Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions (original) (raw)
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Geochimica Et Cosmochimica Acta, 2010
To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K -potassium, REE -Rare Earth Element, and P -phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock d 18 O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock d 18 O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock d 18 O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt d 18 O bulk-rock values. Differences between the low-and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk d 18 O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.
Oxygen isotope constraints on the origin and differentiation of the Moon
2007
We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of δ 18 O values (− 0.5 to 22.9‰) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; λ = 0.5259 ± 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Δ 17 O for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01‰ and are indistinguishable from the TFL. The δ 18 O values of high-and low-Ti lunar basalts are distinct. Average whole-rock δ 18 O values for low-Ti lunar basalts from the Apollo 12 (5.72 ± 0.06‰) and Apollo 15 landing sites (5.65 ± 0.12‰) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 ± 0.11‰). Evolved low-Ti LaPaz mare-basalt meteorite δ 18 O values (5.67 ± 0.05‰) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high-and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in δ 18 O do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine-pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average δ 18 O (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Δ=0.18‰) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the δ 18 O of lunar mantle olivine in this study are only 0.05‰ higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth-Moon system.
Geochimica et Cosmochimica Acta, 1992
It is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. Crystal-liquid and crystal-crystal separations were similarly inefficient during evolution of the lunar magma ocean (LMO), allowing for the trapping of interstitial liquid and entrainment of a small proportion of less-dense plagioclase into the denser mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this for anorthosites (with l-10% olivine). The residual liquid after 80-90% crystallization was very evolved (in fact KREEPy) and, even in smali proportions (l-5%), would have a noticeable effect on the trace-element chemistry of melts generated from these cumulates. This trapped residual liquid would elevate total REE abundances in the cumulate pile, while synchronously deepening the already negative Eu anomaly. Essentially, this trapped liquid will make the cumulate more fertile for melting to generate both KREEP basalt and mare basalt magmas. Plagioclase entrained in the mane cumulate pile adds an essential Al component to the high-Ti basalt source and will moderate the requisite negative Eu anomaly in the cumulate. Early in the evolution of the lunar mantle, when the LMO still was largely liquid, it is likely that vigorous convection was an important factor in crystallization. Such convection would allow crystals to remain suspended and in equilibrium with the LMO liquid for relatively long periods of time. This extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust. Previous authors have proposed a three-component model for the evolution of high-Ti mare basalt source regions. This model includes KREEP, early (olivine-rich, high Mg#) cumulates, and late (ilmeniterich, low Mg#) cumulates in various proportions. However, we propose a model for high-Ti basalt parent magmas which is in accord with studies of terrestrial layered int~sions. This model for the high-Ti source includes trapped ins~~neous residual liquid (TIRL, I-3%) and entrainment of a small (2-5%) prounion of plagioclase into the late-stage cumulate pile in order to account for both the observed Al compositions and trace-element characteristics of high-Ti mare basalts. Melting of this relatively shallow, ilmenite-and clinopyroxene-bearing, late-stage cumulate can generate high-Ti mare basalt magmas. Furthermore, we are in agreement with other workers that only through a process of nonmodal melting will the high Ti values for the parent magmas be realized. Large-scale convective overturn of the cumulate pile and mixing of KREEP with early-and late-stage cumulates is not required. However, localized overturn of the upper tenth of the cumulate pile is likely and, in fact, required to achieve an appropriate major-element balance for the high-Ti mare basalt source region. tNTRODIJCI'ION THE MAGMA WEAN CONCEPT has been an integral part of the lunar literature practically since the return of the first lunar samples (e.g., SMITH et al., 19'70; WOOD et al., 1970; WARREN, 1985). Several authors have indicated problems and inconsistencies inherent in an early Moon-wide magma ocean and have challenged its validity (WALKER, 1983; SHIRLEY, 1983; LONGHI and ASHWAL, 1985). However, the bulk of petrologic, geochemical, and geophysical data for the highlands and mare basins of the Moon point to a common, ancient, global reservoir. The proceeding discussion will assume the existence of this early Moon-wide magma ocean.
