Juliane Gross

The chlorine isotope fingerprint of the lunar magma ocean

The unusually heavy Cl of the Moon is related not to degassing of dry magmas but rather to the loss of Cl from the lunar magma ocean. 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 37Cl/35Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, 37Cl/35Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high 37Cl/35Cl 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.

Phosphate-halogen metasomatism of lunar granulite 79215: Impact-induced fractionation of volatiles and incompatible elements

Abstract In the last decade, it has been recognized that the Moon contains significant proportions of volatile elements (H, F, Cl), and that they are transported through the lunar crust and across its surface. Here, we document a significant segment of that volatile cycle in lunar granulite breccia 79215: impactinduced remobilization of volatiles, and vapor-phase transport with extreme elemental fractionation. 79215 contains ∼1% volume of fluorapatite, Ca5(PO4)3(F,Cl,OH), in crystals to 1 mm long, which is reflected in its analyzed abundances of F, Cl, and P. The apatite has a molar F/Cl ratio of ∼10, and contains only 25 ppm OH and low abundances of the rare earth elements (REE). The chlorine in the apatite is isotopically heavy, at δ37Cl = +32.7 ± 1.6‰. Hydrogen in the apatite is heavy at δD = +1060 ± 180‰; much of that D came from spallogenic nuclear reactions, and the original δD was lower, between +350‰ and +700‰. Unlike other P-rich lunar rocks (e.g., 65015), 79215 lacks abundant K and REE, and other igneous incompatible elements characteristic of the lunar KREEP component. Here, we show that the P and halogens in 79215 were added to an otherwise “normal” granulite by vapor-phase metasomatism, similar to rock alteration by fumarolic exhalations as observed on Earth. The ultimate source of the P and halogens was most likely KREEP, it being the richest reservoir of P on the Moon, and 79215 having H and Cl isotopic compositions consistent with KREEP. A KREEP-rich rock was heated and devolatilized by an impact event. This vapor was fractionated by interaction with solid phases, including merrillite (a volatile-free phosphate mineral), a Fe-Ti oxide, and a Zr-bearing phase. These solids removed REE, Th, Zr, Hf, etc., from the vapor, and allowed the vapor to transport primarily P, F, and Cl, with lesser proportions of Ba and U into 79215. Vapor-deposited crystals of apatite (to 30 μm) are known in some lunar regolith samples, but lunar vapor has not (before this) been implicated in significant mass transfer. It seems unlikely, however, that phosphate-halogen metasomatism is related to the high-Th/Sm abundance ratios of this and other lunar magnesian granulites. The metasomatism of 79215 emphasizes the importance of impact heating in the lunar volatile cycle, both in mobilizing volatile components into vapor and in generating strong elemental fractionations.

Gabbroic Shergottite Northwest Africa 6963: An intrusive sample of Mars

Abstract Meteorite Northwest Africa (NWA) 6963 was classified as a basaltic shergottite based on mineralogy, but here we show that it is a gabbroic rock with a quartz-alkali feldspar intergrowth that represents a late-stage granitic melt. NWA 6963 contains clinopyroxene and maskelynite grains up to 5 mm in length, with minor ferroan olivine, spinel, ilmenite, merrillite, apatite, Fe-sulfides, and high-Si glass. NWA 6963 also contains areas of quartz and alkali-feldspar intergrowths up to ~1 mm in size. Based on mineral abundances and textural analysis, we suggest that NWA 6963 is an intrusive rock similar to a terrestrial gabbro. Infiltration of the martian crust by young gabbroic bodies would suggest that estimates of crustal composition, density, and thickness based on the surface chemistry alone would be problematic and the martian crust may be even more heterogenous than is seen from orbit alone. Investigations of crater walls, where intrusive crustal rocks would be exposed, are needed to discover the launch sites of the shergottites and the full heterogeneity of the martian crust.

Spinel-rich lithologies in the lunar highland crust: Linking lunar samples with crystallization experiments and remote sensing

Abstract Mg-Al spinel is rare in lunar rocks (Apollo and meteorite collections), and occurs mostly in troctolites and troctolitic cataclastites. Recently, a new lunar lithology, rich in spinel and plagioclase, and lacking abundant olivine and pyroxene, was recognized in visible to near-infrared (VNIR) reflectance spectra by the Moon Mineralogy Mapper (M3) instrument on the Chandrayaan-1 spacecraft at the Moscoviense basin. These outcrop-scale areas are inferred to contain 20-30% Mg-Al spinel. Possible explanations for the petrogenesis of spinel-bearing and spinel-rich lithology(s) range from low-pressure near-surface crystallization to a deep-seated origin in the lower lunar crust or upper mantle. Here, we describe 1-bar crystallization experiments conducted on rock compositions rich in olivine and plagioclase that crystallize spinel. This would be equivalent to impact-melting, which is moderately common among lunar plutonic rocks and granulites. To explore possible precursor materials and the maximum amount of spinel that could be crystallized, a lunar troctolitic composition similar to Apollo pink spinel troctolite 65785, and a composition similar to ALHA81005 as analog to the source region of this meteorite have been chosen. The crystallization experiments on the composition of AHLA 81005 did not yield any spinel; experiments on the composition similar to Apollo 65785 crystallized a maximum of ~8 wt% spinel, much less than the suggested 20-30% spinel of the new lithology detected by M3. However, our VNIR spectral reflectance analyses of the experimental run products indicate that the spinel composition of the experimental run products not only appears to be similar to the composition of the spinel lithology detected by M3 (characteristics of the spinel absorption), but also that the modal abundances of coexisting phases (e.g., mafic glass) influence the spectral reflectance properties. Thus, the spinel-rich deposits detected by M3 might not be as spinel-rich as previously thought and could contain as little as 4-5 wt% spinel. However, the effect of space weathering on spinel is unknown and could significantly weaken its 2 μm absorptions. If this occurs, weathered lunar rocks could contain more spinel than a comparison with our unweathered experimental charges would suggest.

A rock fragment related to the magnesian suite in lunar meteorite Allan Hills (ALHA) 81005

Abstract 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 × 300 μm, and has a sub-ophitic texture with olivine phenocrysts, euhedral plagioclase grains (An97-70),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.

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

Northwest Africa (NWA) 6963 was found in Guelmim-Es-Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was resubclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original resubclassification and enable full comparison between this and other Martian samples; here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion). NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites but importantly does not contain any fine-grained mesostasis. Consistent with the mineralogy, the reflectance (visible/near-infrared and thermal infrared) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface. Plain Language Summary This study investigates a new meteorite from Mars, which has different properties than previous Martian meteorites. Specifically, this rock has large crystals that likely formed as the magma ponded in the crust instead of erupting as a lava flow. On Earth, 10 times more magma gets stuck in the crust than erupts on the surface; therefore, we would expect something similar on Mars—yet this rock is the first example of an intrusive magma on Mars. This work shows that this meteorite possibly represents the feeder dike system that fed the lava flow represented by the other shergottite meteorites.