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Wim Van Westrenen - One of the best experts on this subject based on the ideXlab platform.
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A possible high-temperature origin of the Moon and its geochemical consequences
Earth and Planetary Science Letters, 2020Co-Authors: E S Steenstra, Jasper Berndt, Stephan Klemme, Yingwei Fei, Wim Van WestrenenAbstract:Abstract The formation of the Moon is thought to be the result of a giant impact between a Mercury-to-proto-Earth-sized body and the proto-Earth. However, the initial thermal state of the Moon following its accretion is not well constrained by geochemical data. Here, we provide geochemical evidence that supports a high-temperature origin of the Moon by performing high-temperature (1973–2873 K) metal-silicate partitioning experiments, simulating core formation in the newly-formed Moon. Results indicate that the observed Lunar Mantle depletions of Ni and Co record extreme temperatures (>2600–3700 K depending on assumptions about the composition of the Lunar core) during Lunar core formation. This temperature range is within range of the modeled silicate evaporation buffer in a synestia-type environment. Our results provide independent geochemical support for a giant-impact origin of the Moon and show that Lunar thermal models should start with a fully molten Moon. Our results also provide quantitative constraints on the effects of high-temperature Lunar differentiation on the Lunar Mantle geochemistry of volatile, and potentially siderophile elements Cu, Zn, Ga, Ge, Se, Sn, Cd, In, Te and Pb. At the extreme temperatures recorded by Ni and Co, many of these elements behave insufficiently siderophile to explain their depletions by core formation only, consistent with the inferred volatility-related loss of Cr, Cu, Zn, Ga and Sn during the Moon-forming event and/or subsequent magma-ocean degassing.
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Lunar core formation new constraints from metal silicate partitioning of siderophile elements
Earth and Planetary Science Letters, 2014Co-Authors: N Rai, Wim Van WestrenenAbstract:Abstract Analyses of Apollo era seismograms, Lunar laser ranging data and the Lunar moment of inertia suggest the presence of a small, at least partially molten Fe-rich metallic core in the Moon, but the chemical composition and formation conditions of this core are not well constrained. Here, we assess whether pressure–temperature conditions can be found at which the Lunar silicate Mantle equilibrated with a Fe-rich metallic liquid during core formation. To this end, we combine measurements of the metal–silicate partitioning behavior of siderophile elements with the estimated depletion due to core formation in those elements in the silicate Mantle of the Moon. We also explore how the presence of the light element sulfur (suggested by seismic models to be present in the core at concentrations of up to 6 wt%) in the Lunar core affects core formation models. We use published metal–silicate partitioning data for Ni, Co, W, Mo, P, V and Cr in the Lunar pressure range (1 atm–5 GPa) and characterize the dependence of the metal/silicate partition coefficients ( D ) on temperature, pressure, oxygen fugacity and composition of the silicate melt and the metal. If the core is assumed to consist of pure iron, core–Mantle equilibration conditions that best satisfy Lunar Mantle depletions of five siderophile elements—Ni, Co, W, Mo and P—are a pressure of 4.5 ( ± 0.5 ) GPa and a temperature of 2200 K. The Lunar Mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if 6 wt% S is incorporated into the Lunar core. Our results therefore suggest that metal–silicate equilibrium during Lunar core formation occurred at depths close to the present-day Lunar core–Mantle boundary. This provides independent support for both the existence of a deep magma ocean in the Moon in its early history and the presence of significant amounts of sulfur in the Lunar core.
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experimental study of trace element partitioning between Lunar orthopyroxene and anhydrous silicate melt effects of lithium and iron
Chemical Geology, 2011Co-Authors: Mirjam Van Kan Parker, Paul R D Mason, Wim Van WestrenenAbstract:Abstract Orthopyroxene (Opx) was present during a significant portion of crystallisation of the Lunar magma ocean, but its influence on co-existing melt trace element contents is not well quantified. We performed high-pressure ( P, 1.1 to 3.2 GPa), high-temperature ( T, 1400 to 1600 °C) experiments on synthetic Fe-rich compositions at reducing conditions relevant to the Lunar Mantle to constrain trace element partitioning between Opx and anhydrous silicate melts. Opx–melt partition coefficients ( D Opx-melt ) for a wide range of trace elements (LILE: Li, Ba; REE: La, Ce, Nd, Sm, Dy, Eu, Er, Tm Y, Yb, Lu; HFSE: Zr, Nb, Hf, Ta, Th, U); and transition metals (Sc, V, Mn, Co, Mo) show only very minor variations across the considered P – T range. REE partition coefficients increase from D La opx-melt = 0.0014 ± 0.0008 to D Lu opx-melt = 0.051 ± 0.007. D values for highly charged elements vary from D Th opx-melt = 0.0013 ± 0.0008 through D Nb opx-melt = 0.0018 ± 0.0006 and D U opx-melt = 0.0015 ± 0.0006 to D Ti opx-melt = 0.068 ± 0.0010. D Lu opx-melt / D Hf opx-melt values of 6.3 ± 2.4 are at the high end of reported values for minerals that played a role during crystallisation of the Lunar magma ocean, and higher than previously reported for Opx under identical oxygen fugacity conditions, implying Opx-rich cumulates in the Lunar Mantle have highly superchondritic Lu–Hf ratios. Lattice strain modelling of our REE partitioning data suggests that varying the concentration of divalent Fe in Opx very slightly decreases the ideal cation radius for M 3+ elements entering the M2 site, r 0 M2 , whereas the partitioning of M 3+ elements entering the M1 site is unaffected. A subtle increase in the maximum partition coefficient for M 3+ elements entering both the M1 and M2 sites with decreasing T is identified, when experiments carried out at similar reducing oxygen fugacities are considered. The presence of Li at concentrations of up to ~ 350 ppm does not have a measureable effect on the Opx–melt partitioning behaviour of REE or any other element, showing that charge-balancing of M 3+ , M 4+ and M 5+ elements in Opx is likely dominated by a vacancy mechanism.
