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We conducted a petrologic study of apatite within eight unbrecciated, non-cumulate eucrites and two monomict, non-cumulate eucrites. These data were combined with previously published data to quantify the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta (BSV). Using a combination of apatite-based melt hygrometry/chlorometry and appropriately paired volatile/refractory element ratios, we determined that BSV has 3.0–7.2 ppm F, 0.39–1.8 ppm Cl, and 3.6–22 ppm H2O. The abundances of F and H2O are depleted in BSV relative to CI chondrites to a similar degree as F and H2O in the bulk silicate portion of the Moon. This degree of volatile depletion in BSV is similar to what has been determined previously for many moderately volatile elements in 4 Vesta (e.g., Na, K, Zn, Rb, Cs, and Pb). In contrast, Cl is depleted in 4 Vesta by a greater degree than what is recorded in samples from Earth or the Moon. Based on the Cl-isotopic compositions of eucrites and the bulk rock Cl/F ratios determined in this study, the eucrites likely formed through serial magmatism of a mantle with heterogeneous δ37Cl and Cl/F, not as extracts from a partially crystallized global magma ocean. Furthermore, the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites is likely inherited, at least in part, from the precursor materials that accreted to form 4 Vesta and is unlikely to have resulted solely from degassing of a global magma ocean, magmatic degassing of eucrite melts, and/or volatile loss during thermal metamorphism. Although our results can be reconciled with the past presence of wide-scale melting on 4 Vesta (i.e., a partial magma ocean), any future models for eucrite petrogenesis involving a global magma ocean would need to account for the preservation of a heterogeneous eucrite source with respect to Cl/F ratios and Cl isotopes.

The isotopes of chlorine (37Cl and 35Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ37Cl (‰) = [(37Cl/35Clsample/37Cl/35ClSMOC)-1]×1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14 ‰ (Apollo 15), +10 to +19 ‰ (Apollo 12), +9 to +15 ‰ (70017), +4 to +8 ‰ (MIL 05035), and +15 to +22 ‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al., 2015, Barnes et al., 2016, as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.

The relative abundances of chlorine isotopes measured in low-Ti basalts from the Moon appear to reflect mixing between two reservoirs: One component representing the urKREEP—the final product of the crystallization of the lunar magma ocean—with δ 37 Cl = + 25 ‰ (relative to Standard Mean Ocean Chlorine), the other representing either a mare basalt reservoir or meteoritic materials with δ 37 Cl ∼ 0 ‰ . Using the abundances of other KREEP-enriched elements as proxies for the abundance of Cl in low-Ti mare basalts—which is difficult to constrain due to magmatic processes such as fractional crystallization and degassing—we find that the urKREEP contains ∼28 times higher Cl abundance (25–170 ppm Cl) as compared to the low- δ 37 Cl end member in the observed mixing relationship. Chlorine—with an urKREEP/C.I. ratio of 0.2 to 1.5—is 500 to 3400 times less enriched than refractory incompatibles such as U and Th, and is consistent with incomplete loss of Cl species taking place during or prior to the magma ocean phase. The preservation of multiple, isotopically distinct reservoirs of Cl can be explained by: 1) Incomplete degassing pre- or syn-giant impact, with preservation of undegassed chondritic Cl and subsequent formation of an enriched and isotopically fractionated reservoir; or 2) Development of both high-concentration, high- δ 37 Cl and low-concentration, low- δ 37 Cl reservoirs during the degassing and crystallization of the lunar magma ocean. A range of model bulk lunar Cl abundances from 0.3–0.6 ppm allows us to place Cl in the context of the rest of the elements of the periodic table, and suggests that Cl behaves as only a moderately volatile element during degassing. Chlorine isotope fractionation resulting from loss syn- or pre-magma ocean is characterized by 1000 • ln ⁡ [ α ] = − 3.96 to −4.04. Abundance and isotopic constraints are consistent with the loss of Cl being limited by vaporization of mixtures of Cl salts such as HCl, ZnCl2, FeCl2, and NaCl. These new constraints on the chlorine abundance and isotopic values of urKREEP make it a well-constrained target for dynamic models aiming to test plausible conditions for the formation of the Earth–Moon system.

