Clay Mineralogy and Crystallinity as a Climatic Indicator

46th Lunar and Planetary Science Conference (2015)
COLD AND TEMPERATE CONDITIONS ON EARLY MARS. B. Horgan1, A. Rutledge2, and E. B. Rampe3,
Dept. of Earth, Atmospheric, and Planetary Sciences, Purdue University ([email protected]), 2School of Earth
and Space Exploration, Arizona State University, 3Aerodyne - Jacobs JETS contract NASA Johnson Space Center.
Introduction: Surface weathering on Earth is driven by precipitation (rain/snow melt). Here we summarize the influence of climate on minerals produced during surface weathering, based on terrestrial literature
and our new laboratory analyses of weathering products from glacial analog sites. By comparison to minerals identified in likely surface environments on Mars,
we evaluate the implications for early martian climate.
Main points: (1) Warm, arid climates cause surface weathering due to rain, and mainly produce wellcrystalline smectite clays, similar to those found on
Mars at Mawrth Vallis and in widespread clay leaching
profiles. (2) Cold climates cause weathering due to
snow/ice melt, and mainly produce poorly crystalline
minerals, similar to those found in situ at Gale and
Gusev Craters. (3) In any climate, sustained weathering produces highly leached kaolin clays, as observed
at the top of the widespread leaching profiles on Mars.
Together, these observations suggest that Mars experienced both sustained cold climates and one or more
sustained eras of at least seasonally temperate climates.
Weathering in warm climates: In climates where
aqueous surface weathering is driven primarily by rainfall at low to modest precipitation rates (approximately
<1 meter mean annual precipitation), weathering preferentially leaches mobile cations (e.g., Na+, Ca2+,
Mg2+, Fe2+). These are then incorporatd into smectites
of similar composition to their host sediments, often
forming thick clay-rich (up to 95 wt.%) subsurface
horizons. At somewhat higher precipitation rates, or if
arid soils are weathered for sustained periods (100k1M years), mobile cations are removed from the system entirely and kaolin clays are formed [1,2]. Longterm arid weathering is one hypothesis for the origin of
widespread laterites in the Australian Outback [3].
Warm climates on early Mars? On Mars, thick
(>100m) stacks of smectite-bearing layers at sites like
Mawrth Vallis are consistent with soil sequences [4].
Furthermore, thin (<5m) stacks of Al-clays overlying
Fe/Mg-smectites have been identified in diverse ancient settings across Mars, and are consistent with laterite-like leaching profiles [5]. These observations
suggest at least one era of sustained temperate and arid
or wetter climate during the late Noachian.
Weathering in cold climates: Very high leaching
rates tend to favor the rapid precipitation of poorlycrystalline phases, like the aluminosilicate allophane
[6]. High leaching rates occur due to overall high pre-
cipitation rates (e.g., a few meters MAP), seasonal
monsoons in an otherwise arid climate (the deserts of
Hawaii), or weathering of glass in tephra (Iceland) [7].
However, high leaching rates are also caused by the
rapid onset of seasonal melting in cold environments,
where precipitation mainly falls as snow. Thus, alpine
soils are commonly dominated by poorly crystalline
phases, which then mature into kaolin minerals [8].
Laboratory analysis of pro-glacial weathering:
To test our hypothesis that sub-aerial weathering by
snow/ice melt produces mainly poorly crystalline
phases, sediment samples were obtained from a mafic
pro-glacial moraine and lakeshore in the Three Sisters
volcanic complex [9] in central Oregon (44.156°N, 121.78°E; Fig 1). Fluid flow in the proglacial environment is dominated by seasonal ice/snow melt.
VNIR spectra of the bulk samples are dominated
by alteration products (Fig. 2), with broad iron absorptions between 0.9-1.0 µm that may be consistent with
an iron oxide like ferrihydrite, 1.9 µm bands that persist after heating that are most likely due to water in
hydrated minerals and/or nanophase materials, and
shallow bands near 2.20 µm are consistent with an
aluminosilicate (allophane, Al-smectites, etc.). However, 1.33 and 2.07 µm bands are not consistent with
any known phases and require further analysis.
