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Abstract: Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment. We report an approximate 40Ar-39Ar maximum age of 1.13 Ga to 1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket. High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days. Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.

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Abstract: The mineralogy of comet 81P/Wild 2 particles, collected in aerogel by the Stardust mission, has been determined using synchrotron Fe-K X-ray absorption spectroscopy with in situ transmission XRD and X-ray fluorescence, plus complementary microRaman analyses. Our investigation focuses on the terminal grains of eight Stardust tracks: C2112,4,170,0,0; C2045,2,176,0,0; C2045,3,177,0,0; C2045,4,178,0,0; C2065,4,187,0,0; C2098,4,188,0,0; C2119,4,189,0,0; and C2119,5,190,0,0. Three terminal grains have been identified as near pure magnetite Fe3O4. The presence of magnetite shows affinities between the Wild 2 mineral assemblage and carbonaceous chondrites, and probably resulted from hydrothermal alteration of the coexisting FeNi and ferromagnesian silicates in the cometary parent body. In order to further explore this hypothesis, powdered material from a CR2 meteorite (NWA 10256) was shot into the aerogel at 6.1 km s−1, using a light-gas gun, and keystones were then prepared in the same way as the Stardust keystones. Using similar analysis techniques to the eight Stardust tracks, a CR2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel.

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Abstract: Four Itokawa particles collected from the first touchdown site were mineralogically investigated by optical microscopy, micro-Raman (mu-Raman) spectrometry, scanning electron microscopy (SEM), electron microprobe analysis (EPMA), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). Their mineralogy has an affinity to that of LL6 chondrites based on micro-Raman spectroscopy, EPMA, and XAS analyses. However, the space weathering rims on them are less developed than those observed on the Itokawa particles collected from the second touchdown site. Solar flare tracks are rarely observed in the four particles, whose number densities were lower than those observed in the Itokawa particles from the second touchdown site.

Open Access

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Abstract: Transmission electron microscopy and Fe-K X-ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites, NWA 998, Lafayette, Nakhla, GV, Y 000593, Y 000749, MIL 03346, NWA 817, and NWA 5790. The secondary phases are a rapidly cooled, metastable assemblage that has preserved Mg# and Ca fractionation related to distance from the fluid source, for most of the nakhlites, though one, NWA 5790, appears not to have experienced a fluid pathway. All nine nakhlite samples have also been analysed with scanning electron microscopy, electron probe micro analysis, Bright Field high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction. By measuring the energy position of the Fe-K XANES 1s !3d pre-edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites. The crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable Fe3+/RFe values ranging from 0.4 to 0.9. In Lafayette, the central silicate gel parts of the veins are more ferric than the phyllosilicates around it, showing that the fluid became increasingly oxidised. The mesostasis of Lafayette and NWA 817 also have phyllosilicate, which have a higher ferric content than the olivine fracture deposits, with Fe3+/RFe values of up to 1.0. Further study, via TEM analyses, reveal the Lafayette and NWA 817 olivine phyllosilicates to have 2:1 T–O–T lattice structure with a the d001-spacing of 0.96 nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T–O structure with d001-spacings of 0.7 nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca0.2K0.1)P0.3(Mg2.6Fe2+ 1.3Fe3+ 1.7Mn0.1)P5.7[(Si6.7AlIV 0.9Fe3+ 0.4)P8.0O20](OH)4nH2O, and that found in the mesostasis fractures is an Fe-serpentine (Ca0.1Mg0.7Fe3+ 1.0AlVI 0.4)P2.2[Si2O5]OH4, with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.

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Abstract: The identification of saponite and amorphous material in the Sheepbed mudstone of Gale Crater by CheMin XRD on Mars Science Laboratory [1] has highlighted the importance of understanding the nature of martian phyllosilicates and amorphous material, as they have the potential to reveal the fluid history of habitable terrains [2,3]. The nakhlite meteorites are the only martian samples on Earth which contain phyllosilicates and poorly crystalline or amorphous material [4-10], which we can study by new XRD and X-ray Absorption techniques and TEM to provide an accurate Fe3+/ΣFe ratio, composition and structural comparison to the in situ analyses on Mars.