Deep Lunar Interior Research Articles

Sound velocities in lunar mantle aggregates at simultaneous high pressures and temperatures: Implications for the presence of garnet in the deep lunar interior

Recent experimental and theoretical studies on lunar magma ocean crystallisation have suggested the presence of significant proportions of garnet in the deep lunar interior. While phase relation studies indicate a deep lunar mantle consisting of olivine, pyroxene, and garnet, the compatibility of such an assemblage with seismic models of the lunar interior is yet untested. In this study we report compressional and shear wave velocities in an iron-rich assemblage consisting of olivine, orthopyroxene, clinopyroxene, and garnet up to ∼8 GPa and 1300 K, by means of ultrasonic interferometry measurements combined with synchrotron techniques using the multi-anvil press apparatus. Sound velocity and density models of lunar mantle rocks along a selenotherm based on our experimental results find good agreement with the seismic and density profiles at lunar interior depths of 740–1260 km. Further models are constructed, allowing for the variation of chemical composition, phase proportion, and temperature; these suggest that a garnet-rich deep lunar mantle is compatible with present-day lower lunar mantle temperatures of between 1400–1800 K. Our results show that lunar mantle rocks with up to 33 wt.% garnet may provide an explanation for the observed high velocities of the lower lunar mantle. The presence of garnet in the lowermost part of the Moon's mantle has significant implications for the depth and temperature of the Moon's magma ocean as well as the composition, structure and internal dynamics of the solid Moon.

Experimental Evidence Supporting an Overturned Iron‐Titanium‐Rich Melt Layer in the Deep Lunar Interior

AbstractDense Fe‐Ti‐rich cumulates, formed as the last dregs of the lunar magma ocean, are thought to have driven a large‐scale overturn of the lunar mantle over 4 Ga ago. Analysis of lunar seismic data has implied that some of the overturned bodies may have reached the lunar core‐mantle boundary and remained there until the present day as a partially molten layer. However, whether such a molten layer could be stable during >4 Ga of post‐magma‐ocean lunar history and explain lunar seismic observations remains poorly constrained. Here, we report the first sound velocity measurements on a Fe‐Ti‐rich lunar melt up to conditions of the lowermost lunar mantle. Our results suggest that a partial melt layer with at least 20% overturned Fe‐Ti‐rich melt can be trapped atop the lunar core‐mantle boundary until the present day, strongly influencing the thermochemical evolution of the lunar interior.

The Lunar Geophysical Network Landing Sites Science Rationale

Abstract The Lunar Geophysical Network (LGN) mission is proposed to land on the Moon in 2030 and deploy packages at four locations to enable geophysical measurements for 6–10 yr. Returning to the lunar surface with a long-lived geophysical network is a key next step to advance lunar and planetary science. LGN will greatly expand our primarily Apollo-based knowledge of the deep lunar interior by identifying and characterizing mantle melt layers, as well as core size and state. To meet the mission objectives, the instrument suite provides complementary seismic, geodetic, heat flow, and electromagnetic observations. We discuss the network landing site requirements and provide example sites that meet these requirements. Landing site selection will continue to be optimized throughout the formulation of this mission. Possible sites include the P-5 region within the Procellarum KREEP Terrane (PKT; (lat: 15°; long: −35°), Schickard Basin (lat: −44.°3; long: −55.°1), Crisium Basin (lat: 18.°5; long: 61.°8), and the farside Korolev Basin (lat: −2.°4; long: −159.°3). Network optimization considers the best locations to observe seismic core phases, e.g., ScS and PKP. Ray path density and proximity to young fault scarps are also analyzed to provide increased opportunities for seismic observations. Geodetic constraints require the network to have at least three nearside stations at maximum limb distances. Heat flow and electromagnetic measurements should be obtained away from terrane boundaries and from magnetic anomalies at locations representative of global trends. An in-depth case study is provided for Crisium. In addition, we discuss the consequences for scientific return of less than optimal locations or number of stations.

