Silica Retention and Enrichment in Open

46th Lunar and Planetary Science Conference (2015)
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SILICA RETENTION AND ENRICHMENT IN OPEN-SYSTEM CHEMICAL WEATHERING ON MARS.
A. S. Yen1, D. W. Ming2, R. Gellert3, B. C. Clark4, D. W. Mittlefehldt2, R. V. Morris2, L. M. Thompson5, J. Berger6,
MSL and Athena Science Teams, 1JPL/Caltech, 2NASA Johnson Space Center, 3University of Guelph, 4Space Science Institute, 5University of New Brunswick, 6University of Western Ontario.
Introduction: Chemical signatures of weathering
are evident in the Alpha Particle X-ray Spectrometer
(APXS) datasets from Gusev Crater, Meridiani
Planum, and Gale Crater. Comparisons across the landing sites show consistent patterns indicating silica retention and/or enrichment in open-system aqueous alteration.
Spirit: The Spirit rover at Gusev Crater traversed
across 7.7 km over an operating lifetime of 2210 sols
and analyzed with its APXS approximately 225 distinct
targets. Fig. 1 depicts Mg/Si versus Al/Si for all the
Gusev samples. A circular reference zone is plotted to
represent an approximate average Mars crustal composition and the dashed lines bound the expected values
for unaltered martian igneous rocks.
Spirit at Gusev
1
Soils, Plains Basalts
Algonquin
Peace Class
Clovis Class
Wishstone/Watchtower
Home Plate area
Other Gusev
0.9
0.8
Mg/Si
0.7
0.6
Extensive silica-rich deposits are found here, including
soils with >90 wt% SiO2. Acid-sulfate weathering, consistent with the discovery of adjacent fumarolic deposits dominated by ferric sulfates, is likely responsible for
leaching cations and leaving residual Si and Ti [3].
Direct precipitation from hydrothermal, Si-rich solutions may also be a contributor to these chemical signatures [4]. An open-system, chemical weathering environment was clearly once active in the Home Plate
region.
Abundant water was present to mobilize elements
from precursor materials. This process is in sharp contrast to other observations of aqueous weathering at
Gusev Crater where there are only subtle chemical differences between altered samples and precursor rocks.
The Wishstone-Watchtower alteration sequence, for
example, likely occurred with a low ratio of water-torock, allowing only limited spatial redistribution of
chemical constituents [1].
Opportunity: The Opportunity rover continues to
operate at Meridiani Planum after over 40 km of traverse distance and nearly 3900 sols of surface operations. The Mg/Si versus Al/Si relationship for the ~400
APXS analyses is shown in Fig. 2.
0.5
0.4
0.3
Opportunity at Meridiani
0.2
1
0.1
0.9
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.8
0.5
Al/Si
Fig 1: Molar Mg/Si versus Al/Si for APXS measurements at Gusev Crater. Red line shows trend of Si
preservation and coupled Mg-Al depletion for many
samples at Home Plate.
Basaltic soils and rocks from the Gusev plains plot
within the zone of average crustal composition. However, many other samples show significant deviations
from the average (see Fig. 1): (a) Weathered
(Fe3+/FeTOTAL > 0.7), possibly initially Mg-rich, Clovis
class rocks [1], (b) high-Al, isochemically weathered
Wishstone-Watchtower class rocks [1], (c) ultramafic,
olivine-rich Algonquin class deposits [2], (d) Peace
class ultramafic, magnetite-rich sandstones cemented
by sulfates [1].
The lower left quadrant of Fig. 1 consists entirely
of rock and soil samples analyzed in the vicinity of
Home Plate, a ~90 meter diameter pyroclastic deposit.
0.7
Mg/Si
0
Soils
Burns outcrop
Marquette
Esperance
"Island" Rocks
Grasberg
Other Meridiani
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Al/Si
Fig 2: Molar Mg/Si versus Al/Si for APXS measurements at Meridiani Planum.
Departures from the average composition depicted
in the circular reference zone (Fig. 2) include: (a) The
mafic erratic referred to as “Marquette Island,” a likely
impact ejecta block [5], (b) the aqueous precipitates
and exceptionally high Mn coating “Island” rocks at
the rim of Endeavour [6], (c) Al-smectite found as frac-
46th Lunar and Planetary Science Conference (2015)
2380.pdf
ture fill (“Esperance”) [7], and (d) Burns formation
outcrop, where silica is a likely constituent [8].
