The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. ...Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.
To investigate the origin of fine-grained rims around chondrules (FGRs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material ...and individual FGRs in the primitive CO3.0 chondrites Allan Hills A77307, LaPaz Icefield 031117 and Dominion Range 08006. The observation of similar overall O-anomalous (∼155ppm) and C-anomalous grain abundances (∼40ppm) in all three CO3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. The presence of presolar silicate grains in FGRs combined with the observation of similar estimated porosity between interstitial matrix regions and FGRs in LAP 031117 and ALHA77307, as well as the identification of a composite FGR (a small rimmed chondrule within a larger chondrule rim) in ALHA77307, all provide evidence for a formation of FGRs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. Our study also shows systematically lower abundances of presolar silicate grains in the FGRs than in the matrix regions of CO3 chondrites, while the abundances of SiC grains are the same in all areas, within errors. This trend differs from CR2 chondrites in which the presolar silicate abundances are higher in the FGRs than in the matrix, but similar to each other within 2σ errors. This observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a FGR of LAP 031117 suggests that the lower abundance of presolar silicates in FGRs reflects pre-accretionary aqueous alteration of the fine-grained material in the FGRs. This pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26Al-related heating inside precursor planetesimals, followed by aggregation of FGRs into the CO chondrite parent-body.
Laser Raman Spectroscopy has been proposed and is under extensive development for surface exploration missions to planetary bodies of our Solar System. It reveals information on molecular structure ...and chemistry. The spatial distribution of molecular species in natural geological samples and planetary materials has significance for the geological processes by which they formed. Raman imaging is the best way to combine the molecular identification and characterization of geologic materials with their spatial distribution. This paper reports Raman imaging studies of five types of extraterrestrial materials and three terrestrial samples using a state-of-the-art Raman imaging system. The Raman spectral features of major, minor, and trace species in these samples, together with their spatial correlations revealed by these Raman imaging studies indicate the genetic relationships and the geological processes that these materials have been experienced. For robotic planetary surface exploration mission, a simple yet very useful molecular map of a sample can be generated by using line-scan or grid-scan of an in situ Raman system with tightly focused laser beam.
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•Laser Raman imaging provides spatial correlation of molecular species.•Fast Raman imaging with high spatial resolution is attainable with modern Raman instruments.•Study on extraterrestrial materials revealed genetic relationship of various species.
The chemical compositions of relatively young mare lava flows have implications for the late volcanism on the Moon. Here we report the composition of soil along the rim of a 450-m diameter fresh ...crater at the Chang'e-3 (CE-3) landing site, investigated by the Yutu rover with in situ APXS (Active Particle-induced X-ray Spectrometer) and VNIS (Visible and Near-infrared Imaging Spectrometer) measurements. Results indicate that this region's composition differs from other mare sample-return sites and is a new type of mare basalt not previously sampled, but consistent with remote sensing. The CE-3 regolith derived from olivine-normative basaltic rocks with high FeO/(FeO+MgO). Deconvolution of the VNIS data indicates abundant high-Ca ferropyroxene (augite and pigeonite) plus Fe-rich olivine. We infer from the regolith composition that the basaltic source rocks formed during late-stage magma-ocean differentiation when dense ferropyroxene-ilmenite cumulates sank and mixed with deeper, relatively ferroan olivine and orthopyroxene in a hybridized mantle source.
Granitic lunar samples largely consist of granophyric intergrowths of silica and K-feldspar. The identification of the silica polymorph present in the granophyre can clarify the petrogenesis of the ...lunar granites. The presence of tridymite or cristobalite would indicate rapid crystallization at high temperature. Quartz would indicate crystallization at low temperature or perhaps intrusive, slow crystallization, allowing for the orderly transformation from high-temperature silica polymorphs (tridymite or cristobalite). We identify the silica polymorphs present in four granitic lunar samples from the Apollo 12 regolith using laser Raman spectroscopy. Typically, lunar silica occurs with a hackle fracture pattern. We did an initial density calculation on the hackle fracture pattern of quartz and determined that the volume of quartz and fracture space is consistent with a molar volume contraction from tridymite or cristobalite, both of which are less dense than quartz. Moreover, we analyzed the silica in the granitic fragments from Apollo 12 by electron-probe microanalysis and found it contains up to 0.7 wt% TiO2, consistent with initial formation as the high-temperature silica polymorphs, which have more open crystal structures that can more readily accommodate cations other than Si. The silica in Apollo 12 granitic samples crystallized rapidly as tridymite or cristobalite, consistent with extrusive volcanism. The silica then inverted to quartz at a later time, causing it to contract and fracture. A hackle fracture pattern is common in silica occurring in extrusive lunar lithologies (e.g., mare basalt). The extrusive nature of these granitic samples makes them excellent candidates to be similar to the rocks that compose positive relief silicic features such as the Gruithuisen Domes.
