Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite

Miller Range (MIL) 05035 is a lunar gabbroic meteorite. The mineralogy, Fe/Mn ratios in olivine and pyroxene, bulk-rock chemical composition and the bulk oxygen isotope values (δ¹⁷O = 2.86-2.97‰ and δ¹⁸O = 5.47-5.71‰) are similar to those of other mare basalts, and are taken as supporting evidence for a lunar origin for this meteorite. The sample is dominated by pyroxene grains (54-61% by area mode of thin section) along with large plagioclase feldspar (25-36% by mode) and accessory quartz, ilmenite, spinel, apatite and troilite. The bulk-rock major element composition of MIL 05035 indicates that the sample has a very low-Ti (VLT) to low-Ti lunar heritage (we measure bulk TiO₂ to be 0.9 Wt.%) and has low bulk incompatible trace element (ITE) concentrations, akin to samples from the VLT mare basalt suite. To account for these geochemical characteristics we hypothesize that MIL 05035’s parental melt was derived from a mantle region dominated by early cumulates of the magma ocean (comprised principally of olivine and orthopyroxene). MIL 05035 is likely launch paired with the Asuka-881757 and Yamato-793169 basaltic lunar meteorites and the basaltic regolith breccia MET 01210. This group of meteorites (Y/A/M/M) therefore may be a part of a stratigraphic column consisting of an upper regolith environment underlain by a coarsening downwards basalt lava flow.     

Petrology and geochemistry of LaPaz Icefield 02205: A new unique low-Ti mare-basalt meteorite

LaPaz Icefield 02205 (LAP 02205) is a new low-Ti mare-basalt meteorite that was discovered in the LaPaz Ice Field in Antarctica. This is the first crystalline lunar basalt in the US Antarctic collection and the only 5th unbrecciated mare-basalt meteorite to be discovered to date. The rock has a typical basaltic texture with tabular and elongated pyroxene and plagioclase crystals, and minor olivine grains commonly rimmed by pyroxenes. Core- to rim-zoning in terms of Fe and Mg is present in almost all pyroxene grains. Accessory minerals include ilmenite, chromite, ulvöspinel, troilite, and FeNi metal. This rock is highly enriched in late-stage mesostasis. Free silica is also abundant. In terms of texture and mineralogy, LAP 02205 displays features of low-Ti mare basalts, with similarities to some low-Ti Apollo 12 and Apollo 15 basalts. Whole-rock major- and trace-element compositions confirm the highly fractionated nature of this basalt. The whole-rock REE contents of the meteorite are the highest among all known low-Ti mare basalts. The platinum group element (PGE) contents in LAP are also enriched suggesting the possibility of endogenously enriched source regions or the PGEs generally behaved as incompatible elements during crystal fractionation under low fO₂ conditions. Trace-element contents of mineral grains in LAP 02205 display wide variations, suggesting extensive non-equilibrium crystallization. The REE concentrations in the earliest-formed minerals provide constraints on the composition of the parental liquid, which is similar to the measured whole-rock composition. Crystallization modeling of the LAP 02205 bulk composition yields a reasonable fit between predicted and observed mineral phases and compositions, except for the high-Mg olivine cores, which are observed in the rock but not predicted by the modeling. An isochron age of 2929 ± 150 Ma for phosphate minerals makes this rock one of the youngest lunar basalts known to date. The young age and specific geochemical characteristics of LAP distinguish it from those of most other low-Ti mare basalts. However, the low-Ti mare basalt meteorite, NWA 032, has a similar young age, and the two meteorites also appear to be closely related from some geochemical perspectives and might have originated from similar source regions on the Moon.     

Comparative petrology, geochemistry, and petrogenesis of evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo_64-2) and pyroxene (Fs₃₂Wo₈En₆₀ to Fs_84-86Wo₁₅En_2-0) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (∼2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon.     

Correlated δ18O and [Ti] in lunar zircons: a terrestrial perspective for magma temperatures and water content on the Moon

Zircon grains were separated from lunar regolith and rocks returned from four Apollo landing sites, and analyzed in situ by secondary ion mass spectrometry. Many regolith zircons preserve magmatic δ¹⁸O and trace element compositions and, although out of petrologic context, represent a relatively unexplored resource for study of the Moon and possibly other bodies in the solar system. The combination of oxygen isotope ratios and [Ti] provides a unique geochemical signature that identifies zircons from the Moon. The oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in δ¹⁸O (5.61 ± 0.07 ‰ VSMOW) than zircons from Earth’s oceanic crust (5.20 ± 0.03 ‰) even though mare basalt whole-rock samples are nearly the same in δ¹⁸O as oceanic basalts on Earth (~5.6 ‰). Thus, the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the Moon [Δ¹⁸O(WR-Zrc) = 0.08 ± 0.09 ‰] than on Earth (0.37 ± 0.04 ‰). The smaller fractionations on the Moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher [Ti] in lunar zircons. Phase equilibria estimates also indicate high temperatures for lunar magmas, but not specifically for evolved zircon-forming melts. If the solidus temperature of a given magma is a function of its water content, then so is the crystallization temperature of any zircon forming in that melt. The systematic nature of O and Ti data for lunar zircons suggests a model based on the following observations. Many of the analyzed lunar zircons are likely from K, rare earth elements, P (KREEP)-Zr-rich magmas. Zircon does not saturate in normal mafic magmas; igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts. Even if initial bulk water content is moderately low, the late zircon-forming melt can concentrate water locally. In general, water lowers crystallization temperatures, in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk-rock composition. Although lunar basalts could readily lose H₂ to space during eruption, lowering water fugacity; the morphology, large size, and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing. In this case, the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon-forming melt. If the smaller Δ¹⁸O(zircon–mare basalt) values reported here are characteristic of the Moon, then that would suggest that even highly evolved zircon-forming magmas on the Moon crystallized at higher temperature than similar magmas on Earth and that magmas, though not necessarily water-free, were generally drier on the Moon.     

A laser-ablation ICP-MS study of Apollo 15 low-titanium olivine-normative and quartz-normative mare basalts

Apollo 15 low-Ti mare basalts have traditionally been subdivided into olivine- and quartz-normative basalt types, based on their different SiO₂, FeO, and TiO₂ whole-rock compositions. Previous studies have reconciled this compositional diversity by considering the olivine- and quartz-normative basalts as originating from different lunar mantle source regions. To provide new information on the compositions of Apollo 15 low-Ti mare basalt parental magmas, we report a study of major and trace-element compositions of whole rocks, pyroxenes, and other phases in the olivine-normative basalts 15016 and 15555 and quartz-normative basalts 15475 and 15499. Results show similar rare-earth-element patterns in pyroxenes from all four basalts. The estimated equilibrium parental-melt compositions from the trace-element compositions of pyroxenes are similar for 15016, 15555 and 15499. Additionally, an independent set of trace-element distribution coefficients has been determined from measured pyroxene and mesostasis compositions in sample 15499. These data suggest that fractional crystallization may be a viable alternative to compositional differences in the mantle source to explain the 25% difference in whole-rock TiO₂, and corresponding differences in SiO₂ and FeO between the Apollo 15 olivine- and quartz-normative basalts. In this model, the older (3.35 Ga) quartz-normative basalts, with lower TiO₂ experienced olivine, chromite, and Cr-ulvöspinel fractionation at ‘crustal levels’ in magma chambers or dikes, followed by limited near-surface mineral fractionation, within the lava flows. In contrast, the younger (3.25 Ga) olivine-normative basalts experienced only limited magmatic differentiation at ‘crustal-levels’, but extensive near-surface mineral fractionation to produce their evolved mineral compositions. A two-stage mineral-fractionation model is consistent with textural and mineralogical observations, as well as the mineral trace-element constraints developed by this study.     

Antarctic lunar meteorites Yamato-793169, Asuka-881757, MIL 05035, and MET 01210 (YAMM): Launch pairing and possible cryptomare origin

