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: Tridymite, a low-pressure, high-temperature (〉870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.%...

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: Among the lunar samples that were returned by the Apollo missions are many cumulate plutonic rocks with high Mg# [molar Mg/(Mg+Fe) in %] and abundances of KREEP elements (potassium, rare earth elements, phosphorus, U, Th, etc.) that imply KREEP-rich parental magmas. These rocks, collectively called the magnesian suite, are nearly absent from sampling sites distant from Imbrium basin ejecta, including those of lunar highlands meteorites. This absence has significant implications for the early differentiation of the Moon and its distribution of heat-producing elements (K, Th, U). Here, we analyze a unique fragment of basalt with the mineralogy and mineral chemistry of a magnesian suite rock, in the lunar highlands meteorite Allan Hills (ALH) A81005. In thin section, the fragment is 700 x 300 μm, and has a sub-ophitic texture with olivine phenocrysts, euhedral plagioclase grains (An 97-70 ),and interstitial pyroxenes. Its minerals are chemically equilibrated. Olivine has Fe/Mn ~ 70 (consistent with a lunar origin), and Mg# ~80, which is consistent with rocks of the magnesian suite and far higher than in mare basalts. It has a rich suite of minor minerals: fluorapatite, ilmenite, Zr-armalcolite, chromite, troilite, silica, and Fe metal (Ni = 3.8%, Co = 0.17%). The metal is comparable to that in chondrite meteorites, which suggests that the fragment is from an impact melt. The fragment itself is not a piece of magnesian suite rock (which are plutonic), but its mineralogy and mineral chemistry suggest that its protolith (which was melted by impact) was related to the magnesian suite. However, the fragment’s mineral chemistry and minor minerals are not identical to those of known magnesian suite rocks, suggesting that the suite may be more varied than apparent in the Apollo samples. Although ALHA81005 is from the lunar highlands (and likely from the farside), Clast U need not have formed in the highlands. It could have formed in an impact melt pool on the nearside and been transported by meteoroid impact. Lunar highlands meteorites should be searched for rock fragments related to the magnesian-suite rocks, but the fragments are rare and may have mineral compositions similar to some meteoritic (impactor) materials.

Description: The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because 37 Cl/ 35 Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, 37 Cl/ 35 Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high 37 Cl/ 35 Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon’s history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

Description: The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H 2 O inside the CheMin instrument (relative humidity 〈1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences. The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe 2+ in olivine to Fe 3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms.

Description: Clay minerals provide indicators of the evolution of aqueous conditions and possible habitats for life on ancient Mars. Analyses by the Mars Science Laboratory rover Curiosity show that ~3.5–billion year (Ga) fluvio-lacustrine mudstones in Gale crater contain up to ~28 weight % (wt %) clay minerals. We demonstrate that the species of clay minerals deduced from x-ray diffraction and evolved gas analysis show a strong paleoenvironmental dependency. While perennial lake mudstones are characterized by Fe-saponite, we find that stratigraphic intervals associated with episodic lake drying contain Al-rich, Fe 3+ -bearing dioctahedral smectite, with minor (3 wt %) quantities of ferripyrophyllite, interpreted as wind-blown detritus, found in candidate aeolian deposits. Our results suggest that dioctahedral smectite formed via near-surface chemical weathering driven by fluctuations in lake level and atmospheric infiltration, a process leading to the redistribution of nutrients and potentially influencing the cycling of gases that help regulate climate.

Description: Curiosity investigated active eolian sands near linear dunes during Phase 2 of the Bagnold Dunes campaign in Gale crater, Mars. Ogunquit Beach, a sample scooped from a large-ripple trough within the Mount Desert Island ripple field and delivered to the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument, is dominated by basaltic igneous minerals and X-ray amorphous materials. CheMin mineralogy of the Gobabeb sample acquired at a large-ripple crest on the Namib barchan dune during Phase 1 is similar to Ogunquit Beach. Ogunquit Beach, however, contains more plagioclase and Gobabeb contains more olivine. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)-based estimates of mineralogy at the optical surface of Namib Dune and Mount Desert Island demonstrate that surface sands are enriched in olivine and depleted in plagioclase over Mount Desert Island relative to Namib Dune. Differences between CheMin-derived and CRISM-derived mineralogies suggest sorting by grain size on bedform to dune field scales. Crystal chemistry from CheMin suggests contributions from multiple igneous sources and the local bedrock. ©2018. The Authors.

Description: Hydrous phases have been identified to be a significant component of Martian mineralogy. Particularly, prehnite, zeolites, and serpentine are evidence for low-grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the Martian crust is sufficiently hydrated for low-grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible Martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability, and rock properties in the Martian crust. In this study, we use phase equilibria models to explore low-grade metamorphic reactions covering a pressure-temperature range of 0–0.5 GPa and 150–450 °C for several Martian protolith compositions and varying fluid content. Our models replicate the detected low-grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower-grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole-rock composition, we can estimate the amount of water required to precipitate low-grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, suggesting either a geotherm of ~20 °C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity.