Lunar Apatite Research Articles

Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites

The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion. In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study. Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries. The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼940 to 1410 ppm and −3.38 to +7.70‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from −157 to +163‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD −122 ± 20‰, δ37Cl + 1.76 ± 0.66‰) are consistent with the range displayed in other inner Solar System bodies. NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.

Cl-bearing fluorcalciobritholite in high-Ti basalts from Apollo 11 and 17: Implications for volatile histories of late-stage lunar magmas.

Abstract We report the occurrence of a previously unidentified mineral in lunar samples: a Cl-,F-,REE-rich silico-phosphate identified as Cl-bearing fluorcalciobritholite. This mineral is found in late-stage crystallization assemblages of slowly cooled high-Ti basalts 10044, 10047, 75035, and 75055. It occurs as rims on fluorapatite or as a solid-solution between fluorapatite and Cl-fluorcalciobritholite. The Cl-fluorcalciobritholite appears to be nominally anhydrous. The Cl and Fe2+ of the lunar Cl fluorcalciobritholite distinguishes it from its terrestrial analog. The textures and chemistry of the Clfluorcalciobritholite argue for growth during the last stages of igneous crystallization, rather than by later alteration/replacement by Cl-, REE-bearing metasomatic agents in the lunar crust. The igneous growth of this Cl- and F-bearing and OH-poor mineral after apatite in the samples we have studied suggests that the Lunar Apatite Paradox model (Boyce et al. 2014) may be inapplicable for high-Ti lunar magmas. This new volatile-bearing mineral has important potential as a geochemical tool for understanding Cl isotopes and REE chemistry of lunar samples.

An extremely heavy chlorine reservoir in the Moon: Insights from the apatite in lunar meteorites

Chlorine, an extremely hydrophilic volatile element, provides important information regarding the origin of intrinsic volatiles in the Moon. Lunar apatite was found to have a wider spread of δ37Cl (from −1 to +40‰ versus standard mean ocean chloride) than most terrestrial and chondritic ones (0 ± 0.5‰). However, the provenance of the elevated lunar δ37Cl is still enigmatic. Here we report new isotopic data for H and Cl in apatite from three lunar meteorites and discuss possible mechanisms for Cl isotopic fractionation of the Moon. The apatite grain in Dhofar 458 has an average δ37Cl value of +76‰, indicative of an extremely heavy Cl reservoir in the Moon. Volatile loss associated with the Moon-forming Giant Impact and the formation of lunar magma ocean could account for the large Cl isotopic fractionation of the Moon. The observed H2O contents (220–5200 ppm), δD (−100 to +550‰) and δ37Cl values (+3.8 − +81.1‰) in lunar apatite could be understood if late accretion of hydrous components were added to the Moon after the fractionation of Cl isotopes. The heterogeneous distribution of lunar Cl isotopes is probably resulted from complex lunar formation and differentiation processes.

Shock‐induced microtextures in lunar apatite and merrillite

AbstractApatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock‐loading. In this work, we analyzed a set of well‐studied lunar highlands samples (Apollo 17 Mg‐suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock‐pressure barometry using optical microscopy, Raman spectroscopy, and SEM‐based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM‐CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock‐loading. An early shock‐stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal‐plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal‐plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock‐loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages.

Isotopic evidence for volatile replenishment of the Moon during the Late Accretion.

The traditional view of a dry, volatile-poor Moon has been challenged by the identification of water and other volatiles in lunar samples, but the volatile budget delivery time(s), source(s) and temporal evolution remain poorly constrained. Here we show that hydrogen and chlorine isotopic ratios in lunar apatite changed significantly during the Late Accretion (LA, 4.1–3.8 billion years ago). During this period, deuterium/hydrogen ratios in the Moon changed from initial carbonaceous-chondrite-like values to values consistent with an influx of ordinary-chondrite-like material and pre-LA elevated δ37Cl values drop towards lower chondrite-like values. Inferred pre-LA lunar interior water contents are significantly lower than pristine values suggesting degassing, followed by an increase during the LA. These trends are consistent with dynamic models of solar-system evolution, suggesting that the Moon's (and Earth's) initial volatiles were replenished ∼0.5 Ga after their formation, with their final budgets reflecting a mixture of sources and delivery times.

