Determining the composition of olivine from reflectance ... - CiteSeerX
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E6, PAGES 13,675-13,688,JUNE 25, 1998
Determining the composition of olivine from reflectance spectroscopy Jessica M. Sunshine AdvancedTechnologyApplicationsDivision,ScienceApplicationsInternationalCorporation(SAIC), Chantilly,Virginia
Carl6 M. Pieters Departmentof GeologicalSciences,Brown University,Providence,RhodeIsland
Abstract. Reflectancespectraof olivinesspanningthe forsterite-fayalitesolid-solutionseries havebeenanalyzedwith the modifiedGaussianmodel(MGM). The compositionalvariabilityof the threeprimaryabsorption bandsthatconstitute thediagnostic1.0-gmolivinefeaturehasbeen quantifiedby examiningthecenters,widths,andrelativestrengths in 18 spectrarangingin composition from Mg-rich forsteriteto Fe-richfayalite. Theseanalyses havealsorevealedseveral interrelationships amongthe threeabsorption bandsthatprpvidenew insightsinto their crystallographic origins. The primaryolivineabsorptions near 1.0-gin are well behavedand providea meansto remotelyidentifyandestimatethecomposition of olivinefrom reflectance spectra.However,thespectralresolutionandsignal-to-noise of currentremotelyacquireddata, combinedwith the significantoverlapin the 1.0-gmregion,provideenoughuncertaintyto allow modelsbasedon simpleleastsquaresminimizationto reachsolutionsthataremathematically satisfyingyet physicallyunrealistic.More sophisticated modelsusinginversetheorythat incorporate constraints amongtheabsorption bandsasdetermined fromtheselaboratoryMGM analysisare shownto yield meaningfulresultswhichcanbe confidentlyusedto estimate composition in remotedata. The MGM andinversetheoryareusedto analyzethe spectrumof the olivine-richasteroid246 Asporinaandto quantitativelyshowthatthe olivinecomponenton Asporinais magnesium-rich (i.e., forsteritic).In contrastto the systematic behaviorof the three
primaryolivineabsorptions, absorption featuresshortof 0.7 gm arefoundto exhibitno obvious relationship to composition andassucharenotrecommended for usein remoteapplications.
1. Introduction
petrologic history and thermal evolution. Furthermore, estimatingthe chemicalcompositionof olivines, in particular Olivine is one of the most common and petrologically their Fe/Mg ratios, can constrain the fractionation and significant mafic minerals. At visible and near-infrared differentiationpathsthat led to the formation of sucholivinewavelengths, spectra of olivine are dominatedby a broad bearing lithologies [e.g., Basaltic VolcanismStudy Project complex absorptionfeaturenear 1.0-gm. Reflectancespectra (BSVP), 1980]. of several different olivine compositions spanning the The overall position of the 1.0-gm olivine absorption
forsterite-fayalite,(Mg,Fe)2SiO 4, solid-solutionseries are
featureis known to shift toward longer wavelengths with increasing iron content [e.g., Burns, 1970a; Adams, 1975; King and Ridley, 1987; Burns, 1993]. However, direct produced fromthe presence of Fe2+in the olivinestructure extrapolationof chemicalcompositionsfrom olivine spectra [Burns, 1993]. This diagnostic 1.0-gm olivine absorption is complicatedby the fact that the 1.0-gm absorption is a featurehas been recognizedin laboratory spectraof many composite feature, consisting of several overlapping terrestrial[e.g., Burns, 1970a; Hunt et al., 1973; Adams, absorptionbands. These absorptionsresult from crystal field 1975], lunar [e.g., Burns et al., 1972; Hazen et al., 1977], and transitions of Fe2+ ions situatedin two distinct distorted meteorite samples [e.g., Gaffey, 1976]. Similar 1.0-gm octahedralcoordination sites (the M1 and M2 sites) within the absorptions in telescopic spectra have led to the remote olivine structure [Burns, 1993 and references therein]. identification of olivine-rich lithologies on surfaces Detailed assessmentsof the relationship between Fe/Mg throughout the solar system, including the Moon [e.g., composition and olivine spectrarequireexamination of the Pieters, 1982] and several asteroids [e.g., Cruikshank and compositionalvariability of each of the absorptionbands that Hartmann, 1984; Bell et al., 1989; Gaffey et al., 1993; comprise the overall 1.0-gm feature [e.g., Burns, 1970a; Sunshineet al., 1997]. Identifyingthe presenceof olivine on Singer, 1981]. thesevarioussolarsystembodiescan aid in determiningtheir In the studypresentedhere, the individual absorptionbands
shown in Figure 1. The characteristic1.0-gm feature in olivine spectrais a compositeof severalabsorptionsthat are
Copyright1998by the AmericanGeophysicalUnion. Paper number98JE01217. 0148-0227/98/98JE-01217509.00
that form the 1.0-gm olivine absorption feature are quantitatively evaluatedusing the modified Gaussian model (MGM) developedby Sunshineet al. [1990]. The MGM is used to characterizethe compositional variability of each of the primary absorptionbandsin olivine reflectancespectraacross 13,675
13,676 1.75
SUNSHINE ANDPIETERS: OLIVINEREFLECTANCE SPECTRA ......
