Lunar Mare Basalts Research Articles

Are the Earth and the Moon compositionally alike? Inferences on lunar composition and implications for lunar origin and evolution from geophysical modeling

The main objective of the present study is to discuss in detail the results obtained from an inversion of the Apollo lunar seismic data set, lunar mass, and moment of inertia. We inverted directly for lunar chemical composition and temperature using the model system CaO‐FeO‐MgO‐Al2O3‐SiO2. Using Gibbs free energy minimization, stable mineral phases at the temperatures and pressures of interest, their modes and physical properties are calculated. We determine the compositional range of the oxide elements, thermal state, Mg#, mineralogy and physical structure of the lunar interior, as well as constraining core size and density. The results indicate a lunar mantle mineralogy that is dominated by olivine and orthopyroxene (−80 vol%), with the remainder being composed of clinopyroxene and an aluminous phase (plagioclase, spinel, and garnet present in the depth ranges 0–150 km, 150–200 km, and >200 km, respectively). This model is broadly consistent with constraints on mantle mineralogy derived from the experimental and observational study of the phase relationships and trace element compositions of lunar mare basalts and picritic glasses. In particular, by melting a typical model mantle composition using the pMELTS algorithm, we found that a range of batch melts generated from these models have features in common with low Ti mare basalts and picritic glasses. Our results also indicate a bulk lunar composition and Mg# different to that of the Earth's upper mantle, represented by the pyrolite composition. This difference is reflected in a lower bulk lunar Mg# (−0.83). Results also indicate a small iron‐like core with a radius around 340 km.

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 (Fs 32Wo 8En 60 to Fs 84–86Wo 15En 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.

Petrography and geochemistry of the LaPaz Icefield basaltic lunar meteorite and source crater pairing with Northwest Africa 032

Abstract— We report on the bulk composition and petrography of four new basaltic meteorites found in Antarctica—LAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436—and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse-grained (up to 1.5 mm), subophitic low-Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low-Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.

Erosion by flowing Martian lava: New insights for Hecates Tholus from Mars Express and MER data

We have used new compositional information on Martian basaltic rocks from the Mars Exploration Rover Spirit and data from new stereo imaging by the High Resolution Stereo Camera (HRSC) on the Mars Express orbiter to constrain an existing analytical‐numerical computer model to assess the potential of Martian lavas to form lava channels by erosion of substrate. The basaltic rocks studied in Gusev crater by Spirit have compositions consistent with lavas of higher liquidus temperature and lower dynamic viscosity than terrestrial tholeiitic basalts, suggesting that they had a greater potential for turbulent flow and erosion of substrate during flow emplacement than terrestrial basalts, more similar to lunar mare basalts. We modeled a specific case to determine whether these lavas could have formed part of a >66 km long channel on the Martian shield volcano Hecates Tholus. This channel formed on relatively steep slopes (∼2–8°), based on measurements from the HRSC Digital Terrain Model. Our results suggest that erosion rates were of the order of tens to hundreds of centimeters/day, depending on flow rate and ice content of the substrate. Eruption durations required to erode the Hecates channel (∼100–30 m deep over the first ∼20 km, depth decreasing downstream) range from weeks to months, which are consistent with the known eruption durations of terrestrial basaltic lavas and the previous modeling of Wilson and Mouginis‐Mark (2004). Any Hecates lava channels formed from erosion by lava could have served as conduits for later fluvial activity, as recently described by Fassett and Head (2004, 2005).

Reflectance Spectral Characteristics of Lunar Surface Materials

Based on a comprehensive analysis of the mineral composition of major lunar rocks (highland anorthosite, lunar mare basalt and KREEP rock), we investigate the reflectance spectral characteristics of the lunar rock-forming minerals, including feldspar, pyroxene and olivine. The affecting factors, the variation of the intensity of solar radiation with wavelength and the reflectance spectra of the lunar rocks are studied. We also calculate the reflectivity of lunar mare basalt and highland anorthosite at 300, 415, 750, 900, 950 and 1000 nm. It is considered that the difference in composition between lunar mare basalt and highland anorthosite is so large that separate analyses are needed in the study of the reflectivity of lunar surface materials in the two regions covered by mare basalt and highland anorthosite, and especially in the region with high Th contents, which may be the KREEP-distributed region.