American Mineralogist, 2022
We compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited ranges in δ66Zn, δ34S, and δ37ClSBC+WSC values of 1.27 ± 0.71, 0.55 ± 0.18, and 4.1 ± 4.0‰, respectively, compared to the silicate Earth at 0.15, –1.28, and 0‰, respectively. We find that the Zn, S, and Cl isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of a geochemical signature in the mare basalt source region that is inherited from lunar formation and magma ocean crystallization. The uniformity of these compositions implies mixing following mantle overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15‰ in δ66Zn, 60‰ in δ34S, and 30‰ in δ37Cl). This reflects ...
1996
High-Ti basalts from the Apollo collections span a range in age from 3.87 Ga to 3.55 Ga. The oldest of these are the common Apollo 11 Group B2 basalts which yield evidence of some of the earliest melting of the lunar mantle beneath Mare Tranquillitatis. Rare Group D high-Ti basalts from Mare Tranquillitatis have been studied in an attempt to confirm a postulated link with Group B2 basalts (Jerde et al.,1994). The initial Sr isotopic ratio ofa known Group D basalt (0.69916 ± 3 at 3.85 Ga) lies at the lower end of the tight range for Group B2 basalts (87Sr/86Sr = 0.69920 to 0.69921). One known Group D basalt and a second postulated Group D basalt yield indistinguishable initial ENd (1.2 ± 0.6 and 1.2 ± 0.3) and again lie at the lower end of the range for the Group B2 basalts from Apollo 11 (+2.0 ± 0.4 to +3.9 ± 0.6, at 3.85 Ga). A third sample has isotopic (87Sr/86Sr = 0.69932 ± 2; ENd = 2.5 ± 0.4; at 3.59 Ga; as per Snyder et al., 1994b) and elemental characteristics similar to the Group A high-Ti basalts returned from the Apollo 11 landing site. Ages of 40Ar 3 9 Ar have been determined for one known Group D basalt and a second postulated Group D basalt using step-heating with a continuous-wave laser. Suspected Group D basalt, 10002,1006, yielded disturbed, age spectra on two separate runs, which was probably due to 39Ar recoil effects. Using the "reduced plateau age" method of Turner et al. (1978), the ages derived from this sample were 3898 ± 19 and 3894 ± 19 Ma. Three separate runs of known Group D basalt 10002,116 yielded 40Ar/ 39 Ar plateau ages of 3798 ± 9 Ma, 3781 ± 8 Ma, and 3805 ± 7 Ma (all errors 20). Furthermore, this sample has apparently suffered significant 40Ar loss either due to solar heating or due to meteorite impact. The loss of a significant proportion of 40Ar at such a time means that the plateau ages underestimate the "true" crystallization age of the sample. Modelling of this Ar loss yields older, "true" ages of3837 ± 18,3826 ± 16, and 3836 ± 14 Ma. These ages overlap the ages of Group B2 high-Ti basalts (weighted average age = 3850 ± 20 Ma; range in ages = 3.80 to 3.90 Ga). The combined evidence indicates that the Group D and B2 high-Ti basalts could be coeval and may be genetically related, possibly through increasing degrees of melting of a similar source region in the upper mantle of the Moon that formed >4.2 Ga ago. The Group D basalts were melted from the source first and contained 3-5x more trapped KREEP-like liquid than the later (by possibly only a few million years) Group B2 basalts. Furthermore, the relatively LREE-and Rb-enriched nature of these early magmas may lend credence to the idea that the decay of heat-producing elements enriched in the KREEP-like trapped liquid of upper mantle cumulates, such as K, U, and Th, could have initiated widespread lunar volcanism.
Geochimica Et Cosmochimica Acta, 2009
The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Marssized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142 Nd/ 144 Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142 Nd-143 Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262,[505][506][507][508][509][510][511][512][513][514][515][516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.
Geochimica et Cosmochimica Acta, 2008
Apollo 15 low-Ti mare basalts have traditionally been subdivided into olivine-and quartz-normative basalt types, based on their different SiO 2 , FeO, and TiO 2 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 2525% difference in whole-rock TiO 2 , and corresponding differences in SiO 2 and FeO between the Apollo 15 olivine-and quartz-normative basalts. In this model, the older (253.35 Ga) quartz-normative basalts, with lower TiO 2 experienced olivine, chromite, and Cr-ulvö spinel 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.
