Lunar Magma Ocean Research Articles

The isotopic record of lunar volcanism

The timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon. A late, heavy bombardment produced most of the observable lunar basins, easily recognized stratigraphic markers and also may have influenced lunar volcanism. Most of the nearside basins seem to have been produced 3.8–4.0 Ga ago, during a period often referred to as the lunar cataclysm. The pre-cataclysm period was characterized by formation of the lunar crust, plutonic rocks within the crust, and pre-mare volcanism. Isotopic analysis of breccia clasts pre-dating the cataclysm has yielded an outline of this period. The isotopic record of post-cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking. Specifically, the isotopic data show that some lunar anorthosites formed ~4.45 Ga ago, consistent with early formation of an anorthositic lunar crust. Some “Mg-suite” norites, troctolites, and dunites also formed approximately contemporaneously with the oldest anorthosites, but others are up to 200–300 Ma younger. More evolved rocks (granites and quartz-monzodiorites) also formed between ~4.4 and ~4.0 Ga ago. The earliest known pre-mare volcanism at ~4.25 Ga ago included aluminous, mare-like basalts and an olivine gabbronorite similar to Apollo 12 basalts. KREEP basalt volcanism began at least ~4.1 Ga ago, approximately contemporaneously with that of the ancient, mare-like basalts, and continued during the cataclysm. Recorded instances of high-Ti mare basalt volcanism began during or immediately after the cataclysm. Low-Ti mare basalt volcanism is the most recent for which samples are available. The isotopic data record an early primary lunar differentiation during which mare basalt sources formed, probably by crystallization of cumulus minerals from a lunar magma ocean. The isotopic record of post-cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources, but more complex petrogenesis also has been suggested. More complex petrogenetic processes including assimilation and Assimilation-Fractional-Crystallization (AFC) seem to be required for pre-mare volcanics. The isotopic data contain a hint that petrogenesis of the oldest pre-mare volcanics may be linked to formation of the so-called “Procellarum Basin,” possibly the oldest and largest lunar basin. Such a basin would provide a geologic context for the pre-mare volcanics, and its formation may have been a defining moment in lunar history.

Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts?

The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO 2, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO 2 mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

Metasomatic products of the lunar magma ocean: The role of KREEP dissemination

The origin of the incompatible element-rich lunar component, KREEP, is in the Lunar Magma Ocean (LMO). The fractionated residual melt after crystallization of the LMO represents “urKREEP” (after Warren and Wasson, 1979). The percentage of this residual melt is low enough to be within the realm of silicate liquid immiscibility (SLI). This process has the ability to split the KREEP signature into K- and REEP-Fractions, which are manifest as lunar granite (K) and phosphate phases present in highland lithologies or as quartz ferrotroctolite in lunar soils (REEP). We envisage this as a localized, but significant process since only a small portion of urKREEP undergoes SLI. Norms of experimental and Apollo 15 basaltic immiscible glasses suggest that thevREEP-Fraction found in the lunar highlands has undergone post-SLI fractionation of at least fayalite. This significantly reduces the density of the REEP-Fraction and coupled with its low viscosity (10–15 poise), it can percolate upward, metasomatizing the lunar crust. The higher viscosity of the granitic melt (=∼30000 poise) inhibits its mobility, and it forms “pods” in the lower rust (as required for VHK basalt petrogenesis). With the identification of KREEPy components, the composition of urKREEP can be calculated. Using experimental evidence, the KREEP components may be recombined to give the pre-SLI composition, or liquid-liquid Kd's can be used to calculate a pre-SLI composition from lunar granite. Both calculated urKREEP compositions are lower in MgO and Al 2O 3 and higher in FeO and P 2O 5 than reported low- and high-K KREEP compositions. The calculated REE abundances are higher and the REE profiles are slightly more LREE-enriched than the previously reported KREEP compositions. However, the REE profile calculated using liquid-liquid Kd's is concave-upwards, compared to the LREE-enriched profile produced by recombining the KREEP components. Pre-SLI whitlockite/apatite fractionation occurred prior to immiscibility in order to generate the concave-upwards profile. The LREE-enriched profile represents the pre-SLI magma prior to phosphate fractionation. The presence of “superKREEPy” rock types can be accounted for by K- and REEP-Fraction assimilation, and the compositions of olivine vitrophyres can now be modeled using analyzed lunar rock types without the inference of a mythical “high-Mg” component. Other, more widespread KREEPy rocks ( e.g., Apollo 14 breccias, Apollo 15 KREEP basalts, LKFM basalts) are produced by incorporation of the more widespread pre-SLI urKREEP component. The splitting of KREEP into identifiable lithological components allows the petrogenesis and role of KREEP in lunar evolution to be better understood.

