SHERGOTTY METEORITE: MINERALOGY,PETROGRAPHY AND MINOR ELEMENTS

The unusual achondrite Shergotty resembles terrestrial diabases, and textural and chemical evidence indicates pre-settling and post-settling crystallization of zoned augite (En₄₈Fs₁₉Wo₃₃-En₂₅Fs₄₇Wo₂₈) and pigeonite (En₆₁Fs₂₆Wo₁₃-En₂₁Fs₆₁Wo₁₈) coupled with late crystallization of plagioclase (Ab₄₃An₅₆/Or₁-Ab₅₆An₄₁Or₃: now shocked to maskelynite), titanomagnetite-ilmenite composite grains, mesostasis (normative Qz₃₄Ab₂₁An₅Or₃₈Fs₂, assuming Fe as ferrous), whitlockite, pyrrhotite (Fe_0.94S), fayalite (Fo₁₀), baddeleyite and chlorapatite. The oxide compositions (Usp₆₂Mt₃₈, Al₂O₃ 2.4, Cr₂O₃ 0.8 wt %; Ilm₉₅Hm₅) indicate ～ 850 °C and log oxygen fugacity ? 14, while the occurrence of fayalite rims on mesostasis next to ilmenite indicates 890 °C. Bearing in mind experimental uncertainties, these data are consistent with late-stage crystallization under relatively high oxygen fugacity, as indicated by coexistence of fayalite, Ti-magnetite and a silica glass. The high alkali content of the maskelynite and mesostasis, coupled with the redox state, indicates that the Shergotty meteorite resembles terrestrial basalts more than any other meteorites. Nevertheless the absence of H₂O, as shown by the occurrence of phosphorus in whitlockite rather than in hydroxylapatite, distinguish the Shergotty achondrite from typical terrestrial diabases. Whereas the FeO/MnO ratios of pyroxenes from the Moon, Earth and several differentiated meteorites are independent of FeO, the ratio for Shergotty pyroxenes changes from 30 to 40 with increasing FeO, and the linear trend extrapolates to 0.2 MnO for zero iron. Hence caution is needed in using FeO/MnO as a planetary indicator. For pyroxenes, Na is almost independent of Fe/Mg while Ti increases and Cr decreases with increasing Fe/Mg. Maskelynite contains 0.5–0.25 wt % K₂O, 0.6 wt % FeO, 0.04 TiO₂, 0.04–0.07 MgO, ～ 0.01 BaO and 0.02–0.03 P₂O₅. A bulk analysis calculated from the mode and compositions of the minerals matches quite well with two bulk chemical analyses but not with a third.     

Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition

The variations of the bulk composition of the silicate Moon (crust + mantle = Bulk Silicate Moon, BSM) depending on the thermal state are explored based on the joint inversion of gravitational, seismic, and petrologic data within the Na₂O–TiO₂–CaO–FeO–MgO–Al₂O₃–SiO₂ system. The mantle bulk temperature T_mean determining the mineral composition and physical properties of the Moon is adopted as the integral characteristic of thermal state. By parameter T_mean, all thermal models of the Moon can be conventionally broken down into the “cold” with T_mean ~ 690–860°C and the “hot” with T_mean ~ 925–1075°C. The estimations of refractory oxide abundance in lunar rocks depending on the thermal state are included in two different groups. Cold models of BSM are comparable by the bulk content of Al₂O₃ ~ 3.0–4.6 wt % to those for the silicate Earth (Bulk Silicate Earth, BSE), while hot models of BSM are significantly enriched with Al₂O₃ ~ 5.1–7.3 wt % (Al₂O₃ ~ 1.2–1.7 × BSE) as compared with BSE. On the contrary, independent of the temperature distribution, both types of BSM models are characterized by nearly constant values of bulk concentrations of FeO ~ 12–13 wt % and magnesian number MG# 80–81.5 (MG# = [MgO/(MgO + FeO) × 100]), which differ markedly from those for BSE (FeO ~ 8% and MG# 89). It means that for all possible temperature distributions, the silicate fraction of the Moon is FeO-enriched and MgO-depleted in relation to BSE. These arguments discard the possibility of the Moon’s formation out of the material of the Earth’s primitive mantle. In spite of the almost complete coincidence of the isotopic systems, this apparently undeniable fact has no adequate explanation in the existing canonical models of the Moon’s origin and should result in additional constraints on the dynamic processes in models of the formation of the Earth–Moon system. However, the problem of the similarity of and/or difference between compositions of the Moon and the Earth regarding the abundance of refractory elements, which is very important for the geochemistry of the Moon and the Earth’s mantle, remains unresolved and requires further study.     

