A Brief outline of Research Proposal for PhD thesis

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NOrth Pole Dome Hyperspectral Mapping
Project
Report of 2003 Pilbara Fieldwork
Prepared for GSWA by
Adrian Brown
PhD Student, Australian Centre for Astrobiology
2
BROWN ET AL.
Overview
From 15 May – 17 Jun 2003, as part of the Australian Centre for Astrobiology (ACA) and the
GSWA investigations into the Earliest Life on Earth in the Pilbara Craton, fieldwork was
conducted to support the “North Pole Dome Hyperspectral Mapping” project, which is the primary
task of PhD student Adrian Brown. In order to provide feedback on the results of the fieldwork, a
copy of a manuscript that has recently been submitted to the journal Astrobiology is attached,
following an abstract to the upcoming JPL AVIRIS Conference 2004, which has been accepted
for an oral presentation.
The two documents give an overview of the project, in the case of the abstract, and then a
detailed report on a scientific study conducted using Short Wave Infra Red reflectance
spectroscopy at the Trendall locality, a site of immense palaeobiological importance. New
scientific results arising from this research are detailed in the paper. Continuing results are being
published on-line at the project website:
http://aca.mq.edu.au/abrown.htm
In addition, Adrian Brown published a short article with Dr. Kathleen Grey (GSWA) that appears
in the Fieldnotes magazine in the December 2003 edition, providing an introduction to ongoing
collaborative work between the GSWA and ACA.
Adrian Brown
Jan 04
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
3
Hyperspectral Mapping of Earth’s Earliest Hydrothermal
Activity in an Archaean Granite-Greenstone Terrane in
the Pilbara Craton, Western Australia
Adrian Brown
Australian Centre for Astrobiology
Department of Earth and Planetary Sciences
Macquarie University, NSW 2109
+61 29850 6286
http://aca.mq.edu.au/abrown.htm
[email protected]
In October 2002 a VNIR-SWIR airborne hyperspectral study was commenced by the CSIRO and
the Australian Centre for Astrobiology over a 600 km 2 area of the Pilbara Craton termed the North
Pole Dome (NPD). The NPD is largely constituted by volcanic rocks of the 3.5 Ga Warrawoona
Group with some minor, but important interbedded sediments. These sediments have been
reported to hold evidence of the Earth’s earliest biota [1,2] although this has recently been the
subject of much debate in the Astrobiological community [3]. Mapping the extent and spatial
distribution of hydrothermal activity that may have supported this putative biota is the goal of this
hyperspectral survey.
The HyMap instrument [4] was used to collect the airborne hyperspectral dataset. It has similar
capabilities to AVIRIS and has been extensively used in Australia and also recently in the U.S.
The Pilbara coverage was collected as 14 swathes, each 2km wide, covering nearly 600 km 2.
The instrument was flown at approximately 2.5km, or 8200ft AMSL. Spectral coverage was
between 450-2500 nanometers in 126 contiguous bands.
Early hyperspectral mapping and fieldwork results of this project will be presented, including
evidence for two hydrothermal events, one of which resulted in weak mineralization, and one that
resulted in massive T-bedded cherts that are now covered by a veneer of goethite. Applications
and techniques for hyperspectral mapping in low metamorphic Archaean granite-greenstone
terranes with AVIRIS will be discussed.
Results of the project are available
http://aca.mq.edu.au/abrown.htm .
online
and continue to be updated at
[1] Walter, M. R., Buick, R. and Dunlop, J. S. R. (1980) Nature, 284, 443-445. [2] Schopf, J. W.
and Packer, B. M. (1987) Science, 237 (4810), 70-73. [3] Brasier, M. D., Green, O. R., Jephcoat,
A. P., Kleppe, A. K., Van Kranendonk, M. J., Lindsay, J. F., Steele, A. and Grassineau, N. V.
(2002). Nature, 416 (6876), 76-81. [4] http://www.intspec.com
4
BROWN ET AL.
Short Wave Infrared Reflectance Investigation of Sites of
Palaeobiological interest: Applications for Mars Exploration
ADRIAN BROWN1, MALCOLM WALTER1, AND THOMAS CUDAHY1,2
1 Australian Centre for Astrobiology, Macquarie University, NSW 2109, Australia, corresponding email: [email protected]
2 CSIRO Division of Exploration and Mining, ARRC Centre, 26 Dick Perry Ave, Technology Park, WA , 6102 Australia
Keywords: Pilbara, SWIR Spectroscopy, Mars, Stromatolites, Archean
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
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ABSTRACT
Rover missions to the rocky bodies of the solar system and especially to Mars require lightweight, portable
instruments that use minimal power and provide suitably diagnostic mineralogical information to an Earth
based exploration team. Short Wave Infrared (SWIR) spectroscopic instruments such as the Portable Infrared
Mineral Analyser (PIMA) have been proposed to fulfill these requirements. We describe an investigation of a
possible Mars analogue site using a PIMA instrument.
A survey was carried out on an outcrop of the Strelley Pool Chert, a stromatolitic, silicified Archean carbonate
and clastic succession in the Pilbara Craton, previously interpreted as being modified by hydrothermal
processes. The results of this study demonstrate the capability of SWIR techniques to add significantly to the
geological interpretation of such hydrothermally altered outcrops. Minerals including dolomite, white mica and
chlorite were detected in the Strelley Pool Chert. In addition, the detection of pyrophyllite below the succession
suggests acidic, high sulphidation hydrothermal alteration may have affected the footwall rocks of the Strelley
Pool Chert.
