An ancient reservoir of volatiles in the Moon sampled by lunar meteorite Northwest Africa 10989

1A.Stephant et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.045]
1School of Physical Sciences, The Open University, Milton Keynes, UK
Copyright Elsevier

Northwest Africa (NWA) 10989 is a recently found lunar meteorite, we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates. The petrology, bulk geochemistry and mineralogy of NWA 10989 are consistent with it being a lunar meteorite with intermediate-iron bulk composition, composed of 40% of mare basaltic material and ∼60% non-mare material, but with no obvious KREEP-rich basaltic components. It is probable that the source region for this meteorite resides near a mare–highlands boundary, possibly on the farside of the Moon. Analyses of chlorine and hydrogen abundances and isotopic composition in apatite and merrillite grains from NWA 10989 indicate sampling of at least two distinct reservoirs of volatiles, one being similar to those for known mare basalts from the Apollo collections, while the other potentially represents a yet unrecognized reservoir. In situ Th-U-Pb dating of phosphates reveal two distinct age clusters with one ranging from 3.98±0.04 to 4.20±0.02 Ga, similar to the ages of cryptomare material, and the other ranging from 3.32±0.01 to 3.96±0.03 Ga, closer to the ages of mare basalts known from the Apollo collections. This lunar breccia features mixing of material, among which a basaltic D-poor volatile reservoir which doesn’t appear to have been recorded by Apollo samples.

Mid-infrared spectroscopy of laser-produced basalt melts for remote sensing application

1Andreas Morlok,2,3Christopher Hamann,4Dayl Martin,1Iris Weber,4Katherine H.Joy,1Harald Hiesinger,4Roy Wogelius,1Aleksandra Stojic,5Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113410]
1Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
2Museum für Naturkunde, 10115 Berlin, Germany
3Bundesanstalt für Materialforschung und –prüfung, 12489 Berlin, Germany
4School of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
5Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

We obtained mid-infrared spectra and major-element analyses of glasses produced in pulsed laser experiments of basalt. Materials from pits excavated in a basalt slab, as well as of a larger, separated melt droplet were studied. The results of this study show that these glasses exhibits spectral features clearly distinguishable from the unprocessed starting material. Spectra and chemistry show changes, which could be the result of not only melting but also vaporization.

Christiansen Features (CF) for the melt glass in the laser-excavated pits are at 8.3–8.5 μm, and a dominating Reststrahlen Band (RB) at 10.1–10.5 μm in wavelength. The spectra of the powdered glass droplet has a CF at 8.8–8.9 μm and a RB at 10.3–10.5 μm. The spectra are clearly different from the spectra of the surrounding starting material, which shows CF between 8.0 and 8.3 μm, and ample RBs between 9.3 μm and 14.7 μm, typical olivine, plagioclase and pyroxene features.

The results reflect the chemical composition, which shows significant losses of volatiles like K2O and Na2O, as well as of moderate volatiles like FeO, SiO2, and MgO. Refractories TiO2, Al2O3, and CaO tend to be enriched compared to the bulk starting composition. This indicates loss of material through evaporation.

While the spectra of size fractions of the powdered bulk melt glass droplet follow this trend in general, but, because of contamination by the experimental set-up, CaO was found to be strongly enriched in contrast to the other refractories TiO2 and Al2O3.

At least the composition of the glasses in the laser-excavated pits could serve as an ‘endmember’ for the sequence of glassy materials expected to be produced in high energy impact processes involving a basaltic target.

Correlation of CF with SiO2 contents and the SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index show similar behaviour of the pit melts like found in earlier studies. However, when the position of the RB in the pit glass is correlated with the SiO2 content, the result shows a different trend compared with earlier studies. Consequently, the data presented in this study could help distinguishing between surface regions formed by volcanic processes and such modified by high-velocity impacts, where evaporation could play a central role.

This is of high interest for remote sensing studies of Mercury, which, because of its proximity to the Sun, was probably affected by high-velocity impacts to a very high degree.

The composition and structure of Ceres’ interior

1Mikhail Yu.Zolotov
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113404]
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
Copyright Icarus

Results of Ceres’ exploration with the Dawn spacecraft are modeled and discussed in terms of rock/organic/elemental composition, density and porosity in the interior, and formation, migration and geological evolution of the body. Carbon-rich surface composition is used to assess phase and elemental composition of the interior. The consistent bulk density and surface composition suggest an abundant organic matter within the body. Ceres is modeled as a chemically uniform mixture of CI-type carbonaceous chondritic rocks and 12–29 vol% of macromolecular organic matter. Water ice, gas hydrates or high porosity (>10%) are not required to explain bulk density. Ceres may not have a partially differentiated interior structure because gravity and shape could be explained by compaction of chemically uniform materials. Gravity data suggest a two-layer structure with an abrupt density change. Gravity may not reflect the current global density distribution in the interior because the implied bulk porosity >9% and grain density > 2380 kg m−3 disagree with organic-rich compositions. In contrast, Ceres’ polar flattening indicates mild density gradients that could be explained by two-layer and gradual compaction models. The flattening implies grain density of 2200–2350 kg m−3 that is consistent with the organic-rich interior. Viscosity of warmed rock-organic mixtures at depth could account for the observed relaxation of long wavelength topography. The organic-rich composition together with abundant surface carbonates, NH4-bearing phases suggests Ceres’ formation at larger heliocentric distances and later than CI chondrites. Ceres-forming materials could have been more water-rich than parent bodies of CI chondrites and excessive water could have been lost from the body. A majority of Ceres’ surface compounds could have formed through water-rock-organic reactions in a middle interior followed by collisional stripping of an upper interior.