The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Samarium–neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial ε Nd 143 value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87Sr/ 86Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite. Combining the ε Nd 142-ε Nd 143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513 +0.033 −0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios ( 87Rb/ 86Sr = 0.037–0.374; 147Sm/ 144Nd = 0.182–0.285; 176Lu/ 177Hf = 0.028–0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of f O 2 , suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.

Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum

Accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the Moon. We performed new crater size‐frequency distribution measurements in order to investigate the stratigraphy of mare basalts in Oceanus Procellarum and related regions such as Mare Nubium, Mare Cognitum, and Mare Insularum. We used high‐resolution Clementine color data to define 86 spectrally homogeneous units within these basins, which were then dated with crater counts on Lunar Orbiter IV images. Our crater size‐frequency distribution measurements define mineralogical and spectral surface units and offer significant improvements in accuracy over previous analyses. Our data show that volcanism in the investigated region was active over a long period of time from ∼3.93 to 1.2 b.y., a total of ∼2.7 b.y. Volumetrically, most of the basalts erupted in the Late Imbrian Period between ∼3.3 and 3.7 b.y., and we see evidence that numerous units have been resurfaced. During the Eratosthenian Period, significantly less basalt was erupted. Depending on the absolute model ages that one can assign to the lunar chronostratigraphic systems, five units might be of Copernican age. Younger basalts are generally exposed in the center of the investigated area, that is, closer to the volcanic centers of the Aristarchus Plateau and Marius Hills. Older basalts occur preferentially along the northwestern margin of Oceanus Procellarum and in the southeastern regions of the studied area, i.e., in Mare Cognitum and Mare Nubium. Combining the new data with our previously measured ages for basalts in Mare Imbrium, Serenitatis, Tranquillitatis, Humorum, Australe, and Humboldtianum, we find that the period of active volcanism on the Moon lasted ∼2.8 b.y., from ∼4 b.y. to ∼1.2 b.y. On the basis of the basalts dated so far, which do not yet include the potentially young basalts of Mare Smythii e.g.,all investigated basins but probably also is the location of some of the youngest basalts on the lunar surface.

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773

Abstract— The 40Ar‐39Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 ± 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of 39Ar, or the presence of pre‐shock 40Ar incorporated into shock‐melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61–2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ˜2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.

An early lunar core dynamo driven by thermochemical mantle convection.

Although the Moon currently has no internally generated magnetic field, palaeomagnetic data, combined with radiometric ages of Apollo samples, provide evidence for such a magnetic field from approximately 3.9 to 3.6 billion years (Gyr) ago, possibly owing to an ancient lunar dynamo. But the presence of a lunar dynamo during this time period is difficult to explain, because thermal evolution models for the Moon yield insufficient core heat flux to power a dynamo after approximately 4.2 Gyr ago. Here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo. Using a three-dimensional spherical convection model, we show that a dense layer, enriched in radioactive elements (a 'thermal blanket'), at the base of the lunar mantle can initially prevent core cooling, thereby inhibiting core convection and magnetic field generation. Subsequent radioactive heating progressively increases the buoyancy of the thermal blanket, ultimately causing it to rise back into the mantle. The removal of the thermal blanket, proposed to explain the eruption of thorium- and titanium-rich lunar mare basalts, plausibly results in a core heat flux sufficient to power a short-lived lunar dynamo.

