The relationship between geology and geochemistry in the Undarum/Spumans/Balmer region of the Moon

A detailed study of geochemical variation in the Undarum/Spumans/Balmer region identifies major rock components which form the region's regolith. The northern portion of the region contains numerous small mare deposits, which cluster in the Undarum and Spumans areas particularly. In the southern portion of the region lies the Balmer basin chemical anomaly, which correlates with extensive plains deposits. Regional maps of the orbital geochemical variables (Mg, Al, Fe, Ti, and Th) are examined individually, then correlated to one another and to the most recent geological map of the region. Results of the study indicate the presence of basalt components, which show considerable variation in character and which are correlated with both mare and plains features in a region otherwise dominated by ANT suite material. The northeastern area is the most characteristically anorthositic part of the region and mare plains deposits from this area have relatively anorthositic composition. A basalt intermediate in composition between mare basalt and Fra Mauro basalt is proposed as a probable component of the Balmer plains deposits. Southeastern Crisium basalts, and the parts of other mare deposits that are distributed along a structurally-related ring have a similar character. The major basalt deposits around the eastern part of Fecunditatis appear to have a different character. All deposits, particularly those in the northeastern area, appear to be mare basalt contiminated with ANT suite material from the surrounding highlands. In general, the surface geochemistry of the Undarum/Spumans/Balmer region is quite heterogeneous, showing a distinct north/south dichotomy.     

Lunar mare volcanism in the eastern nearside region derived from Clementine UV/VIS data

Abstract— Clementine UV/VIS multi‐spectral data were used to map mare deposits in the eastern lunar nearside region (Mare Tranquillitatis, Mare Fecunditatis, Mare Serenitatis, Mare Crisium, Mare Nectaris) to understand the volcanic history of this region. An array of Clementine and Clementine‐derived data were used to classify mare basalts; these include: 750 nm albedo, UV/VIS ratio, 1 μm absorption signatures, and Clementine derived FeO and TiO₂ contents. We have successfully identified several new geological units and have determined their spectral characteristics. For example, the relatively younger low‐Ti basalts were recognized in the eastern part of Mare Tranquillitatis. The central low‐Ti basalts in Mare Serenitatis, which had been classed as mISP, were divided into 2 groups. In Mare Nectaris, 2 types of mare basalts were identified, while only 1 group was recognized in the previous study. The stratigraphy constructed from the spectral analysis indicates that the mare deposits tend to become younger in the northern maria, including Serenitatis and Crisium, and older in the southern maria, including Tranquillitatis, Fecunditatis, and Nectaris. According to the relationship between the titanium contents of the mare units and their stratigraphy, the titanium content decreases with time in the early stage but increases toward the end of volcanism in the Serenitatis and Crisium region, while it increases with time but finally decreases in the Tranquillitatis and Fecunditatis region. In connection with the distribution of mare basalts, a large amount of high‐Ti mare basalts are found in Mare Tranquillitatis, especially in the western part, while other maria are covered by low‐Ti basalts. The iron contents show a similar distribution to that of titanium.     

The lunar farside: The nature of highlands east of Mare Smythii

Available lunar orbital data were studied in detail to determine the nature and origin of geochemical variation in a portion of the farside highlands east of the Smythii basin. Such data exist for the elements Al, Mg, Fe, Ti, and Th (Clark and Hawke, 1981; Davis, 1980; Metzger et al., 1977). As in our previous studies (Clarke and Hawke, 1981, 1987), averages and ranges of concentrations for these elements are calculated and correlated for photogeologically defined units associated with features such as Babcock, King, Al-Khwarezmi, Langemak, Pasteur, and Sklodowska and with the region as a whole. In addition, comparisons are made between this and other highland regions which have been investigated by other workers in a similar manner (Andre et al., 1977; Haines et al., 1978; Maxwell and Andre, 1981). The region is shown to be distinctively enriched in the anorthositic end-members of the ANT-suite. Anomalies which have been reported for this region (Hawke et al., 1985) are confirmed by this study. An area south of Pasteur shows enrichment in some mafic components, giving evidence for the presence of buried mare basalt, and lending support to the hypothesis that volcanic activity may be fairly widespread even in the farside highlands. The units just southeast of Mare Smythii appear to be geochemically related to the area partly surrounding the Smythii on the west (Clark and Hawke, 1987). Considerable geochemical heterogeneity exists in this area, as in areas of the nearside highlands (Clark and Hawke, 1981, 1982, 1987; Hawke et al., 1985).     

