The Portable Instant Display and Analysis Spectrometer (PIDAS) was taken to southwestern Greenland to investigate in situ the potential application of AVIRIS to estimate the mineralogy and composition of rocks exposed in Archean terranes. The goal was to determine the feasibility of using a high spectral resolution scanner to find and study pristine rocks, those that have not been altered by subsequent deformation and metamorphism. The application of AVIRIS data to the problems in Greenland is logical. However, before a costly deployment of the U-2 aircraft to Greenland is proposed, this study was undertaken to acquire the spectral data necessary to verify that mineralogical mapping in the environmental conditions found there is possible. Although field conditions were far from favorable, all of the major objectives of the study were addressed. One of the major concerns was that lichens would obscure the rock surfaces. It was found that the spectral signature of the lichens was distinct from the underlying rocks. Thus, a spectrum of a rock outcrop, with its partial cover of lichens, can be un-mixed into rock and lichen components. The data acquired during the course of this study supports the conclusion that areas of pristine Archean crust can be differentiated from that which has experienced low grade alteration associated with Proterizoic faulting

Curtiss, Brian

Goetz, Alexander F. H.

English

NASA Technical Reports Server

. ,fS + 1*‘ 69 
Final Report for m3 
NASA Contract NAGW-1183 
Entitled 
MINERALOGY AND COMPOSITION OF ARCHEAN CRUST, GREENLAND: 
A PILOT STUDY 
Principal Investigator: Alexander F. H. Goetz 
Professor and Director 
Center for the Study of Earth from Space (CSES)/CIRES 
Campus Box 449 
University of Colorado 
Boulder, CO 80309-0449 
Telephone: (303) 492-5086 
Co-Inves tigator: Brian Curtiss 
Research Associate, CIRES 
Campus Box 449 
University of Colorado 
Boulder, CO 80309-0449 
Telephone: (303) 492-6718 
Brian Curtiss 
P- T ~ x r n r e ; n ~ f n r  
(BASA-CE-182859) H l b l E R A L O G Y  A A C  CCIIPOSI!IION N89-15480 CE BBCPEdN C t i C S I ,  G R E E L L A I D :  1 EIlCT STUOY 
I i n a l  fiegort (Cclczradc Univ.) 13 KSCL 0 8 G  
Unclas 
G3/46 0 1854EC 
i 
V Principal Investigator 
https://ntrs.nasa.gov/search.jsp?R=19890006109 2020-03-20T04:47:59+00:00Z
Summarv 
The Portable Instant Display and Analysis Spectrometer (PIDAS) (Goetz et al., 
1987) was taken to southwestern Greenland to investigate in situ the potential 
application of AVIRIS to estimate the mineralogy and composition of rocks exposed 
in Archean terranes. The goal was to determine the feasibility of using a high 
spectral resolution scanner to find and study pristine rocks, those that have not been 
altered by subsequent deformatior: and metamorphism. The application of AVIRIS 
data to the problems in Greenland is logical. However, before a costly deployment 
of the U-2 aircraft to Greenland is proposed, this study was undertaken to acquire 
the spectral data necessary to verify that mineralogical mapping in the 
environmental conditions found there is possible. 
Although field conditions were far from favorable, all of the major objectives 
of the study were addressed. One of the major concerns was that lichens would 
obscure the rock surfaces. While lichens do cover a high percentage of many of the 
rock outcrops, it was found that the spectral signature of the lichens was distinct 
from the underlying rocks. Thus, a spectrum of a rock outcrop, with its partial cover 
of lichens, can be un-mixed into rock and lichen components. The data acquired 
during the course of this study supports the conclusion that areas of pristine 
Archean crust can be differentiated from that which has experienced low grade 
alteration associated with Proterizoic faulting. It was initially thought that high soil 
moisture would hinder the usage of remote sensing. In almost all cases it was 
found that wet areas occurred in areas covered by ice rafted material. Because the 
mineralogical composition of this material gives no insight into the geology of the 
bedrock, the increased soil moisture in these areas is of .little concern. Because these 
materials have no relationship to the underlying bedrock, the differentiation of this 
glacial float from the bedrock is one of the major problems that must be addressed 
prior to the deployment of AVIRIS in Greenland. 