Orbital data from the MESSENGER mission to Mercury have facilitated a new view of the planet's structure, chemical makeup, and diverse surface, and have confirmed Mercury's status as a geochemical endmember among the terrestrial planets. In this work, the most recent results from MESSENGER's X-Ray Spectrometer, Gamma-Ray Spectrometer, and Neutron Spectrometer have been used to identify nine distinct geochemical regions on Mercury. Using a variation on the classical CIPW normative mineralogy calculation, elemental composition data is used to constrain the potential mineralogy of Mercury's surface; the calculated silicate mineralogy is dominated by plagioclase, pyroxene (both orthopyroxene and clinopyroxene), and olivine, with lesser amounts of quartz. The range in surface compositions indicate that the rocks on the surface of Mercury are diverse and vary from komatiitic to boninitic. The high abundance of alkalis on Mercury's surface results in several of the nine regions being classified as alkali-rich komatiites and/or boninites. In addition, Mercury's surface terranes span a wide range of SiO 2 values that encompass crustal compositions that are more silica-rich than geochemical terranes on the Moon, Mars, and Vesta, but the range is similar to that of Earth. Although the composition of Mercury's surface appears to be chemically evolved, the high SiO 2 content is a primitive feature and a direct result of the planet's low oxygen fugacity.

Phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of Mercury as measured by the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). The northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6% of the surface area of Mercury. The northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism, making them a prime candidate for experimental study. The bulk composition of the northern volcanic plains is that of an alkali-rich boninite and represents the first silica-enriched crustal terrane identified on an extraterrestrial planet from orbital data. Phase equilibrium experiments were conducted over the pressure range of the mercurian mantle (0.5–5 GPa) at very low oxygen fugacity (∼ΔIW0 to −7) using a piston-cylinder apparatus (P 0.5–1.7 GPa) and a Walker-style multi-anvil device (P ⩾ 2.5 GPa). Our results indicate the origin of the northern volcanic plains lavas (boninites) are best explained by high degrees of partial melting of an olivine-dominant, pyroxene- and plagioclase-bearing mantle source at low pressure (⩽1.4 GPa) and does not require hydrous melting to achieve the silica-enriched melt composition. The formation mechanism for boninites on Mercury contrasts substantially with terrestrial boninites, which typically occur in oxidized and hydrous arc environments associated with subduction zones. Instead, mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials. The NVP lavas represent a novel mechanism by which planetary bodies can form silica-enriched secondary crusts without the aid of water.

Orbital measurements obtained by the MESSENGER Gamma-Ray Spectrometer have been analyzed to determine the surface abundance of chlorine in Mercury’s northern hemisphere. The derived Cl/Si mass ratio is 0.0057 ± 0.001, which for an assumed Si abundance of 24.6 wt% corresponds to 0.14 ± 0.03 wt% Cl. The abundance of Cl is a factor of 2.9 ± 1.3 higher in the north polar region (>80°N) than at latitudes 0−60°N, a latitudinal variation similar to that observed for Na. Our reported Cl abundances are consistent with measured bulk concentrations of neutron-absorbing elements on Mercury, particularly those observed at high northern latitudes. The Cl/K ratio on Mercury is chondritic, indicating a limited impact history akin to that of Mars, which accreted rapidly. Hypotheses for the origin of Mercury’s high metal-to-silicate ratio must be able to reproduce Mercury’s observed elemental abundances, including Cl. Chlorine is also an important magmatic volatile, and its elevated abundance in the northern polar region of Mercury indicates that it could have played a role in the production, ascent, and eruption of flood volcanic material in this region. We have identified several candidate primary mineralogical hosts for Cl on Mercury, including the halide minerals lawrencite (FeCl 2 ), sylvite (KCl), and halite (NaCl), as well as Cl-bearing alkali sulfides. Amphiboles, micas, apatite, and aqueously deposited halides, in contrast, may be ruled out as mineralogical hosts of Cl on Mercury.

The martian meteorite Northwest Africa (NWA) 7034 was examined both petrographically and geochemically using several micro-beam techniques including electron probe microanalysis and secondary ion mass spectrometry. We have identified various clast types of igneous, sedimentary, and impact origin that occur within the breccia, and we define a classification scheme for these materials based on our observations, although our primary focus here is on the petrology of the igneous clasts. A number of different igneous clasts are present in this meteorite, and our study revealed the presence of at least four different igneous lithologies (basalt, basaltic andesite, trachyandesite, and an Fe, Ti, and P (FTP) rich lithology). These lithologies do not appear to be related by simple igneous processes such as fractional crystallization, indicating NWA 7034 is a polymict breccia that contains samples from several different igneous sources. The basalt lithologies are a good match for measured rock compositions from the martian surface, however more exotic lithologies (e.g., trachyandesite and FTP lithologies) show this meteorite contains previously unsampled rock types from Mars. These new rock types provide evidence for a much greater variety of igneous rocks within the martian crust than previously revealed by martian meteorites, and supports recent rover observations of lithologic diversity across the martian surface. Furthermore, the ancient ages for the lithologic components in NWA 7034 indicate Mars developed this lithologic diversity in the early stages of crust formation.