Distribution and stratigraphy of basaltic units in Maria Tranquillitatis and Fecunditatis: A Clementine perspective

Abstract— Maria Tranquillitatis and Fecunditatis have been mapped based on Clementine image mosaics and derived iron and titanium maps. Impact craters served as stratigraphic probes enabling better delineation of compositionally different basaltic units, determining the distribution of subsurface basalts, and providing estimates of total basalt thickness and the thickness of the surface units. Collected data indicate that volcanism in these maria started with the eruption of low‐Ti basalts and evolved toward medium‐ and high‐Ti basalts. Some of the high‐Ti basalts in Mare Tranquillitatis began erupting early and were contemporaneous with the low‐ and medium‐Ti basalts; these units form the oldest units exposed on the mare surface. Mare Tranquillitatis is mostly covered with high‐Ti basalts. In Mare Fecunditatis, the volume of erupting basalts clearly decreased as the Ti content increased, and the high‐Ti basalts occur as a few patches on the mare surface. The basalt in both maria is on the order of several hundred meters thick and locally may be as thick as 1600 m. The new basalt thickness estimates generally fall within the range set by earlier studies, although locally differ. The medium‐ to high‐Ti basalts exposed at the surfaces of both maria are meters to tens of meters thick.     

Similarities between the shergottites and terrestrial ferropicrites

Element abundance ratios have been used to both distinguish terrestrial and martian basalts and make estimates on the bulk planetary chemistry. However, these estimates are based upon ratios that are assumed to have been unaffected by igneous processes. Since the extent to which this is valid is unknown, comparisons of terrestrial and martian rocks are best conducted on rocks with similar mineralogy and petrology, and therefore a good likelihood of similar crystallization histories. When the geochemistry of terrestrial ferropicrites is compared with the olivine-phyric and basaltic shergottites, previously observed differences in chemistry are no longer definitive (i.e. Mg/Si, Al/Si, Ca/Si, Fe/Si, FeO/MnO, Al/Ti, Na/Ti, Na/Al, K/La, K/Rb, K/Th, K/U, Th/U ratios). Since ferropicrites are geochemical terrestrial analogs for the shergottites, their formation history can provide useful information about the formation of the shergottites. This suggests that both ferropicrites and shergottites formed from a heavily processed mantle source region.     

Geochemical characterization of moldavites from a new locality,the Cheb Basin,Czech Republic

Abstract— Twenty‐three moldavites from a new locality, the Cheb Basin in Western Bohemia, were analyzed by instrumental neutron activation analysis for 45 major and trace elements. Detailed comparison of the Cheb Basin moldavites with moldavites from other substrewn fields in both major and trace element composition shows that the Cheb Basin is a separate substrewn field. The geochemical data obtained are discussed with respect to the source materials and processes leading to formation of moldavites. The data show that three groups of Cheb Basin moldavites exist. Ten samples of group 1 are characterized by the lowest content of Al, Fe, Na, and other elements representing phyllosilicate minerals, and by high Ca + Mg contents related probably to carbonates. They resemble the “poisonous green” moldavites, a subgroup of the Southern Bohemian moldavites. Seven samples of group 2 and 6 samples of group 3 are similar to typical moldavites of the Southern Bohemian substrewn field. These two groups differ from each other mainly in Al contents; with higher contents of Al and the elements associated with phyllosilicate minerals (namely Ba and Sr), group 3 also resembles the Moravian moldavites. Significant positive correlations between K, Ca, Mg, and Mn found in group 2 of the Cheb Basin moldavites and the enrichment in these elements observed generally in all moldavites, as well as other facts, e.g., high K/Na and K/Rb ratios and the reduced conditions during formation of moldavites, have been attributed to possible contribution to the moldavite source materials of the ash produced by burning of vegetation and soil organic matter present at the pre‐impact area.     