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compressibility of molten apollo 17 orange glass and implications for density crossovers in the Lunar Mantle
Geochimica et Cosmochimica Acta, 2011Co-Authors: Mirjam Van Kan Parker, C B Agee, M S Duncan, Wim Van WestrenenAbstract:Abstract We performed density measurements on a synthetic equivalent of Lunar Apollo 17 74,220 “orange glass”, containing 9.1 wt% TiO2, at superliquidus conditions in the pressure range 0.5–8.5 GPa and temperature range 1723–2223 K using the sink/float technique. In the Lunar pressure range, two experiments containing pure forsterite (Fo100) spheres at 1.0 GPa and 1727 K, and at 1.3 GPa–1739 K, showed neutral buoyancies, indicating that the density of molten orange glass was equal to the density of Fo100 at these conditions (3.09 ± 0.02 g cm−3). A third tight sink/float bracket using Fo90 spheres corresponds to a melt density of 3.25 ± 0.02 g cm−3 at ∼2.8 GPa and ∼1838 K. Our data predict a density crossover for the molten orange glass composition with equilibrium orthopyroxene at ∼2.8 GPa, equivalent to a depth of ∼600 km in the Lunar Mantle, and a density of ∼3.25 g cm−3. This crossover depth is close to the orange glass multiple saturation point, representing its minimum formation depth, at the appropriate oxygen fugacity (2.8–2.9 GPa). A density crossover with equilibrium olivine is predicted to fall outside the Lunar pressure range (>4.7 GPa), indicating that molten orange glass is always less dense than its equilibrium olivines in the Moon. Our data therefore suggest that that Lunar liquids with orange glass composition are buoyant with respect to their source region at P Fitting the density data to a Birch–Murnaghan equation of state at 2173 K leads to an array of acceptable solutions ranging between 16.1 and 20.3 GPa for the isothermal bulk modulus K2173 and 3.6–8 for its pressure derivative K′, with best-fit values K2173 = 18.8 GPa and K′ = 4.4 when assuming a model 1 bar density value of 2.86 g cm−3. When assuming a slightly lower 1 bar density value of 2.84 g cm−3 we find a range for K2173 of 14.4–18.0 and K′ 3.7–8.7, with best-fit values of 17.2 GPa and 4.5, respectively.
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numerical convection modelling of a compositionally stratified Lunar Mantle
European Planetary Science Congress 2010, 2010Co-Authors: J De Vries, A P Van Den Berg, Wim Van WestrenenAbstract:Full Moon convection models of Lunar interior evolution are usually simplified in their compositional setup. They are often purely thermal convection-diffusion models, using a homogeneous Mantle composition and a heat reservoir to model the core [1, 2]. Other models add only an ilmenite-rich layer at shallow depth [3, 4]. When this compositionally distinct and relatively dense layer is used, the focus is typically on the overturn of the Lunar Mantle, due to the gravitational instability which originated from the crystallisation of an early Lunar magma ocean. In this study, we investigate compositionally more complex models. The initial configuration of our models has a both mineralogically and geochemically layered composition, to determine the influence of a more realistic Mantle stratification on Mantle dynamics and the thermal evolution of the Moon.
J J Papike - One of the best experts on this subject based on the ideXlab platform.
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low abundances of highly siderophile elements in the Lunar Mantle evidence for prolonged late accretion
Earth and Planetary Science Letters, 2004Co-Authors: Richard J Walker, M F Horan, Charles K Shearer, J J PapikeAbstract:The highly siderophile elements (HSE: including Re, Au, Ir, Os, Ru, Pt, Pd, Rh) are strongly partitioned into metal relative to silicates. In the terrestrial planets these elements are concentrated in metallic cores. Earth s Mantle has sufficiently high abundances of the HSE (~0.008 times CI abundances) that it has been hypothesized approximately 0.1-0.5% of the mass of the Earth was added following the last major interaction between the core and Mantle [e.g. 1]. The additional material added to the Earth and Moon has been termed a late veneer , and the process has often been termed late accretion [2]. The timing of the dominant late accretionary period of the Earth and Moon is still poorly known. The abundances of HSE in the Lunar Mantle could provide important constraints on when the late veneer was added. The material that ultimately became the silicate portion of the Moon was likely stripped of most of its HSE prior to and during coalescence of the Moon. Consequently the initial Lunar Mantle likely had very low concentrations of the HSE. Unlike Earth, the generation of permanent Lunar crust by ~4.4 Ga prevented subsequent additions of HSE to the Lunar Mantle via continued accretion. Thus, if a substantial portion of the late veneer was added after 4.4 Ga, the Lunar Mantle should have retained very low HSE concentrations. Conversely, if the late veneer was mostly added prior to 4.4 Ga, HSE abundances in the Lunar Mantle may be roughly similar to abundances in the terrestrial Mantle.