Lunar mare basalt flow units: Thicknesses determined from crater size‐frequency distributions

Accurate lava flow unit thicknesses estimates are necessary to place constraints on volcanic flux estimates. We refine the technique of using the shape of crater size‐frequency distribution (CSFD) curves to estimate the thickness of individual lunar mare flow units. We find that a characteristic knee often observed in CSFD curves is reasonably interpreted to represent the presence of two lava flow units separated in time, and that the diameter at which this knee occurs is related to the thickness of the overlying flow unit. Examination of 58 curves with this characteristic knee in several lunar nearside basins (Oceanus Procellarum, Imbrium, Tranquillitatis, Humorum, Cognitum, Nubium, and Insularum) allowed us to identify flow units that have not been detected in low‐sun images. We found that the range of flow unit thickness is ∼20–220 m and the average is ∼30–60 m. This technique expands considerably the ability to assess lava flow unit thicknesses and volumes on the Moon and planets.

Comparative geochemistry of basalts from the moon, earth, HED asteroid, and Mars: implications for the origin of the moon

Most hypotheses for the origin of the Moon (rotational fission, co-accretion, and collisional ejection from the Earth, including “giant impact”) call for the formation of the Moon in a geocentric environment. However, key geochemical data for basaltic rocks from the Moon, Earth, the howardite-eucrite-diogenite (HED) meteorite parent body (probably asteroid 4-Vesta), and the shergottite-nakhlite-chassignite (SNC) meteorite parent body (likely Mars), provide no evidence that the Moon was derived from the Earth, and suggest that some objects with lunar-like compositions were produced without involvement of the Earth. The source region compositions of basalts produced in the Moon (mare basalts) were similar to those produced in the HED asteroid (eucrites) with regard to volatile-lithophile elements (Na, K, Rb, Cs, and Tl), siderophile elements (Ni, Co, Ga, Ge, Re, and Ir), and ferromagnesian elements (Mg, Fe, Cr, and V), and less similar to those in the Earth or Mars. Mare and eucrite basalts differ in their Mn abundances, Fe/Mn values, and isotopic composition, suggesting that the Moon and HED asteroid formed in different nebular locations. However, previous claims that the Moon and HED parent body differ significantly in the abundances of some elements, such as Ni, Co, Cr, and V, are not supported by the data. Instead, Cr-Mg-Fe-Ni-Co abundance systematics suggest a close similarity between the source region compositions and conditions involved in producing mare and eucrite basalts, and a significant difference from those of terrestrial basalts. The data imply that the Moon and HED asteroid experienced similar volatile-element depletion and similar fractionation of metallic and mafic phases. Among hypotheses of lunar origin, rotational fission, and small-impact collisional ejection seem less tenable than co-accretion, capture, or a variant of giant-impact collisional ejection in which the Moon inherits the composition of the impactor. Both the Moon and HED asteroid may have been derived from a class of objects that were common in the early solar system. “The most plausible model for the origin of the Moon in line with geochemical and cosmochemical constraints is an impact-induced “fission” of the proto-Earth.” — Wänke and Dreibus (1986) “Clearly, the Moon and eucrite parent body resemble each other to a high degree. Nature produced such a composition not once but at least twice. This calls into question an entire class of models that invoke ad hoc processes to explain the Moon by a unique chance event.” — Anders (1977) “The similarity between eucrites and lunar mare basalts are remarkable. Were it not for the differences in age and oxygen-isotope signature, it might be difficult to distinguish them on petrological or geochemical grounds.” — Taylor (1986)

The Longevity of Lunar Volcanism: Implications of Thermal Evolution Calculations with 2D and 3D Mantle Convection Models

Lunar volcanism, as is generally accepted, started well before the emplacement of the mare fill at 3.1–3.9 Ga b.p. and likely extended well into the Eratosthenian (3.1–1.1 Ga b.p.). The early volcanism is relatively easily understood. The extension of the activity to, possibly, 1.5 Ga b.p., albeit at a decreasing rate, poses a problem since a relatively small body such as the Moon may be expected to cool rapidly and freeze a partially molten mantle layer quickly. We present thermal history models in which a region of partial melt forms in the mantle underneath the lithosphere almost immediately after the start of the model at depths between 300 and 700 km depending on the chosen initial depth of the magma ocean. To calculate the thermal histories we use axisymmetric 2D and fully 3D spherical shell convection codes with viscosity depending on the azimuthally averaged temperature. The thermal evolution of the Moon is found to be characterized by the growth of a massive 700- to 800-km-thick lithosphere while the lower mantle and core cool only by 100–200 K. The partial melt zone decreases in thickness with time from top to bottom until it vanishes at times between 3.4 and 2.2 Ga b.p. The maximum degree of partial melting is between roughly 10 and 20%. The partial melt layer is initially global and is disrupted by cold downwellings as cooling proceeds. The melt zone freezes from above due to the thickening of the lithosphere, which implies that the source region of volcanic rock proceeds to increasing depth with time. This corresponds nicely with the variation of titanium with age of lunar mare basalts. Mixing of the melt zone with the underlying mantle chemically rejuvenates the melt zone to some degree and, thus, the source region of the mare basalts.