The points representing apron deposits at the Endeavour Rim, referred to as “Grasberg” have low
Mg/Si and Al/Si in Fig. 2, suggesting loss of Mg and
Al and/or net gain of silica, perhaps by regional
groundwater upwelling [9]. Acid-sulfate leaching processes may also contribute to the observed chemical
signatures. Aqueous weathering in an open system and
transport of soluble phases is further supported by the
depletion of Mn relative to Fe in these samples.
sample found ~300 m northeast of Pahrump. The pattern of lower Mg/Si in these samples suggests dissolution of mafic minerals and the removal of Mg2+. The
similarity of the trend defined by the Pahrump and Bonanza King analyses in comparison to samples at Home
Plate suggest that hydrothermal activity and/or acidsulfate leaching processes could be important at Gale
Crater. The possibility of acid-sulfate weathering at the
base of Mount Sharp is supported by the tentative initial detection of ferric sulfates such as jarosite in the
CheMin x-ray diffraction data [13].
Curiosity: The Curiosity rover continues to operate at Gale Crater after 10 km of traverse distance and
850 sols of surface operations. The Mg/Si versus Al/Si
relationship for the ~180 APXS analyses is shown in
Fig. 3.
Summary: A consistent pattern of Al and Mg removal and relative enrichment of Si is evident at Gusev
Crater, Meridiani Planum and Gale Crater. These
chemical trends reflect open system weathering where
cations are mobilized in fluids and the resulting composition differs significantly from that of the precursor
materials. Similarities in the behavior of Mg/Si and
Al/Si between Home Plate at Gusev Crater and
Pahrump at Gale Crater suggest that hydrothermal alteration, possibly under acidic conditions, could be an
important process in the formation of the deposits at
the base of Mount Sharp.
Curiosity at Gale
1
Soils
JakeM
Windjana
Steven
Bonanza King
Pahrump
Other Gale
0.9
0.8
Mg/Si
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Al/Si
Fig 3: Molar Mg/Si versus Al/Si for APXS measurements at Gale Crater.
Gale Crater soils fall within the zone for average
martian crustal composition (circle in Fig. 3) and deviations along the approximately linear profile from high
Mg/Si to high Al/Si are bounded by the high-Mg
Windjana samples [10] and rocks of the Jake Matijevic
class which have chemical compositions suggestive of
a mugearite [11]. These samples within the dashed
lines, including samples analyzed at Yellowknife Bay
[12], have compositions comparable to pristine martian
rocks and their Mg-Al-Si concentrations alone do not
indicate a history of aqueous alteration.
The Stephen samples adjacent to Windjana, on the
other hand, plot well outside the dashed lines and are
exceptionally high in Mg, Mn, Cl, Ni and Zn. These
materials may have formed as fracture-filling fluid precipitates. Also indicative of open-system weathering
are samples that trend towards low Mg/Si and Al/Si.
These analyses include Pahrump deposits at the base of
Mount Sharp and the arguably related Bonanza King
References: [1] Ming D. W. et al. (2006) JGR,
111, E02S12. [2] Ming D. W. et al. (2008) JGR, 113,
E12S39. [3] Squyres S. W. et al. (2008) Science, 320,
1063-1067. [4] Ruff S. W. et al. (2011) JGR, 116,
E00F23. [5] Mittlefehldt D. W. et al. (2010) 41st LPSC,
#2373. [6] Arvidson R. E. et al. (2014) presented at
2014 Fall AGU. [7] Clark B. C. (2014) 45th LPSC,
#1419. [8] Clark B. C. (2005) EPSL, 240, 73-94. [9]
Ming D. W. et al (2015) 46th LPSC. [10] Gellert R. et
al. (2014) presented at 2014 Fall AGU. [11] Stolper E.
M. et al. (2013) Science, 341, #1239463. [12] McLennan et al. (2014) Science, 343, #1232777. [13] Rampe
E. et al. (2014) presented at 2014 Fall AGU.