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oxygen fugacity of mare basalts and the Lunar Mantle application of a new microscale oxybarometer based on the valence state of vanadium
Lunar and Planetary Science XXXV: Viewing the Lunar Interior Through Titanium-Colored Glassed, 2004Co-Authors: C K Shearer, J J Papike, J M Karner, S R SuttonAbstract:The ability to estimate oxygen fugacities for mare basalts and to extend these observations to the Lunar Mantle is limited using bulk analysis techniques based on buffering assemblages or the valence state of iron. These limitations are due to reequilibration of mineral assemblages at subsolidus conditions, deviations of mineral compositions from thermodynamic ideality, size requirements, and the limits of the iron valence at very low fO2. Still, these approaches have been helpful and indicate that mare basalts crystallized at fO2 between the iron-w stite buffer (IW) and the ilmenite breakdown reaction (ilmenite = rutile + iron). It has also been inferred from these estimates that the Lunar Mantle is also highly reduced lying at conditions below IW. Generally, these data cannot be used to determine if the mare basalts become increasingly reduced during transport from their Mantle source and eruption at the Lunar surface and if there are differences in fO2 among mare basalts or Mantle sources. One promising approach to determining the fO2 of mare basalts is using the mean valence of vanadium (2+, 3+, 4+, 5+) determined on spots of a few micrometers in diameter using synchrotron x-ray absorption fine structure (XAFS) spectroscopy. The average valence state of V in basaltic glasses is a function of fO2, temperature, V coordination, and melt composition. Here, we report the initial results of this approach applied to Lunar pyroclastic glasses.
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melting in the deep Lunar Mantle
Lunar and Planetary Science Conference, 2003Co-Authors: Charles K Shearer, J J Papike, C R Neal, David Draper, C B AgeeAbstract:Introduction: Whereas previous experimental studies [1,2,3] and interpretation of Apollo seismic experiments [4,5] are permissive for garnet in the deep Lunar Mantle, interpretations of Lu-Hf isotopic systematics [6] and trace element data [7] seem to require garnet in the source of selected mare basalts. Understanding the mineralogy of the deep Lunar Mantle is critical to our further interpretation of the structure of the Moon and modeling early Lunar differentiation. Also, as it has been suggested that some mare basalts may be the product of melting that was initiated at depths to 1000 km, the stability of phases such as garnet may affect basalt composition and magma transport. To better understand melting in the deep Lunar Mantle we have initiated a study that focuses upon both the trace element characteristics of mare basalts and pyroclastic glasses and the high pressure (> 2.5 GPa) phase equilibria of pyroclastic glass compositions. Here we report the initial high pressure results on picritic glass compositions and ion probe trace element study of the pyroclastic glasses with an emphasis on elements that could potentially be fractionated by garnet. In addition to continuing this line of experiments, an additional facet of this study will focus upon a comparison of trace elements between mare basalts and pyroclastic glasses using comparable ion probeICP-MS data sets. Analytical Approach: For this study, we initially started with the Apollo 15 and 17 pyroclastic glasses. Prior to trace element analysis, individual glasses were imaged and analyzed using a JEOL 733 superprobe. Selected trace elements (Sc, Y, Zr, Ce, Yb, Hf, Hf) that would be useful in evaluating the presence of garnet in the source were measured using the Cameca ims 4f operated on the University of New Mexico campus by IOM. Analyses were made using primary O ions accelerated through a nominal potential of 10.0 kV. A primary beam current of 20 nA was focused on the sample over a spot diameter of 20 μm. Sputtered secondary ions were energy filtered using a sample offset voltage of 105 V and an energy window of ± 25 V. Analyses involved repeated cycles of peak counting. The analytical procedure included counting on a background position to monitor detection noise. Absolute concentrations of each element were calculated using empirical relationships of Trace Element/ Si ratios (normalized to known SiO2 content) to element concentrations as derived from daily calibration. Calibration curve was constructed using multiple glass standards (> 3) for each element. Calibration curves for each element have correlation coefficients of greater than 0.97. Average values for Hf for the green and orange glasses as determined by ion probe are 0.63 and 7.3 ppm, respectively. The average values for Hf for bulk Apollo 15 green and Apollo 17 orange glasses are 0.6 and 5.8 ppm, respectively All analyses are referenced to a single basalt glass standard that will then be used as a comparative reference to future ICPMS analysis of crystalline mare basalts. Experimental Approach: High pressure experiments are being carried out in the IOM high pressure experimental lab at pressures between 2.0 and 4.5 GPa using synthetic Apollo 15 green C glass and Apollo 14 black glass compositions. These compositions span nearly the entire range of Lunar pristine glasses, and these pressures range to the deepest parts of the Lunar Mantle. Results: High Pressure Experiments. Apollo 15 green glass C is saturated with garnet and clinopyroxene at 4.5 GPa and 1775°C (Fig. 1). Additional experiments will determine which phase is on the liquidus.