a
1.50
Fo 97-A
Olivine Spectra
Fo 92-B Fo 92-C
1.25 Fo 90-D
Fo 89-E
1.00
Fo 86-G
0.75
Fo 84-H
Fo 42-L
0.50 Fo 36-N
0.00
,
0.30
,
,
• 0.70
,
,
,
I 1.10
,
ß ß I 1.50
......
I
1.90
ß
2.30
Wavelength in Microns 1.75
ß' ' i ' ' ' i ß' ßi ß' ßi ß' ' i ß' b
1.50
Kiglipait Olivine Spectra
Fo 89-F Fo 66-I
1.25
Fo 60-J
1.00
The composite1.0-gm absorptionfeaturein olivine spectra can be explicitly separatedinto its constituentabsorption bandsusingthe modifiedGaussianmodel (MGM) of Sunshine et al. [1990], the first accuratemathematicaldescriptionof the shapeof individualcrystalfield absorptions. The MGM was developedand validatedwith empirical studiesof isolated crystal field absorption bands in both transmissionand reflectancespectraof orthopyroxenes [Sunshineet al., 1990]. The MGM can readily be understoodin terms of the physical processes involvedin crystalfield absorptions and has proven to be a robust approach to deconvolving overlapping absorptions in spectra of pyroxene mixtures [Sunshine and Pieters, 1993]. The MGM has been usedto successfullyinfer the composition of pyroxenes from reflectance spectra of basaltic lithologies in both meteorites[Sunshineet al., 1993]
Determining the composition of olivine from reflectance spectroscopy Jessica M. Sunshine AdvancedTechnologyApplicationsDivision,ScienceApplicationsInternationalCorporation(SAIC), Chantilly,Virginia
Carl6 M. Pieters Departmentof GeologicalSciences,Brown University,Providence,RhodeIsland
Abstract. Reflectancespectraof olivinesspanningthe forsterite-fayalitesolid-solutionseries havebeenanalyzedwith the modifiedGaussianmodel(MGM). The compositionalvariabilityof the threeprimaryabsorption bandsthatconstitute thediagnostic1.0-gmolivinefeaturehasbeen quantifiedby examiningthecenters,widths,andrelativestrengths in 18 spectrarangingin composition from Mg-rich forsteriteto Fe-richfayalite. Theseanalyses havealsorevealedseveral interrelationships amongthe threeabsorption bandsthatprpvidenew insightsinto their crystallographic origins. The primaryolivineabsorptions near 1.0-gin are well behavedand providea meansto remotelyidentifyandestimatethecomposition of olivinefrom reflectance spectra.However,thespectralresolutionandsignal-to-noise of currentremotelyacquireddata, combinedwith the significantoverlapin the 1.0-gmregion,provideenoughuncertaintyto allow modelsbasedon simpleleastsquaresminimizationto reachsolutionsthataremathematically satisfyingyet physicallyunrealistic.More sophisticated modelsusinginversetheorythat incorporate constraints amongtheabsorption bandsasdetermined fromtheselaboratoryMGM analysisare shownto yield meaningfulresultswhichcanbe confidentlyusedto estimate composition in remotedata. The MGM andinversetheoryareusedto analyzethe spectrumof the olivine-richasteroid246 Asporinaandto quantitativelyshowthatthe olivinecomponenton Asporinais magnesium-rich (i.e., forsteritic).In contrastto the systematic behaviorof the three
primaryolivineabsorptions, absorption featuresshortof 0.7 gm arefoundto exhibitno obvious relationship to composition andassucharenotrecommended for usein remoteapplications.