Apollo 15 yellow-brown volcanic glass: Chemistry and petrogenetic relations to green volcanic glass and olivine-normative mare basalts

Apollo 15 yellow-brown glass is one of twenty-five, high-Mg, primary magmas emplaced on the lunar surface in pyroclastic eruptions. Forty spherules of this glass were individually analyzed by electron microprobe and INAA for major- and trace-elements ( e.g. Sc, V, Cr, Co, La, Ce, Nd, Sm, Eu, Ho, Tm, Yb, Lu, Hf, Ta, and U). The abundances demonstrate that this primary magma was produced by partial melting of differentiated cumulates in the lunar mantle. Models are developed to explain the possible source-regions of several Apollo 15 and Apollo 12 low-Ti mare magmas as being products of hybridization involving three ancient differentiated components of a primordial lunar magma ocean: (a) early olivine ± orthopyroxene cumulates; (b) late-stage clinopyroxene + pigeonite + ilmenite + plagioclase cumulates; and (c) late-stage inter-cumulus liquid. Isotopic constraints on Sm-Nd fractionation and least-squares tests of potential mixing models require models of Apollo 15 yellow-brown glass, and Apollo 12 and 15 olivine mare basalts, involving low F ~3% cumulate remelting of sources in which olivine and orthopyroxene are residual phases. Models of the Apollo 15 green glass composition indicate higher (~4–7%) degrees of melting, but with only a slight preference for orthopyroxene-rich relative to orthopyroxene -free sources. The modeled dependence on residual orthopyroxene in mare source regions places source depths at ~400 km or greater in order to maintain olivine and orthopyroxene on the liquidus. Hybridization at such depths would likely result from convective overturn of the primordial magma ocean and suggests that separate fractionating masses were positioned at various depths. Our models for developing hybridized source regions of primary low-Ti mare magmas do not require selective contamination of primary liquids. The preferred model for Apollo 15 yellow-brown glass is presented as the commingling of 95% early cumulate olivine + orthopyroxene, 1.9% clinopyroxene, 0.27% pigeonite, 1.7% plagioclase, 0.02% ilmenite, and 0.56% trapped liquid. Respective proportions for the Apollo 15 olivine mare basalt source model are 98.5%, 0.37%, 0%, 0.79%, 0.16%, and 0.13%; and the proportions used for the preferred Apollo 12 olivine mare basalt model are 97.6%, 0.73%, 0.24%, 0.77%, 0.42%, and 0.21%. Our preferred model for the green glass source composition ( F = 0.07) requires the commingling of 97.4% early olivine + orthopyroxene, 0.18% clinopyroxene, 1.32% pigeonite, 0.85% plagioclase, and 0.28% trapped liquid.