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the systematics of light lithophile elements li be and b in Lunar picritic glasses implications for basaltic magmatism on the moon and the origin of the moon
Geochimica et Cosmochimica Acta, 1994Co-Authors: C K Shearer, G D Layne, J J PapikeAbstract:Lunar picrites, represented by high-Mg volcanic glasses, are thought to be products of either partial melting of the deep Lunar Mantle followed by rapid ascent or polybaric partial melting initiated in the deep Lunar Mantle. The near primary compositions of these volcanic glasses provide us with a unique perspective for evaluating basaltic magmatism, the characteristics and evolution of the Lunar Mantle, and the origin of the Moon. The light lithophile elements (LLE = Li, Be, B) in planetary materials have been used to estimate planetary compositions and evaluate magmatic processes. Ion microprobe analyses of these glasses for LLEs were conducted using a Cameca 4f ion microprobe. This suite of glass beads ranged in TiO2 from 0.3 to 17 wt%. Seventy-one individual glass beads were analyzed for the LLEs. In addition, core-rim analyses of individual glass beads were made. The LLEs show a wide range of variability with Li ranging from 1.2 to 23.8 ppm, Be ranging from 0.06 to 3.09 ppm and B ranging from 0.11 to 3.87 ppm. B/Be ranges from 0.40 to 4.6. Li/Be ranges from 2.7 to 41.7, although 90% of the Li/Be values range from 14 to 30. Both B/Be and Li/Be values for the picritic glasses are less than chondrite. Be/Nd for the glasses ranges from .04 to .06 and are similar to chondrite (.058). Traverses across individual beads indicate that they are generally homogeneous with regards to LLEs regardless of TiO2 content. The individual glass groups show limited variations in LLE characteristics. The exceptions to this observation are the A17 VLT and the A15 yellow glasses. At individual Apollo sampling sites, the LLE content is generally correlated to TiO2. The high-Ti glasses are displaced toward higher Li at similar B and Be relative to the very low-Ti glasses. LLE concentrations also parallel the enrichments of other lithophile elements such as Ba, Zr, Sr and REEs. As noted for other trace element characteristics, glasses from each sampling site have similar LLE signatures. For example, the Apollo 14 glasses generally have higher LLE concentrations relative to glasses of similar TiO2 content from other sites. The LLE data support Mantle inhomogeneity and Lunar Magma Ocean (LMO) cumulate overturn models suggested by previous studies. A KREEP component had been incorporated into some of these picritic glasses. This is consistent with other trace elements and probably reflects the recycling of KREEP and/or other late stage LMO cumulates into the deep Lunar Mantle. The picritic glasses are compositionally distinct from the crystalline mare basalts in LLEs. They are not related by either fractional crystallization or partial melting processes. This suggests that they were derived from distinctively different Mantle sources. Estimates of the bulk compositions of the Earth and the Moon have previously been made based on the assumption that the ratio of Li to Be is a direct measure of the ratio of the high temperature condensates (HTC, refractory components) to the Mg-silicates (less refractory components) in a planet. We assert that if Li/Be is to be used to estimate bulk Moon composition, the picritic glasses provide fewer pitfalls and a better estimate than the crystalline mare basalts. Differences in partition coefficients (D) for Li and Be indicate that fractional crystallization and partial melting will modify the LiBe ratio. Estimates based on the picritic glasses infer a higher Li/Be for the bulk Moon than estimated from the mare basalts. This would indicate that the bulk Moon is less refractory than previously calculated by Li/Be and approaches the bulk composition of the Earth.