The oxidation state of sulfur in lunar apatite

Lunar apatites contain hundreds to thousands of parts per million of sulfur. This is puzzling because lunar basalts are thought to form in low oxygen fugacity (f_(O_2)) conditions where sulfur can only exist in its reduced form (S^(2–)), a substitution not previously observed in natural apatite. We present measurements of the oxidation state of S in lunar apatites and associated mesostasis glass that show that lunar apatites and glass contain dominantly S^(2–), whereas natural apatites from Earth are only known to contain S^(6+). It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S^(6+)/S^(2–) ratios of such apatites could be used to quantify the f_(O_2) values at which they crystallized, given information on the portioning of S^(6+) and S^(2–) between apatite and melt and on the S^(6+)/S^(2–) ratios of melts as functions of f_(O_2) and melt composition. Such a well-calibrated oxybarometer based on this the oxidation state of S in apatite would have wide application.

Cryptic metasomatism during late-stage lunar magmatism implicated by sulfur in apatite

The use of volatile-bearing (H 2 O-Cl-F-C-S) minerals as proxies for constraining the volatile content of lunar magmas remains controversial. In this study, we apply published F-Cl-S signature trends in apatite from lunar mare basalts, new experimentally determined apatitemelt partition coefficients for S, and thermodynamic modeling to demonstrate unequivocally that fractional crystallization of lunar silicate melt could not have produced the observed S signatures of lunar apatite. Instead, metasomatic (cryptic) alteration by a S-Cl–bearing, F-poor volatile phase plausibly explains the volatile element zoning in lunar apatite, consistent with some lunar magmas being more volatile rich than previously suggested.

Characterization of mesostasis regions in lunar basalts: Understanding late‐stage melt evolution and its influence on apatite formation

AbstractRecent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile‐bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk‐rock basaltic composition and that of bulk‐mesostasis regions, indicating that bulk‐rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late‐stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO2 abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite‐melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.

New evidence for lunar basalt metasomatism by underlying regolith

Earth-like δD values reported from lunar mare-basalt apatites have typically been interpreted to reflect the intrinsic isotopic composition of lunar-mantle water. New data indicates that some of these basalts are also characterized by having experienced a slow cooling history after their emplacement onto the lunar surface. This suggests that these basalts may have experienced metasomatism by fluxes generated during the degassing of the lunar regolith induced by the long-duration, high-temperature residence times of overlying basalts.

Early degassing of lunar urKREEP by crust-breaching impact(s)

Current models for the Moon's formation have yet to fully account for the thermal evolution of the Moon in the presence of H2O and other volatiles. Of particular importance is chlorine, since most lunar samples are characterised by unique heavy δ37Cl values, significantly deviating from those of other planetary materials, including Earth, for which δ37Cl values cluster around ∼0‰. In order to unravel the cause(s) of the Moon's unique chlorine isotope signature, we performed a comprehensive study of high-precision in situ Cl isotope measurements of apatite from a suite of Apollo samples with a range of geochemical characteristics and petrologic types. The Cl-isotopic compositions measured in lunar apatite in the studied samples display a wide range of δ37Cl values (reaching a maximum value of +36‰), which are positively correlated with the amount of potassium (K), Rare Earth Element (REE) and phosphorous (P) (KREEP) component in each sample. Using these new data, integrated with existing H-isotope data obtained for the same samples, we are able to place these findings in the context of the canonical lunar magma ocean (LMO) model. The results are consistent with the urKREEP reservoir being characterised by a δ37Cl ∼+30‰. Such a heavy Cl isotope signature requires metal-chloride degassing from a Cl-enriched urKREEP LMO residue, a process likely to have been triggered by at least one large crust-breaching impact event that facilitated the transport and exposure of urKREEP liquid to the lunar surface.