The density and compressibility of four synthetic molten lunar picritic glasses was investigated from 0 to 10 GPa and 1748 to 2473 K. The picritic glasses were collected from the lunar surface during the Apollo missions, and they are hypothesized to have rapidly quenched as glass beads during pyroclastic fire fountain eruptions. The specific melt compositions investigated in the present study are the Apollo 15 green glass Type C (A15C, TiO2 = 0.26 wt%), the Apollo 14 yellow glass (A14Y, TiO2 = 4.58 wt%), the Apollo 17 orange glass 74220-type (A17O TiO2 = 9.12 wt%), and the Apollo 14 black glass (A14B, TiO2 = 16.40 wt%). These glasses are reported to represent primary unfractionated melts, making them a prime candidate for experimental studies into lunar basalt density and compressibility during partial melting of the lunar mantle. Sink–float experiments were conducted on the synthetic molten lunar glass compositions using a piston-cylinder apparatus (P 2.5 GPa) in order to bracket the density of the melts. New sink–float data are reported for A15C, A14Y, and A17O, which are combined with previously published density and compressibility data on A15C, A17O, and A14B. Although the Ti-rich liquids are highly compressible at lower pressures, they become nearly incompressible at much higher pressures when compared to the molten low-Ti glasses. Consequently, the melts with the most TiO2 (A14B) are the least dense at higher pressures, a reversal of what is seen at lower pressures. This change in density and compressibility is attributed to changes in coordination of Ti and Fe in the silicate melt structure. As Ti4+ abundances in the silicate melt increase, predominantly [IV]Ti4+ and [IV]Fe2+ change to [VI]Ti4+ and [VI]Fe2+ in the melt structure. All of the data from the present study were used to calculate a Birch–Murnaghan equation-of-state (BM-EOS) for each melt composition. The BM-EOS model for each composition was then combined with previously published estimates for the residual mantle source mineralogy and depth of origin for each of the glasses to assess the density of the partial melt with respect to its point of origin. This information was used to determine whether or not the melt would rise or sink with respect to its source region. We determined that all melt compositions, with the exception of the A17O melt, should have been able to rise to the crust-mantle boundary as a result of buoyancy forces alone, although different mechanisms are likely required for magma ascent through the lunar crust. For the rise of A17O, other modes of ascent through the lunar mantle are required to extract this melt composition from the mantle, and volatiles are not a plausible solution on the grounds of melt density alone.

An interesting characteristic of the pyroclastic glass bead deposits, select impact produced lithologies such as the “rusty rock” 66095, and unique lunar soils from the Apollo 16 landing site, is their unusual enrichments in 204 Pb, Cd, Bi, Br, I, Ge, Sb, Tl, Zn, and Cl which indicates that portions of these sample contain a substantial volatile component. Sample 66095, a fine-grained, subophitic to ophitic polymict melt breccia, also hosts a pervasive low-temperature, volatile-rich, oxyhydrated mineral assemblage. The volatile element enrichments in these assorted lunar lithologies have been attributed to a variety of extra-lunar and lunar processes, whereas the oxyhydration in 66095 has long been thought to represent either terrestrial alteration of lunar chlorides and Fe–Ni metal to βFeO(OH,Cl) or indigenous lunar processes. In 66095, Cl is accommodated in FeO(OH,Cl), phosphates, and chlorides and is heterogeneously distributed. The low-temperature alteration occurs as rims around Fe–Ni metal and sulfide grains, and as dispersed grains in the adjacent matrix. Micro-Raman and transmission electron microscope (TEM) imaging indicate that akaganeite (βFeO(OH,Cl)) is the dominant FeO(OH) polymorph and is intergrown with goethite (αFeO(OH)) and hematite (αFe 2 O 3 ). TEM observations indicate a well-defined “nanometer-scale” stratigraphy” to the alteration. For example, kamacite (body centered cubic) → face-centered cubic (fcc) Fe–Ni alloy → lawrencite (FeCl 2 ) → akaganeite. The lunar lawrencite (Fe,Ni)Cl 2 in 66095 does not react directly to akaganeite on Earth. Rather, lawrencite exposed to terrestrial conditions reacts to form an amorphous Fe- and Cl-bearing phase, nano-crystalline goethite, and hematite. The morphology of these terrestrial alteration products is significantly different than that of the akaganeite occurring in 66095. The chlorine isotopic compositions of these volatile-rich samples are enriched in heavy Cl. For 66095, the δ 37 Cl ranges from +14.0‰ to +15.6‰, whereas the δ 37 Cl for the volatile-rich A16 soils ranges from +5.6‰ to +15.7‰. Based on these data it appears likely that the volatile element enrichments and the Cl isotopic fractionation observed in 66095 and the Apollo 16 soils did not result from extra-lunar additions, but are most likely indigenous to the Moon. Lawrencite was deposited on mineral surfaces at approximately 650 °C to 570 °C from a metal-chloride-bearing, H-poor gas phase. This gas phase was also responsible for the transport of other metals (e.g. Zn, Cu, Pb, Fe). The fractionation of Cl isotopes in the rusty rock can be attributed to fumarole processes in a low-H system. The origin and formation of the akaganeite is more enigmatic. The Cl-isotopes are consistent with it replacing lawrencite. However, numerous nanometer-scale observations are not consistent with a terrestrial origin and indicate multiple episodes of oxyhydration.