Extending Science from Lunar Laser Ranging

The Lunar Laser Ranging (LLR) experiment has accumulated 50 years of range data of improving accuracy from ground stations to the laser retroreflector arrays (LRAs) on the lunar surface. The upcoming decade offers several opportunities to break new ground in data precision through the deployment of the next generation of single corner-cube lunar retroreflectors and active laser transponders. This is likely to expand the LLR station network. Lunar dynamical models and analysis tools have the potential to improve and fully exploit the long temporal baseline and precision allowed by millimetric LLR data. Some of the model limitations are outlined for future efforts. Differential observation techniques will help mitigate some of the primary limiting factors and reach unprecedented accuracy. Such observations and techniques may enable the detection of several subtle signatures required to understand the dynamics of the Earth-Moon system and the deep lunar interior. LLR model improvements would impact multi-disciplinary fields that include lunar and planetary science, Earth science, fundamental physics, celestial mechanics and ephemerides.

The formation and evolution of the Moon’s crust inferred from the Sm-Nd isotopic systematics of highlands rocks

Ages determined for magnesian and ferroan anorthosite crustal rock suites overlap, suggesting they formed contemporaneously about 4.3–4.5 Ga. A notable exception is the Sm-Nd age previously determined on Mg-suite gabbronorite 67667 which is at least 100 Ma younger than the youngest ferroan anorthosite. New chronologic measurements of 67667 presented here yield concordant Sm-Nd and Rb-Sr mineral isochron ages of 4349 ± 31 Ma and 4368 ± 67 Ma, suggesting the sample is older than previous estimates. Furthermore, a whole rock Sm-Nd isochron of Mg-suite rocks from the Apollo 14, 15, 16, and 17 landing sites yields an age of 4348 ± 25 Ma, indicating that Mg-suite magmatism was widespread and roughly contemporaneous on the lunar nearside. Analysis of Sm-Nd internal isochron ages confirms that Mg-suite magmatism was restricted to a period between about 4.33 and 4.35 Ga at the Apollo 14, 15, 16, and 17 landing sites and was synchronous with magmatism at the Apollo 16 site associated with the ferroan anorthosite suite between 4.35 and 4.37 Ga. Magnesian- and ferroan anorthosite suite rocks with ages younger than ∼4.33 Ga appear to have experienced slow cooling in the deep lunar interior, so that the ages record when the samples cooled below the closure temperature of the Sm-Nd isotopic system and not the time they crystallized.The ages determined for Mg-suite and ferroan anorthosite suite rocks are concordant with the age determined for the formation of urKREEP of 4350 ± 34 Ma using the Sm-Nd isotopic systematics of 67667 and measurements completed on norite 78238, troctolite 76535, KREEP basalt 15386, and gabbronorite NWA 773. Crystallization ages of Mg-suite and FAS are also concordant with the average of 146Sm-142Nd ages previously determined for the formation of the mare basalt source region of 4333 ± 30 Ma. The similarity of ages for Mg-suite magmatism, ferroan anorthosite suite magmatism, urKREEP formation, and formation of the mare basalt source regions implies the processes that produced these rocks were petrogenetically linked. It also implies that both early-stage and late-stage lunar magma ocean cumulates formed over a relatively short duration of <40 Ma. Late and somewhat rapid solidification of a lunar magma ocean can account for the concordance of ferroan anorthosite suite rocks, urKREEP, and the mare basalt source regions. However, the major and trace element compositions of Mg-suite magmas preclude them from being a primary differentiation product of the lunar magma ocean. Instead, the Mg-suite could be produced as a result of mixing of magma ocean solidification products during density driven overturn occurring immediately after, or perhaps during, solidification of the lunar magma ocean. This scenario not only accounts for the chronology of the various rock suites, but is consistent with the petrogenesis of the Mg-suite that involves the interaction between pre-existing Mg-rich, plagioclase-rich, and urKREEP-rich cumulates of the magma ocean.

Remote detection of widespread indigenous water in lunar pyroclastic deposits

Laboratory analyses of lunar samples provide a direct means to identify indigenous volatiles and have been used to argue for the presence of Earth-like water content in the lunar interior. Some volatile elements, however, have been interpreted as evidence for a bulk lunar mantle that is dry. Here we demonstrate that, for a number of lunar pyroclastic deposits, near-infrared reflectance spectra acquired by the Moon Mineralogy Mapper instrument onboard the Chandrayaan-1 orbiter exhibit absorptions consistent with enhanced OH- and/or H2O-bearing materials. These enhancements suggest a widespread occurrence of water in pyroclastic materials sourced from the deep lunar interior, and thus an indigenous origin. Water abundances of up to 150 ppm are estimated for large pyroclastic deposits, with localized values of about 300 to 400 ppm at potential vent areas. Enhanced water content associated with lunar pyroclastic deposits and the large areal extent, widespread distribution and variable chemistry of these deposits on the lunar surface are consistent with significant water in the bulk lunar mantle. We therefore suggest that water-bearing volcanic glasses from Apollo landing sites are not anomalous, and volatile loss during pyroclastic eruptions may represent a significant pathway for the transport of water to the lunar surface. Volcanic glasses sampled by Apollo missions display high water contents. Remotely sensed spectral data show that pyroclastic deposits are generally enriched in water across the Moon, suggesting significant amounts of water in the lunar interior.