1. Introduction
petrologic history and thermal evolution. Furthermore, estimatingthe chemicalcompositionof olivines, in particular Olivine is one of the most common and petrologically their Fe/Mg ratios, can constrain the fractionation and significant mafic minerals. At visible and near-infrared differentiationpathsthat led to the formation of sucholivinewavelengths, spectra of olivine are dominatedby a broad bearing lithologies [e.g., Basaltic VolcanismStudy Project complex absorptionfeaturenear 1.0-gm. Reflectancespectra (BSVP), 1980]. of several different olivine compositions spanning the The overall position of the 1.0-gm olivine absorption
forsterite-fayalite,(Mg,Fe)2SiO 4, solid-solutionseries are
featureis known to shift toward longer wavelengths with increasing iron content [e.g., Burns, 1970a; Adams, 1975; King and Ridley, 1987; Burns, 1993]. However, direct produced fromthe presence of Fe2+in the olivinestructure extrapolationof chemicalcompositionsfrom olivine spectra [Burns, 1993]. This diagnostic 1.0-gm olivine absorption is complicatedby the fact that the 1.0-gm absorption is a featurehas been recognizedin laboratory spectraof many composite feature, consisting of several overlapping terrestrial[e.g., Burns, 1970a; Hunt et al., 1973; Adams, absorptionbands. These absorptionsresult from crystal field 1975], lunar [e.g., Burns et al., 1972; Hazen et al., 1977], and transitions of Fe2+ ions situatedin two distinct distorted meteorite samples [e.g., Gaffey, 1976]. Similar 1.0-gm octahedralcoordination sites (the M1 and M2 sites) within the absorptions in telescopic spectra have led to the remote olivine structure [Burns, 1993 and references therein]. identification of olivine-rich lithologies on surfaces Detailed assessmentsof the relationship between Fe/Mg throughout the solar system, including the Moon [e.g., composition and olivine spectrarequireexamination of the Pieters, 1982] and several asteroids [e.g., Cruikshank and compositionalvariability of each of the absorptionbands that Hartmann, 1984; Bell et al., 1989; Gaffey et al., 1993; comprise the overall 1.0-gm feature [e.g., Burns, 1970a; Sunshineet al., 1997]. Identifyingthe presenceof olivine on Singer, 1981]. thesevarioussolarsystembodiescan aid in determiningtheir In the studypresentedhere, the individual absorptionbands
shown in Figure 1. The characteristic1.0-gm feature in olivine spectrais a compositeof severalabsorptionsthat are
Copyright1998by the AmericanGeophysicalUnion. Paper number98JE01217. 0148-0227/98/98JE-01217509.00
that form the 1.0-gm olivine absorption feature are quantitatively evaluatedusing the modified Gaussian model (MGM) developedby Sunshineet al. [1990]. The MGM is used to characterizethe compositional variability of each of the primary absorptionbandsin olivine reflectancespectraacross 13,675
13,676 1.75
SUNSHINE ANDPIETERS: OLIVINEREFLECTANCE SPECTRA ......
a
1.50
Fo 97-A
Olivine Spectra
Fo 92-B Fo 92-C
1.25 Fo 90-D
Fo 89-E
1.00
Fo 86-G
0.75
Fo 84-H
Fo 42-L
0.50 Fo 36-N
0.00
,
0.30
,
,
• 0.70
,
,
,
I 1.10
,
ß ß I 1.50
......
I
1.90
ß
2.30
Wavelength in Microns 1.75
ß' ' i ' ' ' i ß' ßi ß' ßi ß' ' i ß' b
1.50
Kiglipait Olivine Spectra
Fo 89-F Fo 66-I
1.25
Fo 60-J
1.00
The composite1.0-gm absorptionfeaturein olivine spectra can be explicitly separatedinto its constituentabsorption bandsusingthe modifiedGaussianmodel (MGM) of Sunshine et al. [1990], the first accuratemathematicaldescriptionof the shapeof individualcrystalfield absorptions. The MGM was developedand validatedwith empirical studiesof isolated crystal field absorption bands in both transmissionand reflectancespectraof orthopyroxenes [Sunshineet al., 1990]. The MGM can readily be understoodin terms of the physical processes involvedin crystalfield absorptions and has proven to be a robust approach to deconvolving overlapping absorptions in spectra of pyroxene mixtures [Sunshine and Pieters, 1993]. The MGM has been usedto successfullyinfer the composition of pyroxenes from reflectance spectra of basaltic lithologies in both meteorites[Sunshineet al., 1993]