Mid-IR spectral models of bulk samples (Fig. 3)
are a mixture of primary minerals (plagioclase and
minor pyroxene) and alteration products. The models
include a major glass component, but as glass is unlikely, we hypothesize that this is the closest match in
our library to a poorly crystalline alteration mineral,
most likely an aluminosilicate like allophane [10].
Preliminary XRD patterns of the samples (Fig. 4),
dry-sieved to <90 um, show that the crystalline component is dominated by primary minerals, with plagioclase with minor pyroxene and olivine peaks, but the
two broad reflections at low angles indicate poorly
crystalline phases. The broad maximum near ~14 Å is
consistent with a poorly crystalline phyllosilicate. A
smaller hump near ~3.4 Å might be indicative of a
poorly crystalline silicate, like allophane.
The prevalence of alteration products in the VNIR
spectra compared to the prevalence of crystalline primary phases in XRD suggests that the alteration phases
are poorly crystalline, which is supported by the identification of amorphous silicate in our mid-IR models.
Thus, these analyses support our general hypothesis
46th Lunar and Planetary Science Conference (2015)
that sub-aerial weathering by snow/ice melt produces
mainly poorly crystalline alteration phases.
Cold climates on early Mars? On Mars, poorly
crystalline alteration phases have been detected as a
major component of Noachian deposits sampled by
landed missions. Models of MER Mini-TES mid-IR
spectra of altered Clovis and Watchtower class rocks
in Gusev Crater include up to 50% of an unknown
poorly crystalline component [11]. Furthermore, nearly
every unit sampled for CheMin XRD analysis by Mars
Science Laboratory at Gale Crater contains a significant poorly crystalline component of variable composition [12,13]. We hypothesize that these poorly crystalline phases could be the result of weathering by ice/
snow melt, perhaps providing some limited geochemical support for sustained cold climates on early Mars.
A major unknown – subglacial weathering: A
major limitation of our ability to confirm or refute a
dominantly cold climate on early Mars is that it is unknown what alteration minerals can be produced in the
subglacial environment. On Earth, glacial weathering
has been addressed exclusively through analysis of
glacial outwash fluid chemistry. While these analyses
have shown that wet glacial environments promote
significant weathering [15], the mineralogy produced
is poorly understood [16], and therefore currently cannot be used to evaluate a glacial origin for alteration
minerals on Mars. Glacial flour in distal lakes on Earth
contains clay minerals that could be glacially sourced
[17], suggesting that glaciers could have been a source
of alteration minerals in lacustrine deposits on Mars.
However, this has not been confirmed in the glacial
environment on Earth. To rectify this knowledge gap,
we are currently planning extensive field campaigns to
sample glacial weathering products.
References: [1] Retallack et al (1999) GSA Sp. Pap. 344,
1-192. [2] Sheldon & Tabor (2009) Earth Sci Rev 95, 1-52.
[3] Bourman (1993) Aus J Earth Sci 40, 387-401. [4] Horgan
et al. (2014) 8th Mars, #1276. [5] Carter et al. (2015) Icarus
248, 373-382. [6] Ziegler et al (2003) Chem Geo 202, 461478. [7] Arnalds (2004) Catena 56, 3–20. [8] Tsai et al.
(2010) Geoderma 156, 48-59. [9] Hildreth et al (2012) USGS
SIM 3186. [10] Rampe et al (2012) Geology 40, 995-998.
[11] Ruff et al (2006) JGR 111, doi:10.1029/2006 JE002747.
[12] Blake et al (2013) Science 341, doi:10.1126/science.
1239505. [14] Vaniman et al. (2014) Science 343, 10.1126/
science.1243480. [15] Anderson (2007), An Rev Earth Planet
Sci, 35, 375–399. [16] Carrivick & Tweed, (2013) Quat Sci
Rev, 78, 34-52. [17] Menking (1997) GSA SP 317, 25-36.
Figure 3: Mid-IR spectra of bulk proglacial samples,
with a poorly crystalline component modeled as glass.
Figure 1: Collier Glacier pro-glacial moraine. Stars
indicate sampling sites for preliminary lab analysis.
Figure 4: XRD pattern of samples (sieved to <90 µm),
Figure 2: VNIR spectra of bulk proglacial samples, lines showing primary minerals (sharp peaks) and poorly
indicate absorptions most likely due to alteration.
crystalline phases (humps at shorter wavelengths).