Revisiting 26Al‐26Mg systematics of plagioclase in H4 chondrites

Zinner and Göpel ( 1992 , 2002 ) found clear evidence for the former presence of ²⁶Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the ²⁶Al‐²⁶Mg systematics of these chondrites date “metamorphic cooling of the H4 parent body.” Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al. 2011 ). Zinner and Göpel ( 2002 ) used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial ²⁶Al/²⁷Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel ( 2002 ), indicating that ratio bias does not significantly affect ²⁶Al‐²⁶Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial ²⁶Al/²⁷Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAIs. The isochrons for Forest Vale and Beaver Creek also show clear evidence that ²⁶Al was alive when they cooled, but the initial ²⁶Al/²⁷Al ratios are not well constrained. Sena does not show evidence that ²⁶Al was alive when it cooled to below the Al‐Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10–50 °C/Myr at 500 °C (Scott et al. 2014 ), we conclude that the Al‐Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the parent body, instead of undisturbed cooling at depth in the H chondrite parent body, like many have assumed.     

Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in ²⁶Mg and ⁴⁴Ca, which can be attributed to the in situ decay of radioactive and now extinct ²⁶Al and ⁴⁴Ti, respectively. Previous work on presolar SiC and graphite had shown that ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios of approximately 1 × 10^?3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios considerably.     

Searching for nonlocal lithologies in the Apollo 12 regolith: A geochemical and petrological study of basaltic coarse fines from the Apollo lunar soil sample 12023,155

New data from a petrological and geochemical examination of 12 coarse basaltic fines from the Apollo 12 soil sample 12023,155 provide evidence of additional geochemical diversity at the landing site. In addition to the bulk chemical composition, major, minor, and trace element analyses of mineral phases are employed to ascertain how these samples relate to the Apollo 12 lithological basalt groups, thereby overcoming the problems of representativeness of small samples. All of the samples studied are low‐Ti basalts (0.9–5.7 wt% TiO₂), and many fall into the established olivine, pigeonite, and ilmenite classification of Apollo 12 basaltic suites. There are five exceptions: sample 12023,155_1A is mineralogically and compositionally distinct from other Apollo 12 basalt types, with low pigeonite REE concentrations and low Ni (41–55 ppm) and Mn (2400–2556 ppm) concentrations in olivine. Sample 12023,155_11A is also unique, with Fe‐rich mineral compositions and low bulk Mg# (=100 × atomic Mg/[Mg+Fe]) of 21.6. Sample 12023,155_7A has different plagioclase chemistry and crystallization trends as well as a wider range of olivine Mg# (34–55) compared with other Apollo 12 basalts, and shows greater similarities to Apollo 14 high‐Al basalts. Two other samples (12023,155_4A, and _5A) are similar to the Apollo 12 feldspathic basalt 12038, providing additional evidence that feldspathic basalts represent a lava flow proximal to the Apollo 12 site rather than material introduced by impacts. We suggest that at least one parent magma, and possibly as many as four separate parent magmas, are required in addition to the previously identified olivine, pigeonite, and ilmenite basaltic suites to account for the observed chemical diversity of basalts found in this study.     

PETROLOGICAL CHARACTER OF THE LUNA 16 SAMPLE FROM MARE FECUNDITATIS

We have classified 1858 lithic and vitreous fragments from the Luna 16 core-tube sample. They were taken from the soil fractions ranging in size from 150 to 425 μ, at levels A and G (γ). No important differences are observed between the proportions of particle types in levels A and G, nor between the soils of Luna 16 and those from the Apollo 11 landing site in the nearby Mare Tranquillitatis. Luna 16 basalts are texturally and mineralogically similar to Apollo 11 basalts, though the former are characterized by more Fe-rich olivines and pyroxenes and by lower ilmenite contents than are Apollo 11 basalts. The atomic ratio Al/Ti in Luna 16 basalt pyroxenes in about 1.5; Apollo 11 basalt pyroxenes have Al/Ti = 2.0, indicating the possibility of a lower mean valence for Ti in the Luna 16 material than in the Apollo 11 material. Most light-colored lithic fragments are anorthositic rather than noritic in character and are comparable to Apollo 11 anorthosites in mineral chemistry. We believe they are samples of terra regions to the north of the Luna 16 landing site. Triangular diagrams plotting normative plagioclase, normative mafics plus oxides, and normative orthoclase plus apatite neatly separate the three major types of lunar materials — mare basalts, anorthosites, and noritic rocks — and reveal that the Luna 16 regolith is composed of mare basalt and anorthosite, with very little norite component. Colorless-to-greenish glass occurs in the Luna 16 sample, which has high Fe and low Ti; it may represent gabbroic rock related to the anorthosites     

The scientific rationale for the C1XS X-ray spectrometer on India's Chandrayaan-1 mission to the moon

