corresponding
with the large interlayer spacing of a
2:1
smectite
. This broa
d range of 001
diffraction
is common to a variety of phyllosilicates, but the breadth of this peak (and the lack of
other well
-
defined peaks, such as an
002 peak at 5 Å) argue
against
the
presence of well
-
crystallized phyllosil
icates such as mica or illite
.
Well
-
defined diffraction peaks for
kaolinite
or
chlori
te
-
group minerals at 7 Å
are absent.
A
smectite
with similar diffraction properties is
present in C
umberla
nd, although the low
-
angle region includes a second peak ranging from
~12
-
17
Å with a maximum at
~
13.2
Å
.
This
larger interlayer spacing
in the Cumberland
XRD pattern
is a noteworthy
characteristic
.
The interlayer spacing in a smectite
,
revealed by the broad 001 peak
,
is affected by the layer
charge, the nature of the interlayer
cation(s) (typically K, Na, a
nd/or Ca; less commonly Mg),
the
hydration state of the interlayer cations
, and the possible presence of chloritic interlayers
. The
layer charge and interlayer cation content of a smectite are relatively stable in solid samples
, so
changes in interlayer spacing are mostly dependent on relative humidity. Modeling and
experimental studies
(
28
, 29
)
suggest that if exposed at Mars surface conditions, smectites can go
through diurnal and seasonal hydration cycling, with
substantial
dependence of the amount of
hydration on the nature of the interlayer cation
. For example,
Ca
-
smectite will hold more
interlayer H
2
O than Na
-
smectite at the same conditions
(
30
). The John Klein and Cumberland
samples inside the body of
Curiosity
were
expos
ed to higher and less variable temperature (a
diurnal range of 5 to 25
°
C) than they were in situ. These temperatures yield very low relative
humidities and dehydration should be favored. The position and breadth of the 001 diffraction
peak in the John Kle
in sample have not changed over 30 sols of analysis following collection,
but at 10 Å this
smectite
appears to be largely dehydrated and little or no change would be
expected. The larger 001 spacing in Cumberland has also been static, over 28 sols of analy
sis;
the preservation of this wider spacing suggests a difference in the interlayer composition of the
smectite
in Cumberland compared with John Klein.
Possible explanations for persistent larger interlayer spacing in Cu
mberland include
smectite
having hyd
rated interlayers with H
2
O molecules retained by high hydra
tion
-
energy interlayer
cations, possibly
Mg
2+
(
28
)
or
Ca
2+
(
30
)
,
and partial pillaring of the inter
layer by metal
-
hydroxyl
groups, as with
incipient chloritization
(
31
)
,
that would prevent collapse. The nodule
-
bearing
portion
s of the mudstone that were drilled for sampling pass
laterally into mudstone with early
-
digenetic Mg
-
rich raised ridges described above. These could be sources of Mg for cation
exchange or incipient
chloritization, focused more on Cumberland than on John Klein.
Cation
exchange occurs readily, with the interlayer cation largely reflecting the dominant cation in
solution. Incipient chloritization by fixation of Mg
-
hydroxl groups
can occur
under surficia
l
conditions when exposed to Mg
-
rich alkaline fluids, a process observed in some saline lakes
(
32
)
. Hydrothermal fluids may induce this change as well
(
33
)
.
X
-
ray diffraction analysis of
clay minerals
in terrestrial laboratories has the advantage of
additi
onal s
ample processing,
such as preparation of oriented mounts,
controlled
variation of
relative humidity,
treatment with ethylene
glycol, and heat treatment. Th
ese treatments are
not
poss
ible in CheMin on Mars. In addition
, a
substantial
component of smectite classification is in
determination of trioctahedral or diocta
hedral crystal structure (the range from full to
2/3
occupancy of sites in the octahedral sheet), but this is general
ly
accomplished b
y analysis of
the
06
l
diffraction band
at ~1.54 Å (trioctahedral, ~71
2
Co Kα) to ~1.50 Å (dioctahedral, ~73
2
Co Kα). This is
beyond the diffraction range of the CheMin CCD
detector
(~50
2
). However,
other components of t
he diffraction pattern
correlate
similarly
with this structural dif
ference. The
position of the
maximum in the
0
2
l
band
, at ~ 22.5
° to 23.1° Co Kα, corresponds with the range
from trioctahedral to dioctahedral structures (Fig
.