THE COLONY METEORITE AND VARIATIONS IN CO3 CHONDRITE PROPERTIES

The Colony meteorite is an accretionary breccia containing several millimeter-to centimeter-size chondritic clasts embedded in a chondritic host. Colony is one of the least equilibrated CO3 chondrites; it has an unrecrystallized texture and contains compositionally heterogeneous olivine and low-Ca pyroxene, kamacite with low Ni and Co and high Cr, amoeboid inclusions with low FeO and MnO, a fine-grained silicate matrix with very high FeO, and numerous small chondrules with clear pink glass. However, Colony differs from normal CO chondrites in several respects: Although Al, Sc, V, Cr, Ir, Fe, Au and Ga abundances are consistent with a CO chondrite classification, certain lithophiles (Mg and Mn), siderophiles (Ni and Co) and chalcophiles (Se and Zn) are depleted by factors of 10–40%. The shape of Colony's thermoluminescence (TL) glow curve is similar to that of Allan Hills A77307 (another unequilibrated chondrite with CO3 petrological characteristics) and different from those of normal CO chondrites. [ALHA77307 also resembles Colony in having low Mg, Mn, Ni and Co, compared to normal CO chondrites, but it possesses CO-CV levels of Se and Zn and nearly CV levels of Cd.] Colony is badly weathered; it contains 22.7 wt.% Fe₂O₃ and 5.7 wt.% H₂O. Recalculating the analysis on an H₂O-free basis with all Fe₂O₃, NiO and CoO converted to metal, yields an inferred original metallic Fe, Ni abundance of ～ 19 wt.%. This is similar to that of Kainsaz (an unweathered CO3 fall), but much higher than that of all other CO3 chondrites (< 6.3 wt.%). Although it is possible that Colony and either ALHA77307 or Kainsaz constitute distinct CO3 chemical subgroups, the weathered nature of Colony and ALHA77307 preclude the drawing of firm conclusions. Nevertheless, it is clear that CO3 chondrites vary more in compositional and petrological properties than was previously recognized.     

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3-1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt.% SiO₂, 10-35 wt.% Al₂O₃, 1-8 wt.% FeO, and <25 wt.% TiO₂; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.     

Magnesian anorthositic granulites in lunar meteorites Allan Hills A81005 and Dhofar 309: Geochemistry and global significance

Abstract– Fragments of magnesian anorthositic granulite are found in the lunar highlands meteorites Allan Hills (ALH) A81005 and Dhofar (Dho) 309. Five analyzed clasts of meteoritic magnesian anorthositic granulite have Mg′ [molar Mg/(Mg + Fe)] = 81–87; FeO ≈ 5% wt; Al₂O₃ ≈ 22% wt; rare earth elements abundances ≈ 0.5–2 × CI (except Eu ≈ 10 × CI); and low Ni and Co in a non‐chondritic ratio. The clasts have nearly identical chemical compositions, even though their host meteorites formed at different places on the Moon. These magnesian anorthositic granulites are distinct from other highlands materials in their unique combination of mineral proportions, Mg′, REE abundances and patterns, Ti/Sm ratio, and Sc/Sm ratio. Their Mg′ is too high for a close relationship to ferroan anorthosites, or to have formed as flotation cumulates from the lunar magma ocean. Compositions of these magnesian anorthositic granulites cannot be modeled as mixtures of, or fractionates from, known lunar rocks. However, compositions of lunar highlands meteorites can be represented as mixtures of magnesian anorthositic granulite, ferroan anorthosite, mare basalt, and KREEP. Meteoritic magnesian anorthositic granulite is a good candidate for the magnesian highlands component inferred from Apollo highland impactites: magnesian, feldspathic, and REE‐poor. Bulk compositions of meteorite magnesian anorthositic granulites are comparable to those inferred for parts of the lunar farside (the Feldspathic Highlands Terrane): ～4.5 wt% FeO; ～28 wt% Al₂O₃; and Th <1 ppm. Thus, magnesian anorthositic granulite may be a widespread and abundant component of the lunar highlands.     

Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 2—Three-Layer Mantle

Internally consistent models of the thermal state, chemical composition and mineralogy of the three-layer mantle of the Moon are constructed based on the joint inversion of gravity, seismic and petrological-geochemical data within the Na₂O-TiO₂-CaO-FeO-MgO-Al₂O₃-SiO₂ system. Geochemical constraints on the chemical composition and physical properties in three zones of the mantle are obtained in terms of the cold and hot models. Velocities of P-waves in the lower mantle (～8 km/s) are higher than in the upper mantle (～7.7 km/s). The behavior of velocities of S-waves is conservative, they are observed in the interval 4.40–4.45 km/s in all zones of the mantle. It was found that, independently of the temperature distribution, the most probable concentrations of FeO, ～11–14 wt % and MgO, 28–31 wt % and the values of the magnesian number MG# 80–83 are approximately the same in the upper and the lower mantles of the Moon, but drastically differ from those in the bulk composition of the silicate Earth (Bulk Silicate Earth, BSE, FeO 8%, MG# 89). On the contrary, the estimates of Al₂O₃ concentration in the three-layer mantle noticeably depend on the thermal state. The results of solution of the inverse problem indicate the trend towards the gradual increase in the Al₂O₃ content with depth, from the upper to the lower mantle to 4–7% with the higher content of garnet. For the cold models of the lower mantle of the Moon, the bulk content of Al₂O₃ is ～1 × BSE, and for the hot models it can be in the interval of 1.3 × BSE-1.7 × BSE. The abundance of SiO₂ depends, to a lesser degree, on the thermal state and is 50–55% in the upper and 45–50% in the lower mantle. The high pyroxene content of the upper mantle of the Moon is the geochemical consequence of the geophysical models used with the inversion into composition and temperature relations; orthopyroxene, instead of olivine, is the dominant mineral of the upper mantle. Concentrations of SiO₂ in the lower (undifferentiated) mantle showing the bulk composition of the silicate Moon (Bulk Silicate Moon, BSM), are consistent with the geochemical estimates of 45–48% of SiO₂ for the BSM and close to those for the Earth’s mantle (45–47%). The composition of the mantle middle zone remains discussible, since it might be partially overlapped with compositions of the over- and underlying envelopes. The results of the model suggest that the mantle of the Moon is stratified in chemical composition. For the considered thermal state models, the mantle of the Moon is enriched in FeO and depleted in MgO in relation to the primitive Earth mantle, which indicates considerable differences between compositions of the Earth and its satellite.

     

Stratigraphy and evolution of basalts in Mare Humorum and southeastern Procellarum

Abstract— We have studied the mare basalts of Mare Humorum and southeastern Procellarum (30°W–50°W, 0°–40°S). One hundred and nine basaltic units have been identified from differences in their FeO wt% and TiO₂ wt% content, and variations in crater densities. Crater counting and reference to isotopically dated Apollo samples have provided an age for 33 major units. Some evidence for three distinct periods of volcanic activity has been found. We found that the large unit in the middle of Mare Humorum is the oldest in the basin. This supports the suggestion that the oldest central unit sank causing the lithosphere to bend and create dykes through which lava flowed to produce the outer units. No evidence of a trend in FeO wt% and TiO₂ wt% content against time is found within Mare Humorum. There appears to be no lateral trend of basalts in terms of FeO and TiO₂ wt% over the entire area with time. An increase in FeO content with time is found in the 33 major units and there is some evidence for an increase in TiO₂ in the same units. A correlation between FeO wt% and TiO₂ wt% content is evident when all 109 units are compared. A notable feature of this correlation is a sharp increase in gradient of TiO₂ wt% content when the FeO wt% content rises above about 17%.     