INTRODUCTION
Short Wave Infrared (SWIR) reflectance spectroscopy has gained recognition in the mining
exploration community due to its speed, simplicity and ability to characterize alteration
zones around ore bodies. Recent literature on the results of SWIR studies (Huston et al.,
1999; Thompson et al., 1999; Yang et al., 2000; Herrmann et al., 2001; Yang et al., 2001;
Bierwirth et al., 2002; Thomas and Walter, 2002) have capitalized on the ability of SWIR
instruments to identify alteration minerals such as white micas and chlorites. Each of these
studies was conducted using the Australian-built Portable Infrared Mineral Analyser (PIMA)
instrument.
In order to test the SWIR technique in a geologic setting comparable to the ancient flood
basalts of Mars, fieldwork was undertaken in the arid, 3.5 Ga Pilbara Craton of Western
Australia. Over 130 SWIR point spectra were obtained by a hand-held “PIMA II”
spectrometer at an outcrop of a heavily silicified, stromatolitic Archean carbonate-chert
succession in order to simulate the investigation of such an outcrop by a rover on Mars.
To simulate the rover responding to commands from an Earth-based exploration team,
point spectra were taken on the accessible weathered surface of the outcrop only, and care
was taken to target mineralogies that were visually differentiable. This simulated a
situation whereby the remote command team would possess only panoramic color images
from the rover to choose locations for collection of point spectra.
GEOLOGICAL SETTING
The North Pole Dome (NPD) in the East Pilbara Granite Greenstone Terrane (EPGGT) (Van
Kranendonk, 2000), is a structural dome of bedded, dominantly mafic volcanic rocks
(greenstones) and interbedded cherty horizons of the Warrawoona Group that dip gently
away from the central North Pole Monzogranite, interpreted as a syn-volcanic laccolith (Van
Kranendonk, 2000). Minor occurrences of felsic volcanic rocks are interbedded with the
greenstones, and these are capped by cherts associated with hiatuses in volcanism (Van
Kranendonk, 2000). Many stromatolite and putative microfossil occurrences have been
documented within the NPD at three distinct stratigraphic levels (Dunlop et al., 1978;
Walter et al., 1980; Awramik et al., 1983; Lowe, 1983; Ueno, 1998; Hofmann et al., 1999;
Van Kranendonk, 2000; Ueno et al., 2001a; Ueno et al., 2001b; Van Kranendonk et al.,
6
BROWN ET AL.
2003) and it has been interpreted as an early setting for life on this planet (Groves et al.,
1981; Buick, 1990).
Many authors have attempted to determine the depositional setting of the Warrawoona
Group. Early researchers suggested a shallow marine environment (Buick and Dunlop,
1990; Van Kranendonk et al., 2003), but recent work has suggested an Archean mid ocean
ridge (Kitajima et al., 2001) or an oceanic plateau (Van Kranendonk and Pirajno, in press).
The ancient age of the Archean rocks of the North Pole Dome makes them a compelling
analog for similarly aged parts of the Martian surface. In addition, the NPD greenstones
may have similar geochemical characteristics to flood basalts prevalent on Mars (Mustard
and Sunshine, 1995; Christensen et al., 2000).
Hydrothermal activity at the North Pole Dome was first examined by Barley (Barley, 1984),
who suggested that silicification and carbonitisation of NPD greenstones were the result of
low temperature, low pressure (‘epithermal’) hydrothermal alteration. Studies by earlier
researchers determined that silicification in the Dresser Formation in the lower part of the
NPD succession and the Strelley Pool Chert (SPC) higher up, occurred very early after
deposition of a shallow subaqueous to subaerial evaporite sequence (Lowe, 1983; Buick
and Dunlop, 1990).
Trendall Locality. The Trendall Locality, within the SPC in the south-west part of the NPD,
contains a silicified carbonate-clastic succession with well-preserved stromatolites
(Hofmann et al., 1999). These stromatolites are considered to indicate the presence of a
primitive biosphere at 3.4 Ga (Van Kranendonk et al., 2003), although some researchers
have proposed an abiotic depositional environment (Lindsay et al., 2003). It has been
proposed that lit-par-lit morphology of black and white cherts at the Trendall locality is
evidence of hydraulic fracturing caused by syn-depositional hydrothermal activity (Van
Kranendonk and Hickman, 2000). Hofmann et al, (1999) and Van Kranendonk et al, (2003)
provided evidence for normal marine stromatolite precipitation of SPC limestones, and Van
Kranendonk and Pirajno (in press) suggested a model of post-depositional hydrothermal
alteration of the SPC. In contrast, Lindsay et al. (2003) suggest the stromatolites were
abiotic and formed by direct carbonate precipitation from syn-depositional hydrothermal
activity.
In either case, the Trendall locality has been affected by hydrothermal activity, either syndepositional, post-depositional or both, and a hydrothermal alteration model is adopted as
a working hypothesis in this paper. For an on-line introduction to the site, the reader is
encouraged to visit a website dedicated to the Trendall locality (Brown, 2003b).