The UK-built Chandrayaan-1 X-ray Spectrometer (C1XS) will fly as an ESA instrument on India's Chandrayaan-1 mission to the Moon, launched in October 2008. C1XS builds on experience gained with the earlier D-CIXS instrument on SMART-1, but will be a scientifically much more capable instrument. Here we describe the scientific objectives of this instrument, which include mapping the abundances of the major rock-forming elements (principally Mg, Al, Si, Ti, Ca and Fe) in the lunar crust. These data will aid in determining whether regional compositional differences (e.g., the Mg/Fe ratio) are consistent with models of lunar crustal evolution. C1XS data will also permit geochemical studies of smaller scale features, such as the ejecta blankets and central peaks of large impact craters, and individual lava flows and pyroclastic deposits. These objectives all bear on important, and currently unresolved, questions in lunar science, including the structure and evolution of any primordial magma ocean, as revealed by vertical and lateral geochemical variations in the crust, and the composition of the lunar mantle, which will further constrain theories of the Moon's origin, thermal history and internal structure.     

Global Mg/Si and Al/Si Distributions on the Lunar Surface Derived from Chang'E-2 X-ray Spectrometer

The technique of X-ray fluorescence remote sensing plays a significant role in research related to the chemical compositions of the Moon. Here we describe the data analysis method for China's Chang'E-2 X-ray spectrometer in detail and present the preliminary results about the first global Mg/Si and Al/Si maps of the lunar surface. Our results show that the distributions of Mg/Si and Al/Si correlate well with terrains on the Moon. The higher Mg/Si ratio corresponds to the mare regions while the lower value corresponds to the highland terrains. The map of the Al/Si ratio shows a reversed distribution compared with the map of the Mg/Si ratio.     

3‐D elemental and isotopic composition of presolar silicon carbides

Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ～300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with ²⁶Mg‐enrichment from extinct ²⁶Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial ²⁶Al/²⁷Al ratio. A second SN grain exhibits a lateral heterogeneity in ²⁶Al/²⁷Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta.     

26Al–26Mg chronology of high-temperature condensate hibonite in a fine-grained,Ca-Al-rich inclusion from reduced CV chondrite

Al–Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) have revealed the initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, for individual Ca-Al-rich inclusions (CAIs) in meteorites. We find that the relative sensitivity factors of ²⁷Al/²⁴Mg ratio for SIMS analysis of hibonite, one of the major constituent minerals of CAIs, exhibit variations based on their chemical compositions. This underscores the critical need for using appropriate hibonite standards to obtain accurate Al−Mg data. We measured the Al−Mg mineral isochron for hibonite in a fine-grained CAI (FGI) from the Northwest Africa 8613 reduced CV chondrite by SIMS using synthesized hibonite standards with ²⁷Al/²⁴Mg of ~30, ~100, and ~400. The obtained mineral isochron of hibonite in the FGI yields (²⁶Al/²⁷Al)₀ of (4.73 ± 0.09) × 10⁻⁵, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI (Kawasaki et al., 2020). The uncertainties of (²⁶Al/²⁷Al)₀ indicate that the constituent minerals in the FGI formed within ~0.02 Myr in the earliest solar system. The disequilibrium O-isotope distributions of the minerals in the FGI suggest that the O-isotope compositions of the nebular gas from which they condensed underwent a transitional change from ¹⁶O-rich to ¹⁶O-poor within ~0.02 Myr in the earliest solar system. Once formed, the FGI may have been removed from the forming region within ~0.02 Myr and transported to the accretion region of the parent body.     

26Al‐26Mg isotope systematics of the first solids in the early solar system

High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk ²⁶Al‐²⁶Mg isochron with an inferred initial ²⁶Al/²⁷Al ratio of approximately 5.25 × 10^?5, named the canonical ratio. Nonigneous CV CAIs yield well‐defined internal ²⁶Al‐²⁶Mg isochrons consistent with the canonical value. These observations indicate that the canonical ²⁶Al/²⁷Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI‐forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial ²⁶Al/²⁷Al ratios, (4.2–5.2) × 10^?5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial ²⁶Al/²⁷Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in ²⁶Al. At least some of these refractory objects may have formed prior to injection of ²⁶Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of ²⁶Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial ²⁶Al/²⁷Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical ²⁶Al/²⁷Al ratio.     

Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of ²⁶Mg, whereas the primary CAI phases mostly yield correlated excesses of ²⁶Mg with increasing Al/Mg corresponding to “canonical” initial ²⁶Al/²⁷Al ～ 4.5–5 × 10^?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in ²⁶Mg/²⁴Mg with increasing Al/Mg, yielding (²⁶Al/²⁷Al)0 = (4.9 ± 2.8) × 10^?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of ²⁶Mg consistent with initial ²⁶Al/²⁷Al ～ 4.5 × 10^?5. Especially in the compact type A CAI, where ²⁶Mg/²⁴Mg in grossular correlates with increasing Al/Mg, these ²⁶Mg excesses are almost certainly due to in situ decay of ²⁶Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.     

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO₂ content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO₂ content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.     

Questions,questions: Can the contradictions between the petrologic,isotopic, thermodynamic,and astrophysical constraints on chondrule formation be resolved?

Abstract– Here, we show that several geochemical indicators point to number densities during chondrule formation that were far higher than can be accounted for by known nebula processes. The number densities implied by compound chondrules and nonspherical chondrules are shown to be significantly higher than estimated in previous studies. At the implied chondrule number densities, if a chondrule formation region survived a formation event it would have been gravitationally bound and would have collapsed quite rapidly to form an asteroidal‐sized body. The diversity of chondrule compositions and textures in a chondrite group could have formed in a single event in subvolumes of a formation region that were chemically isolated from one another because of slow diffusion in the gas. Within these subvolumes, equilibration between chondrules with different compositions would have been fairly rapid, although small isotopic mass fractionations in elements like Fe, Si, Mg, and O may persist. This could explain the existence of the small isotopic mass fractionations in these elements that have been observed in chondrules. However, the evidence for recycling of chondrules requires that there was more than one chondrule formation event prior to formation of a parent asteroid. Finally, we argue that OC and CO chondrule Mg‐Al systematics are both consistent with single ages or narrow ranges of ages, and that the CO, and possibly the OC, ages date parent body alteration. This would resolve the conundrum of needing to preserve in a turbulent nebula physically and chemically distinct CO and OC chondrule populations for 1–2 Myr.     

银河系铝元素丰度研究

恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关.     

Al‐Mg systematics of hibonite‐bearing Ca,Al‐rich inclusions from Ningqiang

Abstract– Hibonite‐bearing Ca,Al‐rich inclusions (CAIs) usually occur in CM and CH chondrites and possess petrographic and isotopic characteristics distinctive from other typical CAIs. Despite their highly refractory nature, most hibonite‐bearing CAIs have little or no ²⁶Mg excess (the decay product of ²⁶Al), but do show wide variations of Ca and Ti isotopic anomalies. A few spinel‐hibonite spherules preserve evidence of live ²⁶Al with an inferred ²⁶Al/²⁷Al close to the canonical value. The bimodal distribution of ²⁶Al abundances in hibonite‐bearing CAIs has inspired several interpretations regarding the origin of short‐lived nuclides and the evolution of the solar nebula. Herein we show that hibonite‐bearing CAIs from Ningqiang, an ungrouped carbonaceous chondrite, also provide evidence for a bimodal distribution of ²⁶Al. Two hibonite aggregates and two hibonite‐pyroxene spherules show no ²⁶Mg excesses, corresponding to inferred ²⁶Al/²⁷Al < 8 × 10^?6. Two hibonite‐melilite spherules are indistinguishable from each other in terms of chemistry and mineralogy but have different Mg isotopic compositions. Hibonite and melilite in one of them display positive ²⁶Mg excesses (up to 25‰) that are correlated with Al/Mg with an inferred ²⁶Al/²⁷Al of (5.5 ± 0.6) × 10^?5. The other one contains normal Mg isotopes with an inferred ²⁶Al/²⁷Al < 3.4 × 10^?6. Hibonite in a hibonite‐spinel fragment displays large ²⁶Mg excesses (up to 38‰) that correlate with Al/Mg, with an inferred ²⁶Al/²⁷Al of (4.5 ± 0.8) × 10^?5. Prolonged formation duration and thermal alteration of hibonite‐bearing CAIs seem to be inconsistent with petrological and isotopic observations of Ningqiang. Our results support the theory of formation of ²⁶Al‐free/poor hibonite‐bearing CAIs prior to the injection of ²⁶Al into the solar nebula from a nearby stellar source.