The Antarctic lunar meteorite Meteorite Hills (MET) 01210 is a polymict regolith breccia, dominantly composed of mare basalt components. One relatively large (2.7 × 4.7 mm) basalt clast in MET 01210 (MET basalt) shows remarkable mineralogical similarities to the lunar-meteorite crystalline mare basalts Yamato (Y)-793169, Asuka (A)-881757, and Miller Range (MIL) 05035. All four basalts have similar rock texture, mineral assemblage, mineral composition, pyroxene crystallization trend, and pyroxene exsolution lamellae. The estimated TiO₂ contents (∼2.0 wt%) of the MET basalt and MIL 05035 are close to the bulk-rock TiO₂ contents of Y-793169 and A-881757. These similarities suggest that Y-793169, A-881757, MIL 05035, and the MET basalt came from the same basalt flow, which we designate the YAMM basalt. The source-basalt pairing of the YAMM is also supported by their similar REE abundances, crystallization ages (approx. 3.8-3.9 Ga), and isotopic compositions (low U/Pb, low Rb/Sr, and high Sm/Nd). The pyroxene exsolution lamellae, which are unusually coarse (up to a few microns) by mare standards, imply a relatively slow cooling in an unusually thick lava and/or subsequent annealing within a cryptomare. Reported noble gas and CRE data with close launch ages (∼1 Ma) and ejection depths (deeper than several meters) among the four meteorites further indicate their simultaneous ejection from the moon. Despite the marginally close terrestrial ages, pairing in the conventional Earth-entry sense seems unlikely because of the remote recovery sites among the YAMM meteorites.The high abundance (68%) of mare components in MET 01210 estimated from a two-component mixing model calculation could have resulted from either lateral mixing at a mare-highland boundary or vertical mixing in a cryptomare. The proportion of mare materials in MET 01210 is greater than in Apollo core samples at the mare-highland boundary. The burial depth (>several meters deep) inferred from the lack of surface irradiation of MET 01210 exceeds the typical mare regolith thickness (a few meters). Thus, the source of the YAMM meteorites is likely a terrain of locally high mare-highland mixing within a cryptomare. We searched for a possible source crater of the YAMM meteorites within the well-defined cryptomare, based on the multiple constraints obtained from this study and published data. An unnamed 1.4 km-diameter crater (53°W, 44.5°S) on the floor of the Schickard crater is the most suitable source for the YAMM meteorites.The ²³⁸U/²⁰⁴Pb (μ) value of the YAMM basalts is extremely low, relative to those of the Apollo mare basalts, but comparable to those of the Luna 24 very low-Ti basalts. The low-μ source indicates a derivation from a less differentiated mantle with a lack of KREEP components. Although the chemical sources of materials and heat source of melting might be independent, the heat source that generated the source magma of the YAMM and Luna 24 basalts may not be related to KREEP, unlike the case of the Apollo basalts. The distinct chemical and isotopic compositions of mantle sources between the Apollo basalts and the YAMM/Lunar 24 basalts imply differences in mantle composition and thermal evolution between the Procellarum KREEP Terrane (PKT) and non-PKT regions of the nearside.     

Oxygen isotopic signature of 4.4-3.9 Ga zircons as a monitor of differentiation processes on the Moon

We report oxygen isotopic compositions for 14 zircon grains from a sample of sawdust from lunar breccia 14321. The zircons range in age from ∼4.4 to 3.9 Ga and in U and Th content from a few to several hundred ppm. As such these grains represent a range of possible source rocks, from granophyric to mafic composition, and cover the total age range of the major initial lunar bombardment. Nevertheless, results show that the oxygen isotopic compositions of the zircons fall within a narrow range of δ¹⁸O of about 1 per mil and have δ¹⁸O values indistinguishable from those observed for terrestrial mid-ocean ridge basalts confirming the coincidence of lunar and Earth oxygen isotopic compositions. In the δ¹⁷O vs. δ¹⁸O, coordinates data form a tight group with a limited trend on the terrestrial fractionation line. The zircon oxygen isotopes show minimal evidence of the extreme and variable mineral differentiation and element fractionation that have contributed to the formation of their parent rocks.     