METHODOLOGY
In this study, as in other studies utilizing SWIR, the most important absorption bands are
those due to the hydroxyl (OH-) ion bonding with nearby cations in the crystal lattice of the
minerals under study. Minerals containing hydroxyl ions are commonly associated with
alteration by reaction with aqueous fluids. Minerals are commonly altered in this way in
hydrothermal systems, where hot (>50O C) water, entraining solutes, passes through rock
pores or fissures.
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
7
FIGURE 1 displays the SWIR spectra of several alteration minerals typical of hydrothermal
systems. The absorption bands around 2.2 microns are caused by a combination of the ν2
fundamental stretching vibration mode of the O-H hydroxyl ion with the Al-O-H bending
mode in the crystal lattice of each mineral (Hunt, 1979). The wavelength of each absorption
band minimum varies slightly due to the cation (for example Fe2+ or Mg substituting for Al)
ionically bonded to the hydroxyl ion. This characteristic allows the determination of relative
proportions of Mg or Fe2+ to Al in white micas like muscovite-phengite, or the proportion of
Fe to Mg in chlorite. Another significant absorption band utilized in this study is the
carbonate vibration mode at 2.32 microns, mediated by varying amounts of Ca to Mg in
magnesite, dolomite and calcite.
<<insert Figure 1 here>>
TABLE 1 gives a partial list of alteration minerals that are discernable using SWIR spectra,
and also details their common mode of occurrence (Thompson et al., 1999). Of particular
interest in astrobiological investigations, saponite, (an Mg-smectite) has been proposed as
an effective catalyst for abiotic synthesis reactions (Corliss, 1990). Evaporites associated
with gypsum have been proposed to be important in the search for life on Mars (Rothschild,
1990), and these may be detected using characteristic absorptions in the SWIR. Chlorite
and epidote are common alteration minerals in basalts (Reed, 1983) and could be
diagnostic of hydrothermal activity in postulated Martian oceans (Clifford and Parker,
2001). Talc and serpentine are common alteration minerals in ultramafic rocks (Deer et al.,
1992) and may also be detected using SWIR spectroscopy.
<<insert Table 1 here>>
Mapping of alteration systems surrounding ore bodies has long been a pursuit of economic
geologists (Meyer and Hemley, 1967). By recognising distinctive mineralogies that typically
form zones within hydrothermal alteration systems, suitable ‘vectors to ore’ can be
determined (Galley, 1993). For example, the presence of white mica or sericite is often the
result of the breakdown of plagioclase when a formerly igneous rock is altered in a
hydrothermal system. By mapping the occurrence of white mica, effective detection of
hydrothermal fluid flow is possible. Airborne SWIR studies are a particularly effective tool
for regional mapping of these veins (Cudahy et al., 2000; Brown, 2003a) and correlation on
the ground is possible using hand held, or rover-mounted, instruments such as the PIMA.
The PIMA instrument measures reflected light from an internal light source in the
wavelengths between 1.3 and 2.5 microns. It illuminates a small region of approximately
10mm diameter directly on the surface of a mineral in front of the detector. The bandwidth
of the PIMA is approximately 7 m and has a spectral sampling interval of 2 m, though its
spectral resolution in the SWIR is closer to 8m. It is quoted by the manufacturer
(www.intspec.com) to have a Signal to Noise Ratio (SNR) of between 3500 and 4500 to 1.
Measurements typically take less than a minute and the instrument must be in direct
contact with the sample being analysed. Calibration against a known internal standard is
automatically carried out by the instrument prior to each spectra collection cycle. Internal
temperature and battery status are measured and reported to an attached WinCE palm
computer. Spectra from the PIMA instrument can be downloaded to a PC for further
analysis.
8
BROWN ET AL.
PIMA instruments have been manufactured by Integrated Spectronics, Pty Ltd. since 1995.
Approximately 200 units are in service throughout the world. Two models were produced;
the PIMA II and the upgraded PIMA SP model (P. Cocks, pers. comm.).
In this study, “The Spectral Analyst” (TSA, version 4) program was used to obtain a first
order automatic assessment of the acquired SWIR spectra. TSA compares the spectra to a
library of pure endmembers and uses a proprietary algorithm to identify mineral features.
When there are insufficient infrared active features available to make an assessment, the
rock is declared ‘aspectral’. The use of a “state-of-the-art” automatic mineral classifier such
as TSA simulates a scenario where a rover could independently assess the mineralogy of a
landing site and report the results of its survey to the remote science team. Where
necessary, the spectra were then manually inspected using “The Spectral Geologist”
software and visual assessments were used to interpret the mineralogy according to shape
and wavelength minima of detected absorption bands. This technique allowed the
automatic spectral identification techniques of TSA to be tested and assessed for utility as
an onboard mineralogical processor for modern Mars rovers.
TSA uses a database of 500 samples of 42 SWIR active library (or “endmember”) minerals
(Pontual, 1997) which are used to determine the closest match for unknown sample
spectra. Similar databases have been developed for reflectance spectroscopy techniques by
the USGS (Clark et al., 1990).
TSA also provides a method for removal of the spectral continuum in order to better identify
spectral absorption bands. This normalization method, termed “convex hull removal” (Clark
et al., 1987) has been employed when manually investigating spectra and has been used to
display spectra in the figures of this report.