A komatiite component in Apollo 16 highland breccias: implications for the nickel-cobalt systematics and bulk composition of the moon

The lunar crust at the Apollo 16 landing site contains substantial amounts of a “primitive component” in which the ferromagnesian group of elements is concentrated. The composition of this component can be retrieved via an analysis of mixing relationships displayed by lunar breccias. It is found to be a komatiite which is compositionally similar to terrestrial komatiites both in major and minor elements. The komatiite component of the lunar crust is believed to have formed by extensive degrees of melting of the lunar interior at depths greater than were involved in the formation of the lunar magma ocean which was parental to the crust. After formation of the anorthositic crust, it was invaded by extensive flows and intrusions of komatiite magma from these deeper source regions. The komatiites became intimately mixed with the anorthosite by intensive meteoroid impacts about 4.5 b.y. ago, thereby accounting for the observed mixing relationships displayed by the crust. The compositional similarity between lunar and terrestrial komatiites strongly implies a corresponding similarity between the compositions of their source regions in the lunar interior and the Earth's upper mantle. The composition of the lunar interior can be modelled more specifically by combining the komatiite composition with its liquidus olivine composition (as determined experimentally) in proportions chosen so as to produce a cosmochemically acceptable range of Mg/Si ratios for the bulk Moon. Except for higher FeO and lower Na 2O, the range of compositions thereby obtained for the bulk moon is very similar to the composition of the Earth's upper mantle. The effects of meteoritic contamination on the abundances of cobalt and nickel in lunar highland breccias were subtracted on the assumption that the contaminating projectiles were chondritic. The cobalt and nickel residuals thereby obtained were found to correlate strongly with the (Mg + Fe) content of the breccias, demonstrating that the Co and Ni are associated with the ferromagnesian component of the breccias and are genuinely indigenous to the Moon. The lunar highland Co and Ni residuals also display striking Ni/Co versus Ni correlations which follow a similar trend to those displayed by terrestrial basalts, picrites and komatiites. The lunar trends provide further decisive evidence of the indigenous nature of the Co and Ni residuals and suggest the operation of extensive fractionation controlled by olivine-liquid equilibria in producing the primitive component of the lunar breccias. Indigenous nickel abundances at the Apollo 14, 15 and 17 sites are much lower than at the Apollo 16 site, although rocks from all sites follow the same Ni/Co versus Ni trends. It is suggested that the primitive component at the Apollo 14, 15 and 17 sites was generally of basaltic composition, in contrast to the komatiitic nature of the Apollo 16 primitive component.

Anorthosite assimilation and the origin of the Mg/Fe‐related bimodality of pristine moon rocks: Support for the magmasphere hypothesis

The lunar magma ocean (or magmasphere) hypothesis, always controversial, has required considerable revision in recent years. The geochemical bimodality of pristine rocks, epitomized by a diagram of Na/(Na + Ca) versus mg′ ratio, has led to proposals that a major fraction of the crust (the Mg‐rich suite) formed as cumulates in numerous intrusions slightly younger than the magmasphere. The precise origin of the bimodality has been elusive, however. One previous suggestion was that Na was lost as a volatile before the ferroan anorthosites crystallized from the magmasphere, whereas the Mg‐rich intrusives retained all of their original Na. However, this model cannot fully explain the bimodality, because the same bimodal pattern is manifested by plotting Eu/Al (a ratio of involatile elements) versus mg′ ratio. Assimilation probably helped to engender the bimodal patterns. Mass/energy balance calculations indicate that large proportions of plagioclase were probably assimilated from the older (magmasphere‐generated) ferroan anorthosite crust by most of the Mg‐rich intrusive melts. The magmasphere, in the absence of assimilation, probably did not yield appreciable plagioclase (the ferroan anorthosite crust) until fractional crystallization of mafic silicates had diminished the melt mg′ ratio to about 0.42. However, assuming identical initial melt composition, an Mg‐rich intrusion assimilating ferroan anorthosite (and perhaps also a small proportion of urKREEP) would have reached plagioclase saturation at much higher mg′, about 0.66. The current version of the magmasphere hypothesis (ferroan anorthosites = magmasphere flotation cumulates; Mg‐rich rocks = products of slightly younger, localized intrusions) appears to be the only plausible mechanism for engendering the Mg/Fe‐related bimodality.