Geochimica et Cosmochimica Acta, 2019
Lunar mantle overturn caused by gravitational instability of the Fe-Ti rich KREEP layer (formed as the last 5% of a crystallizing magma ocean, and emplaced between the overlying anorthitic crust and the underlying lunar mantle) is a process that would introduce Fe-Ti enriched bodies deep inside the lunar interior. These chemical heterogeneities in the lunar mantle may be the source of the very Fe-Ti enriched nearprimary Apollo basalts. Also, the Fe-Ti and KREEP enriched layer deep inside the Moon may be responsible for the 5-30% partial melt seismically detected close to the core-mantle boundary (CMB). This is assuming that that the partial melt is neutrally buoyant at P-T conditions of the CMB. Here, we experimentally investigate the phase equilibria of the overturned Fe-Ti rich layer mixed with the mantle, at P-T conditions deep inside the lunar interior, focusing on the partial melt compositions formed. Our aim is to test (a) whether potential partial melt compositions formed near the CMB are neutrally buoyant with respect to the surrounding mantle, hence, stable; (b) if the partial melts formed within the lunar interior are positively buoyant and ascend, whether they can reproduce chemical characteristics of Apollo basalts. The densities calculated for the Fe-Ti rich partial melts from this study, using the physical parameters from previous studies, range from lower to higher values compared to that of the lunar mantle. This provides a basis for future investigations to experimentally constrain better the densities of these partial melts. Depending on the buoyancy of the partial melts, the following two scenarios are likely to happen. Firstly, if the partial melts are neutrally buoyant at the CMB, 5-30% partial melt would constrain the CMB temperatures between 1330(±1)-1470(±19) °C. This can be used by future studies to derive the selenotherm better. Secondly, if the partial melts are positively buoyant, they should ascend and react with the mantle along their path. Upon reaching shallow depths below the crust, they may likely assimilate any Fe-Ti rich layer that was left over from the gravitational overturn, as well as undergo olivine fractionation upon pooling in a shallow magma chamber. We modeled assimilation-fractional crystallization of the partial melts using the Gibb's-free minimization algorithm alphaMELTS. Our results show that reactive ascent of Fe-Ti rich partial melts through the lunar mantle and subsequent olivine fractionation in a shallow magma chamber is a promising way to evolve the melt compositions to converge with the lunar basalts better. Shallow level assimilation of Fe-Ti rich lithology post reactive-ascent through the mantle is also feasible, but only for low degrees of assimilation. 1.
Meteoritics & Planetary Science, 2005
available online at http://meteoritics.org 679 Abstract-Major element and sulfur concentrations have been determined in experimentally heated olivine-hosted melt inclusions from a suite of Apollo 12 picritic basalts (samples 12009, 12075, 12020, 12018, 12040, 12035). These lunar basalts are likely to be genetically related by olivine accumulation (Walker et al. 1976a, b). Our results show that major element compositions of melt inclusions from samples 12009, 12075, and 12020 follow model crystallization trends from a parental liquid similar in composition to whole rock sample 12009, thereby partially confirming the olivine accumulation hypothesis. In contrast, the compositions of melt inclusions from samples 12018, 12040, and 12035 fall away from model crystallization trends, suggesting that these samples crystallized from melts compositionally distinct from the 12009 parent liquid and therefore may not be strictly cogenetic with other members of the Apollo 12 picritic basalt suite. Sulfur concentrations in melt inclusions hosted in early crystallized olivine (Fo 75 ) are consistent with a primary magmatic composition of 1050 ppm S, or about a factor of 2 greater than whole rock compositions with 400-600 ppm S. The Apollo 12 picritic basalt parental magma apparently experienced outgassing and loss of S during transport and eruption on the lunar surface. Even with the higher estimates of primary magmatic sulfur concentrations provided by the melt inclusions, the Apollo 12 picritic basalt magmas would have been undersaturated in sulfide in their mantle source regions and capable of transporting chalcophile elements from the lunar mantle to the surface. Therefore, the measured low concentration of chalcophile elements (e.g., Cu, Au, PGEs) in these lavas must be a primary feature of the lunar mantle and is not related to residual sulfide remaining in the mantle during melting. We estimate the sulfur concentration of the Apollo 12 mare basalt source regions to be ∼75 ppm, which is significantly lower than that of the terrestrial mantle.