ALBERT

Description: Many studies exist on magmatic volatiles (H, C, N, F, S, Cl) in and on the Moon, within the last several years, that have cast into question the post-Apollo view of lunar formation, the distribution and sources of volatiles in the Earth-Moon system, and the thermal and magmatic evolution of the Moon. However, these recent observations are not the first data on lunar volatiles. When Apollo samples were first returned, substantial efforts were made to understand volatile elements, and a wealth of data regarding volatile elements exists in this older literature. In this review paper, we approach volatiles in and on the Moon using new and old data derived from lunar samples and remote sensing. From combining these data sets, we identified many points of convergence, although numerous questions remain unanswered. The abundances of volatiles in the bulk silicate Moon (BSM), lunar mantle, and urKREEP [last ~1% of the lunar magma ocean (LMO)] were estimated and placed within the context of the LMO model. The lunar mantle is likely heterogeneous with respect to volatiles, and the relative abundances of F, Cl, and H 2 O in the lunar mantle (H 2 O 〉 F 〉〉 Cl) do not directly reflect those of BSM or urKREEP (Cl 〉 H 2 O F). In fact, the abundances of volatiles in the cumulate lunar mantle were likely controlled by partitioning of volatiles between LMO liquid and nominally anhydrous minerals instead of residual liquid trapped in the cumulate pile. An internally consistent model for lunar volatiles in BSM should reproduce the absolute and relative abundances of volatiles in urKREEP, the anorthositic primary crust, and the lunar mantle within the context of processes that occurred during the thermal and magmatic evolution of the Moon. Using this mass-balance constraint, we conducted LMO crystallization calculations with a specific focus on the distributions and abundances of F, Cl, and H 2 O to determine whether or not estimates of F, Cl, and H 2 O in urKREEP are consistent with those of the lunar mantle, estimated independently from the analysis of volatiles in mare volcanic materials. Our estimate of volatiles in the bulk lunar mantle are 0.54–4.5 ppm F, 0.15–5.3 ppm H 2 O, 0.26–2.9 ppm Cl, 0.014–0.57 ppm C, and 78.9 ppm S. Our estimates of H 2 O are depleted compared to independent estimates of H 2 O in the lunar mantle, which are largely biased toward the "wettest" samples. Although the lunar mantle is depleted in volatiles relative to Earth, unlike the Earth, the mantle is not the primary host for volatiles. The primary host of the Moon’s incompatible lithophile volatiles (F, Cl, H 2 O) is urKREEP, which we estimate to have 660 ppm F, 300–1250 ppm H 2 O, and 1100–1350 ppm Cl. This urKREEP composition implies a BSM with 7.1 ppm F, 3–13 ppm H 2 O, and 11–14 ppm Cl. An upper bound on the abundances of F, Cl, and H 2 O in urKREEP and the BSM, based on F abundances in CI carbonaceous chondrites, are reported to be 5500 ppm F, 0.26–1.09 wt% H 2 O, and 0.98–1.2 wt% Cl and 60 ppm F, 27–114 ppm H 2 O, and 100–123 ppm Cl, respectively. The role of volatiles in many lunar geologic processes was also determined and discussed. Specifically, analyses of volatiles from lunar glass beads as well as the phase assemblages present in coatings on those beads were used to infer that H 2 is likely the primary vapor component responsible for propelling the fire-fountain eruptions that produced the pyroclastic glass beads (as opposed to CO). The textural occurrences of some volatile-bearing minerals are used to identify hydrothermal alteration, which is manifested by sulfide veining and sulfide-replacement textures in silicates. Metasomatic alteration in lunar systems differs substantially from terrestrial alteration due to differences in oxygen fugacity between the two bodies that result in H 2 O as the primary solvent for alteration fluids on Earth and H 2 as the primary solvent for alteration fluids on the Moon (and other reduced planetary bodies). Additionally, volatile abundances in volatile-bearing materials are combined with isotopic data to determine possible secondary processes that have affected the primary magmatic volatile signatures of lunar rocks including degassing, assimilation, and terrestrial contamination; however, these processes prove difficult to untangle within individual data sets. Data from remote sensing and lunar soils are combined to understand the distribution, origin, and abundances of volatiles on the lunar surface, which can be explained largely by solar wind implantation and spallogenic processes, although some of the volatiles in the soils may also be either indigenous to the Moon or terrestrial contamination. We have also provided a complete inventory of volatile-bearing mineral phases indigenous to lunar samples and discuss some of the "unconfirmed" volatile-bearing minerals that have been reported. Finally, a compilation of unanswered questions and future avenues of research on the topic of lunar volatiles are presented, along with a critical analysis of approaches for answering these questions.

