Optical and Raman spectroscopical properties of the shocked olivines from ALHA 77005 sample.

Sz. Nagy*1(ringwoodit@yahoo.com), S. Józsa1, A. Gucsik2, Sz. Bérczi1, K. Ninagawa3, H. Nishido4, and M. Veres5 1Eötvös Lóránd University of Budapest, H-

1117 Budapest, Pázmány Péter sétány 1/c., Hungary; 2Max Planck Institute for Chemistry, Department of Geochemistry, Becherweg 27, D-55128, Mainz,

Germany; 3Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama, 700-0005, Japan; 4Research Institute of Natural Scinces,

Okayama University of Science, 1-1 Ridai-cho, Okayama, 700-0005, Japan; 5MTA-SZFKI, H-1121 Budapest, Konkoly-Thege M. út 29-33., Hungary.

07.06.2007

The main purpose

In this study, we focus on the shock effects of olivine, and their micro-Raman spectroscopicalproperties to characterize the crystalline background of the shock-induced microdeformations.

The sample:

- The ALHA 77005 sample is a lherzolotic type Martian meteorite.- A preliminary examination of this sample reported that it is ~55% olivine, ~35% pyroxene,~8% maskelynite, and ~2% opaques.

- The olivine (Fo72) occurs as anhedral to subhedral grains up to 2 mm in lenght (Fig. 1).- The olivine grains show brown color because of the hydrous magma [5] (Fig. 1).- The ALHA 77005 contains melt pockets, which are show as spinifex texture (Fig. 2).- The intersticial regions contain two phases: pigeonite and residual glass [6].

Fig. 1. Brown olivines in the ALHA 77005 sample.The image diameter is 4 mm. (N)

Fig. 2. Melt pocket in the ALHA 77005 sample. (+N) The image diameter is 2 mm

Optical properties of shocked olivine in ALHA 77005.

- The olivine exhibit distinct mosaicism (Fig. 3). Some olivine grains have reduced interference color, and undulatory extinction.- The birefringence is also reduced.

Fig. 3. Olivine mosaicism in ALHA 77005. The image diameter is 0.5 mm+N, The dark grains of upper part of image are maskelynite.

Planar microstructures in olivine.

- Shorter than in quartz and feldspar minerals because of the mineral structure.- The planar microstructures occuring up to 2 rational crystallographic orientation (Fig. 4.)

Fig. 4. Planar microdeformations in olivine from ALHA 77005 sample.

-- Slighly curved

Three different shock region in the olivine

- Host grain with low interference color (yellowishwhite).- Transition zone with PDFs, PFs and some isotropic area.- Recrystallized area (Fig. 5 and 6.)

Fig. 5. Three parts of the shocked olivine in ALHA 77005 sample

Shock wave propagation

Fig. 6. Higher resolution image about the three different region in shocked olivine. The red arrow marks the shock wave propagation.

Characteristics of the three region

- The interference color change demonstrates the shock intensity.

- The boundary between the transition zone and host grain is relatively sharp.

- Howewer, the transition between the transition zone and the recrystallyzed area is continous (Fig. 6).

- As a function of the increasing shock pressure, the olivine grains have lost their intense brown color which indicates loss of Fe3+.

- Fe3+ is not presented in the olivine structure, because the grains have not pleochroism effects, consequently its nature rather diffusion type.

- The recrystallized area shows numerous irregular olivine grains (Fig. 6).

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- Near the shock melt pocket, some olivine grains have recrystallized area from the olivine melt and these grains optically different from grains, that recrystallized between the melt pocket and the transition zone.

- High pressure polymorphs of olivine are not presented in this sample.

Raman spectra of olivine (in general)

Olivine spectra can be divided into three parts:

1, less than 400 cm-1

2, between 400-800 cm-1

3, between 800-1100 cm-1

Most characterize peaks at 820 and 850 cm-1 (olivine doublet peaks)

The three analyzing points for the Raman investigation (Fig. 7).

Host grain (A)Isotropic area (B)

Recrystallized area (C)

Raman spectra of three region

The A-area shows the following peaks: 595, 820, 852, and 955 cm-1. The B-area contains: 580, 594, 820, 852,and 950 cm-1. The peaks of the C-area are: 597, 820, 852,and 951 cm-1. All of these peaks correspond to the olivinestructure.

Results

- The doublet peak positions are unchanged in the three region.

- In the B-spectra, the 595 and 597 cm-1 peaks modified to a broadening peak but this change is not significant.

- Because no significant changes we think that olivine collapsed to numerous domains or crystallites during the shock wave propagation in the isotropic area.

- We have not evidence for olivine diaplectic on the base of Raman spectra.

- The A and C-area could represent a higher range order condition than B-area.

- The irregular grains formation in the C-area may have been due to the crystallization around the domains or crystallites.

- It was not found neighter (Mg,FeO) magnesiowüstite nor (SiO2) stishovite phases using Raman spectroscopy.

Conclusion

- We can identify/characterize three region in these olivine grains, being near the melt pocket.

- In the C-area was not hot enough the temperature for the completed recrystallization as in the case of the melt.

- According to the Raman investigation, the nature of the isotropic area is neighter olivine diaplectic nor exolution product.