Introduction 
Areas of preserved Archean crust provide windows into the Earth's earliest 
recorded geologic history. On the other hand, Archean terranes are complex, and 
the original disposition and composition of their rock units have commonly been 
modified by deformation and metamorphism. Therefore, a detailed inventory of 
the mineral assemblages and chemistry present in these rocks is an initial constraint 
essential to unraveling crustal history. The terrane is extensive and difficult to 
traverse. The Airborne Visible Infrared Radiometer Imaging Spectrometer 
(AVIRIS) and The Thermal Infrared Multispectral Scanner (TIMS) are instruments 
that provide high enough spectral and spatial resolution to make possible the direct 
identification of minerals present in exposed rocks, under favorable conditions, 
allowing rapid estimation of rock mineralogy from which compositions can be 
inferred over broad areas. This study was undertaken to determine if adequate 
information can be acquired from reflectance spectra of outcrops which, in some 
cases, are lichen covered and/or water saturated. 
The Portable Instant Display and Analysis Spectrometer (PIDAS) (Goetz et al., 
1987) was used to investigate in situ the potential application of AVIRIS to estimate 
the mineralogy and composition of rocks exposed in Archean terranes. Greenland 
is an unique area because the 3600-3700 Ma white and grey Amitsoq gneisses and the 
3800 Ma Isua supracrustral belt are the most extensive exposures known of earliest 
Archean rocks (Baadsgaard et al., 1986). In addition, although deformation is 
responsible for the predominance of strongly banded Amitsoq gneisses, the least 
deformed Amitsoq gneisses are reported to be among the best preserved of early 
Archean plutonic rocks (Nutman and Bridgwater, 1986). The special characteristics 
of this terrane which make it particularly suitable for eventual AVIRIS/TIMS 
coverage include the presence of extensive outcrop, a broad spectrum of rock 
compositions, the low surface coverage of lichens, and the absence of rock coatings 
and extensive oxidation of the rocks' surfaces. The study was carried out in 
cooperation with Dr. Robert F. Dymek, Washington University, who has 
accomplished pioneer metamorphic studies in the area of Isua (Dymek, 1978; Boak 
et al., 1983; Dymek et al., 1983; Dymek, 1984). Dr. Dymek has spent three field 
seasons in Greenland and gathered an impressive rock sample collection and 
chemical data base to which we will have access. 
Nutman and Rosing (1985) have stressed the importance of distinguishing 
primary geochemical characteristics of Archean crust from perturbations due to 
tectonometamorphic and metasomatic events. For example, some metasomatic 
changes in the grey Amitsoq gneisses can be shown to be due to the intrusion of 
younger white Amitsoq gneisses. In fact, the disturbance of the major and trace 
element chemistry and isotopic systematics of the grey gneisses caused by this 
metasomatic event have been the subject of in-depth studies by Baadsgaard et a!. 
(1986) and Nutman and Bridgwater (1986). Thus, finding pristine rocks is essential 
to characterize the protolith of the grey gneisses. Our efforts were focused in an area 
of Isukasia where the grey gneisses are reported to be least deformed (McGregor ef 
al., 1985, Nutman et al., 1983). The metasomatic alteration imposed on the grey 
gneisses by the intrusion of the white gneisses is reported to have modified the 
primary petrography of the grey gneisses (Baadsgaard et al., 1986). The first signs of 
alteration result in the alteration of plagioclase to zoisite and muscovite, with a 
concurrent increase in water content of the rocks. Wh-ere more intense alteration 
has occurred in association with deformation along faults, the mafic minerals biotite 
and hornblende are altered to phengite and chlorite, while plagioclase is converted 
to zoisite, phengite, quartz and carbonate. If it can be demonstrated that these 
mineralogical changes are detectable spectrally, it may be possible to guide the field 
geologist to areas where more adequate sampling can be accomplished. In other 
words, AVIRIS/TIMS data might be used to "forage for pristine Archean crust," Le., 
the data would be used to pick out candidate pristine zones for further detailed field 
and laboratory work. 
