21002-63 - NWA Possible Achondrite Meteorite Porphyritic Basalt. In study. 244.1 g

Petrographic description

Under petrographic microscope, the sample is a fine-grained volcanic rock with olivine and pyroxene euhedral phenocrysts, surrounded by fine-grained matrix (Figs. 2 and 3). Olivine shows a thin rim (Figs. 2 and 3) of iddingsite (reddish border). The rock matrix is formed by tabular euhedral plagioclase and anhedral pyroxene crystals and, in a minor amount, by ilmenite grains.

Tabular plagioclase crystals display certain orientation, suggesting flow during magma crystallization. Opaque minerals are Fe-Ti oxides, mainly ilmenite.
Olivine has 0.5-1.3 mm μm in size whereas pyroxene varies from 200 to 400 μm. In the matrix, plagioclase, pyroxene and ilmenite microcrysts have a crystal size in the range of 50-120 μm. The percentage in the sample of olivine, pyroxene and plagioclase is respectively 12, 10 and 40%.

The texture is porphyritic, glomeroporphyritic and trachytic.

Figure 4 shows olivine and pyroxene phenocrysts surrounded by plagioclase and pyroxene microcrysts. Olivine and pyroxene are quite similar under the petrography and electronic microscope images. The main difference between them is the formation of iddingsite rim in olivine grains. Alteration processes develop the rim of the sample. Chemical zonations are not appreciable in the crystals.

Figure 5 shows the EDS analyses and indicates that pyroxene is augte, with Ca = 0.78 apfu (atoms per formula unit), proportion of magnesium Mg# = 0.83, and low aluminum (Al = 0.22 apfu). Olivine is magnesium rich (Fo = 0.72) and plagioclase corresponds to labradorite (XAn= 0.56).

The rock is porphyritic basalt (Le Maître et al., 2005). Possible achondrite meteorite.

Mineralogy and textural properties of samples were studied on (30m-thick) polished thin sections using transmitted and reflected light microscopy at LPA (UA) and UGR.

Photomicrographs were performed by using:
o Petrographic microscope: ZEISS Assioskop (LPA-UA).
o Digital camera: USB UI-1490SE
o Image capture Software: uEye Cockpit (IDS).

Samples (polished thin-sections) also were studied under scanning electron microscope (Research Technical Services – UA). Minerals were located and identified using BSE mode on Hitachi S3000N SEM coupled with an X-ray detector Bruker XFlash 3001 for microanalysis (EDS) and mapping.

Semi-quantitative EDS X-ray microanalysis was performed using EDS spot analysis with acquisition time 60s and maximum process time to achieve best resolution of peaks in spectra.

ANNEX II. REFERENCES
Le Maître, R.W. (Editor), Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., Bellieni, G., Dudek, A., Efremova, S., Keller, J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H. & Woolley, A.R. 2005. Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press. Second Edition, revised, 236 p. Cambridge, UK.