ALBERT

Description: 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 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 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 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 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 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 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 D in the residual magmas that reach the lunar surface or near-surface environment. Apatites within some mare basalts exhibit elevated D of 1000 depending on the initial value ( Tartese and Anand 2013 ) in addition to the 37 Cl values, but elevated 37 Cl values are accompanied by only modest enrichments in D in apatites from samples of the highlands crust ( McCubbin et al. 2015a ).

Description: 〈p〉The sources and nature of organic carbon on Mars have been a subject of intense research. Steele 〈i〉et al.〈/i〉 (2012) showed that 10 martian meteorites contain macromolecular carbon phases contained within pyroxene- and olivine-hosted melt inclusions. Here, we show that martian meteorites Tissint, Nakhla, and NWA 1950 have an inventory of organic carbon species associated with fluid-mineral reactions that are remarkably consistent with those detected by the Mars Science Laboratory (MSL) mission. We advance the hypothesis that interactions among spinel-group minerals, sulfides, and a brine enable the electrochemical reduction of aqueous CO〈sub〉2〈/sub〉 to organic molecules. Although documented here in martian samples, a similar process likely occurs wherever igneous rocks containing spinel-group minerals and/or sulfides encounter brines.〈/p〉

Description: The Mg-suite represents an enigmatic episode of lunar highlands magmatism that presumably represents the first stage of crustal building following primordial differentiation. This review examines the mineralogy, geochemistry, petrology, chronology, and the planetary-scale distribution of this suite of highlands plutonic rocks, presents models for their origin, examines petrogenetic relationships to other highlands rocks, and explores the link between this style of magmatism and early stages of lunar differentiation. Of the models considered for the origin of the parent magmas for the Mg-suite, the data best fit a process in which hot (solidus temperature at ≥2 GPa = 1600 to 1800 °C) and less dense ( ~3100 kg/m 3 ) early lunar magma ocean cumulates rise to the base of the crust during cumulate pile overturn. Some decompressional melting would occur, but placing a hot cumulate horizon adjacent to the plagioclase-rich primordial crust and KREEP-rich lithologies (at temperatures of 〈1300 °C) would result in the hybridization of these divergent primordial lithologies, producing Mg-suite parent magmas. As urKREEP (primeval KREEP) is not the "petrologic driver" of this style of magmatism, outside of the Procellarum KREEP Terrane (PKT), Mg-suite magmas are not required to have a KREEP signature. Evaluation of the chronology of this episode of highlands evolution indicates that Mg-suite magmatism was initiated soon after primordial differentiation (〈10 m.y.). Alternatively, the thermal event associated with the mantle overturn may have disrupted the chronometers utilized to date the primordial crust. Petrogenetic relationships between the Mg-suite and other highlands suites (e.g., alkali-suite and magnesian anorthositic granulites) are consistent with both fractional crystallization processes and melting of distinctly different hybrid sources.

Description: Apatite sensu lato, Ca 10 (PO 4 ) 6 (F,OH,Cl) 2 , is the tenth most abundant mineral on Earth, and is fundamentally important in geological processes, biological processes, medicine, dentistry, agriculture, environmental remediation, and material science. The steric interactions among anions in the [0,0, z ] anion column in apatite make it impossible to predict the column anion arrangements in solid solutions among the three end-members. In this work we report the measured atomic arrangements of synthetic apatite in the F-Cl apatite binary with nominal composition Ca 10 (PO 4 ) 6 (F 1 Cl 1 ), synthesized in vacuum at high temperature to minimize both hydroxyl- and oxy-component of the apatite. Four crystals from the high-temperature synthesis batch were prepared to assess the homogeneity of the batch and the precision of the location of small portions of an atom in the apatite anion column by single-crystal X-ray diffraction techniques. Crystals were ground to spheres of 80 μm diameter, and full-spheres of Mo K α diffraction data were collected to = 33º, with average redundancies 〉16. Final R 1 values ranged from 0.0145 to 0.0158; the lattice parameters ranged from a = 9.5084(2)–9.5104(3), c = 6.8289(3)–6.8311(2) Å. Based on this study, solid solution in P 6 3 / m apatites along the F-Cl join is attained by creation of an off-mirror fluorine site at (0,0,0.167), a position wherein the fluorine atom relaxes away from its normal position within the {00 l } mirror plane in P 6 3 / m apatites; that relaxation is coupled with relaxation of a chlorine atom at the adjacent mirror plane away from the off-mirror fluorine, allowing acceptable F-Cl distances in the anion column. There are a total of four partially occupied anion positions in the anion column, including two for fluorine [(0,0,1/4) and (0,0,0.167)] and two for chlorine [(0,0,0.086) and (0,0,0)]; the chlorine site at the origin was previously postulated but not observed in calcium apatite solid solutions.