Fieldwork 
The primary objective of the fieldwork in the Isua area was to evaluate the 
feasibility of using remote sensing data to solve a broad range of geologic problems. 
This was accomplished by observing the amount of bedrock exposure, the extent to 
which lichen obscured the various rock units, the amount of vegetation cover and 
the spectral contrast between units. Our group (B. Curtiss (CU), B. Rivard (WU) ) 
joined Dr. Dymek's expedition and spent seven days in the eastern part of the Isua 
supracrustal belt followed by eight days in the south-west extremity of the belt. 
Initial work in the eastern part of the belt consisted of elaborating a rock and 
vegetation classification scheme which was then used for the description of surfaces 
on which spectral measurements were acquired. Reflectance measurements in the 
450 nm to 2500 nm region of the spectrum were measured using the Portable 
Instantaneous Display and Analysis Spectrometer, (PIDAS). Because of poor 
weather conditions during these seven days, spectral data was acquired on only two 
days. 
Reflectance spectra and samples were obtained for the two upper members of 
sequence B supracrustals, the felsic gneiss and garnet-biotite schists. Three transects 
approximately 100 meters in length, and 30 meters apart, were laid out 
perpendicular to the strike of these units. Reflectance spectra were measured at  
approximately 5 meter intervals along each transect. A wide variety of surfaces, 
including unvegetated weathered rock surfaces, weathered rock surfaces with 
variable lichen covers, and wet rock surfaces were included. Additionally, field 
reflectance spectra and samples were obtained for a wide range of boulder lithologies 
and lichen types. While the weather did not permit the use of PIDAS to make 
additional field spectral measurements, samples of the various lithologic units, 
including the garbenschiefer and carbonate units, were collected. 
The second camp site was located in the western segment of the Isukasia belt 
in the vicinity of the sequence A type section. Once again, poor weather conditions 
limited the days during which PIDAS data could be acquired. Because the poor 
weather conditions did not allow the measurement of the reflectance spectra in the 
field, samples were collected along the sequence A type section of the supracrustal 
rocks (Nutman 1984). PIDAS reflectance measurements were made at a series of 
sites located in the vicinity of the proterizoic Ataneq fault. These data were used to 
examine the spectral changes associated with alteration along the fault. 
Sky conditions and performance of the field spectrometer during the course 
of the fieldwork were far worse than anticipated. There was one day of clear blue 
skies, and three half days with some clouds. Additionally, many problems were 
encountered in the operation of PIDAS under the environmental conditions found 
in Greenland. These problems include IR spectrometer gains that were optimized 
for mid-latitude summers, IR spectrometer grating drives that did not properly step 
the gratings when the instrument was cold, and drift in the visible spectrometer 
dark current. One of the major problems was associated with the thermoelectric 
"coolers" mounted on each of the detector arrays. These coolers are intended to 
provide temperature stabilization for the detectors. To maximize battery life and 
because warm climates were assumed, the thermostats on the T.E.C.s were set to 
about 23OC. Thus, when the ambient temperature is less than 23OC (as it was the 
majority of the time in Greenland), the T.E. coolers are actually heating the detector 
arrays. Because of the erratic nature of the T.E.C.s in heating mode, the signal / 
noise under these conditions is about half of normal. 
While the overall quality of the reflectance data acquired in the field was low, 
it was sufficient to accomplish several of the objectives of the study. In many cases, 
samples were collected in the field to assure that adequate reflectance measurements 
could later be made in the laboratory. Additional reflectance measurements of rock 
surfaces, both lichen covered and lichen free, were made in the laboratory using 
PIDAS on return from Greenland. 