Petrology, geochemistry, and age of low-Ti mare-basalt meteorite Northeast Africa 003-A: A possible member of the Apollo 15 mare basaltic suite

Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and 2001. The main portion (∼75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion (∼25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si-K-rich glass, apatite, and a rare SiO₂ phase. Other accessory minerals include ulvöspinel, chromite, and trace Fe-Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite.The Fe/Mn values of the whole rock, olivines and pyroxenes, and the bulk-rock oxygen isotopic composition provide evidence for the lunar origin of NEA 003-A meteorite. This is further supported by the presence of Fe-Ni metal and the anhydrous mineral association.NEA 003-A is geochemically and petrographically distinct from previously described mare-basalt meteorites and is not paired with any of them. The petrography and major element composition of NEA 003-A is similar to the composition of low-Ti olivine mare basalts from Apollo 12 and olivine-normative basalts from Apollo 15. The NEA 003-A meteorite shows obvious geochemical similarities in trace elements contents with Apollo 15 olivine-normative basalts and could represent a yet unknown geochemically primitive member of the olivine-normative basalt series. The meteorite is depleted in rare earth elements (REE) and incompatible trace elements indicating a primitive character of the parental magma. The bulk-rock chemical composition demonstrates that the parent melt of NEA 003-A was not contaminated with KREEP components as a result of magma mixing or assimilation processes. Results of crystallization modelling and low minimum cooling rate estimates (∼0.07 °C/h) suggest that the parent melt of NEA 003-A crystallized in the lower part of a lava flow containing cumulate olivine (∼10%) and was probably derived from more primitive picritic magma by fractional crystallization processes.Sm-Nd dating yields an age of 3.09 ± 0.06 Ga which corresponds to the period of lower Eratosthenian lunar volcanic activity, and the near-chondritic ε_Nd value of −0.4 ± 0.3 indicates that the meteorite could be derived from a slightly enriched mantle source similar to the Apollo 15 green glasses. Ar-Ar step release results are inconsistent with Sm-Nd ages suggesting that NEA 003-A was exposed to one or more impact events. The most extensive event took place at 1.8 Ga and the shock intensity was likely between 28 and 45 GPa. The absence of solar Ar suggests that NEA 003-A has not been directly exposed at the lunar surface but the cosmic ray exposure age of 209 ± 6 Ma suggests that NEA 003-A resided in the upper regolith for part of its history.     

Nb/Zr fractionation on the Moon and the search for extinct Nb

A comprehensive Zr isotopic study was conducted on eleven lunar basalts and highland rocks to search for evidence of the extinct nuclide ⁹²Nb, which decays to ⁹²Zr with a half-life of 36 Ma. Internal isochrons were determined for two early highland rocks, 77215 and 60025. No resolvable Zr isotopic variations were detected in this wide range of lunar samples and thus there is no evidence for the former existence of live ⁹²Nb on the Moon. The Nb/Zr ratios of lunar ilmenites and bulk rock samples vary by only a factor of two to three relative to the chondritic Nb/Zr ratio. No evidence for larger Nb/Zr fractionation was found. This limited fractionation and late isotopic closure of the source region prevents the formation of measurable ⁹²Zr anomalies in high-Ti mare basalts. As a consequence, it is not possible to draw conclusions from the ⁹²Nb-⁹²Zr chronometer about the timing of early lunar differentiation and to constrain the role of ilmenite in the source region of high-Ti mare basalts. However, the fractionation is still sufficient to deduce an upper limit for the initial ⁹²Nb/⁹³Nb ratio of the solar system of <5 × 10⁻⁴.     

Testing the relation between UV–vis color and TiO2 content of the lunar maria

Reflectance spectroscopy is important for placing lunar samples into a regional and global geologic context. To this end, the ultraviolet–visible (UV–vis) color ratio, used to estimate the TiO₂ composition of mature mare basalts, has been one of the most widely used spectral parameters in lunar exploration. We examine the correlation between UV–vis color and TiO₂ content using a combination of Clementine, Lunar Prospector, and sample data to document the extent to which color is dependent upon TiO₂. Examination of the remotely sensed data reveals that the correlation between UV–vis ratio and TiO₂ composition is best represented by a sigmoidal trend rather than the canonical linear or curvilinear correlation. With this information, we are then able to evaluate between two models that propose different explanations for the relationship between UV–vis color and TiO₂. The first model attributes the correlation between TiO₂ and UV–vis color to spectrally neutral opaques (i.e., ilmenite), while the other emphasizes the effect of Fe–Ti charge-transfer in lunar glasses and dual scattering mechanisms between high- and low-Ti basalts. We do not find evidence in the spectral data to support the occurrence of Fe–Ti charge-transfer in lunar glass as the principal cause for color in high-Ti basalts. The data also do not substantiate the existence of different scattering mechanisms (e.g., volume v. surface scattering) between high- and low-TiO₂ basalts. Instead, our analyses substantiate that the spectral effects of ilmenite exhibit a major influence over the UV–vis ratio. By including sample data we find that in addition to ilemenite/TiO₂ content, factors such as FeO content, ilmenite grain size, modal abundance of plagioclase, and the olivine-to-pyroxene ratio in a mare soils can influence the UV–vis continuum. These findings point to promising avenues of research that future UV–vis spectral techniques can exploit in order to yield more accurate TiO₂ estimates and potentially additional petrologic information.     