Description: Recent re-analyses of lunar samples have undoubtedly measured indigenous water, challenging the paradigm of a "dry" Moon, and arguing that some portions of the lunar interior are as wet as some regions of the Earth’s mantle and that water in both planetary bodies likely share a common origin. Mare basalts indirectly sample the lunar mantle and are affected by petrogenetic processes such as crystallization and degassing that can modify characteristics of indigenous water in primary mantle melts. Analyses of apatite in phosphorus-rich KREEP (K + REE [rare earth elements] + P) basalts may provide more reliable estimates for the water content of lunar magmas, as some apatites likely crystallized before substantial degassing occurred. In lunar KREEP basalt sample 15386, apatite H 2 O content and H isotopic composition suggest that degassing occurred during apatite crystallization, the lowest D value of 90 ± 100 representing an upper limit for the isotopic composition of water in the parental magma. Interpretation of the data for KREEP basalt 15386 suggests that this basalt is characterized by relatively elevated H 2 O contents and CI chondrite–type D values, similar to those proposed for other mare basalts and pyroclastic glasses. On the other hand, most of the apatites in lunar KREEP basalt 72275 and lunar meteorite NWA 773 crystallized before degassing and H isotope fractionation, and their D/H ratios thus directly reflect those of their source regions. These apatites have an average D value of –130 ± 50, suggesting the presence of a water reservoir in the Moon characterized by moderate H 2 O contents and H isotopic composition similar to that of Earth’s interior. These findings imply that significant amounts of water in the Moon were inherited from the proto-Earth, surviving the purported Moon-forming impact event.

Description: Fluorapatite has been synthesized through ion-exchange between NIST hydroxylapatite SRM 2910a and optical-grade fluorite. The latter end-member and additional intermediate F-OH apatite samples made through ion-exchange between the newly synthesized fluorapatite and the original hydroxylapatite have provided materials for an investigation of thermal expansion in the F-OH apatite system. Unit-cell volumes determined from X-ray powder diffraction data collected between 22 and 928 °C have been utilized to compute the coefficients of thermal expansion at a pressure of 1 bar for seven apatite samples having various F:OH ratios, as well as for natural samples of fluorapatite and hydroxylapatite. Results show that the thermal expansion coefficient for volume averages 41.0 ± 1.4 (1) x 10 –6 deg –1 for all samples and is thus little affected by F:OH ratio. This study extends the thermodynamic characterization of this important mineral system, with potential applications for the geological, planetary, biological, medical, and materials-science communities.

Description: A collection of 35 diamondite samples (polycrystalline diamond aggregates, sometimes referred to as framesites), assumed to be from southern Africa, have been studied to investigate their infrared (IR) spectroscopic characteristics. Due to the abundance of sub-micrometer, interlocking diamonds (polycrystalline) with mineral and fluid inclusions within the diamond material affecting their transparency, only fragments from 10 of the samples provided high-quality data. The IR spectra showed a wide range of generally high-nitrogen concentrations (386–2677 ppm), with a full range of nitrogen aggregation states, from pure IaA to pure IaB. Platelet characteristics were interpreted as being regular (i.e., not having been affected by deformation and/or heating events), meaning the nitrogen aggregation data could be interpreted with confidence. Surprisingly, the platelet data showed a positive correlation between their intensity (integrated area) and peak position. The primary hydrogen band (at 3107 cm –1 ) and secondary band (at 1405 cm –1 ) are both often present in the samples’ spectra, but show no correlation with any other characteristic. There is also no correlation between the samples’ paragenesis (as defined by their garnet chemistry) and any of the IR characteristics. While we have no independent determination of the samples mantle residence age, nor the temperature they resided at, we infer that diamondite formation has occurred episodically over a large time frame in single and distinct growth events (as opposed to over a short time frame but over a large depth/temperature range). This idea is more in keeping with the theory that C-O-H diamond- (and diamondite-) forming fluids are the result of localized small volume processes. Interestingly, one sample contained fluid inclusions that exhibited a water:carbonate molar ratio (~0.8), similar to the saline and silicic end-members of the monocrystalline diamond-forming fluid chemical spectrum.

Description: The sources and nature of organic carbon on Mars have been a subject of intense research. Steele et al. (2012) showed that 10 martian meteorites contain macromolecular carbon phases contained within pyroxene- and olivine-hosted melt inclusions. Here, we show that martian meteorites Tissint, Nakhla, and NWA 1950 have an inventory of organic carbon species associated with fluid-mineral reactions that are remarkably consistent with those detected by the Mars Science Laboratory (MSL) mission. We advance the hypothesis that interactions among spinel-group minerals, sulfides, and a brine enable the electrochemical reduction of aqueous CO 2 to organic molecules. Although documented here in martian samples, a similar process likely occurs wherever igneous rocks containing spinel-group minerals and/or sulfides encounter brines.

Description: Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4–6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3–6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H 2 O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.