Description: Apatite-melt partitioning experiments were conducted in a piston-cylinder press at 1.0–1.2 GPa and 950–1000 °C using an Fe-rich basaltic starting composition and an oxygen fugacity within the range of IW-1 to IW+2. Each experiment had a unique F:Cl:OH ratio to assess the partitioning as a function of the volatile content of apatite and melt. The quenched melt and apatite were analyzed by electron probe microanalysis and secondary ion mass spectrometry techniques. The mineral-melt partition coefficients ( D values) determined in this study are as follows: D F Ap-Melt = 4.4–19, D Cl Ap-Melt = 1.1–5, D OH Ap-Melt = 0.07–0.24. This large range in values indicates that a linear relationship does not exist between the concentrations of F, Cl, or OH in apatite and F, Cl, or OH in melt, respectively. This non-Nernstian behavior is a direct consequence of F, Cl, and OH being essential structural constituents in apatite and minor to trace components in the melt. Therefore mineral-melt D values for F, Cl, and OH in apatite should not be used to directly determine the volatile abundances of coexisting silicate melts. However, the apatite-melt D values for F, Cl, and OH are necessarily interdependent given that F, Cl, and OH all mix on the same crystallographic site in apatite. Consequently, we examined the ratio of D values (exchange coefficients) for each volatile pair (OH-F, Cl-F, and OH-Cl) and observed that they display much less variability: K d Cl-F Ap-Melt = 0.21 ± 0.03, K d OH-F Ap-Melt = 0.014 ± 0.002, and K d OH-Cl Ap-Melt = 0.06 ± 0.02. However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low ( X F 〈 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H 2 O content of a silicate melt at the time of apatite crystallization (apatite-based melt hygrometry), the H 2 O abundance of the apatite, an apatite-melt exchange K d that includes OH (either OH-F or OH-Cl), and the abundance of F or Cl in the apatite and F or Cl in the melt at the time of apatite crystallization are needed (F if using the OH-F K d and Cl if using the OH-Cl K d ). To determine the H 2 O content of the parental melt, the F or Cl abundance of the parental melt is needed in place of the F or Cl abundance of the melt at the time of apatite crystallization. Importantly, however, exchange coefficients may vary as a function of temperature, pressure, melt composition, apatite composition, and/or oxygen fugacity, so the combined effects of these parameters must be investigated further before exchange coefficients are applied broadly to determine volatile abundances of coexisting melt from apatite volatile abundances.

Description: Apatite grains in lunar mare basalts contain hydrogen that ranges in D/H ratio by more than a factor of two. For most of these basalts, the D/H ratios in their apatite grains decrease with measures of the host basalts’ time spent at elevated temperature, specifically the Fe-Mg homogenization of their pyroxenes. Most basalts with homogeneous pyroxenes (i.e., with constant Fe/Mg ratio) have apatite grains with low D/H (D –100), whereas most basalts with heterogeneous pyroxenes (i.e., varying or zoned Fe/Mg) have apatite with high D/H (D up to ~ +1100). This relationship suggests that low D/H values were acquired during thermal processing, i.e., during Fe-Mg chemical equilibration, during or after emplacement. This light hydrogen is likely derived from solar wind implanted into the lunar regolith (with D from –125 to –800), and could enter basalts either by assimilation of regolith or by vapor transport from regolith heated by the flow. If a basalt could not interact with regolith rich in solar wind (e.g., it was emplaced onto other fresh basalts), its apatite could retain a magmatic D/H signature. The high D/H component (in the apatites of unequilibrated basalts) is most reasonably that indigenous magmatic hydrogen, i.e., representing hydrogen in the basalt’s source mantles, or magmatic hydrogen that was residual after partial degassing of H 2 .