Results 
The presence of extensive lichen cover is one of the major problems that 
must be addressed prior to the application of remote sensing to geological mapping 
in Greenland. While lichen cover is pervasive in the coastal regions, the areas 
closer to the ice sheet typically have only ten to fifty percent of the surface covered by 
lichen. Laboratory reflectance measurements of lichen free (Figure l a  and b) and 
lichen covered regions (Figure 2a and b) of the same sample confirm that the 
various forms of lichens all have a similar spectral shape in the short wave infrared 
portion of the spectrum. While the rock substrate cannot be seen through the 
lichen cover, the spectrum of the lichens is sufficiently distinct from any of the rock 
spectra. Thus, the mixed signature of a partially lichen covered outcrop could be 
unmixed into "rock" and "lichen" components. 
The presence of moisture in surface soils regolith was determined to be of 
minor consequence. All of the observed rock outcroppings were found to have very 
600 600 1000 3200 1400 1600 IS00 2000 2200 2430 
WAVELENGTH ( nm ) 
Figure la .  Reflectance spectra of lichen free surfaces of the same samples shown 
in Figure 1. Samples IS40 and IS11 are both from the Garbenschiefer unit of the 
Isua supracrustal belt; sample RG47, a pegmatite, is an example of ice rafted 
material. As can be seem from a comparison to Figure 2, the presence of the 
lichen totally obscures the absorption features seen in these reflectance spectra. 
The spectra have been offset vertically for better viewing. 
I I I I I 
I- 1 
I I I I I I I I I I I 
2000 2200 2-100 1400 1G00 1800 
WAVELENGTH ( i m  ) ' - -  
Figure lb. This figure shows the detail in the short wave infrared of the spectra 
show in Figure la.  The spectra have been offset vertically for better viewing. 
Page 7 
.3 
-,/(encrusting) 
- 
I I I I  I ,  I 1 l I I  # / , I  I I I 
\YAVELENGTl-I ( 11111 ) 
Reflectance spectra of five lichen covered surfaces, sample numbers are 
noted on each spectrum. These spectra are representative of the four dominant 
type of lichen found in the Isua area: yellow, black, grey, and encrusting black. 
While there are slight differences in the visible region of the spectrum, the 
spectra are almost identical in the short wave infrared region. 
600 800 7000 1200 1400 1600 i sm 2000 2200 2400 
Figure 2a. 
h 
t 1 
I I I I I , I I I 
1400 1 600 1600 2000 2200 2400 
WAVELENGTH ( 11111 ) 
Figure 2b. This figure shows the detail in the short wave infrared of the spectra 
show in Figure 2a. The three absorption features in thie 2100 - 2300 nm region are 
probably due to the presence of cellulose in the lichens. The spectra have been 
offset vertically for better viewing. 
low porosity, and thus do not hold moisture for an appreciable period of time. 
Those surfaces which were found to be wet were most often comprised of glacially 
deposited material. While the reduction in the spectral contrast of these materials 
resulting from surface moisture will hinder the spectral determination of their 
mineralogy, these materials are totally unrelated to the underlying bedrock. 
The ability to differentiate glacially deposited materials from bedrock outcrops 
will be essential to the interpretation of any remotely sensed dataset. The surface 
textures of these glacially deposited units is quite distinct from the bedrock 
outcroppings. The outcroppings are topographically higher and tend to form broad 
smooth surfaces without much small scale shadowing, while the glacial units 
consist of close packed rounded boulders ( 0.5 - 2 meters in diameter) and have 
considerably more small scale shadowing. The use of mixing models that utilize 
shade as a component and a classification scheme that uses the spatial variation in 
shading should provide a means to differentiate these two types of surfaces. 
