Complexities in pyroxene compositions derived from absorption band centers: Examples from Apollo samples, HED meteorites, synthetic pure pyroxenes, and remote sensing data

We reexamine the relationship between pyroxene composition and near‐infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two‐part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simp...

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Published inMeteoritics & planetary science Vol. 51; no. 2; pp. 207 - 234
Main Authors Moriarty III, D. P., Pieters, C. M.
Format Journal Article
LanguageEnglish
Published Hoboken Blackwell Publishing Ltd 01.02.2016
Wiley Subscription Services, Inc
Subjects
Absorption
Absorption bands
Absorption spectra
Algorithms
Basalt
Basins
Bearing materials
Bearings
Complexity
Composition
Gaussian
Infrared absorption
Lunar soil
Meteorites
Meteors & meteorites
Pyroxenes
Remote sensing
Weathering
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Summary:We reexamine the relationship between pyroxene composition and near‐infrared absorption bands, integrating measurements of diverse natural and synthetic samples. We test an algorithm (PLC) involving a two‐part linear continuum removal and parabolic fits to the 1 and 2 μm bands—a computationally simple approach which can easily be automated and applied to remote sensing data. Employing a suite of synthetic pure pyroxenes, the PLC technique is shown to derive similar band centers to the modified Gaussian model. PLC analyses are extended to natural pyroxene‐bearing materials, including (1) bulk lunar basalts and pyroxene separates, (2) diverse lunar soils, and (3) HED meteorites. For natural pyroxenes, the relationship between composition and absorption band center differs from that of synthetic pyroxenes. These differences arise from complexities inherent in natural materials such as exsolution, zoning, mixing, and space weathering. For these reasons, band center measurements of natural pyroxene‐bearing materials are compositionally nonunique and could represent three distinct scenarios (1) pyroxene with a narrow compositional range, (2) complexly zoned pyroxene grains, or (3) a mixture of multiple pyroxene (or nonpyroxene) components. Therefore, a universal quantitative relationship between band centers and pyroxene composition cannot be uniquely derived for natural pyroxene‐bearing materials without additional geologic context. Nevertheless, useful relative relationships between composition and band center persist in most cases. These relationships are used to interpret M3 data from the Humboldtianum Basin. Four distinct compositional units are identified (1) Mare Humboldtianum basalts, (2) distinct outer basalts, (3) low‐Ca pyroxene‐bearing materials, and (4) feldspathic materials.
Bibliography:SSERVI - No. NNA13AB01A
NASA LASER - No. NNX12AI96G
istex:4549C411813867F8DFCBF65856DA5502AFAF51D5
ArticleID:MAPS12588
Table S1: PLC parameter values for the example spectra. Fig S1: Example continuum fit (left), continuum-removed spectrum, and parabola fits (right) for synthetic pure orthopyroxene En35,Fs65 (RELAB #DL_CMP-025) and synthetic pure clinopyroxene En39, Fs34, Wo27 (RELAB #DL_CMP-051). A flat 2 μm continuum is used across the 2 μm region of the clinopyroxene spectrum rather than the negative continuum that would be derived from a long-wavelength tiepoint at 2595 nm. Fig S2: This histogram summarizes the distribution of non-(Ca,Mg,Fe) cations for a collection of lunar pyroxenes derived from various mare and nonmare samples. While both mare and nonmare pyroxenes typically exhibit <6% non(Ca,Mg,Fe) octahedral cations, mare pyroxenes are slightly more enriched in these nonquadrilateral elements. Compositional information was compiled in Papike et al. () from the following sources: Agrell et al. (), Bence et al. (), Brown et al. (), Hollister and Hargraves (), Kushiro and Nakamura (), Smith et al. (), Boyd and Smith (), Dence et al. (), Gancarz et al. (), Hollister et al. (), Newton et al. (), Albee et al. (), Bence and Papike (), Hargraves and Hollister (), Weigand and Hollister (), Hodges and Kushiro (), Papike et al. (), Dixon and Papike (), Dymek et al. (), McCallum and Mathez (), Sclar and Bauer (), Smyth (), Vaniman and Papike (), Laul et al. (), Vaniman and Papike (), and Shervais et al. (). Fig S3: These example LRMCC Apollo 17 Bulk Rock spectra exhibit the effects of high ilmenite content, namely a steep red-sloped continuum and distorted 1 and 2 μm pyroxene absorption bands. Fig S4: Spectra for all HED samples from the collection with grain size <75 μm. (A-E) Howardites. (F) Polymict eucrites. Spectra for the <25 grain size fraction are available from RELAB at http://www.planetary.brown.edu/relab/. Fig S5: Example M3 target spectra for the four material types identified in the Humboldtianum Basin subregion (A) inter-ring basalts, (B) central Mare Humboldtianum basalts, (C) nonmare materials with high pyroxene content, and (D) nonmare materials with low pyroxene content. The y-axis scale for (D) is different that for (A-C).
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ISSN:1086-9379
1945-5100
DOI:10.1111/maps.12588