RESULTS
The area covered by this survey included a 50m x 30m section of outcrop of the Strelley
Pool Chert. To support the collection of spectra, 1500 digital photographs were taken of the
outcrop; these are available online at http://aca.mq.edu.au/abrown.htm. FIGURE 2 displays
a geological map of the region of the Trendall locality.
<< insert Figure 2 here >>
A visual assessment was made of rock types at the Trendall locality before spectra were
taken. This simulated the classification of rocks in the vicinity of a robotic rover by remote
scientists using panoramic images. Such panoramic images featured heavily in NASA’s
Pathfinder (McSween et al., 1999) and MER (Squyres, 2001) missions. On the basis of this
assessment, rocks were broadly categorized according to color and texture, as indicated in
TABLE 2.
<< insert Table 2 here>>
Following broad categorization, the area was surveyed with the PIMA spectrometer in the
manner a rover might examine the outcrop. This was done by taking a number of point
spectra of rocks of each visually-assessed category. Between 3 and 15 spectra were taken
of each rock type, sufficient in order to determine its broad spectral characteristics.
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
9
<< insert Table 3 here>>
The spectral features of each unit are discussed below. They are presented in approximate
stratigraphic order, from youngest to oldest. Apart from the overlying basalt unit and the
underlying pyrophyllite schist unit, all other units are part of the Strelley Pool Chert.
BASALT. The overlying Euro Basalt is characterized by light brown weathering, fine-grained,
green chlorite-rich mineralogy (Van Kranendonk and Pirajno, in press). Many samples are
present at this locality as partially buried boulders rather than as competent outcrop,
although outcrop occurs 25 meters away.
The TSA analysis of the PIMA spectra of the basalt unit consistently identified Mg-chlorite
within the unit (TABLE 3). An example spectrum is shown in FIGURE 3. The chlorite can be
identified by its diagnostic absorption band centered at 2.25 microns, lack of a feature at
1.55 microns (typical of epidote) and associated chlorite features at 2.0 and 2.33 microns
(McLeod et al., 1987). Amongst the units studied in this paper, the dominance of chlorite
features in the spectra was sufficient to discriminate the basalt unit uniquely.
<< insert Figure 3 here>>
MUDSTONE. The mudstone unit is characterized by a grey fine-grained groundmass,
containing millimeter sized white clasts, commonly showing a sugary texture indicative of
silicification. It lies near the top of the Trendall locality, among strata that have been
classified as part of a clastic sequence by previous research at the locality (Van Kranendonk
and Hickman, 2000).
The spectra obtained from the mudstone unit show variable signatures with most
displaying a symmetric absorption band at 2.2 microns, diagnostic of illite-muscovite. They
also demonstrate variable development of bound water at 1.91 microns, possibly related to
more illitic samples. A typical spectrum of the mudstone unit displaying these features
appears in FIGURE 3. The TSA similarly identified muscovite within four samples and illite in
another four (TABLE 3). Manual interpretation of the spectra suggests a white mica is
definitely present within the unit, as shown by the regular appearance of the Al-OH
absorption band at 2.2 microns.
SANDSTONE. The sandstone unit consists of a light-colored coarse-grained and crossbedded groundmass. It lies within the clastic sequence at the top of the Trendall locality
(Van Kranendonk and Hickman, 2000).
The spectra obtained from the sandstone unit predominantly show evidence for white mica
as shown by a predominant absorption band at 2.2 microns. An example spectrum is
shown in FIGURE 3.
PEBBLE CONGLOMERATE. The pebble conglomerate unit is characterized by a grey coarsegrained rudite groundmass, containing large (up to 2cm) clasts. The clasts are commonly
derived from underlying units, including black chert. This unit is also part of the clastic
succession at the SPC (Van Kranendonk and Hickman, 2000). The presence of rip up clasts
of underlying units argues for a high energy subaqueous depositional environment.
10
BROWN ET AL.
The spectra taken of the pebble conglomerate unit were similar to the mudstone unit but
possibly contain less water shown by a weaker 1.9 micron absorption and flatter hull shape.
TSA predominantly identified illite (5 out of 6 samples). The remaining spectra were
declared aspectral (TABLE 3).
BOULDER CONGLOMERATE. This unit consists of large (up to 5-10cm) clasts, commonly of
black chert, but also including black and white layered chert clasts and carbonate. The
clasts often display red to purple Fe staining. The black color of the chert is possibly due to
kerogen (Kato and Nakamura, 2003).
Broad spectral characteristics of this unit include overall low reflectance (typically < 20%).
Most spectra show water absorption at 1.9 microns with both bound water (centered at
1.915 microns) and unbound water (centered at 1.93 microns) present. The unbound water
is typical of free water in fluid inclusions (as expected for chert with ~1% H2O) whereas the
bound water can be associated with water within minerals like illite and smectites (Aines
and Rossman, 1984). Mineral absorptions include symmetric 2.2 microns of illitemuscovite and indications of coupled 2.25 and 2.33 micron absorptions of chlorite.
TSA found that 50% of the spectra were aspectral (TABLE 3). That is not surprising given the
fact that chert is typically inactive in the infrared. TSA suggested the presence of opaline
silica in one sample as shown by the presence of a broad Si-OH absorption band at 2.2152.25 microns. One sample was identified by TSA as illite and one spectrum identified
kaolin.
Although the Fe staining distinguishes this unit visually from the underlying black chert unit
(discussed below), Fe is not specifically SWIR active and thus the spectra of this unit are
indistinguishable from those of the black chert unit.