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basaltic magmatism on the moon a perspective from volcanic picritic glass beads
Geochimica et Cosmochimica Acta, 1993Co-Authors: C K Shearer, J J PapikeAbstract:Abstract It is widely accepted that basaltic magmas are products of partial fusion of periodotite within planetary Mantles. As such, they provide valuable insights into the composition, structure, and processes of planetary interiors. Those compositions which approach primary melt compositions provide the most direct information about planetary interiors and serve as a starting point to understand basaltic evolution. Within the collection of Lunar samples returned by the Apollo and Luna missions are homogeneous, picritic glass beads of volcanic origin. These picritic glasses are our closest approximations to primary magmas. As such, these glass beads provide a unique perspective concerning the origin of mare basalts, the characteristics of the Lunar interior, and processes in the early differentiation of the Moon. We have obtained trace element data for these picritic glasses using SIMS techniques. These data and literature isotopic and experimental data on the picritic glasses are placed within the framework of mare basaltic magmatism. The volcanic glasses are very diverse in their trace element characteristics, for example, they have a wide range of REE pattern shapes and concentrations. Like the crystalline mare basalts, all picritic glasses have a negative Eu anomaly. Unlike the crystalline mare basalts, there is little correlation between the size of the Eu anomaly and overall REE concentrations. Trace element differences among the various glasses suggests that a KREEP component was incorporated into their Mantle source. This implies large scale mixing of the “Lunar Magma Ocean”-derived cumulate pile. Subtle differences among glasses suggest that local mixing of sources may also have been an important process. Preservation of subtle chemical differences in the picritic glasses and crystalline basalts may be interpreted as indicating that they were produced by small to moderate degrees of partial melting and that the Lunar Mantle did not experience extensive melting during episodes of mare volcanism. Several lines of evidence are consistent with the view that the picritic glasses were derived from Mantle sources that were compositionally distinct from the sources for crystalline mare basalts. These are parallel, but no common, liquid lines of descent; chemical differences between picritic glasses and the more primitive crystalline mare basalts; experimental studies indicating that the picritic glasses are multiply saturated at depths greater than that of the mare basalts; differences in lead isotopic data; and the mode of eruption (i.e., fire fountaining for glass beads). These data also provide circumstantial evidence that suggests that the picritic glasses were derived from a source somewhat more volatile-rich than that of the mare basalts. Several petrogenetic models are suggested by the trace element characteristics of the picritic glasses: 1. (1) Partial melting of heterogeneous Lunar Mantle at depths greater than 300 km to produce the parental magmas (picritic) for both the mare basalts and picritic glasses. Picritic magmas represented by glass beads were erupted to the surface with small degrees of fractional crystallization while mare basalts were produced by larger degrees of fractional crystallization (15–30%) of similar (but not identical) picritic magmas. 2. (2) Picritic magmas represented by the glass beads were generated at depths greater than 400 km in a volatile-enriched (relative to the mare basalt source) heterogeneous Mantle while mare basalts are fractional crystallization products of picritic magmas generated at depths of less than 400 km. 3. (3) The picritic magmas represented by the glass beads represent polybaric melting that initiated at depths of at least 1000 km. A primitive Mantle component or less processed cumulate Mantle components may have been involved in the generation of the picritic glasses in any of these models.
M Anand - One of the best experts on this subject based on the ideXlab platform.
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a dry Lunar Mantle reservoir for young mare basalts of chang e 5
Nature, 2021Co-Authors: Yangting Lin, Romain Tartese, M Anand, Hejiu Hui, Yihong Yan, Jialong Hao, Qian Guo, Ziyuan OuyangAbstract:The distribution of water in the Moon’s interior carries implications for the origin of the Moon1, the crystallisation of the Lunar magma ocean2, and the duration of Lunar volcanism2. The Chang’E-5 (CE5) mission returned the youngest mare basalt samples, dated at 2.0 billion years ago (Ga)3, from the northwestern Procellarum KREEP Terrane (PKT), providing a probe into the spatiotemporal evolution of Lunar water. Here we report the water abundances and hydrogen isotope compositions of apatite and ilmenite-hosted melt inclusions from CE5 basalts. We derive a maximum water abundance of 283 ± 22 μg.g-1 and a δD value of -330 ± 190‰ for the parent magma. Accounting for a low degree partial melting of the depleted Mantle followed by extensive magma fractional crystallisation4, we estimate a maximum Mantle water abundance of 1-5 μg.g-1, suggesting that the Moon’s youngest volcanism was not driven by abundant water in its Mantle source. Such modest water contents for the CE5 basalt Mantle source region is at the low end of the range estimated from mare basalts that erupted from ca. 4.0-2.8 Ga5,6, suggesting that the Mantle source of CE5 basalts had become dehydrated by 2.0 Ga through previous melt extraction from the PKT Mantle during prolonged volcanic activity.
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trace element modelling of mare basalt parental melts implications for a heterogeneous Lunar Mantle
Geochimica et Cosmochimica Acta, 2014Co-Authors: M Anand, L J Hallis, Stanislav StrekopytovAbstract:The heterogeneous-source model of mare basalt formation indicates that Lunar Magma Ocean (LMO) overturn produced an uneven mixture of early-formed olivine and pyroxene, and late-formed, ilmenite-rich cumulates, which subsequently partially melted to give rise to mare magmas. These heterogeneous cumulate source regions would not only have been characterised by different mineral modal abundances, but also by different trace element compositions. The aim of this work was to investigate the petrology and geochemistry of a diverse suite of Apollo mare basalts, and utilise trace-element modelling in order to understand their petrogenetic history. Chemical modelling confirms that the mare basalts were produced by relatively small degrees of partial melting (<10%) of the LMO cumulates, and that the dominant melting type (batch vs. fractional) varies among different basalt groups. Similarly, single-source mineralogy cannot be applied to all mare basalt types, confirming that the Lunar Mantle was heterogeneous at the time of generation of mare magmas. Plagioclase is not required in the source of most mare basalts, with the notable exception of the Apollo 14 high-Al basalts. Addition of more than 1% plagioclase to the source of other basalts produces weaker negative Eu anomalies than those observed in the samples. AFC calculations demonstrate the compositional differences between materials assimilated into the Apollo 14 high-Al and Apollo 11 high-K mare basalt partial melts, highlighting the complexities of mare basalt petrogenesis.