The geochemistry and provenance of Apollo 16 mafic glasses

The regolith of the Apollo 16 lunar landing site is composed mainly of feldspathic lithologies but mafic lithologies are also present. A large proportion of the mafic material occurs as glass. We determined the major element composition of 280 mafic glasses (>10 wt% FeO) from six different Apollo 16 soil samples. A small proportion (5%) of the glasses are of volcanic origin with picritic compositions. Most, however, are of impact origin. Approximately half of the mafic impact glasses are of basaltic composition and half are of noritic composition with high concentrations of incompatible elements. A small fraction have compositions consistent with impact mixtures of mare material and material of the feldspathic highlands. On the basis of major-element chemistry, we identified six mafic glass groups: VLT picritic glass, low-Ti basaltic glass, high-Ti basaltic glass, high-Al basaltic glass, KREEPy glass, and basaltic-andesite glass. These glass groups encompass 60% of the total mafic glasses studied. Trace-element analyses by secondary ion mass spectroscopy for representative examples of each glass group (31 total analyses) support the major-element classifications and groupings. The lack of basaltic glass in Apollo 16 ancient regolith breccias, which provide snapshots of the Apollo 16 soil just after the infall of Imbrium ejecta, leads us to infer that most (if not all) of the basaltic glass was emplaced as ejecta from small- or moderate-sized impacts into the maria surrounding the Apollo 16 site after the Imbrium impact. The high-Ti basaltic glasses likely represent a new type of basalt from Mare Tranquillitatis, whereas the low-Ti and high-Al basaltic glasses possibly represent the composition of the basalts in Mare Nectaris. Both the low-Ti and high-Al basaltic glasses are enriched in light-REEs, which hints at the presence of a KREEP-bearing source region beneath Mare Nectaris. The basaltic andesite glasses have compositions that are siliceous, ferroan, alkali-rich, and moderately titaniferous; they are unlike any previously recognized lunar lithology or glass group. Their likely provenance is within the Procellarum KREEP Terrane, but they are not found within the Apollo 16 ancient regolith breccias and therefore were likely deposited at the Apollo 16 site post-Imbrium. The basaltic-andesite glasses are the most ferroan variety of KREEP yet discovered.     

Petrogenesis of lunar meteorite EET 96008

Lunar meteorite EET 96008 is a fragmental breccia that predominantly consists of basaltic mineral clasts (0.5-2 mm), along with minor lithic fragments and breccia clasts. The matrix consists mainly of smaller mineral fragments (<0.5 mm), bound by glassy cement, the majority of which are pyroxene and plagioclase. The pyroxene possesses extensive exsolution lamellae. These lamellae, up to 1 μm in width, are atypical for mare-basalts. One of the distinguishing textures of EET 96008 is the presence of small pockets (∼400 × 500 μm) of mesostasis areas consisting of coarse (∼20 μm) intergrowths of ferroaugite, fayalite and Si-rich glass. Laths of ilmenite, armalcolite, apatite and whitlockite are also distributed in these areas. Ilmenite grains are abundant and dispersed throughout the thin sections. Chromite and ulvöspinel are present but in minor abundance. Troilite, generally rare in this rock, occurs as several grains in one pyroxene crystal. FeNi metal is conspicuously absent from this meteorite.The molar Fe/Mn ratio in olivines and pyroxenes and the age of the meteorite are evidence for a lunar origin. The mineralogy of EET 96008 shows close affinity to a mare-basalt source, albeit with possible minor highland/non-mare components. The bulk-rock, major-, trace- and rare-earth-element (REE) contents are similar to that of very low-titanium (VLT) basalts, which have experienced extreme fractional crystallization to the point of silicate liquid immiscibility. Mineralogical and textural features of this sample suggest that at least some of the breccia components were derived from a slow-cooled magma. The mineralogy and petrology of EET 96008 is strikingly similar to the lunar meteorite EET 87521, and we support the conclusion that EET 96008 and EET 87521 should be paired.Isochron ages of 3530 ± 270 Ma for apatite and 3519 ± 100 Ma for whitlockite of this rock are consistent with derivation from a mare-basalt precursor. These ages are within error of the low-Ti basalts, dated from the Apollo 12 and 15 sites. The whole-rock, platinum-group-element (PGE) contents of EET 96008 overlap with pristine low-Ti mare basalts, suggesting the presence of only a minimal extraterrestrial component.     