3). The
02
l
two
-
dimensional diffraction band is
asymmetric, is often overlapped by diffraction
peaks from other phases (
e.g
., augite), and
therefore
is not as easy to measure as 06
l
. However,
this band
provide
s
similar information, as its
position is related to the
b
unit
-
cell
parameter in the same way as
the 06
l
band. The 02
l
diffraction band
maxi
mum
for the John Klein
and Cumberland
sample
s (Fig
.
3
)
is at 22.5
°,
indica
tive of a trioctahedral clay mineral
such as saponite
or Fe
-
saponite
a
nd not of dioctahedral
forms
such as mon
tmorillonite. Some Fe
-
bearing d
ioctahedral smectites su
ch as nontronite
have
similar 02
l
bands, but the fit is not as good as with saponite.
Oxide
and Sulfide
Minerals
M
agnetite is the promine
nt oxide phase in
John Klein
and Cumberland,
as it is at Rocknest
(
23
)
.
Magnetite
is present at
3.8
wt%
in the John Klein sample
and
4.4
wt
% in Cumberland
(Table 1
)
. These
abundance
s are
significantly higher than in
Roc
knest (1.5
wt%
). As a basic
observation
,
concentration
of detrital magnetite in sedimentary mudstones
is surprising.
G
rains
of magnetite (ρ
~5
g/cm
3
) are not expected to be enriched in very fine grained detrital
sedimentary rocks
otherwise
composed of olivine, pyroxene, and feldspar (ρ
~2.7
-
3.7
).
By
reason of hydraulic equivalence
,
grains of higher density may be present but are expected to be
sma
lle
r in size, and
enrichment should not occur
(
34
)
.
Other factors are required for selective
enrichment such as free settl
ing of grains in turbulent flow
, selective entrainment of grains from
a
granular bed by flowing water, and shearing of grains in a
moving granular dispersion
(
35
)
.
However, because
the
Sheepbed mudstone likely formed by non
-
turbulent settling of fines
from suspension in a body of standing water
(
8
)
, we expect that
none of these processes would
have been influential in causing a
hydraulic enrichment of h
eavy minerals.
T
he relative
ly
high
abundance of magnetite in the Sheepbed mudstone
may have been caused by
authigenesis.
Authigenesis o
f magnetite is further suggested by the observation that high magnetite abundance
is
associated
with
los
s of Fe
-
forsterite and the appearance
of
smectite
s
.
U
nit
-
cell parameters of magnetit
e in
the mudstone are about 0.2% smaller th
an for ideal
magnetite, with a unit
-
cell edge
of 8.38
versus 8.39 Å. A possible explanation of the s
maller cell
size is
partial (~20
%) oxidation
of the ferrous iron
, toward the ferric defect
-
spinel maghemite
(8.33 Å)
in which
some
Fe sites are vacated to preserve charge balance
. Alternatively,
substitution of smaller cations such as Cr, Mg, or Al could a
ccount for a smaller
unit cell
,
although it is not clear whether sufficient amounts of these are present
.
In addition to magnetite,
the Rietveld refinements are consistent with
small amounts of
ilmen
i
te and hematite in the mudstone samples. Also present is akaganeite, β
-
FeO
(
OH
,Cl)
,
which
is a possible oxide host for Cl.
It has been previously suggested
(
36
)
, based on mid
-
inf
rared and visible/near
-
infrared spectra,
that akaganeite may be a
precursor to hematite
observed from orbit
on Mars, but
the same study
concluded
that
goethite was a more likely
precursor.
However, the mudstone at Yellowknife Bay is not a typical martian surface material
and the colors of the mudstone beneath i
ts reddish dust mantle
are
substantially
different (Fig
.
1).
Akaganeite was
detected in both
mudstone
sample
s, but not
at Rocknest. Akaganeite at
its type
locality
on Earth
(
3
7
)
occurs
as an alteration product of pyrrhotite
, a sulfide that
is also found in
the Yellowknife Bay mudstone
but not in the Rocknest sample
(Table 1).
Occurrence of this
association at Yellowknife Bay may be evidence of a similar altera
tion relationship. Somewhat
h
igher abundance of akaganeite in Cumberland than in John Klein
(Table 1)
sug
gests that it
could be a
component of concretion formation
, especially of
hollow
nodules
that appear to be
twice as abundant at Cumberland as at John Klein.
Sulfate Minerals
Veins of
Ca
-
sulfate, believed to be
gypsum
,
have been detected by the MER rover
Opportunity
at the western edge of Cape York on
the rim of Endeavour Crater
(
38
)
.