BLACK CHERT. The black chert unit is massive microgranular black chert. It occurs typically
as smooth fine grained chert layers 5-10 cm thick when bedding conformable (often draped
over the mudstone unit) or as massive crosscutting veins, generally normal to the
stratigraphic layering and 30-50 cm thick. For the purposes of this study, this unit does not
include samples of black chert associated with laminar layered white chert – these are
grouped in the “black and white chert” unit below.
The PIMA spectra of the black chert unit are dark (generally <10% reflectance) and show
much weaker (if present) water and mineral absorption bands compared to the other units
studied. TSA found that 11 out of 13 sample spectra were aspectral (TABLE 3). The spectra
were generally noisy because of the low albedo. One spectrum was identified by TSA as
containing illite and one indicated the presence of Mg-chlorite.
BLACK AND WHITE CHERT. The black and white chert unit consists of microgranular black
chert interstratified at a millimeter scale with microgranular white chert. Both display
similar smooth texture. In most cases, the black chert surrounds the white chert. The unit
shows sub-parallel lit-par-lit morphology. It is generally found below the clastic upper units
and above the carbonate and siliceous planar laminar units.
The PIMA spectra of the black and white chert unit show higher albedo than the black chert
unit though the water and mineral absorption bands are generally of similar relative
intensity. TSA classified most of the spectra as aspectral (5 out of 6, see TABLE 3) even
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
11
though absorptions at 2.2 and 2.3 microns are apparent. These small features are most
likely due to small amounts of white mica and carbonate.
QUARTZ. Where quartz occurs in small (less than 4cm wide) veins, and displays irregular
surficial textures (unlike the smooth chert units), it was identified as a separate unit for the
purposes of this study. The veins are not mapped in FIGURE 2 due to their small size, but
they all occurred within the black and white chert unit and in most cases represent vug
filling quartz that grew into cavities.
The PIMA spectra of these quartz veins show much stronger development of the unbound
water as shown by the broad absorption centered at 1.93 microns. This is demonstrated by
an example spectrum in FIGURE 3. This contrasts with the weak and generally bound water
typical of the black cherts. Some of the quartz spectra are very dark with albedos ~10%,
similar to the black cherts. Three of the samples were identified by TSA as opaline silica
(TABLE 3). The opaline silica in these samples is chiefly identified by the 2.25 micron
absorption. Some spectra show symmetric Al-OH absorption, possibly related to illitemuscovite but most are aspectral. The presence of opal is most likely due to late stage
weathering, evidenced by the sinuous, crosscutting and vug filling nature of the quartz
veins.
RADIATING CRYSTAL SPLAYS. Radiating crystal splays were identified by earlier
researchers (Van Kranendonk and Hickman, 2000) and assessed as similar to beds of
aragonite deposited in modern day travertine (Jones et al., 1997). Recent trace element
geochemical studies suggest the unit represents a dolomitized replacement of radiating
crystal fans, interpreted as secondary crystal growth below the sediment-water interface
(Van Kranendonk et al., 2003).
A valuable contribution of the SWIR analysis described here is its ability to clearly recognize
the radiating crystal splays unit as part of the carbonate lithology of this outcrop. Although
visibly silicified, the spectra of the unit were identified by TSA as containing dolomite due to
a wide absorption band at 2.31 microns (TABLE 3).
PLANAR LAYERED CARBONATE. Planar laminated carbonate beds with pervasive conical
stromatolites (Hofmann et al., 1999) are present in the south-east (bottom right of FIGURE
2) part of the outcrop. At point A on FIGURE 2, the unit is laterally crosscut by a sub-vertical
black chert, which divides well preserved carbonate outcrop on the south side from heavily
silicified carbonate on the north side. Previous researchers (Van Kranendonk and Hickman,
2000) attributed this to hydrothermally controlled silicification of the carbonate unit. This
has been supplemented by a recent study of the geochemistry of a sample from the
carbonate unit (Van Kranendonk et al., 2003).
Half of the PIMA spectra of the planar laminated carbonate unit were identified by TSA as
containing carbonate (TABLE 3). On closer manual inspection, all carbonate unit samples
were assessed as containing at least some dolomite. A typical spectrum appears in FIGURE
3, showing the dominant carbonate absorption band in the 2.3-2.33 micron region (Gaffey,
1986).
SILICEOUS PLANAR LAMINATE. This unit was visually assessed as carbonate partially
replaced by silica based on a characteristic mottled yellow coloring. The yellowed coloring
appeared to be spatially confined to an area between minor and major areas of
silicification and the intensity of coloration was used to identify degrees of partial
12
BROWN ET AL.
replacement. The silica-rich laminated unit contains Fe stained conical laminae interpreted
as stromatolites. This feature suggests a common origin for the carbonate and siliceous
planar layers (Van Kranendonk et al., 2003). The dominant color of the unit is dark grey to
white with smooth microcrystalline texture between the stromatolitic layers.
The partially silicified nature of this carbonate unit is confirmed by the obtained spectra,
many of which show minor bands around 2.3 microns. One was identified as containing
white mica, and manual inspection of the spectra confirmed this interpretation. Three were
designated aspectral. (TABLE 3).