A dynamic origin for the global asymmetry of lunar mare basalts

We propose that the hemispheric asymmetry ( Fig. 1) of mare basalts may be explained as a result of hydrodynamic instabilities associated with a layer of mixed ilmenite-rich cumulates (IC) and olivine–orthopyroxene (OPx) overlying a metallic core. This mixed layer (MIC) should form shortly after the solidification of the magma ocean (P.C. Hess and E.M. Parmentier, Earth Planet. Sci. Lett. 134 (1995) 501–514) because of gravitational differentiation of chemically dense IC material that is expected to form below the anorthositic crust in the final stages of magma ocean crystallization (A.E. Ringwood and S.E. Kesson, Proc. Lunar Planet. Sci. Conf. 7 (1976) 1697–1722). IC material is rich in heat producing elements, and thermal expansion due to radiogenic heating causes the MIC layer to become less dense than overlying mantle. The time required for the MIC layer to become thermally buoyant may explain a delay of mare volcanism until about 500 Ma after solidification of the magma ocean. Our analyses of the resulting Rayleigh–Taylor instability and numerical modeling of thermo-chemical convection show that the instabilities produce spherical harmonic degree 1 thermal and compositional structure if a lunar metallic core is sufficiently small, less than 250 km in radius.

The titanium contents of lunar mare basalts

Abstract—Lunar mare basalt sample data suggest that there is a bimodal distribution of TiO2concentrations. Using a refined technique for remote determination of TiO2, we find that the maria actually vary continuously from low to high values. The reason for the discrepancy is that the nine lunar sample return missions were not situated near intermediate basalt regions. Moreover, maria with 2–4 wt% TiO2are most abundant, and abundance decreases with increasing TiO2. Maria surfaces with TiO2>5 wt% constitute only 20% of the maria. Although impact mixing of basalts with differing Ti concentrations may smear out the distribution and decrease the abundance of high‐Ti basalts, the distribution of basalt Ti contents probably reflects both the relative abundances of ilmenite‐free and ilmenite‐bearing mantle sources. This distribution is consistent with models of the formation of mare source regions as cumulates from the lunar magma ocean.

Systematics of Ni and Co in olivine from planetary melt systems; lunar mare basalts

The systematics of Co and Ni in olivine from six Apollo 12 olivine basalts were studied by SIMS techniques and correlated with major and minor element data obtained with the electron microprobe. Our results, together with previous studies, demonstrate that one of these basalts (12009) was extruded, as a liquid, onto the lunar surface and was parental to five other cumulates (12075, 12020, 12018, 12040, and 12035). The concentrations of Ni in zoned olivine phenocrysts behave as expected for a highly compatible element with initial estimated D Ni = 9.9. On the other hand, Co concentrations in olivine vary hardly at all and show flat patterns across crystals that retain normal zoning trends for Mg, Fe, Mn, and Ni. The explanation for this behavior is not that D Co ≈ 1 (our estimated D Co = 4) or that Co zoning has been erased by rapid diffusion of Co compared to the other elements (e.g., Mg and Fe) that still show normal zoning. The explanation for the behavior, as originally suggested by Kohn et al. (1989), is that the increase in D Co with crystallization exactly balances the depletion of Co in the melt. This decoupling of the behavior of Ni and Co in olivine results in significant increases in Co/Ni with crystallization.

