Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts

1,2,3Jessica J.Barnes, 1Ian A.Franchi, 2Francis M.McCubbin,1,4Mahesh Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
2ARES, NASA Johnson Space Center, Houston, Texas 77058, USA
3Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
4Department of Earth Sciences, Natural History Museum, London, UK
Copyright Elsevier

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 alteration history of the Jbilet Winselwan CM carbonaceous chondrite: An analog for C‐type asteroid sample return

1A. J. King, 1S. S. Russell, 1P. F. Schofield, 2E. R. Humphreys‐Williams, 2S. Strekopytov, 3F. A. J. Abernethy, 3A.B. Verchovsky, 3M. M. Grady
Meteoritics & Planetary Science (in Press) Link to Article []
1Planetary Materials Group, Department of Earth Sciences, Natural History Museum, , London, SW7 5BD UK
2Imaging and Analysis Centre, Natural History Museum, , London, SW7 5BD UK
3Department of Physical Sciences, The Open University, , Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Jbilet Winselwan is one of the largest CM carbonaceous chondrites available for study. Its light, major, and trace elemental compositions are within the range of other CM chondrites. Chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. Calcium‐ and aluminum‐rich inclusions (CAIs) are present at ≤1 vol% and at least one contains melilite. Jbilet Winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. In most lithologies, the chondrules and CAIs are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7–2.4 on the Rubin et al. (2007) scale. However, chondrules and CAIs in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. Following hydration, some lithologies suffered thermal metamorphism at 400–500 °C. Bulk X‐ray diffraction shows that Jbilet Winselwan consists of a highly disordered and/or very fine‐grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). Some aliquots of Jbilet Winselwan also show significant depletions in volatile elements such as He and Cd. The heating was probably short‐lived and caused by impacts. Jbilet Winselwan samples a mixture of hydrated and dehydrated materials from a primitive water‐rich asteroid. It may therefore be a good analog for the types of materials that will be encountered by the Hayabusa‐2 and OSIRIS‐REx asteroid sample‐return missions.

Reaction of Q to thermal metamorphism in parent bodies: Experimental simulation

1. B. Verchovsky, 2S. A. Hunt, 3W. Montgomery, 3M. A. Sephton
Meteoritics & Planetary Science (in Press) Link to Article []
1School of Physical Sciences, The Open University, , Walton Hall, Milton Keynes, MK7 6AA, UK
2Department of Earth Sciences, University College London, , London, WC1E 6BT, UK
3 Geochemistry Laboratory, Imperial College London, , London, SW7 2AZ, UK
Published by arrangement with John Wiley & Sons

Planetary noble gases in chondrites are concentrated in an unidentified carrier phase, called “Q.” Phase Q oxidized at relatively low temperature in pure oxygen is a very minor part of insoluble organic matter (IOM), but has not been separated in a pure form. High‐pressure (HP) experiments have been used to test the effects of thermal metamorphism on IOM from the Orgueil (CI1) meteorite, at conditions up to 10 GPa and 700 °C. The effect of the treatment on carbon structural order was characterized by Raman spectroscopy of the carbon D and G bands. The Raman results show that the IOM becomes progressively more graphite‐like with increasing intensity and duration of the HP treatment. The carbon structural transformations are accompanied by an increase in the release temperatures for IOM carbon and 36Ar during stepped combustion (the former to a greater extent than the latter for the most HP treated sample) when compared with the original untreated Orgueil (CI1) sample. The 36Ar/C ratio also appears to vary in response to HP treatment. Since 36Ar is a part of Q, its release temperature corresponds to that for Q oxidation. Thus, the structural transformations of Q and IOM upon HP treatment are not equal. These results correspond to observations of thermal metamorphism in the meteorite parent bodies, in particular those of type 4 enstatite chondrites, e.g., Indarch (EH4), where graphitized IOM oxidized at significantly higher temperatures than Q (Verchovsky et al. 2002). Our findings imply that Q is less graphitized than most of the macromolecular carbonaceous material present during parent body metamorphism and is thus a carbonaceous phase distinct from other meteoritic IOM.