Areas of intense brown discoloration within this unit were sampled and found to be due to
the presence of goethite. A typical spectrum in FIGURE 3 shows the distinctive bands at
approximately 1.65 and 1.75 microns indicative of goethite.
The PIMA spectra of the Fe stained stromatolitic laminae show weak evidence for kaolin
with features at 2.165 and 2.209 microns. Generally the spectra of the stromatolites are
relatively featureless but with moderately high albedo.
PYROPHYLLITE SCHIST UNIT. Stratigraphically beneath (to the south east) of the area
depicted in FIGURE 2 lies a golden weathering, heavily foliated unit that extends for
approximately 50m beneath the Strelley Pool Chert. Unfortunately, there is no outcrop at
the contact between this and the overlying silicified succession, and so the samples were
collected approximately 10m stratigraphically below (east of) the carbonate unit. This is
shown in the inset to FIGURE 2.
The spectra taken of the pyrophyllite schist unit clearly show the presence of pyrophyllite
with the diagnostic right asymmetric and sharp absorption bands centered at 2.165
microns. A good example of this appears in FIGURE 3.
The processes that form pyrophyllite are varied. Early research (Van Kranendonk and
Hickman, 2000) interpreted this unit to reflect the effects of low sulphidation epithermal
alteration, however the presence of pyrophyllite could also indicate acidic high sulphidation
hydrothermal alteration (White and Hedenquist, 1990).
TSCHERMAK EXCHANGE MAP
Tschermak substitution in phyllosilicate minerals, where two Al molecules substitute for a
Si and Fe or Mg molecule in the crystal lattice [Al 2Si-1(Fe,Mg)-1], can be mapped in the SWIR
by examination of the wavelength position of the 2.2 micron Al-OH absorption band (Duke,
1994). An increase in Al content is reflected by a decrease in the wavelength of the feature,
typically from 2.217 down to 2.197 microns.
<< insert Figure 4 here>>
All spectra collected at the Trendall locality displaying the 2.2 micron Al-OH feature were
analysed for the wavelength position. The results appear at FIGURE 4. Those spectra
displaying a feature below 2.204 microns are indicated in blue, between 2.204 and 2.209
microns in yellow, and those above 2.21 microns are displayed in red. Spectra displaying a
pyrophyllite 2.16 micron feature are plotted in green. The size of each plotting point is
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
13
enlarged to cover 1x1m on the map and is not necessarily indicative of all the mineralogy in
the colored square.
This method of plotting Tschermak exchange alerts the observer to areas of low Al rocks on
the north (to right on map) of the outcrop. Discrimination of some pyrophyllite-type
alteration appears in the central area of the map. This may be loosely associated with rising
hydrothermal veins from the stratigraphically lower, pyrophyllite schist unit, but these
require further investigation. High-Al phyllosilicate units do appear in the south (top left of
map) and some are scattered elsewhere. The spectral coverage of more outcrop is
necessary to enable definitive statements to be made concerning Tschermak substitution
processes at this locality, but the utility of such maps to assess large areas quickly would
make them a valuable addition to a Mars surface investigation.
GEOCHEMICAL ANALYSIS
In order to support the identifications of minerals present in the samples studied, XRD
spectra were taken of selected samples from the Trendall locality. Due to the historical
importance and current state of preservation of the site, sampling is discouraged by the
Geological Survey of Western Australia and so only limited sampling was undertaken. In
order to support the findings of this paper, a small sample of the planar carbonate unit was
analysed using XRD. The analysis was carried out by Sietronics Pty Ltd using a Bruker AXS
D4 X ray diffractometer.
The results showed the presence of quartz and dolomite. No other minerals were detected.
Rietveld analysis (Rietveld, 1969) was carried out using the “Siroquant” program (Taylor,
1991), which estimated the sample composition to be 45 wt. % quartz, 55 wt. % dolomite.
The XRF analysis thereby supported the SWIR detection of dolomite in the planar carbonate
unit.
DISCUSSION
Past studies have shown the ability of SWIR techniques to identify alteration mineralogy in
hydrothermally altered terrain (Thompson et al., 1999). This study has shown the ability of a
SWIR instrument to obtain useful information in a hydrothermally altered, silicified,
stromatolitic carbonate and clastic succession.
Highly silicified environments are typical of Archean greenstone terrains that have
undergone moderate hydrothermal alteration (Gibson et al., 1983; Duchac and Hanor,
1987; Van Kranendonk and Pirajno, in press). These often develop cherts that preserve
textures and fabrics indicative of past biological activity such as stromatolites (De Wit et al.,
1982). Some workers have indicated that these chert horizons may be ideal places to
search for previous life on Mars (Walter and Des Marais, 1993).
Similarities between hydrothermal events in the Pilbara and Mars. It has been postulated
that impact craters (Newsom et al., 2001) and sites of gully formation (Brakenridge et al.,
1985; Gulick, 1998) may be sites on Mars where hydrothermal waters have reached the
Martian regolith. Alteration minerals and silicification may be exposed at such sites. If they
occur in the heavily cratered Martian southern highlands, these deposits may be similar in
14
BROWN ET AL.
age to the Archean (c. 3.4 Ga) Strelley Pool Chert. The tholeiitic basalts of the North Pole
Dome may provide a good analogue to Martian flood basalts (Baird and Clark, 1981; Reyes
and Christensen, 1994).