ACHONDRITE ALHA77005: ALTERATION OF CHROMITE AND OLIVINE

The olivine crystals of the 77005 achondrite are brown except for colorless shock lamellae, mottled patches, and grains adjacent to pools of impact melt. Sporadic dark alteration patches in brown olivine and Cr-rich spinel gave the following average electron-microprobe analyses: (olivine) P₂O₅ 0.9, SiO₂ 57.9, TiO₂ 0, Al₂O₃ 0.7, Cr₂O₃ 0.4, V₂O₃ 0, Fe₂O₃ (assumed oxidation state) 17.0, MgO 1.6, CaO 0.2, Na₂O 0, K₂O 1.8, SO₃ (assumed oxidation state) 9.2, Cl 0.1, sum 89.8 wt. %; (spinel) P₂O₅3.5, SiO₂2.1, TiO₂.2.2, Al₂O₃2.1, Cr₂O₃ 13.4, V₂O₃ 0.8, Fe₂O₃ 40.7, MgO 0.9, CaO 0.1, Na₂O 0, K₂O 2.0, SO₃ 11.1, Cl 0.1, sum 79.0 wt.%. Ion-microprobe analyses revealed H in both. Rare orange patches in brown olivine from another area gave SiO₂ 33–35, FeO 30-28, MgO 28–32, sum 93 wt. %. Thermal metamorphism under dry oxidizing conditions is discussed as a possible alternative to shock-induced oxidation for generation of the brown olivine (McSween and Stöffler). Because alteration patches transgress shock lamellae, and because sulfatic alteration occurs in fusion crusts of Antarctic meteorites (Gibson et al., 1983), alteration of the 77005 achondrite at the Antarctic surface is preferred to a complex series of processes needed for pre-terrestrial alteration.     

Compositional and temporal investigation of exposed lunar basalts in the Mare Imbrium region

This paper presents an updated stratigraphical and compositional study of the exposed maria within the Imbrium basin on the Moon. Clementine multispectral data were employed to derive TiO₂ and FeO wt% abundance estimates of potentially distinct basaltic flows. Additionally, NASA Lunar Orbiter images were used to estimate flow ages using crater count statistics. Mare Imbrium shows evidence of a complex suite of low to high-Ti basaltic lava units infilling the basin over an 800 million year timescale. More than a third (37%) of identified mare basalts were found to contain 1-3 wt% TiO₂. Two other major mare lithological units (representing about 25% of the surface each) show TiO₂ values between 3-5 and 7-9 wt%. The dominant fraction (55%) of the sampled maria contain FeO between 16 and 18 wt%, followed by 27% of maria having 18-20 wt% and the remaining 18%, 14-16 wt% FeO. A crater frequency count (for diameters >500 m) shows that in three quarters of the sampled mare crater counts range between 3.5 and 5.5×¹⁰⁻² per km², which translates, according to a lunar cratering model chronology, into estimated emplacement ages between ∼3.3 and 2.5 Ga. A compositional convergence trend between the variations of iron and titanium oxides was identified, in particular for materials with TiO₂ and FeO content broadly above 5 and 17 wt%, respectively, suggesting a related petrogenesis and evolution. According to these findings, three major periods of mare infill are exposed in the Imbrium basin; despite each period showing a range of basaltic compositions (classified according to their TiO₂ content), it is apparent that, at least within these local geological settings, the igneous petrogenesis generally evolved through time towards more TiO₂- and FeO-rich melts.     