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late delivery of chondritic hydrogen into the Lunar Mantle insights from mare basalts
Earth and Planetary Science Letters, 2013Co-Authors: Romain Tartese, M AnandAbstract:Recent analytical advances have enabled first successful in-situ detection of water (measured as OH) in Lunar volcanic glasses, and, melt inclusions and minerals from mare basalts. These in-situ measurements in Lunar materials, coupled with observations made by orbiting spacecraft missions have challenged the traditional view of the Moon as an anhydrous body. By synthesizing and modeling of previously published data on OH contents and H isotope compositions of apatite from mare basalts, we demonstrate that a model of hydrogen delivery into the Lunar interior by late accretion of chondritic materials adequately accounts for the measured “water” content and its hydrogen isotopic composition in mare basalts. In our proposed model, “water” in the Lunar interior was mostly constituted by hydrogen, delivered by the late accretion of chondrite-type materials. Our model is also consistent with previously proposed models to account for other geochemical characteristics of the Lunar samples.
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the oxygen isotope composition petrology and geochemistry of mare basalts evidence for large scale compositional variation in the Lunar Mantle
Geochimica et Cosmochimica Acta, 2010Co-Authors: M Anand, L J Hallis, R C Greenwood, Martin F Miller, I A Franchi, S S RussellAbstract: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 delta O-18 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 delta O-18 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 delta O-18 values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt delta O-18 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 delta O-18 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. Consistent with the results of previous studies, our data reveal no detectable difference between the Delta O-17 of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.
Richard J Walker - One of the best experts on this subject based on the ideXlab platform.
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tungsten isotopic evidence for disproportional late accretion to the earth and moon
Nature, 2015Co-Authors: Igor S. Puchtel, Mathieu Touboul, Richard J WalkerAbstract:Examination of three Lunar samples reveals that the Moon’s Mantle has an excess of the tungsten isotope 182W of about 20 parts per million relative to the present-day Earth’s Mantle; this suggests that the two bodies had identical compositions immediately following the formation of the Moon, and that the compositions then diverged as a result of disproportional late accretion of chondritic material to the Earth and Moon. Two papers published in this issue of Nature present precise measurements of tungsten isotope composition in Lunar rocks that are best explained by the Earth and Moon having had similar composition immediately following formation of the Moon, and then having diverged as a result of disproportional late accretion of material to the two bodies. Mathieu Touboul et al. found small 182W excess of about 21 parts per million relative to the present-day Earth's Mantle in metals extracted from two KREEP-rich Apollo 16 impact-melt rocks, while Thomas Kruijer et al. measured tungsten isotopes in seven KREEP-rich whole rock samples that span a wide range of cosmic ray exposure ages, and found a 182W excess of about 27 parts per million over the present-day Earth's Mantle. Characterization of the hafnium–tungsten systematics (182Hf decaying to 182W and emitting two electrons with a half-life of 8.9 million years) of the Lunar Mantle will enable better constraints on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-Mantle-like W isotopic compositions have been reported1,2 among crystallization products of the Lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the Lunar magma ocean after 182Hf was no longer extant—that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three Lunar samples that are more precise by a factor of ≥4 than those previously reported1,2. The new data reveal that the Lunar Mantle has a well-resolved 182W excess of 20.6 ± 5.1 parts per million (±2 standard deviations), relative to the modern terrestrial Mantle. The offset between the Mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon3,4. One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth’s Mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal–silicate equilibration.
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estimation of trace element concentrations in the Lunar magma ocean using mineral and metal silicate melt partition coefficients
Meteoritics & Planetary Science, 2015Co-Authors: Miriam Sharp, Kevin Righter, Richard J WalkerAbstract:This study uses experimentally determined plagioclase-melt D values to estimate the trace element concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing Lunar magma ocean at the point of plagioclase flotation. Similarly, experimentally determined metal-silicate partition experiments combined with a composition model for the Moon are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the Lunar magma ocean at the time of core formation. The metal-silicate derived Lunar Mantle estimates are generally consistent with previous estimates for the concentration of these elements in the Lunar Mantle. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may be concentrated in the residual liquid during fractional crystallization due to their incompatibility. Alternatively, the apparent enrichment could reflect the inappropriate use of bulk anorthosite data, rather than data for plagioclase separates.
