Thermal Metamorphism on the Moon as Recorded by the Granulite Suite

Thermally metamorphosed rocks on the Moon are an important, yet under-studied suite of lithologies that have been identified within the Apollo and lunar meteorite collections. These rocks, with granoblastic and poikilitic textures, are generally referred to as granulites. However, unlike their terrestrial counterparts which are the metamorphic products of both high temperatures and pressures, lunar granulites are thought to be the products of only high-temperature (> 1000 oC) thermal metamorphism that completely recrystallised their protoliths. We summarise the range of textures and chemical systematics reported from lunar granulites. These data enable constraints to be placed on the thermal conditions in the lunar crust required for high-temperature metamorphism to have taken place. Most studies indicate that impact melt sheets have the relevant thermal properties to sustain high temperatures over the time scales required to fully recrystallise surrounding crustal lithologies. However, the roles of alternative heat sources, such as magmatic intrusions into the crust, have not been extensively investigated and, as such, cannot be ruled out. Additionally, chemical data yields important insights into the protoliths of the granulite suite. By identifying protoliths, we greatly enhance our understanding of the range of lithologies that make up the primary lunar crust. In turn, this enables crustal formation models to be better constrained.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5623326

NanoSIMS and EPMA dating of lunar zirconolite

Zirconolite is a common Zr-rich accessary mineral in mafic rocks. It is also an ideal U–Pb/Pb–Pb chronometer because it commonly contains high U content (mostly 0.1–10 wt%) and negligible initial Pb. However, zirconolite is usually very small (e.g., ~ 1 μm in width) in lunar rocks, requiring a high spatial resolution analysis. We analyzed a single, large (25 μm × 20 μm) grain of zirconolite in lunar meteorite NWA 4485 using Pb–Pb dating by NanoSIMS and U–Th–Pb dating by EPMA. The resultant U–Th–Pb age is 4540 ± 340 Ma (2σ) with a spatial resolution of 1.3 μm. The Pb–Pb age by NanoSIMS is 4348.5 ± 4.8 Ma (2σ) with a spatial resolution of ~ 2 μm, consistent with the age of 4352 ± 10 Ma and 4344 ± 14 Ma reported in the same meteorite and its paired meteorite NWA 4472. Although U–Th–Pb age is somewhat older, it still includes the NanoSIMS results within the analytical uncertainty. This work demonstrates the potential application of the combined EPMA dating and REE analysis of lunar zirconolite, with the benefits of high spatial resolution, non-destructive, and readily accessibility of the instrument. The precision of the EPMA dating (7.6%, 2σ) can be improved by increasing the counting time for Pb, U and Th. We expect to apply this EPMA technique for a quick and non-destructive age survey and geochemical study of zirconolite grains from the lunar mare basalts newly returned by Chang’E-5 mission which landed on a very young (1.2–2.0 Ga by crater-counting chronology) basalt unit in Procellarum KREEP Terrain.

Trace and minor element variations in lunar granulites: Insights into lunar metamorphic conditions.