Reflectance spectra of outcroppings of the grey Amitsoq gneisses were 
measured along a transect perpendicular to the Proterizoic Ataneq fault zone to 
determine the spectral changes associated with low grade alteration which occurs 
along the Ataneq and similar Proterizoic faults throughout Greenland. While the 
quality of the reflectance spectra was low, the relative intensities of the absorption 
features in the 2000 - 25000 nanometer region of the spectrum change as a function 
of the distance form the fault (see Figures 3 and 4). The absorption feature at 2220 
nm is indicative of muscovite and the feature at 2350 nm is consistant with the 
presence of chlorite. The presence of muscovite in these rocks is indicative of the 
first stages of alteration. The formation of chlorite is a result of more intense 
alteration, commomly found in association with deformation along faults. 
. 
m 
2 0.3- 
0 
Lo 
0 - 
I I I I 
page 10 
c 
Recommendations 
Reflectance spectra acquired in both the field and in the laboratory support the 
conclusion that a high spectral resolution scanner, such as AVIRIS, would provide a 
useful tool in the mapping of bedrock lithologies in southwest Greenland. The 
problems introduced by extensive lichen cover and by the presence of large regions 
of glacially deposited material can be overcome through the use of sophisticated 
image processing techniques such as mixed signature modeling and texture analysis. 
While lichen cover in the Isua region is relatively low, other areas in Greenland, in 
particular the coastal regions, have much higher coverage. Traditional sensors, 
such as Thematic Mapper, should be used to asses the degree of lichen cover prior to 
the planning stage of a full scale deployment of AVIRIS. 
References cited 
Baadsgaard, H., A. P. Nutman, and D. Bridgwater, 1986. Geochronology and isotopic 
variation of the early Archean Amitsoq gneisses of the Isukasia area, southern 
West Greenland. Geochem Cosmochem Acta, 50, 2173-2183. 
Boak, J. L., R. F. Dymek, and L. P. Gromet, 1983. Petrology and rare earth element 
geochemistry of classic metasedimentary rocks from the Isua supracrustal belt, 
West Greenland, Rapp.. Gronlands geol. Unders. 112, 23-33. 
Dymek, R. F., 1978. Metamorphism of the Archean Malene supracrustals, Gothab 
District, West Greenland, Proc. 1978 Archean Geochem. Conf., edited by E. M. 
Smith and J. G. Williams, pp. 339-342, University of Toledo Press. 
Dymek, R. F., 1984. Supracrustal rocks, polymetamorphism, and evolution of the 
SW Greenland Archean gneiss complex, in Patterns of Change in Earth 
Evolution, edited by H. D. Holland and A. F. Trendall, pp. 313-343, Springer- 
Verlag, Berlin. 
Dymek, R. F., R. Weed, and L. P. Gromet, 1983. The Malene metasedimentary rocks 
on Rypeo, and their relationship to Amitsoq gneisses, Gronl. Geol. Unders. 
Rapp., 53-69. 
Goetz, A. F. H., N. Page, and B. Curtiss, A high-resolution, self- contained reflectance 
spectrometer for field use, Applied Optics, in preparation. 
page 11 
McGregor, V. R., A. P. Nutman, and C. R. L. Friend, 1985. The Archean geology of 
the Godthabsford region, southern West Greenland, in Workshop on Early 
crustal genesis: the world's oldest rocks, LPI Techn. Rep. 86-04, pp. 113-169. 
Nutman, A. P. and M. Rosing, 1985. Isukasia area-Regional geological setting, in 
Workshop on Early crustal genesis: the world's oldest rocks, LPI Techn. Rep. 
85-04, pp. 171-18. 
Nutman, A. P. and D. Bridgwater, 1986. Early Archean Amitsoq tonalities and 
granites of the Isukasia area, southern West Greenland: development of the 
oldest-known sial. Contr Mineral Petrol 94, 137-148. 
page 12 


Mineralogy and composition of Archean Crust, Greenland: A pilot study

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server