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Osmium isotope and highly siderophile element systematics of the Lunar crust
Earth and Planetary Science Letters, 2010Co-Authors: Richard J Walker, Odette B. James, Igor S. PuchtelAbstract:Abstract Coupled 187 Os/ 188 Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine Lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted Lunar Mantle. At the time of formation, the Lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g − 1 Os, 1.5 ± 0.6 pg g − 1 Ir, 6.8 ± 2.7 pg g − 1 Ru, 16 ± 15 pg g − 1 Pt, 33 ± 30 pg g − 1 Pd and 0.29 ± 0.10 pg g − 1 Re (∼ 0.00002 × CI) and Re/Os ratios that were modestly elevated ( 187 Re/ 188 Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (∼ 0.00007 × CI) compared with FAN. This either reflects contrasting Mantle-source HSE characteristics of FAN and MGS rocks, or different Mantle–crust HSE fractionation behaviour during production of these lithologies. Previous studies of Lunar impact-melt rocks have identified possible elevated Ru and Pd in Lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the Lunar Mantle are compared with FAN and MGS averages, crust–Mantle concentration ratios ( D -values) must be ≤ 0.3. Such D -values are broadly similar to those estimated for partitioning between the terrestrial crust and upper Mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary Lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust–Mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the Lunar crust, coupled with estimates of HSE concentrations in the Lunar Mantle implies there may be a ‘missing component’ of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial Mantle HSE abundances were established by late accretion.
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low abundances of highly siderophile elements in the Lunar Mantle evidence for prolonged late accretion
Earth and Planetary Science Letters, 2004Co-Authors: Richard J Walker, M F Horan, Charles K Shearer, J J PapikeAbstract:The highly siderophile elements (HSE: including Re, Au, Ir, Os, Ru, Pt, Pd, Rh) are strongly partitioned into metal relative to silicates. In the terrestrial planets these elements are concentrated in metallic cores. Earth s Mantle has sufficiently high abundances of the HSE (~0.008 times CI abundances) that it has been hypothesized approximately 0.1-0.5% of the mass of the Earth was added following the last major interaction between the core and Mantle [e.g. 1]. The additional material added to the Earth and Moon has been termed a late veneer , and the process has often been termed late accretion [2]. The timing of the dominant late accretionary period of the Earth and Moon is still poorly known. The abundances of HSE in the Lunar Mantle could provide important constraints on when the late veneer was added. The material that ultimately became the silicate portion of the Moon was likely stripped of most of its HSE prior to and during coalescence of the Moon. Consequently the initial Lunar Mantle likely had very low concentrations of the HSE. Unlike Earth, the generation of permanent Lunar crust by ~4.4 Ga prevented subsequent additions of HSE to the Lunar Mantle via continued accretion. Thus, if a substantial portion of the late veneer was added after 4.4 Ga, the Lunar Mantle should have retained very low HSE concentrations. Conversely, if the late veneer was mostly added prior to 4.4 Ga, HSE abundances in the Lunar Mantle may be roughly similar to abundances in the terrestrial Mantle.
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the stability and major element partitioning of ilmenite and armalcolite during Lunar cumulate Mantle overturn
Geochimica et Cosmochimica Acta, 2009Co-Authors: Carla Thacker, Yan Liang, Qinglan Peng, Paul C HessAbstract:Abstract Ilmenite has played an important role in the petrogenesis of Lunar high-Ti picritic magmas, and armalcolite is another high-Ti oxide that was first discovered on the moon. In this study, we examined the thermodynamic stability of ilmenite and armalcolite in the context of Lunar cumulate Mantle overturn. Two starting compositions were explored, an ilmenite-bearing dunite (olivine + ilmenite) and an ilmenite-bearing harzburgite (olivine + orthopyroxene + ilmenite). Experiments were conducted using a 19.05 mm piston-cylinder apparatus at temperatures of 1235–1475 °C and pressures of 1–2 GPa. In runs with the ilmenite-bearing dunite mixture, ilmenite is stable in the subsolidus assemblage at least up to 1450 °C and 2 GPa. In runs with the ilmenite-bearing harzburgite starting mixture, ilmenite is stable at pressures greater than 1.4 GPa, and armalcolite is stable at lower pressures. Solidi for both starting compositions were determined, and the phase boundary between ilmenite- and armalcolite-bearing harzburgite was shown to have little dependence on temperature. During Lunar cumulate overturn, sinking ilmenite formed near the end of Lunar magma ocean solidification transforms into armalcolite when in contact with harzburgite cumulates at depths of less than 280 km in the Lunar Mantle. Inefficient overturn could leave isolated, inhomogeneously distributed pockets of armalcolite-bearing harzburgite in the upper Lunar Mantle, underlain by an ilmenite-bearing lower Lunar Mantle. These high-Ti oxide-bearing harzburgitic pockets can serve as potential sources for the generation of high-Ti magmas through partial melting or through assimilation of high-Ti minerals during transport of low-Ti picritic magmas in the Lunar Mantle. FeO–MgO exchange between olivine and either ilmenite or armalcolite was also examined in this study. We found the FeO–MgO distribution coefficient to be effectively independent of temperature for the pressures, temperatures, and compositions explored, with an average value of 0.179 ± 0.008 for olivine/ilmenite and 0.319 ± 0.021 for olivine/armalcolite. Given the bulk composition of an overturned Lunar cumulate Mantle, our measured FeO–MgO distribution coefficients can be used to estimate the Mg# of coexisting minerals in armalcolite- or ilmenite-bearing harzburgite and dunite in the overturned Lunar Mantle. Finally, the transformation from ilmenite-bearing harzburgite to armalcolite-bearing harzburgite results in a density increase of up to 2%. Large armalcolite-bearing cumulate bodies in the upper Lunar Mantle may be detectable in future Lunar geophysical experiments.