Calcium
s
ulfate hydrates, including
both gypsum and bassanite, have
been inferred from OMEGA and
CRISM orbital spectroscopy in m
ultiple locations on Mars
(
39
,
40
)
,
but with a lack of hydration
bands
at visible
-
near infrared wavelengths
anhydrite has been
elusive.
C
heMin XRD data
show that the
Sheepbed mudstone contains bassanite
and anhydrite (Table
1)
. Anhydrite was also detected in the Roc
knest eolian deposit
.
We have found no XRD evidence
for gypsum in either Rocknest or the two mudstone samples.
However,
Mastcam hydration index
measurements are consistent wit
h the presence of gypsum
in some of the veins crossing the
mudstone, showing that the vein system might contain all three of the principal Ca
-
sulfate
phases.
Specifically, Mastcam’s longest
-
w
avelength filter (1013 ± 21 nm) can detect the 2
1
+
3
H
2
O combination absorption band and/or the 3
OH overtone absorption
band in specific
hydrated minerals
(
16,
27
,
41
)
. Calibrated Mastcam
spectra show evidence for hydration
associated with some light
-
toned, Ca
-
sulfate bearing features in the Sheepbed unit,
including
some
veins
(Fig
.
4A,B)
and
some
fillings within hollow nodules
. However, the hydrati
on
signature is not universal in
these light
-
toned features; several narrow veins observed in the John
Klein vicinity show no evidence fo
r hydration. From comparisons with
laboratory reflectance
spectra of Ca
-
sulfate minerals convolve
d to Mastcam bandpasses (Fig. 4C
), the hydration
signa
ture
near 1013 nm
is consistent with the presence of gypsum, but not bassanite or anhydrite
(
24
). The presence of some Ca
-
sulfate veins that exhibit the Mastcam hydration signature and
others that do not, with apparent lower hydration in thin
ner veins, is
in accord with
XRD
observation
in the drill samples
of bassanite and anhydrite but n
ot gypsum
.
Before the John Klein
drill
sample was collected, observations by LIBS
, supported by
APXS
analyses of some veins,
had indicated widespread association of Ca
and S in light
-
toned
veins
and filling hollow nodules
in Yellowknife Bay. The LIBS
and APXS
data and Mastcam spectral
interpretations suggest hydrogen associated with some but not all of the
se light
-
toned materials.
The drill locations for John Klein and
Cumberland were deliberately targeted to collect samples
of the mudstone matrix with as little sulfate veining as possible (Figure
s
1A
and 1C
).
Nevertheless,
hairline fractures and
fillings within hollow nodules
were obse
rved on borehole
walls (
24
) and
these are likely the
principal or sole
hosts of
Ca
-
sulfate minerals in the
John Klein
and Cumberland
samples.
B
assanite does not have a stability field at
pressures less than 235 MPa
(
42
)
, far in excess of
the maximum pressure (~50 MPa) that
would be attained if the Sheepbed mudstone
had been
buried under ~5 km of sediment
(a possibility because the mudstone could be exhumed from
beneath the 5
-
km
-
high stratigraphy of Mt. Sharp)
.
B
assanite in the
mudstone is not in
equilibrium, but it may persi
st for long periods because of the unique surface conditions on
Mars
. Bassanite is
relatively
rare on Earth because it readily hydrates to for
m gypsum, even at
low
relative
humidity. H
owever
,
the very low vapor pressure of H
2
O in the atmosphere
of Mars
may
favor persistence of bassanite
(
43
,
44
)
. Although nominal near
-
equatorial surface
conditions are unlikely to desicca
te gypsum to
form bassanite
(
44
)
, moderate increase in
temperature or decrease in P
H2O
could lead to destabilization of gypsum
and formation of
bassanite
(
45
,
46
)
.
B
assanite form
s in many different ways
on Earth
. E
xamples include dissolution
-
reprecipitation after gy
psum in sabkha environments
(
47
)
, gypsum dehydration in endoevaporitic
microbial communities under s
lightly alkaline conditions
(
48
)
, alteration of carbonat
es in acid
-
sulfate systems
(
49
)
, and
dehydration of gypsum dunes
(
50
)
or arid
sedimentary rocks
(
51
)
in
desert environments
. Bassanite of undetermined origin also occurs along with gypsum in soil of
t
he Transantarctic Mountains
(
52
)
.
I
n most of these bassanite occurrences on Earth, the
associated
or precursor
Ca
-
sulfate is gypsum
because bassanite is
often a product of
gypsum
dehy
dration. In these representative studies, a
ssociation of bassanite w
ith
anhydrite
, as in the
John Klein sample,
does not occur and is apparently rare
. This is probably
because temperatures
of anhydrite formation
are generally high enough not to favor a
metastable bassanite precursor
,
and hydration of anhydrite
is likely to go
directly to gypsum.
Anhydrite is a common mineral on Earth, altho
ugh it
hydrate
s
to form gypsum in sufficiently
humid environments. Hydration rates for “soluble anhydrite” (having remnant channel structure
similar to bassanite) are relatively rapid; hydrat
ion rates are much slower for insolubl
e anhydrite
(
53
)
. Where
activity
of pore waters is above ~0.9
and up to 1.0
, anhydrite is the stable Ca
-
sulfate
mineral at burial depths where
temperatures rise above ~50
-
58
°C
(
e.g
.,
(
54
)
)
. The
tempe
rature
of this transition
decreases as
H
2
O
activity decreases, and thus the o
ccurrence of anhydrite, in the
absence of other information, can be
a poor guide to past temperatures.
However, if an anhydrite
occurrence carries other information that
constrains the a
ctivity of water
,
it is a re
asonable
indicator of
elevated temperature. Moreover
, persistence of anhydrite, as of bassanite, indicates a
lack of post
-
formation hydration.
Low
-
pH acid
-
sulfate weathering has long been proposed for many locations on Mars (
e.g.,
(
55
)
).
Acid
-
sulfate weathering was likely to have been much more pervasive in the Noachian,
when major impacts had substantial influence on hydrosphere chemistry
(
56
)
.
In the well
-
studied
Burns formation
of Meridiani Planum
the
occurrence of Fe
-
sulfate phases such as jarosite is
evidence of s
uch conditions, with diagenesis related to persistent groundwater of high ionic
strength
(
57
)
.
The
absence of Fe
-
sulfates
at John Klein and Cumb
erland
,
and the presence
of Ca
-
sulfates
instead,
is evidence of a
n environment with
low ionic strength and
circum
-
neutral pH.
The X
-
ray Amorphous Component
The amor
phous component of the mudstone
may represent soil or eolian
fines accumulated
along with crystalline detritus in the mudstone, but the nature and origin of the amorphous
component is poorly known. E
stimated
composition of the amor
phous component in the
mudstone
(
24
) varies
depending on the assumed
composition of t
he phyllosilicates
, but generally
indicate
s
a relatively Si
-
poor material enriched in Fe, S, Cl, and P. The estimated compositions
of amorphous material in the mudstone
are
approximately
similar to the amorphous component
of th
e Rocknest eolian deposit
(
24
,
25
)
,
but
possibly
modified during
diagenesis in the mudstone
,
including smectite formation and subsequent cation exchange or other interlayer
adjustments
.
Implications of the
Sheepbed Mudstone
Mineral Assemblage
D
etrital plagioclase, clinopyroxenes, and
Fe
-
forsterite
identified
by CheMin
are
generally
sim
ilar
in composition
for Rocknest and the mudstone samples John Klein and Cumberland
(Table 3
)
. This suggests a common
basal
tic source
for
much of the crystalline detritus in
both
the
eolian
and mudstone samples
.
The abundance of magnetite relative to other
crystalline phases in
the mudstone
, however, is in excess of what would be expected for
likely
basaltic
source rocks
;
normalized to the
igneous detrital minerals the magnetite abundance
rises
from 2.1 wt% in
Rocknest to 8.7
wt% in
Jo
hn Klein and 9.5
wt
% in Cumberland
. Abundant magnetite in the
mudstone
could indicate either authigenic formation or a mechanism o
f sedimentary
accumulation. The
XRD data alone
cannot distinguish
between these
origins, but
th
e mudstone
sedimentary context
(
8
)
argues against detrital
accumulation of heavy minerals
.
O
ccurrence of gy
psum,
bassanite
, and anhydrite
in
veins transecting
the
Yellowknife Bay
formation
is a disequilibrium association
. Persi
stence of
bassanite and anhydrite
places limits on
post
-
diagenesis
hydration.
T
he
Sheepbed mudstone
mineralogy
favors
both formation and
preservation of the markers of habitability
,
having
been formed in an aqueous
depositional
environment with late diagenesis limit
e
d to fractures that are
isolated from the sediment matrix
and
with
little or no evidence of hydrous alte
ration following late
diagenesis
.
The
phyllosilicate
in John Klein is
trioctahedral and likely
a
sapo
nitic
smectite
.