The deep lunar interior with a low-viscosity zone: Revised constraints from recent geodetic parameters on the tidal response of the Moon

We revisit the constraints on the deep lunar interior with a possible low-viscosity zone at the core-mantle boundary obtained from our previous forward modeling of the tidal response of the Moon, by comparing a numerical model with several tidal parameters (i.e., k2, k3, h2, and Q) that have been improved or are newly determined by recent geodetic observations and analyses from GRAIL (gravity), LRO (shape), and LLR (rotation). Our results are in principle consistent with these data and suggest a low-viscosity layer (with an outer radius of about 540–560 km) which possibly extends inside the region where deep moonquakes occur.

Geophysical evidence for melt in the deep lunar interior and implications for lunar evolution

Analysis of lunar laser ranging and seismic data has yielded evidence that has been interpreted to indicate a molten zone in the lowermost mantle overlying a fluid core. Such a zone provides strong constraints on models of lunar thermal evolution. Here we determine thermochemical and physical structure of the deep Moon by inverting lunar geophysical data (mean mass and moment of inertia, tidal Love number, and electromagnetic sounding data) in combination with phase-equilibrium computations. Specifically, we assess whether a molten layer is required by the geophysical data. The main conclusion drawn from this study is that a region with high dissipation located deep within the Moon is required to explain the geophysical data. This region is located within the mantle where the solidus is crossed at a depth of ∼1200 km (≥1600°C). Inverted compositions for the partially molten layer (150–200 km thick) are enriched in FeO and TiO2 relative to the surrounding mantle. The melt phase is neutrally buoyant at pressures of ∼4.5–4.6 GPa but contains less TiO2 (<15 wt %) than the Ti-rich (∼16 wt %) melts that produced a set of high-density primitive lunar magmas (density of 3.4 g/cm3). Melt densities computed here range from 3.25 to 3.45 g/cm3 bracketing the density of lunar magmas with moderate-to-high TiO2 contents. Our results are consistent with a model of lunar evolution in which the cumulate pile formed from crystallization of the magma ocean as it overturned, trapping heat-producing elements in the lower mantle.

Neutral buoyancy of titanium-rich melts in the deep lunar interior

The absence of very deep moonquakes implies that the lower mantle of the Moon is partially molten. An analysis of the density range of lunar melts at high pressures suggests that only titanium-rich melt is neutrally buoyant deep within the Moon.

Lunar Outgassing, Transient Phenomena, and the Return to the Moon. I. Existing Data

Herein the transient lunar phenomena (TLP) report database is subjected to a discriminating statistical filter robust against sites of spurious reports, and produces a restricted sample that may be largely reliable. This subset is highly correlated geographically with the catalog of outgassing events seen by the Apollo 15, 16 and Lunar Prospector alpha-particle spectrometers for episodic Rn-222 gas release. Both this robust TLP sample and even the larger, unfiltered sample are highly correlated with the boundary between mare and highlands, as are both deep and shallow moonquakes, as well as Po-210, a long-lived product of Rn-222 decay and a further tracer of outgassing. This offers another significant correlation relating TLPs and outgassing, and may tie some of this activity to sagging mare basalt plains (perhaps mascons). Additionally, low-level but likely significant TLP activity is connected to recent, major impact craters (while moonquakes are not), which may indicate the effects of cracks caused by the impacts, or perhaps avalanches, allowing release of gas. The majority of TLP (and Rn-222) activity, however, is confined to one site that produced much of the basalt in the Procellarum Terrane, and it seems plausible that this TLP activity may be tied to residual outgassing from the formerly largest volcanic ffusion sites from the deep lunar interior. With the coming in the next few years of robotic spacecraft followed by human exploration, the study of TLPs and outgassing is both promising and imperiled. We will have an unprecedented pportunity to study lunar outgassing, but will also deal with a greater burden of anthropogenic lunar gas than ever produced. There is a pressing need to study lunar atmosphere and its sources while still pristine. [Abstract abridged.]