The presence of H2O within differentiated terrestrial bodies in the inner Solar System is well established; however, the source(s) of this H2O and the time of its arrival to the inner Solar System is an area of active study. At present, the prevailing model for the origin of inner Solar System H2O calls upon carbonaceous chondrites as the source. This is largely based on reported observations that H- and N-isotopic compositions of differentiated planetary bodies are largely the same and within a range of values that overlaps with carbonaceous chondrites as opposed to comets or the Sun. In this contribution, we evaluate the efficacy of this model and other models for the origin of inner Solar System H2O by considering geochronological constraints on early Solar System history, constraints on primary building blocks of differentiated bodies based on nucleosynthetic isotope anomalies, and constraints from dynamical models of planet formation. In addition to H- and N-isotopic data, these constraints indicate that an interstellar source of H2O was present in the inner Solar System within the first 4 Ma of CAI formation. Furthermore, the most H2O-rich carbonaceous chondrites are unlikely to be the source of H2O for the earliest-formed differentiated bodies based on their minimally overlapping primary accretion windows and the separation of their respective isotopic reservoirs by Jupiter in the timespan of about 1–4 Ma after CAI formation. The presence of deuterium-rich, non-nebular H2O sources in the inner Solar System prior to the formation of carbonaceous chondrites or comets implies early contributions of interstellar ices to both the inner and outer Solar System portions of the protoplanetary disk. Evidence for this interstellar ice component in the inner Solar System may be preserved in LL chondrites and in the mantle of Mars. In contrast to the earlier-formed bodies within the inner Solar System, Earth's protracted accretion window may have facilitated incorporation of H2O in its interior from both the inner and outer Solar System, helping the Earth to become a habitable planet.

Article history: It is now generally accepted that the Moon formed by collision of a Mars-sized impactor with the Earth (Hartmann and Davis, 1975), a process termed "the Giant Impact". The oxygen isotope compositions of both bodies are indistinguishable (Wiechert et al., 2001), a result that would require extensive chemical homogenization. Recent simulations have been developed that predict intimate mixing immediately following the impact (Canup, 2012; ´ Cuk and Stewart, 2012; Pahlevan and Stevenson, 2007), minimizing the oxygen isotope problem. Nevertheless, striking chemical differences remain. We propose that the process of hydrogen degassing and loss to space during magma cooling can explain differences in the water content, oxygen fugacity ( f (O2)), and isotopic composition of Cl and H in the Earth-Moon system. At low f (O2), H2 gas is the stable O-H phase in basalts. The low solubility and rapid diffusivity of H2 explain the presently dry character of most lunar samples. Many of the apparently discrepant observations regarding the hydrous character of the Moon are reconciled by H2 degassing. Early H2 degassing also explains the high f (O2) of Earth's mantle. A loss of only 1/3 'ocean equivalent' water from Earth by hydrodynamic escape of H2 would shift the f (O2 )o f the upper mantle from the very low oxidation state equivalent to the Moon and other primitive differentiated bodies to its present oxidized state (near the fayalite-magnetite-quartz buffer). No other processes, such as late addition of material to Earth or injection of Fe 3+ -rich deep mantle materials to the upper mantle are required to explain the early elevated oxidation state of Earth's upper mantle.