Description: In the last decade, it has been recognized that the Moon contains significant proportions of volatile elements (H, F, Cl), and that they are transported through the lunar crust and across its surface. Here, we document a significant segment of that volatile cycle in lunar granulite breccia 79215: impact-induced remobilization of volatiles, and vapor-phase transport with extreme elemental fractionation. 79215 contains ~1% volume of fluorapatite, Ca 5 (PO 4 ) 3 (F,Cl,OH), in crystals to 1 mm long, which is reflected in its analyzed abundances of F, Cl, and P. The apatite has a molar F/Cl ratio of ~10, and contains only 25 ppm OH and low abundances of the rare earth elements (REE). The chlorine in the apatite is isotopically heavy, at 37 Cl = +32.7 ± 1.6. Hydrogen in the apatite is heavy at D = +1060 ± 180; much of that D came from spallogenic nuclear reactions, and the original D was lower, between +350 and +700. Unlike other P-rich lunar rocks (e.g., 65015), 79215 lacks abundant K and REE, and other igneous incompatible elements characteristic of the lunar KREEP component. Here, we show that the P and halogens in 79215 were added to an otherwise "normal" granulite by vapor-phase metasomatism, similar to rock alteration by fumarolic exhalations as observed on Earth. The ultimate source of the P and halogens was most likely KREEP, it being the richest reservoir of P on the Moon, and 79215 having H and Cl isotopic compositions consistent with KREEP. A KREEP-rich rock was heated and devolatilized by an impact event. This vapor was fractionated by interaction with solid phases, including merrillite (a volatile-free phosphate mineral), a Fe-Ti oxide, and a Zr-bearing phase. These solids removed REE, Th, Zr, Hf, etc., from the vapor, and allowed the vapor to transport primarily P, F, and Cl, with lesser proportions of Ba and U into 79215. Vapor-deposited crystals of apatite (to 30 μm) are known in some lunar regolith samples, but lunar vapor has not (before this) been implicated in significant mass transfer. It seems unlikely, however, that phosphate-halogen metasomatism is related to the high-Th/Sm abundance ratios of this and other lunar magnesian granulites. The metasomatism of 79215 emphasizes the importance of impact heating in the lunar volatile cycle, both in mobilizing volatile components into vapor and in generating strong elemental fractionations.

Description: 〈span〉〈div〉Abstract〈/div〉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 (〈span〉f〈/span〉〈sub〉O〈sub〉2〈/sub〉〈/sub〉) conditions where sulfur can only exist in its reduced form (S〈sup〉2–〈/sup〉), 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〈sup〉2–〈/sup〉, whereas natural apatites from Earth are only known to contain S〈sup〉6+〈/sup〉. It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S〈sup〉6+〈/sup〉/S〈sup〉2–〈/sup〉 ratios of such apatites could be used to quantify the 〈span〉f〈/span〉〈sub〉O〈sub〉2〈/sub〉〈/sub〉 values at which they crystallized, given information on the portioning of S〈sup〉6+〈/sup〉 and S〈sup〉2–〈/sup〉 between apatite and melt and on the S〈sup〉6+〈/sup〉/S〈sup〉2–〈/sup〉 ratios of melts as functions of 〈span〉f〈/span〉〈sub〉O〈sub〉2〈/sub〉〈/sub〉 and melt composition. Such a well-calibrated oxybarometer based on this the oxidation state of S in apatite would have wide application.〈/span〉