Mineralogy of Antarctic basaltic shergottite Queen Alexandra Range 94201: Similarities to Elephant Moraine A79001 (Lithology B) martian meteorite

Abstract— Antarctic meteorite QUE 94201 is a new basaltic shergottite that is mainly composed of subequal amounts of maskelynite and pyroxenes (pigeonite and augite) plus abundant merrillite and accessory phases. It also contains impact melt. Complex zoning patterns in QUE 94201 pyroxenes revealed by elemental map analyses using an electron microprobe suggest a crystallization sequence from Mg‐rich pigeonite (En62Fss30Wog) to extremely Fe‐rich pigeonite (En5Fs81Wo14) via {110} Mg‐rich augite bands (En44Fs20Wo36) in a single crystal. These textures, along with the abundant plagioclase (maskelynite), indicates single‐stage rapid cooling (>5 °C/year) of this rock from a supercooled magma. Transition from Mg‐rich augite to Fe‐rich pigeonite reflects the onset of plagioclase crystallization. Enrichment of late‐stage phases in QUE 94201 implies crystallization from an evolved magma and suggests a different parent magma composition from the other basaltic shergottites. Lithology B of EETA79001 basaltic shergottite contains pyroxenes that show complex zoning with augite bands similar to those in QUE 94201 pyroxene, which suggests similar one‐stage rapid cooling. Lithology B of EETA79001 also resembles QUE 94201 in its coarse‐grained texture of silicates and its high abundance of maskelynite, although QUE 94201 probably crystallized from a more fractionated magma. We also note that some Apollo lunar mare basalts (e.g., 12020 and 12021) have similar mineralogy and petrology to QUE 94201, especially in pyroxene zoning. All these basaltic rocks with complex pyroxene zoning suggest rapid metastable crystallization from supercooled magmas.

Is the Composition of the Moon Consistent with the Giant Impact Hypothesis?

Current versions of the giant impact hypothesis for the origin of the Moon call for the impact to occur when the Earth is about two thirds of its present size. The material in the Moon is derived mostly from the mantle of the impactor, although less than 10% of the impactor finishes up in the Moon. The metallic core of the impactor accretes to the Earth. Accretion of the last third of the Earth occurs subsequently to lunar formation. The Moon suffers nett erosion rather than accumulation during this stage. This model is examined in the light of the lunar composition. The Moon has a density indicative of a low content of metallic iron, is bone-dry (the water ice detected by Clementine and Prospector is a trivial late addition from comets) and is depleted in the very volatile elements (e.g. Bi,TI) relative to abundances in the solar nebula. The alkali elements are depleted relative both to the Earth and the solar nebula. The FeO content of 13% in the bulk Moon is intermediate between that of Mars with 18% and the terrestrial mantle with 8% FeO. There is no sign of a late veneer on the Moon that probably contributed the excess siderophile signature to the terrestrial mantle. Lunar trace siderophile elements are very low in abundance, consistent with the low metallic iron content. Relative to the Earth, there is a probable enrichment of refractory elements (eg. Ca, A1, Ti, REE, U, Th) in the Moon, based on heat flow data, seismic velocity profiles, their high near-surface concentrations and the presence of a thick aluminous crust. Those elements with condensation temperatures above 1100-1200 K (e.g. REE) are present in the Moon in their solar nebular ratios. The volatile element depletion and enrichment of refractory elements is thus generally consistent with the giant impact hypothesis, although the material now in the Moon was not subjected to extreme temperatures. The lunar FeO content is too high to come from the terrestrial mantle. The lack of fractionation effects in the potassium isotopes rule out evaporative loss of potassium by Raleigh-type distillation processes during the giant collision. The formation of tektites during much smaller scale collisional events provides some analogues. Relative to their source material, tektites have lost H20, and elements more volatile than Cs (e.g. Pb, T1, Bi), but display no fractionation of the potassium isotopes. The depletion of the bone-dry Moon in these elements thus may have occurred during the giant collision, but not by Raleigh fractionation as indicated by the absence of variations in isotope ratios. Possible compositions for the impactor include a body with an iron-rich mantle, intermediate between that of Earth and Mars, but with a low Rb/ Sr ratio and a low volatile content. A possible analogue for the impactor is the Eucrite Parent Body (4 Vesta), because the depletions in the siderophile elements, alkali elements and more volatile elements in low-Ti lunar mare basalts are similar to their abundances in eucrites.