Anatomy of individual agglutinates from a lunar highland soil

Abstract— We report results of our investigation of the relationship between values of I_s/FeO (relative concentration of nanophase Fe⁰ divided by total FeO content), glass abundance, total Fe content, and degree of digestion of <20 μm clasts for 22 individual agglutinates (250–1000 μm) from the mature Apollo 16 soil 61181 (I_s/FeO = 82 units in the <250 μm fraction). Agglutinates are important products of space weathering on the Moon, and they influence spectral observations at visible and near-IR wavelengths. Values of I_s/FeO for individual agglutinates (250–1000 μm) within this single soil span a range from 3 to 262 units which is larger than the range observed for all Apollo 16 bulk soils (～0 to 110 units). No correlation was observed between I_s/FeO and glass abundance and FeO concentrations for either agglutinitic glass or whole agglutinate particles under investigation. Our results suggest that the variation in I_s/FeO for agglutinates from a single soil may be in part a consequence of natural mixing processes on the Moon that produce highly-variable environments (with respect to surface exposure) for agglutinate formation and in part to variable kinetics of reactions in an agglutinate melt, which are influenced by a variety of factors including melt composition, temperature, impactor velocity, and quench rate. We cannot exclude but do not see evidence for other processes including addition of exotic agglutinates, micrometeoritic bombardment into compositionally-diverse microtargets, recycling of agglutinates, preferential melting of very fine soil particles, and production of nanophase Fe⁰ in amorphous rims of very fine irradiated lunar grains contributing to the observed variation of I_s/FeO.     

Lunar surface agglutinates: Mapping composition anomalies

From the Clementine UVVIS imagery of the lunar surface, the abundance of agglutinates in the lunar regolith and their composition in terms of FeO and Al₂O₃ oxides have been predicted. Data on the spectral, chemical, and mineralogic measurements of about 30 lunar soil samples from the Lunar Samples Characterization Consortium (LSCC) collection were used. The fulfilled prognosis confirms that the mare agglutinates are enriched in Al₂O₃ and depleted of FeO, while the highland agglutinates are depleted of Al₂O₃ and enriched in FeO. This behavior can be caused by the global transport of the lunar surface material induced by cosmogenic factors.     

THE LOOP CHONDRITE: PETROLOGY,MINERAL CHEMISTRY,AND OPAQUE MINERALOGY

The Loop meteorite was found in 1962 in Gaines County, Texas, at a location very close to that where the Ashmore chondrite was found in 1969. The two specimens were assumed to be fragments of the same meteorite. The Loop meteorite is a type L6 chondrite composed of olivine (Fo_75.4Fa_24.6), orthopyroxene (En_77.6Wo_1.5Fs_20.9), clinopyroxene (En_47.5Wo_45.1Fs_7.4), plagioclase (Ab_84.3Or_5.5An_10.2), Fe-Ni metal, troilite, and chromite. Fe-Ni metal is represented by kamacite (5.8-6.4 wt % Ni, 0.88-1.00 wt % Co), taenite (30.0–52.9 wt % Ni, 0.16-0.34 wt % Co), and plessite (16.8–28.5 wt % Ni, 0.38-0.54 wt % Co). Native copper occurs as rare inclusions in Fe-Ni metal. Both chondrules and matrix have similar mineral compositions. The mineral chemistry of the Loop meteorite is quite different from that of the Ashmore, which was classified as an H5 chondrite by Bryan and Kullerud (1975). Therefore, the Ashmore and Loop meteorites are two different chondrites, even though they were recovered from the same geographic location.     

Evidence in CO3.0 chondrules for a drift in the O isotopic composition of the solar nebula

Abstract— Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Δ¹⁷O (=δ¹⁷O ? 0.52 × δ¹⁸O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Δ¹⁷O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low‐FeO (≤Fa₁) type I and high‐FeO (≥Fa₁₅) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato‐81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Δ¹⁷O tends to increase with increasing FeO. We find that Δ¹⁷O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high‐FeO chondrules, the difference between Δ¹⁷O of the late‐formed, high‐FeO phenocryst olivine and those in the low‐FeO cores of relict grains is well‐resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule‐forming period, there was a small upward drift in the Δ¹⁷O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.     