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preferential assimilation of armalcolite and ilmenite during melt migration and melt rock reaction in the Lunar Mantle an experimental study
LPI, 2007Co-Authors: Yan Liang, Lo M Cascio, Paul C HessAbstract:Introduction: The primitive Lunar picritic magmas are not saturated with pyroxene and ilmenite at their liquidi at pressures below multi-saturation. Once segregated from their source regions, the picritic magmas will have a strong tendency to interact, both thermally and chemically, with their surrounding Lunar Mantle at shallower depth [1,2]. Such thermochemical interaction between a through-going magma and its surrounding Mantle is referred to as melt-rock reaction. The processes of melt-rock reaction during basaltic magma transport in the Earth’s Mantle are relatively well understood. They involve preferential dissolution of pyroxene and precipitation of olivine, as olivine normative basalts percolate through a harzburgite or lherzolite matrix [3-5]. Evidences of melt migration and melt-rock reaction in the Earth’s upper Mantle were preserved in the Mantle section of ophiolites where tabular to elongated dunite bodies formed by reactive dissolution of harzburgite or lherzolite are exposed [3]. Melt-rock reaction will inevitably lead to assimilation that can significantly modify the through-going melt composition. Assimilation has been called upon in several occasions to explain the high Ti content of some Lunar picritic glasses. It has been suggested that assimilation of ilmenite-bearing late cumulates (and KREEP) or even armalcolite to the low Ti primary magmas at shallow levels can produce the high Ti magmas observed on the Lunar surface [6-11]. Nevertheless, the processes that gave rise to assimilation in the Lunar Mantle are still not well understood, in part because the mineralogy of the Lunar Mantle is not well constrained. In a companion study, we determined the thermodynamic stability of ilmenite and armalcolite in a dunite and a harzburgite in the context of Lunar cumulate overturn [12]. We have shown that subsolidus ilmenite and olivine mixture (ilmenite-bearing dunite) is stable over a wide range of temperatures (T) and pressures (P), whereas armalcoite-bearing harzburgite is more stable than ilmenite-bearing harzburgite at pressures less than 1.4 GPa. This raises an important question: is assimilation of armalcolite, instead of ilmenite, in the shallow Lunar Mantle feasible? And if so, what are the consequences and mechanisms of armalcolite and ilmenite assimilation during melt transport and meltrock reaction in the Lunar Mantle? Experiments: In order to better understand the processes of assimilation and melt-rock reaction during magma transport in the Lunar Mantle, we conducted a serious of dissolution experiments using dissolution couples formed by juxtaposing a presynthesized armalcolite-bearing or ilmenite-bearing harzburgite against a pre-synthesized Apollo 15 yellow glass in either Ni or C-Pt-Mo capsules. Dissolution experiments were conducted at 12501350°C and 1 GPa. Starting compositions and experimental procedures were described in [1,4,5,12].
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Dunite channels as viable pathways for mare basalt transport in the deep Lunar Mantle
Geophysical Research Letters, 2006Co-Authors: Alyssa R. Beck, Yan Liang, Zachary Morgan, Paul C HessAbstract:[1] Lunar picritic glasses are multisaturated with olivine and orthopyroxene at pressures up to 2.45 GPa. This corresponds to a depth of approximately 490 km in the Lunar Mantle and represents a minimum estimate of the depth of melt generation. Models that propose a mechanism to move these melts through the Mantle and crust generally involve the creation of a network of fractures through which melt can rise very rapidly, minimizing its interaction with shallower Mantle. We carried out an experimental study of harzburgite dissolution in a synthetic high Ti red glass. Our results show that during ascent of olivine saturated melts, dissolution of wallrock orthopyroxene and precipitation of olivine leads to the formation of high porosity, high permeability dunite channels that efficiently shield subsequent melts from reaction with the Mantle. These dunite channels are similar to dunite dikes observed in ophiolite sequences which are believed to be channels for mid-ocean ridge basalts. Models for Lunar melt migration that require brittle fracturing extending to the depths of multisaturation need not be invoked.
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mineralogy of the mafic anomaly in the south pole aitken basin implications for excavation of the Lunar Mantle
Geophysical Research Letters, 1997Co-Authors: C M Pieters, S Tompkins, J W Head, Paul C HessAbstract:Mineralogy of South Pole-Aitken Basin (SPA) (the largest confirmed impact basin on the Moon) is evaluated using five-color images from Clementine. Although olivine-rich material as well as basalts rich in clinopyroxene are readily identified elsewhere on the farside, the dominant rock type observed across the interior of SPA is of a very noritic composition. This mineralogy suggests that lower crust rather than the Mantle is the dominant source of the mafic component at SPA. The lack of variation in observed noritic composition is probably due to basin formation processes, during which extensive melting and mixing of target materials are likely to occur.