If a Martian rover were to encounter such an environment on Mars, it would be necessary to
characterize the mineralogy of the outcrop, especially any superimposed hydrothermal
alteration. In this event, the availability of an instrument such as the PIMA would be of
great utility to supplement other onboard instruments. Although a SWIR instrument would
be limited in ability to differentiate variations in the silicate minerals present, it has been
shown here that accessory minerals such as carbonates and phyllosilicates can be used to
characterize the alteration mineralogy of the outcrop, a critical precursor to the discovery of
fossil life.
In preparation for the 2003 Mars Exploration Rover missions, NASA science and
engineering teams used an instrument called IPS, which is similar to PIMA, on its
evaluation rovers (Haldemann et al., 2002). The instrument was found to add considerably
to the ability of geologists at a remote site to characterize encountered geological
sequences (Jolliff et al., 2002).
The ability of SWIR instruments to identify minerals makes them ideal support instruments
for an instrument which provides point sample elemental abundance measurements, such
as the Alpha Proton X-ray Spectrometer (APXS) (Golombek, 1998) or Laser Induced
Breakdown Spectrometer (LIBS) (Wiens et al., 2002). The minerals detected by a PIMA
could be broadly correlated and possibly quantified by the use of these two semi-orthogonal
techniques.
Utility of “The Spectral Analyst” for onboard mineralogical analysis. TSA has shown its
ability to identify many of the minerals encountered at the Trendall Locality. The TSA
algorithm attempts to match the spectrum of the rock to over 500 samples of 42 SWIR
active library minerals. The low albedo of black cherts, which increases apparent noise of
the signal and decreases the depth of absorption bands, provides a challenge for the
current TSA algorithm, however this could be overcome in future releases. This study shows
that automatic rock classification using a Visible or SWIR instrument such as PIMA
(Pedersen et al., 1999; Moody et al., 2001; Gulick et al., 2003) may prove useful in future
extraterrestrial robotic missions.
CONCLUSION
This study has demonstrated the ability of SWIR reflectance spectroscopy to identify
minerals in an Archean hydrothermally-altered environment. Minerals identified included
carbonates such as dolomite, white micas such as illite-muscovite, opaline silica, clays and
chlorite. These identifications supplemented visual assessments and aided in the
geological interpretation of the outcrop under study.
The SWIR point spectroscopic method is ideal for mounting on a small rover for remote
field analysis of collected samples. It is lightweight and requires low power. With an
instrument such as the PIMA, calibration and spectrum collection can be completed in
around 2 minutes. The availability of a large body of research and spectral libraries coupled
with advanced algorithms in packages such as “The Spectral Analyst” make the technique
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
15
a low- risk supplementary instrument suitable for mineral detection even in highly silicified
environments.
The results of this study show that SWIR spectroscopy is able to contribute to the
classification and interpretation of the geology of the Strelley Pool Chert. Previous studies
of this area have now been augmented with the following additional information:
a.)
b.)
the presence and extent of carbonate and partially replaced carbonate has been
confirmed, and it was further found that most of the carbonate was dolomitic,
which has been confirmed by XRD, and
pyrophyllite has been detected in an extensively altered and foliated unit
(pyrophyllite schist) beneath the silicified outcrop, suggesting acidic high
sulphidation hydrothermal alteration has taken place beneath the Strelley Pool
Chert.
ACKNOWLEDGEMENTS
This research would not have been possible without the generous assistance of the
Geological Survey of Western Australia (GSWA). Assistance in the field and reviews
contributed by M. Van Kranendonk (GSWA) were immensely valuable. Abigail Allwood is
thanked for her comments on the manuscript. CSIRO Division of Exploration and Mining are
thanked for the loan of their PIMA II field spectrometer. Jonathon Huntington at CSIRO is
thanked for his assistance with processing and interpretation of spectra.
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SWIR SPECTROCOPY - APPLICATIONS FOR MARS
Mineral
Chlorite
Illite-Muscovite
Pyrophyllite
Saponite
Calcite
Dolomite
Gypsum
Epidote
Serpentine
Talc
Standard Formula
(Mg,Al,Fe)12[(Si,Al)8O20](OH)16
K2Al4(Si6Al2)O20(OH)4
Al2Si4O10(OH)2
(Ca,Na)0.67(Mg,Fe)6(Si,Al)8O20(OH)4·8H2O
CaCO3
CaMg(CO3)2
CaSO4·2H2O
Ca2Fe3+Al2Si3O12(OH)
Mg3Si2O5(OH)4
Mg3(Si4O10)(OH)2
19
Example Mode of Occurrence
Diagenesis, metamorphism and hydrothermal alteration
Diagenesis, metamorphism and hydrothermal alteration
Diagenesis and hydrothermal alteration
Weathering, sedimentary and hydrothermal alteration
Ca-rich Hydrothermal alteration and marine sedimentary
Mg alteration and diagenesis
Evaporite
Ca-rich hydrothermal alteration
Alteration of ultramafic rocks
Alteration of ultramafic and carbonate rocks
TABLE 1 - PARTIAL LIST OF MINERALS DETECTABLE USING SWIR INSTRUMENTS. STANDARD FORMULA FROM (DEER ET AL., 1992), EXAMPLE MODE OF
OCCURRENCE MODIFIED FROM (THOMPSON AND THOMPSON, 1996).