In light of global remotely sensed data, the igneous crust of the Moon can no longer be viewed as a simple, globally stratified cumulus structure, composed of a flotation upper crust of anorthosite ...underlain by progressively more mafic rocks and a residual‐melt (KREEP) sandwich horizon near the base of the lower crust. Instead, global geochemical information derived from Clementine multispectral data and Lunar Prospector gamma‐ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique: (1) the Procellarum KREEP Terrane (PKT), (2) the Feldspathic High‐lands Terrane (FHT), and (3) the South Pole‐Aitken Terrane (SPAT). The PKT is a mafic province, coincident with the largely resurfaced area in the Procellarum‐Imbrium region whose petrogenesis relates to the early differentiation of the Moon. Here, some 40% of the Th in the Moon's crust is concentrated into a region that constitutes only about 10% of the crustal volume. This concentration of Th (average ∼5 ppm), and by implication the other heat producing elements, U and K, led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust. Lower‐crustal materials within the PKT likely interacted with underlying mantle materials to produce hybrid magmatism, leading to the magnesian suite of lunar rocks and possibly KREEP basalt. Although rare in the Apollo sample collection, widespread mare volcanic rocks having substantial Th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources. The FHT is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation. Basin impacts into this region do not excavate significantly more mafic material, suggesting a thickness of tens of kilometers of anorthositic crust. The feldspathic lunar meteorites may represent samples from the anorthositic central region of the FHT. Ejecta from deep‐penetrating basin impacts outside of the central anorthositic region, however, indicate an increasingly mafic composition with depth. The SPAT, a mafic anomaly of great magnitude, may include material of the upper mantle as well as lower crust; thus it is designated a separate terrane. Whether the SPA basin impact simply uncovered lower crust such as we infer for the FHT remains to be determined.
Merrillite, Ca18(Ca,)Mg2(PO4)14-Ca18 Na2Mg2(PO4)14-Ca16REE2(Mg,Fe)2(PO4)14 occurs as a primary phosphate along with apatite, in lunar and martian rocks, and in meteorites. It is nominally anhydrous, ...but attempts to directly measure H in this mineral have not previously been reported. Because of the occurrence on Earth of whitlockite, Ca18(Mg,Fe2+)2(PO4)12HPO42, and the apparent incorporation in whitlockite of a merrillite component, the lack of a whitlockite component in extraterrestrial merrillite could be taken as an indicator of low hydrogen fugacity, and this implication has been applied to lunar merrillite. On the other hand, for martian rocks, where magmatic OH or H2O contents were likely higher, apatite accordingly contains higher OH contents, yet coexists with anhydrous, Na-rich merrillite. With direct measurements by SIMS, McCubbin et al. (2014), which is in the July issue of American Mineralogist (p. 1347-1354), show that Shergotty merrillite is anhydrous and infer that the high T of crystallization of Shergotty precluded incorporation of a whitlockite component. The mineral pair apatite-merrillite in extraterrestrial rocks constitutes a powerful pair for recording magmatic conditions; however, as McCubbin et al. show, the implications of these minerals and their compositions must be interpreted in light of careful and complete analyses and crystal chemical constraints.
The ages of zircons from high-Th impact-melt breccias (IMBs) from meteorite Sayh al Uhaymir (SaU) 169 and from rock fragments in soil samples from Apollo 12 have been determined using the SHRIMP-II ...ion microprobe. The IMBs are very similar to each other in chemistry, mineralogy and texture, and the zircons from the KREEP-rich (high-Th) crystalline impact melt have similar U and Th contents and identical ages, within uncertainties, of 3920±13 (2σ) Ma (SaU 169) and 3914±7 (2σ) Ma (Apollo 12). The age results support the idea that the high-Th IMBs (Apollo 12 and SaU 169) formed in the same impact event. The similarity of composition and age suggest that SaU 169 and the high-Th IMB fragments of Apollo 12 originated from the same area of the Procellarum KREEP Terrane. We interpret the age of zircon grains in the Apollo 12 high-Th IMB as a precise and direct determination of the age of the Imbrium impact. This age is significantly older than the commonly cited age of 3.85Ga but is similar to recent determinations from SIMS U–Pb dating of Apollo 14 apatite grains and with anticipated revision of ages by 40Ar–39Ar and 87Rb–86Sr. The present zircon 207Pb–206Pb age is the first direct zircon age determination of the Imbrium impact event from an Apollo sample. Previous measurements of zircon ages of Apollo IMBs have recorded events pre-dating the Imbrium basin-forming event.
► Zircons in Apollo 12 high-Th impact breccia yield a 3914±7 Ma crystallization age. ► The Apollo 12 breccias are related to impact-melt breccia in lunar meteorite SaU 169. ► Zircons crystallized in the Apollo 12 and SaU 169 melt breccias have the same ages. ► These breccias formed by solidification of melt produced during a basin impact. ► The high-Th impact-melt breccias most likely originated in the Imbrium event.
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