Lu‐Hf constraints on the evolution of lunar basalts

Very low Ti basalts and green glass samples from the moon show high Lu/Hf ratios and low Hf concentrations. Low‐Ti lunar basalts show high and variable Lu/Hf ratios and higher Hf concentrations, whereas high‐Ti lunar basalts show low Lu/Hf ratios and high Hf concentrations. KREEP basalts have constant Lu/Hf ratios and high but variable Hf concentrations. Using the Lu‐Hf behavior as a constraint, we propose a model for the mare basalts evolution. This constraint requires extensive crystallization of the primary lunar magma ocean prior to formation of the lunar mare basalt sources and the KREEP basalts. Mare basalts are produced by the melting of the cumulate rocks, and KREEP basalts represent the residual liquid of the magma ocean.Lu and Hf concentrations and the Hf isotopic data of lunar rocks suggest that assimilation cannot be accepted as a major process to explain the diversity of the lunar mare basalts. The urKREEP hypothesis is also unnecessary. Both high‐ and low‐Ti basalts show enough iron enrichment to be regarded as melting products of the last stage cumulate rocks from the lunar magma ocean. The KREEP basalts are also rich in iron and may be regarded as the final, residual liquid left after the crystallization of the major portion of the primary lunar magma ocean.

Lunar and terrestrial crust formation

Planetary crusts may be accreted, produced in primordial differentiation, or built up piecemeal by serial magmatism. The existence of old, polygenetic, laterally heterogeneous, partial melt rocks in the lunar highlands suggests that the moon produced its early crust by serial magmatism. This view can be reconciled with lunar Eu anomalies, previously thought to support the magma ocean model of crust formation, if complications in the fractionation of mare basalts are recognized. Phase equilibrium and magmatic density information for mare basalts suggest a model in which plagioclase fractionation can occur even though plagioclase is not a near‐liquidus phase. The cryptic fractionation of clinopyroxene in MORB provides a precedent for this model. The necessity for a lunar magma ocean is questioned, but a role for a terrestrial magma ocean of sorts at depth is suggested.

Isotopic and REE studies of lunar basalt 12038: implications for petrogenesis of aluminous mare basalts

We report Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts. A precise internal Rb-Sr isochron yields a crystallization age of 3.35±0.09 AE and initial 87Sr/ 86Sr=0.69922∓2 (2σ error limits, 1AE=10 9 years, λ( 87Rb)=0.0139AE −1 ). An internal Sm-Nd isochron yields an age of 3.28±0.23AE and initial 143Nd/ 144Nd=0.50764∓28. Present-day 143Nd/ 144Nd is less than the “chondritic” value, i.e. ɛ(Nd, 0)=−2.3±0.4 where ɛ(Nd) is the deviation of 143Nd/ 144Nd from chondritic evolution, expressed as parts in 10 4. At the time of crystallization ɛ(Nd, 3.2AE)=1.5±0.6. We have successfully modeled the evolution of the Sr and Nd isotopic compositions and the REE abundances within the framework of our earlier model for Apollo 12 olivine-pigeonite and ilmenite basalts. The isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb=2.2 for this hypothetical magma ocean pattern. A plot of I(Sr) versus ɛ(Nd) for the Apollo 12 basalts clearly shows the influence of varying proportions of olivine, clinopyroxene, orthopyroxene, and plagioclase in the basalt source regions. A small percentage of plagioclase (∼5%) in the 12038 source apparently is responsible for low I(Sr) and ɛ(Nd) in this basalt. Aluminous mare basalts from Mare Crisium (Luna 24) and by inference Mare Fecunditatis (Luna 16) occupy locations on the I(Sr)-ɛ(Nd) plot similar to that of 12038, implying that some basalts from three widely separated lunar regions came from plagioclase-bearing source regions. A summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when REE abundances in the lunar magma ocean increased from ∼20× chondritic to >100× chondritic. Although there is a general trend from olivine to clinopyroxene-dominated source regions with progressive magma ocean evolution, significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution, as evidenced in particular by the three Apollo 12 magma types.

