Micro&petrological-Analysis-of-Shergottite Meteorite-4468Galena-M.C.-Roots-and-Richard-A.-Cox

Introduction

Methodology

References

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Fig 4. X-ray map of pyroxene crystal showing distinct zoning patterns.

Fig 5. Pyroxene ternary diagram with Wollastanite (Ca2Si2O6) Enstatite (Mg2Si2O6) and Ferrosilite (Fe2Si2O6) end members illustration placement of point analyses along line traverse in pyroxene crystal.

Results

Fig 6. Line scan graph displaying changes in end members cross cutting different zones in pyroxene crystal.

Fig 3. End-member zoning profile across quantitative line scan analyses in olivine crystal, with little variation in endmember percentages.

Fig 2. X-ray maps of olivine crystal with phosphorous zoning and with no magnesium, iron zoning.

Four grains of Shergottite 4468 were mounted in a 1 part hardener, 4 part epoxy to form a 1 inch diameter disc. Fine grit (600) was used to grind through the epoxy to expose the Shergottite grain’s surfaces. 1um alumina followed by 0.3 micro alumina was used to polish the sample. A Nikon Eclispe 50iPOL, a digital camera was attached to microscope to capture images of the grains. This map was used primarily to determine the general composition of the sample. The sample was then carbon coated prior to electron microprobe analysis. The JEOL JXA-8200, fifth generation SuperProbe, located in the Robert M. MacKay Electron Microprobe Laboratory at Dalhousie University was used to create Backscattering Electron Images (BSE), detailed 2-D elemental maps and do quantitative point analysis. PTGUiwas used to stitch together microscope and back scattering images.

Fig 1. Backscattering image of Martian meteorite NWA4468.

Shergottite Northwest Africa (NWA) 4468 is a Martian meteorite that was discovered in 2006 in the Western Sahara of Morocco. It is classified as a olivine basaltic Shergottite . It is a coarse grained olivine-phyric with a large clinopyroxene oikocrysts, maskelynite, chromite, ilmenite Ca-polymorphs, troilite , and pyrrhotite (Irving et. Al., 2007). NWA 4468 is of Amazonian-age, has been dated between 150 – 200 Ma (Cassata and Borg, 2015). During this time period there has been no observed tectonic activity on Mars, although large lava flows continued (Wise et. Al., 1979). Large scale primitive mantle melting continues on Mars today. Most of the recorded igneous activity on mars is recorded by the formation of cumulate rocks. Studying the variations within this sample’s mineralogical and geochemical compositions have helped give insight into the meteorites cooling history.

About 20% of the olivine crystals in NWA 4468 have skeletal phosphorous zoning. Phosphorous has a high residence time and due to it’s early presence in crystalizing olivines it can be used to determine the early stages of olivine crystallization (Milman-Barris et. Al., 2008). The skeletal pattern seen in this sample suggests rapid growth at a high temperature not preserved in the iron and magnesium.

The pyroxene grains show a clear distinction between pigeonite (Ca-rich) and Augite (Ca-poor) zones. Due to the immiscibility gap (Aoki, 1964) within the pyroxene cooling series this suggests the crystallization of the pyroxenes occurred in two phases with the pigeonite forming at a high temperatures and the augite forming at lower temperatures.

The geochemical and chemical changes observed within the crystals of NWA 4468 suggest the sample underwent a multiphase cooling history with olivine’s forming at high temperatures and rapid cooling a disequilibrium. Then pyroxene formed in at least two separate temperatures.

Conclusion

Aoki,=K.I.,=1964.=Clinopyroxenes=from=alkaline=rocks=of=Japan. American)Mineralogist:)Journal)of))))))))))))))))))))))))))))))))Earth)and)Planetary)Materials, 49(9T10),=pp.1199T1223.

Cassata,=W.S.,=and=Borg,=L.E.,=2015,=March.=40Ar/39Ar=Systematics=of=Shergottite NWA=4468.=======Lunar)and)Planetary)Science)Conference vol.=46,=pp.=2742.

Irving,=A.J.,=Kuehner,=S.M.,=Korotev,=R.L.=and=Hupé,=G.M.,=2007.=Petrology=and=bulk=composition=of=primitive=enriched=olivine=basaltic=shergottite Northwest=Africa=4468.=Lunar=and=Planetary=Science=Conference vol.=38,=pp.1526

MilmanTBarris,=M.S.,=Beckett,=J.R.,=Baker,=M.B.,=Hofmann,=A.E.,=Morgan,=Z.,=Crowley,=M.R.,=…….Vielzeuf,=D.=and=Stolper,=E.,=2008.=Zoning=of=phosphorus=in=igneous=olivine. Contributions)to)Mineralogy)and)Petrology, 155(6),=pp.739T765.

Wise,=D.U.,=Golombek,=M.P.=and=McGill,=G.E.,=1979.=Tectonic=evolution=of=Mars. Journal)of) …….)Geophysical)Research:)Solid)Earth, 84(B14),=pp.7934T7939.