Mechanisms for incompatible-element enrichment on the Moon deduced from the lunar basaltic meteorite Northwest Africa 032

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb-Sr and Sm-Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb-Sr and Sm-Nd ages. Rubidium-Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial ⁸⁷Sr/⁸⁶Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an ⁸⁷Rb/⁸⁶Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium-neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial ε_Nd of +9.71 ± 0.74 corresponding to a source region with ¹⁴⁷Sm/¹⁴⁴Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts.Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.     

Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: Insights into the source of a large igneous province

The nature of the source of continental flood basalts (CFB) is a highly debated topic. Proposed mantle sources for CFBs, including both high- and low-Ti basalts, include subcontinental lithospheric mantle (SCLM), asthenospheric mantle, and deep, plume-related mantle. Re-Os isotope systematics can offer important constraints on the sources of both ocean island basalts (OIB) and CFB, and may be applied to distinguish different possible melt sources. This paper reports the first Re-Os isotope data for the Late Permian Emeishan large igneous province (LIP) in Southwest China. Twenty one CFB samples including both low- and high-Ti basalts from five representative sites within the Emeishan LIP have been analyzed for Os, Nd, and Pb isotopic compositions. The obtained Os data demonstrate that crustal assimilation affected Os isotopic compositions of some Emeishan basalt samples with low Os concentrations but not all of the samples, and the Emeishan basalts with high Os contents likely experienced the least crustal contamination. The low and high-Ti basalts yield distinct Os signatures in terms of ¹⁸⁷Os/¹⁸⁸Os and Os content. The low-Ti basalt with the highest Os concentration (400 ppt) has a radiogenic Os isotopic composition (γOs(t), +6.5), similar to that of plume-derived OIB. Because the Os isotopic composition of basalts with relatively high Os concentrations (typically >50 ppt) likely represents that of their mantle source, this result implies a plume-derived origin for the low-Ti basalts. On the other hand, the high-Ti basalts with high Os concentration (over 50 ppt) have unradiogenic Os isotopic signatures (γOs(t) values range from −0.8 to −1.4), suggesting that a subcontinental lithosphere mantle (SCLM) component most likely contributed to the generation of these magmas. Combining Pb and Nd isotopic tracers with the Os data, we demonstrate that the low-Ti basaltic magmas in the Emeishan CFB were mainly sourced from a mantle plume reservoir, whereas the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plume-related magma ascent through the SCLM.     

On the elusive isotopic composition of lunar Pb

Highly radiogenic Pb isotope compositions determined for volcanic glass beads from the Apollo 14 soil sample 14163 are similar to those commonly determined for mare basalts and are correlated with chemical variations observed in the beads. This indicates that Pb unsupported by in-situ U decay has a similar origin in both glass beads and mare basalt samples and is likely to reflect variations of ²³⁸U/²⁰⁴Pb (μ) in the lunar mantle. An alternative explanation that this Pb is a result of late equilibration with the radiogenic Pb present in soil is less likely as it would imply that all other characteristics of glass beads such as their chemistry must also be a consequence of equilibration near the lunar surface. Regardless of the origin of unsupported Pb, observed variations of Pb isotope compositions in the glass beads and mare basalts appear to be a result of two component mixing between a primitive reservoir with a μ-value similar to the Earth’s mantle and KREEP with a μ-value in excess of several thousand. This range cannot be explained by the fractionation of major rock forming minerals from the crystallising Lunar Magma Ocean and instead requires substantial extraction of sulphide late in the crystallisation sequence. The proportion of sulphide required to produce the inferred range places limits on the starting μ of the Moon prior to differentiation, demanding a relatively high value of about 100-200. Low μ indicated by several basalt samples and previously analysed volcanic glass beads can be explained by the preservation of an early (but post Ferroan Anorthosite) sulphide rich reservoir in the lunar mantle, while a complete range of Pb isotope compositions observed in the glass beads and mare basalts can be interpreted as mixing between this sulphide rich reservoir and KREEP.     