Effects of dust enrichment on oxygen fugacity of cosmic gases

The degree to which dust enrichment enhances the oxygen fugacity (fO₂) of a system otherwise solar in composition depends on the dust composition. Equilibrium calculations were performed at 10^?3 bar in systems enriched by a factor of 10⁴ in two fundamentally different types of dust to investigate the iron oxidation state in both cases. One type of dust, called SC for solar condensate, stopped equilibrating with solar gas at too high a temperature for FeO or condensed water to be stabilized in any form, and thus has the composition expected of a nebular condensate. The other has CI chondrite composition, appropriate for a parent body that accreted from SC dust and low‐temperature ice. Upon total vaporization at 2300 K, both systems have high fO₂, >IW. In the SC dust‐enriched system, FeO of the bulk silicate reaches ~10 wt% at 1970 K but decreases to <1 wt% below 1500 K. The FeO undergoes reduction because consumption of gaseous oxygen by silicate recondensation causes a precipitous drop in fO₂. Thus, enrichment in dust having the composition of likely nebular condensates cannot yield a sufficiently oxidizing environment to account for the FeO contents of chondrules. The fO₂ of the system enriched in water‐rich, CI dust, however, remains high throughout condensation, as gaseous water remains uncondensed until very low temperatures. This allows silicate condensates to achieve and maintain FeO contents of 27–35 wt%. Water‐rich parent bodies are thus excellent candidate sources of chondrule precursors. Impacts on such bodies may have created the combination of high dust enrichment, total pressure, and fO₂ necessary for chondrule formation.     

Correlation between crystallographic structure and infrared spectra of silicon oxide films containing iron or magnesium atoms

Abstract— Films condensed from vapors containing SiO, Fe, or Mg showed an amorphous structure. Infrared (IR) spectra and electron microscopic characterization have been carried out on these films. After the heat treatment of these films in air, IR peaks at approximately 18–23 μm appeared, in addition to peaks attributable to SiO₂. These peaks can be attributed to metallic oxides such as FeO, Fe₂O₃, and MgO. It can be concluded that Fe‐ or Mg‐bearing silicate minerals cannot be produced by the rapid cooling of SiO, Fe, or Mg vapors. Although IR spectra of FeO have been discussed in order to match some spectra obtained with the Infrared Space Observatory, the identification of FeO as the impurity would be very important because the IR spectra of FeO grains are very dependent on the shape and size of the grains. These impurities can also influence the IR spectral feature of SiO₂.     

Geochemistry of Darwin glass and target rocks from Darwin crater,Tasmania, Australia

Abstract— Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro‐Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO₂ (85%), Al₂O₃ (7.3%), TiO₂ (0.05%), FeO (2.2%), MgO (0.9%), and K₂O (1.8%). Group 2 has lower average SiO₂ (81.1%) and higher average Al₂O₃ (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.9–10; Eu/Eu* = 0.55–0.65) and overlapping trace element abundances. 87Sr/⁸⁶Sr ratios for the glasses (0.80778–0.81605) fall in the range (0.76481–1.1212) defined by the rock samples. ε‐Nd results range from –13.57 to –15.86. Nd model ages range from 1.2–1.9 Ga (CHUR) and the glasses (1.2–1.5 Ga) fall within the range defined by the target samples. The ⁸⁷Sr/⁸⁶Sr versus 8⁷Rb/⁸6Sr regression age (411 ± 42 Ma) and initial ratio (0.725 ± 0.016), and the initial ⁴³Nd/144Nd ratio (0.51153 ± 000011) and regression age (451 ± 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elementsexcept FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile.     

A relict‐grain‐bearing porphyritic olivine compound chondrule from LL3.0 Semarkona that experienced limited remelting