Degassing pathways of Cl-, F-, H-, and S-bearing magmas near the lunar surface: Implications for the composition and Cl isotopic values of lunar apatite

Experimental degassing of H-, F-, Cl-, C-, and S-bearing species from volatile-bearing magma of lunar composition at low pressure and f O 2 close to the quartz-iron-fayalite buffer (QIF) indicates that the composition of the fluid/vapor phase that is lost changes over time. A highly H-rich vapor phase is exsolved within the first 10 min of degassing leaving behind a melt that is effectively dehydrated. Some Cl, F, and S is also lost during this time, presumably as HCl, HF, and H 2 S gaseous species; however much of the original inventory of Cl, F, and S components are retained in the melt. After 10 min, the exsolved vapor is dry and dominated by S- and halogen-bearing phases, presumably consisting of metal halides and sulfides, which evolves over time toward F enrichment. This vapor evolution provides important constraints on the geochemistry of volatile-bearing lunar phases that form subsequent to or during degassing. The rapidity of H loss suggests that little if any OH-bearing apatite will crystallize from surface or near surface (~7m) melts and that degassing of lunar magmas will cause the compositions of apatites to evolve first toward the F-Cl apatite binary and eventually toward end-member fluorapatite during crystallization. During the stage of loss of primarily H component from the melt, Cl would have been lost primarily as HCl, which is reported not to fractionate Cl isotopes at magmatic temperatures (Sharp et al. 2010). After the loss of H-bearing species, continued loss of Cl would result in the degassing of metal chlorides, which have been proposed as a mechanism to fractionate Cl isotopes (Sharp et al. 2010). After the onset of metal chloride degassing, the δ 37 Cl of the melt would necessarily increase to +6 (82% Cl loss), +8 (85% Cl loss), and +20‰ (95% Cl loss) at 1, 4, and 6 h, respectively, which was approximated using a computed trajectory of δ 37 Cl values in basalt during degassing of FeCl 2 . This strong enrichment of 37 Cl in the melt after metal chloride volatilization is fully consistent with values measured for the non-leachates of a variety of lunar samples and would be reflected in apatites crystallized from a degassing melt. Our results suggest that a range in δ 37 Cl from 0 to >20‰ is expected in lunar apatite, with heavy enrichment being the norm. While 95% loss in the initial Cl content of the melt (280 ppm Cl left in the melt) would cause an increase to +20‰ in δ 37 Cl, the ability to measure this increase in a lunar sample is ultimately dependent upon the starting Cl abundances and whether or not a mechanism exists to concentrate the remaining Cl such that it can be subsequently analyzed with sufficient accuracy. Therefore, the higher the starting Cl abundances in the initial melts, the heavier δ 37 Cl values that can be measurably preserved. Importantly, such enrichments can occur in spite of high initial hydrogen contents, and therefore, our experiments demonstrate that elevated values of δ 37 Cl cannot be used as supporting evidence for an anhydrous Moon. Furthermore, if the H-bearing vapor has a significant H 2 component, this process should also result in strong enrichment of δD in the residual magmas that reach the lunar surface or near-surface environment. Apatites within some mare basalts exhibit elevated δD of 1000 ‰ depending on the initial value (Tartese and Anand 2013) in addition to the δ 37 Cl values, but elevated δ 37 Cl values are accompanied by only modest enrichments in δD in apatites from samples of the highlands crust (McCubbin et al. 2015a).

Estimating the lunar mantle water budget from phosphates: Complications associated with silicate-liquid-immiscibility