UV luminescence characterisation of organics in Mars-analogue substrates

Icarus (in Press) Link to Article []
1School of Earth and Environmental Sciences, North Street, University of St Andrews, St Andrews, Fife, UK, KY16 9AL
2Institute for Mathematics and Physics, Aberystwyth University, Aberystwyth
Copyright Elsevier

Detection of organic matter is one of the core objectives of future Mars exploration. The ability to probe rocks, soils, and other geological substrates for organic targets is a high priority for in situ investigation, sample caching, and sample return. UV luminescence – the emission of visible light following UV irradiation – is a tool that is beginning to be harnessed for planetary exploration. We conducted  UV photoluminescence analyses of (i) Mars analogue sediments doped with polyaromatic hydrocarbons (PAHs; <15 ppm), (ii) carbonaceous CM chondrites and terrestrial kerogen (Type IV), and (iii) synthetic salt crystals doped with PAHs (2 ppm). We show that that detection of PAHs is possible within synthetic and natural gypsum, and synthetic halite. These substrates show the most apparent spectral modifications, suggesting that the most transparent minerals are more conducive to UV photoluminescence detection of trapped organic matter. Iron oxide, ubiquitously present on Mars surface, hampers but does not completely quench the UV luminescence emission. Finally, the maturity of organic carbonaceous material influences the luminescence response, resulting in a reduced signal for UV excitation wavelengths down to 225 nm. This study demonstrates the utility of UV luminescence spectroscopy for the analysis of mixed organic-inorganic materials applicable to Mars exploration.

Late-Stage Diagenetic Concretions in the Murray Formation, Gale Crater, Mars

1Vivian Z. Sun et al. (>10)
Icarus (in Press) Link to Article []
1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109
Copyright Elsevier

Concretions are prevalent features in the generally lacustrine deposits of the Murray formation in Gale crater. In this work, we document the morphologic, textural, and chemical properties of these concretions throughout 300 meters of Murray formation stratigraphy from Mars Science Laboratory observations between Sols 750-1900. We interpret these observations to constrain the timing and composition of post-depositional fluid events at Gale crater. We determine that the overall diversity of concretion morphology, size, texture, and chemistry throughout the Murray formation indicates that concretions formed in multiple, likely late diagenetic, episodes with varying fluid chemistries. Four major concretion assemblages are observed at distinct stratigraphic intervals and approximately correlate with major distinct chemical enrichments in Mg-S-Ni-Cl, Mn-P, and Ca-S, among other local enrichments. Different concretion size populations and complex relationships between concretions and veins also suggest multiple precipitation events at Gale crater. Many concretions likely formed during late diagenesis after sediment compaction and lithification, based on observations of concretions preserving primary host rock laminations without differential compaction. An upsection decrease in overall concretion size corresponds to an inferred upsection decrease in porosity and permeability, thus constraining concretion formation as postdating fluid events that produced initial cementation and porosity loss. The combined observations of late diagenetic concretions and distinct chemical enrichments related to concretions allow constraints to be placed on the chemistry of late stage fluids at Gale crater. Collectively, concretion observations from this work and previous studies of other diagenetic features (veins, alteration halos) suggest at least six post-depositional events that occurred at Gale crater after the deposition of the Murray formation.

Pressure-induced amorphization in plagioclase feldspars: A time-resolved powder diffraction study during rapid compression

1Melissa Sims et al.(>10)
Earth and Planetary Science Letters 507, 166-174 Link to Article []
1Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100, USA
Copyright Elsevier

The pressure-induced amorphization of the two endmembers of the plagioclase ((Na1−xCax)Al1+xSi3−xO8) solid-solution, anorthite (CaAl2Si2O8) and albite (NaAlSi3O8), has been studied as a function of compression rate by means of time-resolved powder diffraction. Anorthite and albite were compressed in a diamond anvil cell to 80 GPa at multiple rates from 0.05 GPa/s to 80 GPa/s. The amorphization pressure decreases with increasing compression rate. This negative strain rate sensitivity indicates a change in deformation mechanism in the plagioclase solid-solution endmembers from brittle to ductile with increasing compression rate. The presented data support the previously proposed shear deformation mechanism for the amorphization of plagioclase. Furthermore, amorphization progresses over a wide pressure range suggesting heterogeneous amorphization, similar to observations based on recovered material from shock-compression experiments of plagioclase. Our experiments support the contention that amorphization pressures for plagioclase may occur at lower pressures than usually considered.