Exposure history of glass and breccia phases of lunar meteorite EET87521

Abstract— Glass‐rich separates were prepared from a sample of the basaltic lunar meteorite EET87521 rich in dark glass. Noble gas isotopic abundances and 26Al and 10Be activities were measured to find out whether shock effects associated with lunar launch helped to assemble these phases. Similar 10Be and 26Al activities indicate that all materials in EET87521 had a common exposure history in the last few million years before launch. However, the glass contains much higher concentrations of trapped gases and records a much longer cosmic‐ray exposure, 100 Ma–150 Ma, in the lunar regolith than does the bulk sample. The different histories show that the glass existed long before the ejection of EET87521. The trapped 40Ar/36Ar ratio of 1.6 ± 0.1 implies that the lunar exposure that produced most of the stable cosmogenic noble gases began 500 Ma ago.Cosmogenic and trapped noble gas components correlate strongly in various temperature‐release fractions and phases of EET87521, which is probably because the glass contains most of the gas. The trapped solar ratios, 20Ne/22Ne = 12.68 ± 0.20 and 36Ar/38Ar = 5.24 ± 0.05 can be understood as resulting from a mixture consisting of ∼60% solar wind and 40% solar energetic particles (SEP). All EET87521 phases show a 40K‐40Ar gas retention age of ∼3300 Ma, which is in the range of typical lunar mare basalts.

A glass spherule of questionable impact origin from the Apollo 15 landing site: Unique target mare basalt

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 μm-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al 2O 3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basalts. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the 40Ar/ 39Ar laser incremental heating technique, is 1647 ±11 Ma (2 δ); it is expressed as an age spectrum of seventeen steps over 96% of the 39Ar released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; 38Ar cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50 °C s −1 is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive. The 15434,28 glass is distinct from the common yellow impact glasses at the Apollo 15 landing site, in particular in its lower abundances of incompatible elements and much younger age. If we accept an impact origin, then the trace element relative abundances preclude both typical KREEP and the common Apollo 15 yellow impact glass from contributing more than a few percent of the incompatible elements to potential mixtures. The melted part of any target must have consisted almost entirely of a variety (or varieties) of mare basalt or glass distinct from any known mare basalts or glasses, including Apollo 15 yellow volcanic glass, or mixtures of them. However, the rind inclusions, similar to materials of local origin, do suggest a source near the Apollo 15 landing site. An impact melt cannot have dissolved much, if any, of such inclusions. A lack of regolith materials in the rind and in the melt component suggest an immature source terrain. Thus, even for an impact origin, there is the possibility (though not requirement) that the volcanic target is younger than most mare plains. The crater Hadley C, 25 km away, is a potential source. If the 15434,28 glass is instead directly of volcanic origin, it represents an extremely young mare magma of a type previously undiscovered on the Moon.

Hafnium–tungsten chronometry and the timing of terrestrial core formation

THE accretion of the Earth and Moon within the solar nebula is thought1–3 to have taken 50 to 100 million years. But the timing of formation of the Earth's core has been controversial, with some4,5 proposing that it took place within the first 15 Myr of Earth's accretion history and others6,7 proposing that it occurred after 50 Myr of accretion. Meteorite chronometry based on the 182Hf–182W system has the potential to resolve this debate, as segregation of a metal core from silicates should induce strong fractionation of hafnium from tungsten. Here we report tungsten isotope compositions for two iron meteorites, two carbonaceous chondrites, and a lunar mare basalt. We see clear 182W deficits in both iron meteorites, in agreement with previous results4,5. But the data for chondrites are inconsistent with the hypothesis of early core formation, suggesting that both this event and the formation of the Moon must have occurred at least 62 ± 10 Myr after the iron meteorites formed.