This paper represents the start of a campaign to identify possible source regions for pyroxene-phyric “enriched” and “depleted” martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids. Our first experiments focus upon mineralogical and crystal chemical aspects of the basalt–eclogite transformation on Mars. It is anticipated that like Earth, eclogites are not the dominant upper mantle assemblage. However, like Earth they may be important hosts for P, Cl, F, OH, Ti, REE, Sr, Y, high-field-strength elements, Hf, and Zr in the upper mantle. This initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition (QUE 94201). This composition is appropriate for this study because it is evolved, so it is pyroxene-phyric, contains abundant phosphate, which is important for storage of REE, and is very well studied. In the high pressure experiments, garnet and omphacitic pyroxene are the dominant phases. The garnet has a compositional range from Al 56.9 Gr + An 24.6 Py 18.5 to Al 41.5 Gr + An 28.1 Py 30.4 to Al 40.3 Gr + An 22.6 Py 37.1 (where Al = almandine, Gr+An = grossular + andradite, and Py = Pyrope) and the pyroxene has jadeite and acmite components of up to 26.0 and 9.9%, respectively. The garnet is enriched in REE over omphacite; this is especially true for the HREE which likely remain sequestered in residual garnet during melting. This observation suggests that garnet may play a key role in producing the relatively flat REE patterns of enriched martian basalts. The prime substitution couple that incorporates REE into garnet is: REE 3+ X Site + Mg 2+ Y Site ↔ R 2+ X Site + Al 3+ Y Site , where R 2+ = Ca, Mn 2+ , Fe 2+ , and Mg. This is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites. The prime substitution couple that incorporates REE into omphacite is: REE 3+ M2 Site + Na 1+ M2 Site for 2 Ca 2+ M2 Site . This is a less effective couple than that for garnet because Na is pulled in two directions in omphacite; it must balance the Al in the M1 site in the jadeite component and it also has to provide a charge deficiency for each REE 3+ cation in the M2 site. The potential for the presence of eclogite in the martian mantle, has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions. Further, the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas, and the major element composition of those magmas.

Despite the very low chromium concentrations in its cumulus olivine (∼140 ppm), lunar troctolite 76535 contains large amounts of Cr sporadically, but highly concentrated, in symplectite assemblages consisting of Mg–Al-chromite and two pyroxenes. Previously proposed symplectite formation mechanisms include crystallization of trapped interstitial melt, diffusion of Cr from cumulus olivine, and/or remobilization of cumulus chromite grains. These mechanisms would imply that the highly Cr-depleted nature of Mg-suite parental magmas and their source materials inferred from cumulus olivine may be illusory. We have conducted a detailed petrologic and textural study of symplectites, as well as chromite veins, intercumulus assemblages, olivine-hosted melt inclusions and clinopyroxene-troilite veins in 76535 with the goals of constraining the origin of the symplectites, and the degree of Cr-depletion in Mg-suite magmas relative to other lunar basalts. Orthopyroxene and clinopyroxene in melt inclusions are depleted in Cr relative to their symplectite counterparts, averaging 900 and 1200 ppm vs. 7400 and 8100 ppm Cr 2 O 3 , respectively. Olivine in contact with symplectite assemblages may exhibit a diffusion profile of Cr going into olivine, whereas olivine boundaries away from symplectites show no diffusion profile. There is also a distinct lack of primary chromite as inclusions in cumulus phases and melt inclusions. Multiple textural observations, melt inclusion chemistry, and modeling of chromite–olivine equilibrium rule out previously proposed symplectite formation mechanisms, and strongly suggest that chromite was not a primary crystallization product of the 76535 parental magma. Accordingly, the post-cumulus addition of Cr and Fe is required to produce the symplectites. After considering multiple models, the addition of Cr and Fe to 76535 via infiltration metasomatism by an exogenous chromite-saturated melt is the model most consistent with multiple textural and geochemical observations. Failure of models that call upon Cr diffusion out of olivine grains imply that the observed Cr-depleted nature of olivine observed in many Mg-suite lithologies is a primary feature of the Cr-depleted nature of the Mg-suite parental magmas and their source materials. This substantial depletion of Cr in the magma relative to mare basalt magmas still requires a satisfactory explanation in order to be consistent with Mg-suite petrogenetic models and currently accepted bulk-Moon compositions. Additionally, if the intimate interaction of migrating melts with early lunar crustal lithologies was a widespread phenomenon after LMO solidification, it provides another mechanism by which to reset or delay closure of radiogenic isotopic systems and explain the Mg-suite-ferroan anorthosite age overlap.