The discovery of water within the lunar mantle has broad implications for the formation of the Earth–Moon system, differentiation of the Moon, and the magmatic evolution of lunar basalts, as well as the highland rocks. Recently, there has been considerable interest in using combined water abundances and H-isotope systematics of lunar apatites from mare basalts to quantify the origin and extent of water within the Moon’s mantle. However, the petrologic and geochemical conditions that govern apatite crystallization are not well-constrained, especially for high-FeO basaltic melts that crystallize at fO2 values below the iron–wüstite buffer. Apatites are typically located within the late-stage interstitial regions. In this contribution, we present detailed textural descriptions of late-stage inter-cumulus, residual-liquid pockets (i.e., mesostasis pockets), in order to understand the petrogenesis of lunar apatite. Results from five mare basalts demonstrate that the majority of the residual liquids in mesostasis regions have undergone silicate-liquid immiscibility (SLI) splitting into Si–K-rich (felsic) and Fe-rich (Fe-basaltic) conjugate liquids. We demonstrate the complexity of these residual liquids by documenting a wide range of water contents for apatites in several mesostasis pockets within a single mare basalt, a complexity common to many basalts. These data illustrate that individual apatite-hosting mesostasis pockets behave as independent sub-systems, even within a single rock. Furthermore, we present water concentrations for another phosphate phase, merrillite, indicating additional uncertainties during considerations of water partitioning into apatite.Fractional crystallization trends have been used in order to assess the conditions under which magmas are likely to undergo SLI. Predicted liquid lines of descent indicate that it is the late-stage residual liquids of lunar basalts with relatively low-Mg# (e.g., <∼50) that intersect the two-liquid field during crystallization, forming conjugate liquids that are chemically and mineralogically distinct. However, residual liquids for basaltic magmas with high-Mg# (e.g., >∼50) may not intersect the two-liquid field depending upon the fractionation trends in the late-stage mesostasis pockets. For samples that undergo SLI, the apatite/melt-partition coefficients required for back-calculating water abundances of the parental melts are compromised by the generation of two populations of apatites-merrillites from conjugate immiscible liquids. This process highlights an important complexity inherent to all water back-calculations that use apatite, as this requires an additional set of partition coefficients. We emphasize that the complex petrologic nature and common development of SLI of apatite-bearing, late-stage mesostasis pockets have not been considered in published apatite-volatile data. These factors, in additional to other considerations, illustrate why water back-calculations to model the primary melts from such data must be viewed with caution.

An intrinsic volatility scale relevant to the Earth and Moon and the status of water in the Moon

AbstractThe notion of a dry Moon has recently been challenged by the discovery of high water contents in lunar apatites and in melt inclusions within olivine crystals from two pyroclastic glasses. The highest and most compelling water contents were found in pyroclastic glasses that are not very common on the lunar surface. To obtain more representative constraints on the volatile content of the lunar interior, we measured the Zn content, a moderately volatile element, of mineral and rock fragments in lunar soils collected during Apollo missions. We here confirm that the Moon is significantly more depleted in Zn than the Earth. Combining Zn with existing K and Rb data on similar rocks allows us to anchor a new volatility scale based on the bond energy of nonsiderophile elements in their condensed phases. Extrapolating the volatility curve to H shows that the bulk of the lunar interior must be dry (≤1 ppm). This contrasts with the water content of the mantle sources of pyroclastic glasses, inferred to contain up to approximately 40 ppm water based on H2O/Ce ratios. These observations are best reconciled if the pyroclastic glasses derive from localized water‐rich heterogeneities in a dominantly dry lunar interior. We argue that, although late addition of 0.015% of a chondritic veneer to the Moon seems required to explain the abundance of platinum group elements (Day et al. 2007), the volatile content of the added material was clearly heterogeneous.

The Lunar Apatite Paradox

Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

Accurate and precise measurements of the D/H ratio and hydroxyl content in lunar apatites using NanoSIMS

This paper describes the development of a technique using NanoSIMS to make precise and accurate determination of the hydrogen isotopic composition and the hydroxyl content of apatite grains. An experiment was also performed to determine the effects, if any, of using secondary electron microscopy (SEM) and electron probe micro-analysis (EPMA) on the mobility of volatile elements (H) in apatite grains, prior to analysis by secondary ion mass spectrometry (SIMS). The results of the experiment show that using a relatively low beam current, for a short exposure time, did not have adverse effects on the mobility of H with no apparent change in the measured D/H ratio from or within a grain. Thus, based on our study, we suggest that routine SEM analysis can indeed be used to locate suitable apatite grains for SIMS analyses without jeopardising the hydrogen isotopic measurements.As such, apatite grains were located in the studied sections using SEM and the hydrogen isotopic compositions of the lunar apatite grains from two Apollo mare basalts were determined using NanoSIMS. The δD values for the samples analysed ranges from ~600 to 1000‰ and OH content from 1200 to 4400ppm. The measurements of δD and OH contents in apatites, performed in this study, are consistent with previous results for 10044 but the new dataset for 12064 consists of a larger range in δD values and OH contents than previously documented. We used a raster size of 10×10μm (with data collected from a 5×5μm area) for each analysis and obtained typically ±40 to 80‰ precision at the 2σ level. This demonstrates the capability of using the NanoSIMS 50L to accurately measure D/H ratios and OH contents in apatites containing more than 300–400ppm OH. The coupled hydroxyl abundance and hydrogen isotopic composition data permitted an assessment of the potential D/H fractionation which may have occurred during magmatic degassing, which may ultimately be responsible for the relatively high δD signatures of apatites from mare basalts.

