Raman Spectroscopic Studies of Lunar Basaltic Meteorite Northwest Africa 4734

Jian Chen, Zongcheng Ling, Yuheng Ni, Yihang Huang, Zhongchen Wu and Bo Li Institute of Space Sciences, Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment

Shandong University, Weihai, China (merchenj@mail.sdu.edu.cn, zcling@sdu.edu.cn)

Introduction Lunar meteorites as specimens from random locations on the lunar surface, provide crucial supplementary to the returned Apollo and Luna samples.

Northwest Africa (NWA) 4734 is the largest mare basalt among the lunar meteorite collections to date.

http://meteorites.wustl.edu/lunar/moon_meteorites_list_alumina.htm

The petrography, mineralogy, geochemistry and chronology of this meteorite have been reported in many previous studies, e.g., [1], [2], etc.

The purpose of this work is to apply Raman spectroscopy, which has been proved robust to identify mineral compositions, polymorphisms and phase transitions within geologic materials [e.g., 3-5], to characterize variations of mineral chemistry and extensive shock products in this lunar piece.

Shock effects Pyroxene grains are heavily fractured and majority of plagi-

oclase is transformed to maskelynite.

Cristobalite grows and cuts pyroxene and olivine grains and seems to be dissociation products of pyroxene [10].

Raman peak of cristobalite is broaden due to shock while high pressure silica phases (stishovite, coesite and seifertite reported by [11-13]) haven’t been found in our study.

Zircon might have been transformed into baddeleyite and silica supported by the observations of baddeleyite-silica in-tergrowth.

Future Work Preliminary point-counting measurements exhibit the potentials of Raman spectroscopy in

mineral identifications and chemical characterizations as well as examinations of polymor-phisms and phase transitions (e.g., shock metamorphism in meteorites).

Our future work will be concentrated on the overall scanning and mapping to derive modal mineralogy and further searches for shock products in this meteorite to evaluate its impact history.

Acknowledgement: We thank Prof. Weibiao Xu and Jianyun Tan for their assistance with sample preparations in Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China. This work is supported by the National Natural Science Foundation of China (41473065, 41373068), Natural Science Foundation of Shandong Province (JQ201511), Qilu Young Scholar (TANG SCHOLAR) Program of Shandong University, Weihai (2015WHWLJH14).

References: [1] Wang Y. et al. (2012) GCA, 92, 329-344. [2] Elardo S. M. et al. (2014) MAPS, 49, 261-291. [3] Wang A. et al. (1995) JGR 100, 21189–21199. [4] Wang A. et al. (2003) JGR 108, 5005. [5] Ling Z. C. et al. (2011) Icarus 211, 101-113. [6] Wang A. et al. (2001) AM 86, 790-806. [7] Kuebler K. E. et al. (2006) GCA 70, 6201-6222. [8] Freeman J. J. et al. (2008) CM 46, 1477-1500. [9] Wang A. et al. (2004) AM 89, 665-680. [10] Connolly H. C. et al. (2007) MAPS 43, 571-632. [11] Aoudjehane H. C. et al. (2008) AMSM 71, Abstract #5058. [12] Miyahara M. et al. (2013) Nature Communications 4, 1737. [13] Wang Y. et al. (2016) AMSM 79, Abstract #6337.

Analytical Methods Renishaw inVia® Raman Microscope in Shandong University, Weihai, China

Green laser (532 nm), Raman shift range: 100~1400 cm−1, Spectral resolu-

tion < 1 cm−1, Wavelength calibration standard: Si wafer (520.7 cm−1)

5× objective for overall scans, 50× and 100× objectives for detailed analyses

Spatial resolution better than 1 μm with tightly focused beam under the 100× objective

Figure 2 Phases identified in NWA 4734 and their Raman spectra

Figure 5 Pyroxene and olivine chemis-try (accuracy of ±10, refer to [6-7])

Figure 1 The montage image of NWA 4734 section captured by the optical microscope equipped with Raman system

Mineral chemistry

Pyroxene exhibits extensive compositions (Fs25–90Wo7–48En2–54), ranging from orthopyroxene to clinopyroxene, and then continu-ously increasing in Fe toward pyroxferroite and ferrosilite.

Olivine occurs as two clusters, relatively forsteritic phenocrysts (Fo40-70) and fayalitic small patches (Fo4-30) in the mesostasis over-lapped by glassy and transparent spherules known as “Swiss cheese” pattern [1].

Distinct structural types of feldspar and oxide phases can be iden-tified [8-9]. Plagioclase crystals are anorthitic implying its lunar origin and the alkali depleted nature of the Moon. K-feldspar usu-ally emerges in the mesostasis.

Spinel group minerals consist of chromite and ulvospinel, and chromite is surrounded by ulvospinel.

Figure 3 Raman spectra of a pheno-crystic pyroxene grain from rim to core (from top to bottom)

Peak 1987655

Peak 2Peak 3

303372

Pyroxene (pos 1)

313

661 990

379

324 382

666 992

324 390669

996

329671

1001

Pyroxene (pos 2)

Pyroxene (pos 3)

Pyroxene (pos 4)

Pyroxene (pos 5)

Pyroxene

Plagioclase

Olivine

Ilmenite

Baddeleyite

Merrillite

Maskelynite

Cristobalite

Sulfide

Glass

Ulvospinel

1005660

314 352 386

504

817 848

678

954 970

Pyroxene

Plagioclase

Olivine

Ilmenite

Baddeleyite

Merrillite

Maskelynite

Cristobalite

Sulfide

Glass

Ulvospinel

1005660

314 352 386

504

817 848

678

954 970

Figure 6 Cristobalite between clinopyrox-ene and Fayalitic olivine

En Fs

HdDi

100 80 60 40 20 0

Pyroxene

Plagioclase

Olivine

Ilmenite

Baddeleyite

Merrillite

Maskelynite

Cristobalite

Sulfide

Glass

Ulvospinel

1005660

314 352 386

504

817 848

678

954 970

505

508

513

Albite

Anorthite

K-feldspar

Figure 4 Raman spectra of feld-spar end-members K-feldspar, al-bite, and anorthite

Cpx

Fa

Cristobalite

Cpx