Petrology,isotope characteristics,and K-Ar ages of the Maranhão,northern Brazil,Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this 700x250 km Maranhão province (5°–8°S) has low-Ti basalts (1.1 wt% TiO₂) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO₂) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb_(n) 4.2 and 8.8, where La_(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher ¹⁸O than the high-Ti basalts. Respectively, low- vs high-Ti: _Sr26–54 vs 15–18; ²⁰⁶Pb/²⁰⁴Pb=18.25–.78 vs 18.22–.24; and ¹⁸O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: _Nd–1.6 to –3.8 vs –2.1 to –3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb 7/4=4.6–11, and Pb 8/4=72–87; Serra Geral has Pb 7/4=10–13, and Pb 8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.     

40Ar/39Ar age of the onset of high-Ti phase of the Emeishan volcanism strengthens the link with the end-Guadalupian mass extinction

Precise time constraints of the main extrusive phase of the Emeishan large igneous province (ELIP) remain unresolved because basalts commonly do not contain suitable minerals for U–Pb dating, whereas previous ⁴⁰Ar/³⁹Ar studies on basalts yielded tectonothermal overprint ages. The timing for the ELIP was deduced from indirect dating of minor intrusions of ultramafic/mafic and felsic compositions by geochronological methods and geological correlations. The extrusive part of the ELIP consists of an older low-Ti and younger high-Ti basalt phases. We have found fresh samples of plagioclase-phyric rocks at the lower Qiaojia extrusive section (the Yunnan province of China), which belong to the ELIP unit of the high-Ti basalt series. ⁴⁰Ar/³⁹Ar dating on plagioclase from two samples conducted at two different laboratories using different age standards yielded statistically indistinguishable results with the weighted mean age of 260.1 ± 1.2 Ma for five individual measurements. This provides the direct constraints on the onset of the ELIP high-Ti basalt extrusive phase. The obtained age is within the error or slightly older than the age of the Guadalupian–Lopingian boundary and felsic ignimbrite capping the ELIP lava succession (both dated at 259.1 ± 0.5 Ma). Our new data are strengthening the short duration of the, at least, high-Ti phase of the ELIP volcanism and its temporal link with the end-Guadalupian mass extinction. Estimation of the total duration of the ELIP volcanism awaits finding of suitable for dating low-Ti basalts.     

新疆托里蛇绿岩中两种不同玄武岩的地球化学特征及成因

托里蛇绿岩中分布着高Ti玄武岩与低Ti玄武岩,在地球化学特征上存在明显的差异;表明二者之间不存在过渡关系,有着不同的形成条件。它们分别代表岛弧拉张不同阶段之产物,本文讨论了玄武岩的地球化学特征,从而揭示了蛇绿岩形成于弧后盆地的微扩张环境。     

Lunar volcanic glasses and their constraints on mare petrogenesis

Volcanic glasses from the Apollo 11, 14, 15, and 16 landing sites have been analyzed for major elements and Ni by electron microprobe. The 19 varieties of volcanic glass define two distinct chemical arrays that provide new insights into (a) the petrogenesis of mare basalts and (b) the structure of the deep lunar interior. A simple model is proposed whereby mare basaltic liquids may have been derived from two isolated, cumulate systems occurring at depths of ~300 km and ? 400 km. Each system was itself composed of two lithologic components (low-Ti vs high-Ti) that underwent hybridization, assimilation, or mixing to generate the large compositional range of magmas observed within each array. While the distribution of the two components within each system was locally heterogeneous, data indicate that the components themselves were chemically uniform on at least a regional scale. The surface-correlated volatiles associated with the lunar volcanic glasses seem to have come from some other reservoir within the Moon.The simplicity of the chemical relationships observed among the lunar volcanic glasses allow specific-predictions to be made. We believe that they should readily reveal any strengths or weaknesses of this new model.