Electromagnetic heating of minor planets in the early solar system

Electromagnetic processes occurring in the primordial solar system are likely to have significantly affected planetary evolution. In particular, electrical coupling of the kinetic energy of a dense T-Tauri-like solar wind into the interior of the smaller planets could have been a major driver of thermal metamorphism. Accordingly a grid of asteroid models of various sizes and solar distances was constructed using dc transverse magnetic induction theory. Plausible parameterizations with no requirement for a high environmental temperature led to complete melting for Vesta (and others with sizes down to 50 km diameter and distance out to 2.8 AU thus approximately reproducing the observed distributions of S objects) with no melting for Pallas and Ceres. Fairly high temperatures were reached in the Pallas model, perhaps implying nonmelting thermal metamorphosis as a cause of its anomalous spectrum (somewhat similar to but distinct from C type). A reversal of this temperature sequence seems implausible, suggesting that the Ceres-Pallas-Vesta dichotomy is a natural outcome of the induction mechanism. Highly localized heating is expected to arise due to an instability in the temperature-controlled current distribution. Localized metamorphosis resulting from this effect may be relevant to the production and evolution of pallasites, the large presumed metal component of S object spectra, and the formation of the lunar magma ocean.

Pyroxene-melt equilibria

Using a literature survey of analyses of high-Ca pyroxene and co-existing silicate melt pairs and analyses of low-Ca pyroxene-silicate melt pairs, we have performed a thermodynamic analysis of pyroxene-melt equilibria. Three sets of mixing model pairs have been considered, based on two mixing models for liquid silicate solutions and two for pyroxene solid solutions. A modified version of a model developed by Bottinga and Weill (1972) for the mixing properties of silicate melts, in which the melt is considered to be composed of independent network-forming and network-modifying quasi-lattices, more successfully accounts for variations in melt composition than does a model which considers the melt to be composed of simple oxides which mix ideally. An empirical model for the mixing properties of pyroxenes, in which the M1 and M2 sites are considered to be equivalent and are combined as a hypothetical ‘M’ site, is as successful in accounting for variations in pyroxene composition at high temperatures as an ideal multisite mixing model. Using a variety of pyroxene-melt relations, and combinations of the mixing models outlined above, we have developed several pyroxene-melt and low-Ca pyroxene-high-Ca pyroxene geothermometers which have internally-consistent precisions of approximately ±20°C (1σ). One of the two-pyroxene geothermometers has been used to calculate ‘quench’ temperatures for a number of eucrites. Computed temperatures are subsolidus, and are consistent with independent geothermometers and with petrographic observations. The equations may also be used to calculate the composition of pyroxene crystallizing from a silicate melt of known composition, with or without independent knowledge of temperature. Internally consistent precisions vary, but are approximately ± 3 mol% Fs, ± 5 mol% En, and ±4 mol% Wo (all 1σ). These equations may have application in modeling the evolution of mineral compositions during differentiation of basaltic magmas, particularly terrestrial layered intrusions and the lunar magma ocean.

Zr and Nb partition coefficients: Implications for the genesis of mare basalts, KREEP and sea floor basalts