20
BROWN ET AL.
Category Name
Description
Basalt
Mudstone
Sandstone
Pebble conglomerate
Boulder Conglomerate
Black Chert
Black and White Chert
Quartz
Overlying basaltic sequence, green-grey chlorite rich
Partially silicified layer with predominantly white clasts in microcrystalline grey chert
Coarse grained cross bedded light-colored predominantly quartz clastic unit
Conglomeratic with clasts of black chert, some up to 2cm across, with some fine white clasts in coarse-grained grey groundmass
Black microgranular quartz clasts with surficial red to purple iron staining
Black microgranular quartz, smooth textured, displaying conchoidal fracture, possibly kerogenous
As for black chert, but with discontinuous bedding conformable white chert layers
Quartz vein, usually cutting through black chert layering. Textures commonly radiate in towards the centre of veins and display
chalcedonic texture typical of vug filling quartz
Silicified radiating crystal splays
Brown dolomite, roughly textured with fine grain size, planar layering evident with pervasive conical stromatolites
As for carbonate layer, but dolomite partially replaced by silica. Color of unit varies from light brown to yellow, presumably varying with
degree of silicification. Stromatolitic conical planar layering is pervasive throughout the unit.
Advanced argillic altered unit, golden weathered schist. Quartz grains (relict amygdales) in white to orange fine grained pyrophyllite
Radiating crystal splays
Carbonate
Partially Silicified Carbonate
Pyrophyllite schist
TABLE 2 – VISUALLY DETERMINED CHARACTERISTICS OF MINERALS AT TRENDALL OUTCROP
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
UNIT
Basalt
Mudstone
Sandstone
Pebble Conglomerate
Boulder Conglomerate
Black Chert
Black and White Chert
Quartz
Radiating Crystal Splays
Planar Carbonate
Siliceous Planar Laminate
Pyrophyllite Schist
SILCA
CARB
CARB
CARB
CARB
SULPH
21
SULPH
WMICA
WMICA
1
4
WMICA
CHLOR
CHLOR
1
CHLOR
9
MONT
4
KAOL
1
5
1
1
KAOL
KAOL
1
1
1
1
1
3
2
1
1
4
1
1
1
1
2
Opal
Dolm
Magn
Sid
1
1
Ank
Na-Alu
1
1
K-Alu
1
4
Pyro
Musc
Ill
2
Fe-Chl
Int-Chl
Mg-Chl
Mont
Halloy
Kaol
Nac
0
1
TOTAL
11
11
1
4
6
7
11
5
7
7
0
5
1
Aspectral
13
6
11
14
6
10
7
TABLE 3 - MAP OF SPECTRAL VERSUS VISUAL CATEGORIES AT TRENDALL LOCALITY. ALL ASSESSMENTS WERE MADE AUTOMATICALLY USING “THE
SPECTRAL ANALYST” (TSA) COMPUTER CODE. SEE TEXT FOR MANUAL SPECTRA IDENTIFICATION AND INTERPRETATION. MAGN = MAGNESITE,
DOLM=DOLOMITE, SID=SIDERITE, ANK = ANKERITE, K-ALU=K-ALUNITE, NA-ALU = NA ALUNITE, PYRO = PYROPHYLLITE, MUSC = MUSCOVITE, ILL =
ILLITE, INT-CHL=INTERMEDIATE CHLORITE, MG-CHL = CLINOCHLORE, MONT = MONTMORILLONITE, HALLOY = HALLOYSITE, KAOL=KAOLINITE, NAC =
NACRITE
22
BROWN ET AL.
FIGURE 1 – LIBRARY REFLECTANCE SPECTRA OF SWIR-ACTIVE MINERALS, COURTESY OF WWW.INTSPEC.COM. THE SPECTRA HAVE BEEN STACKED BUT
RETAIN THEIR RELATIVE INTENSITIES FOR CLEARER DISPLAY OF THEIR ABSORPTION FEATURES.
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
23
FIGURE 2 – GEOLOGICAL MAP OF TRENDALL LOCALITY. ADAPTED FROM VAN KRANENDONK AND HICKMAN (2000). INSET FIGURE AT TOP RIGHT
INDICATES LOCATION OF THE PYROPHYLLITE SCHIST UNIT RELATIVE TO THE MAIN DIAGRAM. SAMPLE POINTS ARE SHOWN AS CIRCLES WITH CROSSES.
24
BROWN ET AL.
FIGURE 3 – SELECTED HULL REMOVED REFLECTANCE SPECTRA FROM TRENDALL LOCALITY. PREDOMINANT MINERALS ARE LISTED NEXT TO EACH
SPECTRUM ALONG WITH THE UNIT WITHIN WHICH THE SPECTRA WAS TAKEN. INDIVIDUAL SPECTRA ARE NOT NECESSARILY REPRESENTATIVE OF ENTIRE
UNIT BUT TYPICAL DIAGNOSTIC MINERALS HAVE BEEN CHOSEN WHERE POSSIBLE. SEE TEXT FOR A DISCUSSION OF ABSORPTION BANDS OF MINERALS.
SWIR SPECTROCOPY - APPLICATIONS FOR MARS
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FIGURE 4 - MAP OF TSCHERMAK EXCHANGE AT THE TRENDALL LOCALITY. SEE TEXT FOR A DISCUSSION AND IMPLICATIONS.
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