Differential degassing of H2O, Cl, F, and S: Potential effects on lunar apatite

Recent SIMS analysis of water, F, and Cl in lunar apatite suggests significantly higher volatile abundances in lunar magmas than previously considered. However, apatite is commonly a latecrystallizing mineral and its volatile content may reflect late-stage open-system processes that have perturbed the magmatic volatile content and obscured direct information regarding the volatile contents of the parental magmas and magmatic source region. Degassing during magma ascent has the potential to perturb not only the absolute but also the relative magmatic volatile abundances. A set of evacuated silica tube degassing experiments were conducted that simulate ascent of high-Al basalt 14053 with 0.5 wt% Cl, 0.5 wt% F, 0.3 wt% S, and 2.2 wt% (and 2.5 wt%) water (in addition to dissolved C-O-H species) from 100 km to within ~20 m of the surface followed by degassing (at an fO₂ of ~QIF). Extensive degassing occurred within 6 h during which 99-100% of the initial water, 89-84% of the initial Cl, 60-61% of the initial F, and 94-92% of the initial S was lost. During degassing, the relative volatile contents showed a strong decrease in water content and an increase in F:Cl ratio. In reflection of the changes in melt volatile contents, apatites crystallizing from the degassed melt would have much lower OH contents and higher F:Cl ratio than apatites crystallized from the non-degassed melt. These results confirm the possibility of significant underestimation of primary magma volatile contents, especially water and Cl, through use of apatite volatile contents and the assumption of simple increases in volatile abundance during magma differentiation.

Fluorine and chlorine abundances in lunar apatite: Implications for heterogeneous distributions of magmatic volatiles in the lunar interior

Apatite has been analyzed from mare basalts, the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks using an electron probe microanalysis routine developed specifically for apatite. We determined that all the lunar apatite grains analyzed are predominantly fluorine rich; however, they also contain varying concentrations of chlorine and a missing structural component that, after ruling out other possibilities, we attribute to OH. Apatite grains from mare basalts are compositionally distinct from the apatite grains in the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks, which all had similar apatite compositions. Apatite grains in mare basalts are depleted in chlorine, and many of the analyzed grains have stoichiometry that suggests a significant OH component (i.e., >0.08 structural formula units), whereas apatite grains in the magnesian suite, alkali suite, and KREEP-rich impact melts are enriched in chlorine and do not typically have a missing structural component that could be attributed to OH − (within the detection limit of 0.08 sfu). From these data, we infer that residual liquids in the mare basalts were enriched in H 2O and fluorine relative to chlorine at the time of apatite crystallization, whereas residual liquids in magnesian-suite, alkali-suite, and KREEP-rich impact melts were enriched in chlorine relative to H 2O and fluorine at the time of apatite crystallization. The relative volatile abundance that we determined for the mare basalts is identical to the previously determined relative volatile abundance for the lunar picritic glasses. This result indicates that the observed relative volatile abundance signature of the picritic glass source is the same as that in the mare basalt source regions. The magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks likely reflect a volatile source with different volatile abundances than the sources of mare volcanics. Moreover, the magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks may reveal the relative volatile abundance of urKREEP, the residual melt of the magma ocean. This difference in relative magmatic volatile abundance among the lithologic groups investigated cannot be explained by degassing of a single source composition (relative to magmatic volatiles). The most reasonable explanation for the compositional disparity is a difference in the relative volatile abundances in the magmatic source regions of the Moon. Therefore, we conclude that the Moon has a heterogeneous distribution of magmatic volatiles within its interior, with a chemical divide (with respect to magmatic volatiles) existing between magmas that arise by partial melting of the lunar mantle and magmas that have seen significant contamination by a KREEP component.