Are the Moon’s nearside‐farside asymmetries the result of a giant impact?

1,2,3Meng‐Hua Zhu,2Kai Wünnemann,4Ross W.K. Potter,5Thorsten Kleine,6Alessandro Morbidelli
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005826]
1Space Science Institute, Macau University of Science and Technology, Taipa, Macau
2Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
3CAS Center for Excellence in Comparative Planetology, China
4Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
5Institut für Planetologie, University of Münster, Münster, Germany
6Département Lagrange, University of Nice–Sophia Antipolis, Nice, France
Published by arrangement with John Wiley & Sons

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30‐50 km and on top a more mafic‐rich layer ~ 10 km thick; and (2) the nearside exhibits a large area of low‐Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside‐farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega‐basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside’s mafic‐rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep‐seated KREEP (potassium, rare‐earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry‐forming impact, the Moon may have had an 182W anomaly compared to the immediate post‐giant impact Earth’s mantle, as predicted if the Moon was created through a giant collision with the proto‐Earth.

CaCl and CaF emission in LIBS under simulated Martian conditions

1D.S.Vogt,1S.Schröder,1 K.Rammelkamp,1P.B.Hansen,1S.Kubitza,1,2H.-W.Hübers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113393]
1Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Institut für Optische Sensorsysteme, Berlin, Germany
2Humboldt-Universität zu Berlin, Institut für Physik, Berlin, Germany
Copyright Elsevier

Chlorine and fluorine play an important role in the geological history of Mars due to their high concentration in Martian magmas and their influence on the generation and evolution of Martian basalts. Chlorine-bearing salts could also facilitate the formation of eutectic brines that could be important for the fluvial history of Mars. The LIBS instruments of ChemCam and SuperCam can detect emission lines of Cl and F, but the intensity of these emission lines is comparatively low, making it difficult to quantify them correctly. A promising alternative is the quantification by molecular emission of diatomic molecules like CaCl and CaF, which can be observed as intense molecular bands in LIBS spectra if Ca is also present. However, the nonlinear dependence of the band intensity on the concentrations of both elements needs to be considered. In this study, we expand upon our previous analysis of molecular bands by investigating samples which produce CaCl bands, CaF bands, or both. We find that the highest CaCl band intensities are found in samples containing more Ca than Cl, while the strongest CaF bands are found in samples with roughly equal concentrations of Ca and F. Both observations can be described by the model that we present here. We also find that the CaCl band is significantly stronger for a sample containing CaCl2 than it is for a sample containing the same concentrations of Ca and Cl in separate bonds. The opposite is true for the CaF band, which is significantly weaker for the sample containing CaF2 bonds than it is for the sample that does not contain CaF2 bonds. These matrix effects are partially attributed to fragmentation during the ablation process and differences in the dissociation energies. Furthermore, we observe that CaF formation is not affected by competing CaCl formation, while CaCl is strongly affected by competing CaF formation. All measurements are done in simulated Martian atmospheric conditions in order to assist the analysis of Martian LIBS data.

Metal-silicate partitioning systematics of siderophile elements at reducing conditions: A new experimental database

1,3E.S.Steenstra,2A.X.Seegers,2 R.Putter,3 J.Berndt,3S.Klemme,4S.Matveev,1 E.Bullock,2W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113391]
1The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
2Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
3Institute of Mineralogy, University of Münster, Germany
4Department of Geosciences, Utrecht University, the Netherlands
Copyright Elsevier

The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO2) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO2 and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO2 and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB).

To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO2 based on the Si-SiO2 buffer (ΔSi-SiO2) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (εMSi) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, εMSi values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

Geochemistry constrains global hydrology on Early Mars

1Edwin S.Kite,1Mohit Melwani Daswani
Earth and Planetary Science Letters 524, 1157118 Link to Article [https://doi.org/10.1016/j.epsl.2019.115718]
1University of Chicago, Chicago, IL, USA
Copyright Elsevier

Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars’ Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from 2004–2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars’ CO2 atmosphere implies subsurface formation of carbonate. We find that the CO2 sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30–5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO2 sequestration is 0.3–400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.