High-pressure phase equilibria and element partitioning experiments on Apollo 15 green C picritic glass: Implications for the role of garnet in the deep lunar interior

We report results of nominally anhydrous near-liquidus experiments on a synthetic analog to very low-titanium Apollo 15 green C lunar picritic glass from ∼2 to 5 GPa. Apollo 15 green C glass (A15C) is saturated with garnet and pyroxene on the liquidus at ∼3 GPa. However, such an assemblage is unlikely to represent the lunar-mantle source region for this glass, and instead an olivine + orthopyroxene-dominated source is favored, in accord with earlier lower-pressure experiments on A15C. Near-liquidus garnet has a slight but significant majorite component at ∼5 GPa in this iron-rich bulk composition, as expected from our previous work in ordinary-chondritic bulk compositions. Ion microprobe measurements of partitioning of Sr, Ba, Sc, Nd, Sm, Dy, Yb, Y, Zr, Hf, and Th between garnet and coexisting melt in these experiments are the first garnet partition coefficients ( D values) available that are directly relevant to lunar compositions. D values for these garnets differ significantly compared to D values for garnets grown in more magnesian, terrestrial bulk compositions, which until now are all that have been available in modeling the possible role of garnet in the lunar interior. For example, D values for heavy rare earth elements are lower than are those from terrestrial basaltic systems. These partitioning values are well-described by the lattice-strain partitioning model, but predictive relationships for garnet partitioning using that model fail to match the measured values, as was the case in our earlier work on chondritic compositions. Using our new D values in place of the “terrestrial” values in a variety of models of lunar petrogenesis, we suggest that garnet is unlikely to be present in the source regions for very titanium-poor lunar liquids despite its appearance on the liquidus of A15C.

Further constraints on the deep lunar interior

[1] We have inverted the most recent set of geophysical observations pertinent to lunar interior properties, including mass M and moment of inertia I as determined by Lunar Prospector, the two Love numbers k2 and h2 as well as monthly tidal dissipation Q obtained from more than 35 years of lunar laser ranging data. In wanting to assess the ability of these parameters to constrain lunar mantle and core structure, we have used a stochastically based sampling algorithm to invert the geophysical data to obtain radial density and shear wave velocity profiles. The results indicate a small liquid Fe core (r 1, thereby favouring the fluid core hypothesis. In addition, shear wave velocities for the lower mantle region (depths > 1100 km) are generally found to be lower than upper mantle velocities and can be interpreted as implying the presence of partial melt, which can explain the unusually low lunar monthly tidal Q of ∼30.

Does the Moon possess a molten core? Probing the deep lunar interior using results from LLR and Lunar Prospector

It is the main purpose of this study to examine the deeper structure of the Moon in the light of four numbers. These are lunar mass M, mean moment of inertia I, second degree tidal Love number k2, and the quality factor Q, accounting for tidal dissipation within the solid body of the Moon. The former two have been measured by Lunar Prospector to high precision, and more than 30 years of lunar laser ranging (LLR) data have led to an estimate of the second degree tidal Love number and quality factor. The inverse problem dealt with here of obtaining information on the lunar density and S wave velocity profile from the four numbers follows our earlier investigations by employing an inverse Monte Carlo sampling method. We present a novel way of analyzing the outcome using the Bayes factor. The advantage lies in the fact that rather than just looking at a subset of sampled models, we investigate all the information sampled in different runs, i.e., take into account all samples, in order to estimate their relative plausibility. The most likely outcome of our study, based on the data, their uncertainties, and prior information, is a central core with a most probable S wave velocity close to 0 km/s, density of ∼7.2 g/cm3 and radius of about 350 km. This is interpreted as implying the presence of a molten or partially molten Fe core, in line with evidence presented earlier using LLR regarding the dissipation within the Moon.