Nominally hydrous magmatism on the Moon

For the past 40 years, the Moon has been described as nearly devoid of indigenous water; however, evidence for water both on the lunar surface and within the lunar interior have recently emerged, calling into question this long-standing lunar dogma. In the present study, hydroxyl (as well as fluoride and chloride) was analyzed by secondary ion mass spectrometry in apatite [Ca(5)(PO(4))(3)(F,Cl,OH)] from three different lunar samples in order to obtain quantitative constraints on the abundance of water in the lunar interior. This work confirms that hundreds to thousands of ppm water (of the structural form hydroxyl) is present in apatite from the Moon. Moreover, two of the studied samples likely had water preserved from magmatic processes, which would qualify the water as being indigenous to the Moon. The presence of hydroxyl in apatite from a number of different types of lunar rocks indicates that water may be ubiquitous within the lunar interior, potentially as early as the time of lunar formation. The water contents analyzed for the lunar apatite indicate minimum water contents of their lunar source region to range from 64 ppb to 5 ppm H(2)O. This lower limit range of water contents is at least two orders of magnitude greater than the previously reported value for the bulk Moon, and the actual source region water contents could be significantly higher.

Partitioning in REE-saturating minerals: Theory, experiment, and modelling of whitlockite, apatite, and evolution of lunar residual magmas

We present compositions, including REEs determined by ion microprobe, of apatite and whitlockite in lunar rock assemblages rich in incompatible trace elements. Total concentrations of REE oxides in whitlockites range from 9–13 wt%, and those in apatites range from 0.15 to 1 wt%. Ratios of REE concentrations in whitlockite to those in coexisting apatite range from ~ 10 to 60. The distribution of Mg and Fe between apatite and whitlockite is correlated to that of coexisting mafic silicates: Magnesium is strongly preferred by whitlockite, and Fe is preferred by apatite. Incorporation of REEs in whitlockite is dominated by the coupled substitution of 2REE 3+ in Ca(B) sites + vacancy in Ca(IIA) for 2Ca +2 in Ca(B) sites and (Ca 2+,Na +) in Ca(IIA). Other substitutions account for only a small portion of the REEs in whitlockite over the observed concentration range; thus, REE concentrations become partially saturated as the primary substitution approaches its stoichiometric limit of two REEs per fifty-six oxygens, leading to reduced whitlockite/melt distribution coefficients e.g., decreasing from twenty-five to ten for Nd. The REE concentrations of lunar residual melts are not depleted by whitlockite crystallization in assemblages consisting mainly of other minerals in typical proportions. Distribution coefficients for the REEs in lunar apatite appear to be low and variable e.g., ~0.2–0.8 for Nd. Variations in the modal ratio of whitlockite to apatite, specifically the abundance of whitlockite, lead to a range of REE concentrations in the phosphates and variations in the magnitude of REE concentration ratios between whitlockite and apatite. If apatite crystallizes before whitlockite or in the absence of whitlockite, as textures in several samples indicate, then apatite zoned in REEs and apatite crystals of different REE concentrations may occur in a given sample, provided there is some amount of fractional crystallization and apatite does not later equilibrate. This may occur because, in the absence of whitlockite in the crystallizing assemblage, the REEs are highly incompatible relative to the crystalline assemblage, so REE concentrations in lunar residual melts increase strongly during small increments of late-stage crystallization. Once whitlockite begins to crystallize, bulk distribution coefficients for the REEs, although still < 1, are only mildly incompatible, so the change in REE concentrations of residual melts with further crystallization is small, consistent with the lack of REE zoning in whitlockite. The REE concentrations in lunar whitlockites are modelled as resulting mainly from equilibrium crystallization of the assemblages in which they occur; metasomatism or other secondary metamorphic processes are not indicated. Local melt-pocket equilibrium at advanced stages of crystallization may lead to variable REE concentrations and variable whitlockite /apatite concentration ratios within the same sample. Parent melts with extremely high REE concentrations are not required in order to crystallize REE-rich lunar whitlockite if modal proportions of whitlockite are low.