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 2 O 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 2 O 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.

The composition of Mercury's surface is poorly known, but the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission has provided a wealth of new data from three flybys. In particular, MESSENGER Neutron Spectrometer (NS) observations reveal a surface enriched in neutron absorbing elements, consistent with interpretations of color and albedo observations suggesting a surface composition enriched in Fe–Mg–Ti oxides. In this study, we have computed the neutron absorption cross sections for all of the available proposed surface compositions of Mercury and evaluated the plausibility of each surface composition based on the neutron absorption cross section observed by MESSENGER. For identified plausible compositions, the implications for the thermal and magmatic evolution of Mercury are discussed. The measured macroscopic neutron absorption cross section of Mercury is inconsistent with a crust formed from partial melting of plausible bulk mantle compositions, flotation in a magma ocean or adiabatic melting of upwelling cumulates during magma ocean overturn. However, the observed neutron absorption is consistent with model compositions of late-stage magma-ocean cumulates and some proposed compositions from spectral modeling and equilibrium modeling. This suggests that the enrichment of neutron absorbing elements may be indicative of the processes that acted to form Mercury's crust. The enrichment in neutron absorbing elements, in combination with spectral observations that constrain FeO in silicates (

A significant opaque component in Mercury’s crust is inferred based on albedo and spectral observations. Previous workers have favored iron–titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury’s surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury’s surface. However, spectral observations have not detected a diagnostic 1 μm absorption band and have thus limited the FeO in coexisting silicates to

There is an overwhelming amount of evidence supporting the past existence of abundant flowing water on the martian surface, however the source for this water is less understood. Estimates of water in the deep martian interior from SNC meteorites yield low values (1–36 ppm H2O) that are incompatible with magmatic degassing as the primary source for this water. We have analyzed hydrous amphibole in the Chassigny martian meteorite, finding much higher water contents than previously reported. These values are consistent with a much wetter martian mantle (minimum range of 140–250 ppm H2O for the Chassigny source region), allowing for significant contributions of water to the martian surface by magmatic degassing. Furthermore, these results indicate the possibility of young (early–mid Amazonian), water-rich hydrothermal activity at the surface and subsurface, which could have been responsible for intermittent replenishment of water to these regions even after the onset of the cold dry climate that exists today.

Low-temperature aqueous processes have been implicated in the generation of jarosite and hematite on the martian surface, but little is known regarding the role that high-temperature magmatic fluids may have played in producing similar assemblages on Mars. We have identified jarosite and hematite in a clinopyroxene-hosted melt inclusion in martian meteorite MIL 03346 that shows evidence of having been hydrothermally precipitated. In addition to jarosite and hematite, the melt inclusion contains titanomagnetite, pyrrhotite, potassic-chlorohastingsite, an iron-rich silicate glass and possibly goethite. These phases were identified and characterized using scanning electron microscopy (SEM), con-focal Raman-spectroscopy and electron probe microanalysis (EPMA). Based on observed textural relationships and the compositions of the hosted phases, we report that the jarosite-bearing melt inclusion in MIL 03346 has recorded a fluid-rich history that began in the magmatic stage and continued to low-temperatures. This history begins at entrapment of a volatile-rich silicate melt that likely reached fluid-saturation after only minor crystallization within the melt inclusion. This fluid, rich in chlorine, reacted with surrounding silicate material to produce the potassicchlorohastingsite. As cooling proceeded, the liquid phase eventually became more oxidized and reacted with the pyrrhotite. Sulfide oxidation resulted in SO4 2 formation and concomitant acid production, setting the stage for jarosite formation once the fluid cooled beyond the upper thermal stability of jarosite (200 C). As the fluid cooled below 200 C, jarosite continued to precipitate with hematite and/or goethite until equilibrium was established or reactions became kinetically unfavorable. This work suggests an additional jarosite–hematite formation pathway on Mars; one that may be important wherever magmatic-hydrothermal fluids come into contact with primary sulfide grains at the martian surface or subsurface. Moreover, hydrothermal fluids rich in chlorine, sulfur, and iron are important for ore-forming processes on Earth, and their indirect identification on Mars may have important implications for ore-formation on Mars.