New information on the deep lunar interior from an inversion of lunar free oscillation periods

We have obtained crucial information on the deep lunar interior through a Monte Carlo inversion of a number of fundamental lunar spheroidal free oscillations. The results indicate a homogeneous upper mantle with a velocity and density of 3.75±1.1 km/s and 3.3±0.5 g/cm³. A transition between the upper and middle mantle in the range 520‐580 km depth also seems to be implied. Middle mantle velocities and densities are 5.4±1.2 km/s and 3.5±0.2 g/cm³. Moreover, a velocity decrease is seen in the lower mantle (1150–1450 km depth) consistent with earlier inferences of partial melt in this region. The density seems to increase gradually from 1250 km depth to the center of the moon, where it reaches values indicative of an FeS or silicate composition.

Ion and electron microprobe study of troctolites, norite, and anorthosites from Apollo 14: evidence for urKREEP assimilation during petrogenesis of Apollo 14 Mg-suite rocks

Most of the Moon’s highland crust formed during the period 4.65–4.45 Ga ago from a vast magma ocean up to 800 km deep (Hess and Parmentier, 1995). This early lunar crust comprises Fe-rich anorthosites with calcic plagioclase compositions. Subsequent evolution of the highland crust was dominated by troctolites, anorthosites, and norites of the Mg-suite. This plutonic series is characterized by calcic plagioclase, and mafic minerals with high mg# (=100∗Mg/[Mg + Fe]). These rocks evidently formed by partial melting of ultramafic rocks of the lunar mantle, but their bulk rock incompatible element characteristics are too enriched to represent such a primitive source. Previous studies have suggested that this enrichment in incompatible trace elements is the result of metasomatism of the crust by fluids rich in REE and P. The products of this suggested metasomatic event are REE-rich phosphates (typically whitlockite) deposited interstitially. Alternatively, the incompatible element-rich nature of these plutonic rocks may represent a characteristic of their parent magma, acquired prior to crystallization of the plutons. In an effort to distinguish the origin of this important lunar rock series, we have analyzed the REE content of primary cumulus phases in ten Mg-suite cumulates using SIMS, along with their major and minor element compositions by electron microprobe analysis. Nine of these samples have high mg#s, consistent with their formation from the most primitive parent melts of the Mg-suite. The data presented here show that Mg-suite troctolites and anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks and that these characteristics can be used to reconstruct related aspects of the parent magma composition. Our data show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents similar to high-K KREEP in both concentration and relative abundance. The highly enriched nature of this parent magma contrasts with its primitive major element characteristics, as pointed out by previous workers. This enigma is best explained by the mixing of residual magma ocean urKREEP melts with ultramagnesian komatiitic partial melts from the deep lunar interior. The data do not support earlier models that invoke crustal metasomatism to enrich the Mg-suite cumulates after formation, or models which call for a superKREEP parent for the troctolites and anorthosites.

Chronology and petrogenesis of the lunar highlands alkali suite: Cumulates from KREEP basalt crystallization

Alkalic rocks from the highlands of the Moon, though relatively minor in volume, yield important information in understanding the later development of the lunar crust. However, until recently little information has been available on the crystallization ages of samples from this diverse suite of rocks. Previous workers suggested a link between pristine KREEP basalts and lunar quartz monzodiorites and granites. Through mineral and chemical modelling of all known pristine highlands alkali suite (HAS) rocks, and radiogenic isotopic analyses of rather large HAS clasts from the Apollo 14 landing site, we explore further this potential link.Two clasts of alkalic affinity from the western highlands of the Moon have been analyzed for their neodymium and strontium isotopic compositions. An alkali anorthosite (14304,267) yields a SmNd mineral isochron of 4108 ± 53 Ma (MSWD = 0.06) and an εNd(T) of −1.0 ± 0.2. Two splits of the same alkali norite clast (14304,270 and 14304,272) yield similar εNd values when calculated for this age. RbSr systematics for alkali anorthosite 14304,267 scatter about a line which yields an “age” of 4336 ± 81 Ma. However, the large mean square of weighted deviates (MSWD) for this line (12.3) indicates that the age is suspect and that RbSr sytematics may have been disturbed.The SmNd age for 14304,267, in conjunction with UPb zircon ages for two other highlands alkali suite (HAS) rocks from the Apollo 14 landing site and one from the Apollo 16 landing site indicate production of HAS rocks over an extended time period spanning at least 300 million years from 4.34 to 4.02 Ga. Since the last dregs of the lunar magma ocean (LMO) likely crystallized prior to 4.3 Ma, all rocks from this alkalic period cannot represent direct remnants of the late LMO and may include material resulting from the remelting of evolved portions of the Moon by upward-moving basaltic melts from the deep lunar interior. These “contaminated” melts may be represented by pristine KREEP basalts from the Apollo 15 landing site.These KREEP basalt melts could have crystallized within the crust to form cumulate gabbros, norites, anorthosites, monzodiorites, and possibly granites, with the proportion of trapped KREEPy residual liquid determining the large-ion lithophile element enrichment of the rock. Generally, the proportion of trapped KREEPy liquid is small (2–15%). The broad age range for the lunar HAS indicates that parental KREEP basalt magmatism was not a unique event, but was an important process possibly repeated several times throughout the first 600 to 700 million years of lunar history.