The distribution coefficients for Zr and Nb have been determined between armalcolite, ilmenite, clinopyroxene, rutile, plagioclase and a coexisting synthetic high Ti mare basalt melt at 11C5°, 1113° and 1128°C and at f O 2 = IW × 10 0.5. Recommended values are D (Zr) ilm/ liq = 0.28, D (Zr) arm/ liq = 1.2, D (Zr) cpx/ liq = 0.05 – 0.22, D (Zr) plag/ liq < 0.01, D (Nb) ilm/ liq = 0.80, D (Nb) arm/ liq = 1.4, D (Nb) cpx/ liq = 0.01 – 0.03, D (Nb) rut/ liq = 16 and D (Nb) plag/ liq < 0.01. Henry's law is obeyed over the compositional range studied. The measured distribution coefficients for clinopyroxene are strongly dependent on melt and crystal compositions with the activity of Al 2O 3 in the melt being an important controlling parameter. On the basis of the measured distribution coefficients and existing analytical data, the following conclusions can be drawn: 1. (1) Apollo 11 (low K) and Apollo 17 high Ti mare basalts have been generated by the partial melting of an ilmenite-rich cumulate. 2. (2) Apollo 11 (high K) mare basalts have been generated by a small degree of partial melting of a more fractionated ilmenite-rich cumulate. 3. (3) KREEP basalt magmas may have formed as residual melts produced by extensive fractional crystallization of the lunar magma ocean. 4. (4) Anomalous (Type II) mid-ocean ridge basalts may have been generated by small degrees of partial melting of a relatively undepleted mantle with clinopyroxene remaining in the residuum.

Evolution of major mineral compositions and trace element abundances during fractional crystallization of a model lunar composition

The evolution of major mineral compositions and trace element abundances during fractional crystallization of a model lunar magma ocean have been calculated. A lunar bulk composition consistent with petrological constraints has been selected. Major mineral compositions have been calculated using published studies of olivine-melt, plagioclase-melt, and pyroxene-olivine equilibria. Trace element abundances have been calculated using experimentally-determined partition coefficients where possible. In the absence of experimental determinations, published partition coefficients obtained by analyzing phase separates from porphyritic volcanic rocks have been used. Trace elements studied are La, Sm, Eu, Lu, Rb, Sr( Eu 2+), Ni, Co, and Cr. The first mineral to crystallize is olivine, which varies in composition from Fo 98 at the liquidus to Fo 95 at 50% solidification. Orthopyroxene crystallizes from 50 to 60% solidification with a restricted composition range of En 95-En 93. Plagioclase and Ca-rich clinopyroxene ( X Wo arbitrarily set equal to 0.5) co-crystallize during the final 40% solidification. Plagioclase changes in composition from An 97 to approximately An 93, while clinopyroxene evolves from En 46 to approximately En 40. The concomitant evolution of major element abundances in the melt is also discussed. The concentration of Ni in the melt decreases rapidly because solid-melt partition coefficients are significantly greater than unity at all stages of crystallization. The concentration of Cr in the melt increases slowly during olivine crystallization, then drops precipitously during the crystallization of orthopyroxene and clinopyroxene. The concentration of Co in the melt decreases slowly during olivine and orthopyroxene crystallization, after which it returns slowly to its initial concentration. Rubidium and Sr are not fractionated relative to one another until the onset of plagioclase crystallization. Ratios of Rb/Sr, normalized to their initial concentrations in the magma, do not rise above 10 until 95% of the magma has solidified. The ratios of Eu/Sm and La/Lu, normalized to their initial concentrations in the magma, remain essentially unfractionated until the onset of crystallization of clinopyroxene plus plagioclase, at which point the normalized La/Lu ratio increases to approximately 1.3 at 100% solidification and the normalized Eu/Sm ratio decreases to approximately 0.2 at 100% solidification. The model calculations are used to place approximate constraints on the bulk composition of the primitive Moon. Consideration of the effect on plagioclase composition of the activities of NaO 0.5 and SiO 2 in the melt suggests that the primitive Moon contained less than 0.4 wt % NaO 0.5 and approximately 42–43 wt % SiO 2. Concentrations of the REE in model lunar anorthosites are consistent with the returned samples. Concentrations of the REE in several model ‘highland basalts’ (considered to be representative of the average lunar terrae) are too low when compared with returned samples. Several possible explanations of this discrepancy are considered. The possible role of spinel in a twostage geochemical evolution of mare basalt liquids is discussed.