The clay
mineral in Cumberland
appears to be genetically related
,
with an almost identical 02
l
band,
although
its
interlayer constituents
are different
.
The greater basal spacing of the Cumberland
smectite may reflect intercalation of Mg
-
hydroxy interlayers.
Tendency toward interlayer
modification may be widespread on Mars, as indicated by spectral studies that point to the
common occurrence of smectite/chlorite mixed
-
layer clay minerals
(
58
)
.
Smectite
s
in the mudstone could
be
detrital
, neoformed, or formed
from primary phases by
authigenic alteration
(
59
)
.
Any of these origins
could be compatible with a habitable
environment.
R
elative to other
basaltic
de
trital minerals
,
Fe
-
forsterite
is disproportionately
reduce
d in John Klein and is almost
absent
in
Cumberland
. Assuming an initial presence of
olivine
in proportions
consistent with typical martian basalt
ic compositions (as at Rocknest
(
9
)
)
,
the loss of
Fe
-
forsterite
in the mudstone is likely to be a consequence of
alteration during
authigenic formation
of
clay minerals
.
This conclusion is supported by
evidence of isochemical
alteration
(
19
)
as well as evidence of diminished
Fe
-
forsterite
abundance associated with
proportional
increase in magnetite and appearance of
clay minerals
(Table 1
)
.
A
nalogous
a
lteration of
Fe
-
forsterite
is
t
he central process in forming
saponitic, trioctahedral clay mineral
s
plus magnetite
in
chondr
itic meteorites at temperatures <100
°
C
(
60
)
.
In the Sheepbed mudstone
t
his
process
may be related to co
ncretion formation
, perhaps associated
also
with
formation of
akaganeite.
C
onsequences of such a reaction could include
lower Eh, higher pH that favors
intercalation of Mg
-
hydroxy
interlayers
in the clay minerals, and possibly production of H
2
gas
that
might
accoun
t for the
voids in the hollow nodules
. This scenario of
Fe
-
forsterite
“saponitization” is conjectural but worth c
onsideration.
The possible formation of H
2
gas as part
of this process could be another component of habitability, providing a potential energy source
for chemolithoautotrophs.
The cl
ay mineral in John Klein has a
diffraction pattern
suggestive
of a smectite that retains
swelling capacity, but the
signature of the clay mineral in Cumberland is less definitive.
Indeed,
the larger basal spacing of the clay mineral in Cumberland suggests that
it
is either hydrated or
expanded by some form of intercalation. Further
more
, the persistence of hydration
over 30
sols
in the warm body of the rover
(5
-
25
°
C)
at very low RH is
unlikely, so we favor the
interpretation of a structural modification.
D
ifferences in clay mineralogy over such a short
distance
between two samples
indicate
variable
diagenetic
modific
ation in
a
mineralogically
immature
sedimentary rock.
T
he lack of collapsed and highly ordered illite or chlorite
in the Sheepbed member mudstone
argues against prolonged, deep burial at elevated temperature.
In terrestrial shales
development
of corrensite
or chlorite generally requires
alteration temperature in excess of
~
60
-
80
°
C (
e.g
.,
(
61
)
). Absence of
such
clay mineral modification
,
beyond
the proposed
i
ncipient chloritization
and partial intercalation of Mg
-
hydroxy interlayers
in
clay minerals of
the
Cumberland sample
,
suggests alteration at temperatures lower th
an this. This is a fairly loose
constraint at Gale Crater,
as
complete buri
al of the crater may have resulted in
a
maximum burial
temperature
of
only
~75
°
C
(
62
)
. As noted above, formation of
late
-
diagenetic anhy
drite from solutions of low
salinity
(
19
)
may indicate temperatures above ~50
°
C
; however, these solutions probably originated at
depth from zones at higher temperature
. In summary
,
e
vidence from the mudstone miner
alogy
supports modest
authigenesis
temperatures but does not constrain depth of burial.
The preponderance of cla
y mineral formation on Mars, with
associated habitable
environme
nts, has been attributed to
Noachian
processes
(
63
)
.
Estimated ages for the
Sheepbed
m
udstone are poorly constrained
but
sediments in the
Gale Crater
mound
are
no older than
Late
Noachin/E
arly Hesperian
(
64
)
and
the
Yellowknife Bay for
mation is likely no older than E
arly
Hesperian
(
8
)
.
T
he Sheepbed member provides an example of a
n
environment where clay
mineral formation continued
to occur
beyond the end of the Noachia
n
Epoch
.
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