Ultramafic outcrop in the centre of Vredefort: Possible exposure of upper mantle?

Most of the Moon’s highland crust formed during the period 4.65–4.45 Ga ago from a vast magma ocean up to 800 km deep (Hess and Parmentier, 1995). This early lunar crust comprises Fe-rich anorthosites with calcic plagioclase compositions. Subsequent evolution of the highland crust was dominated by troctolites, anorthosites, and norites of the Mg-suite. This plutonic series is characterized by calcic plagioclase, and mafic minerals with high mg# (=100∗Mg/[Mg + Fe]). These rocks evidently formed by partial melting of ultramafic rocks of the lunar mantle, but their bulk rock incompatible element characteristics are too enriched to represent such a primitive source. Previous studies have suggested that this enrichment in incompatible trace elements is the result of metasomatism of the crust by fluids rich in REE and P. The products of this suggested metasomatic event are REE-rich phosphates (typically whitlockite) deposited interstitially. Alternatively, the incompatible element-rich nature of these plutonic rocks may represent a characteristic of their parent magma, acquired prior to crystallization of the plutons.In an effort to distinguish the origin of this important lunar rock series, we have analyzed the REE content of primary cumulus phases in ten Mg-suite cumulates using SIMS, along with their major and minor element compositions by electron microprobe analysis. Nine of these samples have high mg#s, consistent with their formation from the most primitive parent melts of the Mg-suite. The data presented here show that Mg-suite troctolites and anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks and that these characteristics can be used to reconstruct related aspects of the parent magma composition. Our data show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents similar to high-K KREEP in both concentration and relative abundance. The highly enriched nature of this parent magma contrasts with its primitive major element characteristics, as pointed out by previous workers. This enigma is best explained by the mixing of residual magma ocean urKREEP melts with ultramagnesian komatiitic partial melts from the deep lunar interior. The data do not support earlier models that invoke crustal metasomatism to enrich the Mg-suite cumulates after formation, or models which call for a superKREEP parent for the troctolites and anorthosites.

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.

Solid state convection models of the lunar internal temperature

Thermal models of the Moon, which include cooling by subsolidus creep and consideration of the creep behaviour of geologic material, provide estimates of 1500- 1600 K for the temperature, and 10 21-1022 cm2/s for the viscosity of the deep lunar interior.

Experimental Petrology of Lunar Highland Basalt Composition and Applications to Models for the Lunar Interior

Lunar Highland Basalt (gabbroic anorthosite) composition transforms with increasing pressure through assemblages of spinel + clinopyroxene + anorthite, and garnet + clinopyroxene + anorthite ± quartz to eclogitic assemblages of garnet + clinopyroxene + kyanite + quartz (p = 3.51 - 3.52). Estimates of the lunar thermal gradient allow evaluation of the mineralogical variation with depth in the moon and thus allow calculation of mean lunar density and coefficient of moment of inertia for several models for the moon. These calculations show that the simplest models of Highland Basalt as the mean lunar composition and Highland Basalt as comprising the outer 250-270 km (formed as a partial melt from a Ca, Al-rich lunar interior) fail to meet the lunar density constraint but could do so if the hypotheses were modified towards more Fe-rich compositions. The geochemical constraints are much stronger and show that it is not possible for Highland Basalt to act as a source rock for mare basalt magmas under any conditions of melting in the lunar interior. It is further shown that Highland Basalt cannot be a partial melt from a mean lunar composition matching the Ca, Al-rich inclusions of the Allende meteorite (Anderson 1973) leaving the residual deep lunar interior of diopside + merwinite + spinel mineralogy.