Abstract— Chondrule D8n in LL3.0 Semarkona is a porphyritic olivine (PO) chondrule, 1300 times 1900 μm in size, with a complicated thermal history. The oldest recognizable portion of D8n is a moderately high‐FeO, PO chondrule that is modeled as having become enmeshed in a dust ball containing a small, intact, low‐FeO porphyritic chondrule and fine‐grained material consisting of forsterite, kamacite, troilite, and possibly reduced C. The final chondrule melting event may have been a heat pulse that preferentially melted the low‐FeO material and produced a low‐FeO, opaque‐rich, exterior region, 45–140 μm in thickness, around the original chondrule. At one end of the exterior region, a kamacite‐ and troilite‐rich lump 960 μm in length formed. During the final melting event, the coarse, moderately ferroan olivine phenocrysts within the original chondrule appear to have been partly resorbed (These relict phenocrysts have the highest concentrations of FeO, MnO, and Cr₂O₃—7.5, 0.20, and 0.61 wt%, respectively—in D8n.). Narrow olivine overgrowths crystallized around the phenocrysts following final chondrule melting; their compositions seem to reflect mixing between melt derived from the exterior region and the resorbed margins of the phenocrysts. During the melting event, FeO in the relict phenocrysts was reduced, producing numerous small blebs of Ni‐poor metallic Fe along preexisting curvilinear fractures. The reduced olivine flanking the trails of metal blebs has lower FeO than the phenocrysts but virtually identical MnO and Cr₂O₃ contents. Subsequent parent‐body aqueous alteration in the exterior region of the chondrule formed pentlandite and abundant magnetite.     

NIOBIAN RUTILE IN AN APOLLO 14 KREEP FRAGMENT

Niobian rutile was found in a KREEP lithic fragment of basaltic texture. The niobian rutile contains 85.3% TiO₂, 7.1% Nb₂O₅, 2.65% Cr₂O₃, 0.70% ZrO₂, 0.61% SiO₂, 0.82% Al₂O₃ 0.61% FeO, 0.52% CaO, 0.22% V₂O₃ in addition to minor amounts of MnO, MgO, and CeO₂. Rare earth elements were not detected, in contrast with lunar niobian rutile of Marvin (1971). Coexisting minerals in the KREEP fragment are major amounts of plagioclase and orthopyroxene, and minor amounts of olivine, ilmenite, augite, barian K-feldspar, whitlockite, troilite, Ni-Fe, zirkelite and chromite     

The influence of multivariate analysis methods and target grain size on the accuracy of remote quantitative chemical analysis of rocks using laser induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was used to quantitatively analyze 195 rock slab samples with known bulk chemical compositions, 90 pressed-powder samples derived from a subset of those rocks, and 31 pressed-powder geostandards under conditions that simulate the ChemCam instrument on the Mars Science Laboratory Rover (MSL), Curiosity. The low-volatile (<2 wt.%) silicate samples (90 rock slabs, corresponding powders, and 22 geostandards) were split into training, validation, and test sets. The LIBS spectra and chemical compositions of the training set were used with three multivariate methods to predict the chemical compositions of the test set. The methods were partial least squares (PLS), multilayer perceptron artificial neural networks (MLP ANNs) and cascade correlation (CC) ANNs. Both the full LIBS spectrum and the intensity at five pre-selected spectral channels per major element (feature selection) were used as input data for the multivariate calculations. The training spectra were supplied to the algorithms without averaging (i.e. five spectra per target) and with averaging (i.e. all spectra from the same target averaged and treated as one spectrum). In most cases neural networks did not perform better than PLS for our samples. PLS2 without spectral averaging outperformed all other procedures on the basis of lowest quadrature root mean squared error (RMSE) for both the full test set and the igneous rocks test set. The RMSE for PLS2 using the igneous rock slab test set is: 3.07 wt.% SiO₂, 0.87 wt.% TiO₂, 2.36 wt.% Al₂O₃, 2.20 wt.% Fe₂O₃, 0.08 wt.% MnO, 1.74 wt.% MgO, 1.14 wt.% CaO, 0.85 wt.% Na₂O, 0.81 wt.% K₂O. PLS1 with feature selection and averaging had a higher quadrature RMSE than PLS2, but merits further investigation as a method of reducing data volume and computation time and potentially improving prediction accuracy, particularly for samples that differ significantly from the training set. Precision and accuracy were influenced by the ratio of laser beam diameter (∼490 μm) to grain size, with coarse-grained rocks often resulting in lower accuracy and precision than analyses of fine-grained rocks and powders. The number of analysis spots that were normally required to produce a chemical analysis within one standard deviation of the true bulk composition ranged from ∼10 for fine-grained rocks to >20 for some coarse-grained rocks.