<p>Metamorphic rocks on the Moon are an important yet under-studied suite of lunar lithologies that have been identified in the Apollo and lunar meteorite collections [1]. These rocks, with granoblastic textures, are generally referred to as granulites; however, unlike their terrestrial counterparts, they are considered to represent the products of only high-temperature (> 1000 <sup>o</sup>C) thermal metamorphism that completely re-crystallised their protolith(s). Lunar granulites are commonly sub-divided into two main compositional groups related to their protolith lithologies. The Fe-granulites, found at most Apollo landing sites, are generally accepted to derive from metamorphosed plagioclase-rich igneous cumulates, termed the ferroan anorthosite (FAN) suite. The FAN suite are important lithologies as they represent products of the primary lunar crust. The Mg-granulites are found mostly at the Apollo 16 landing site and within lunar meteorite samples; the protolith(s) of this latter group is not well understood [2].  Early studies have linked the protolith to secondary magmatic intrusions into the primary anorthositic crust (termed the Mg-suite); however, recent studies have tentatively connected the protolith to a Mg-rich variation of the primary crustal plagioclase cumulates (termed the MAN suite). The occurrence of MANs is controversial, it is unclear how the MAN suite fits into canonical lunar crustal formation models [3]. To investigate the protoliths of the granulite suites, we report in situ trace- and minor-element abundances for olivine and pyroxene grains within Fe- and Mg-granulites, determined by LA-ICP-MS and EPMA respectively. Trace-element data presented here indicate that the Mg-granulites are compositionally similar to the MAN suite. Furthermore, by comparing plagioclase trace-element data with peak metamorphic temperatures (calculated using two-pyroxene thermometers [4]), we find no relationship between metamorphic temperature and diagnostic trace-element signatures suggesting that both granulite suites experienced similar thermal metamorphic conditions. Additionally, we estimate the duration of metamorphic heating using experimentally derived diffusion rates of minor elements in minerals,  (such as Ca in olivine [5]). Both the calculated cooling rates and peak metamorphic temperatures can set constraints on the metamorphic heat source responsible for thermally annealing the Fe- and Mg-granulites. Specifically, we are able to assess whether the granulites formed as a result of shallow (<1 km) burial of the protolith by impact melt sheets or hot, impact-generated fall-back breccias [6]; or deep (> 1km) contact metamorphism of the protolith due to the emplacement of magma chambers or upwelling plutons within the lunar crust [7].</p><p> </p><p>[1] Lindstrom & Lindstrom, 1986, JGR, 91(B4), 263-276 [2] Treiman et al. 2010. MaPS, 45, 163-180. [3] Gross et al. 2014, EPSL, 388, 318-328. [4] Brey & Köhler, 1990, J Pet, 31, 1353-1378. [5] Dohmen et al, 2007, PCM, 34, 389-407. [6] Cushing et al. 1999, MaPS, 34, 185-195. [7] Hudgins et al. 2011, Am Min, 96, 1673-1685.</p>

Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3; Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth's mantle. Donwilhelmsite forms in the Earth's mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements

“Cr-Zr-Ca armalcolite” is a mineral originally found in Apollo samples five decades ago. However, no structural information has been obtained for this mineral. In this study, we report a new occurrence of “Cr-Zr-Ca armalcolite” and its associated mineral assemblage in an Mg-suite lithic clast (Clast-20) from the brecciated lunar meteorite Northwest Africa 8182. In this lithic clast, plagioclase (An = 88–91), pyroxene (Mg#[Mg/(Mg+Fe)] = 0.87–0.91) and olivine (Mg# = 0.86–0.87) are the major rock-forming minerals. Armalcolite and “Cr-Zr-Ca armalcolite” are observed with other minor phases including ilmenite, chromite, rutile, fluorapatite, merrillite, monazite, FeNi metal, and Fe-sulfide. Based on 38 oxygen atoms, the chemical formula of “Cr-Zr-Ca armalcolite” is (Ca0.99Na0.01)Σ1.00(Ti14.22Fe2.06Cr2.01 Mg1.20Zr0.54Al0.49Ca0.21Y0.05Mn0.04Ce0.03Si0.03La0.01Nd0.01Dy0.01)Σ20.91O38. Electron backscatter diffraction (EBSD) results reveal that the “Cr-Zr-Ca armalcolite” has a loveringite R3 structure, differing from the armalcolite Bbmm structure. The estimated hexagonal cell parameters a and c of “Cr-Zr-Ca armalcolite” are 10.55 and 20.85 Å, respectively. These structural and compositional features indicate that “Cr-Zr-Ca armalcolite” is loveringite, not belonging to the armalcolite family. Comparison with “Cr-Zr-Ca armalcolite” and loveringite of other occurrences implies that loveringite might be an important carrier of rare earth elements in lunar Mg-suite rocks. The compositional features of plagioclase and mafic silicate minerals in Clast-20 differ from those in other Mg-suite lithic clasts from Apollo samples and lunar meteorites, indicating that Clast-20 represents a new example of diverse lunar Mg-suite lithic clasts.

Advances in Cosmochemistry Enabled by Antarctic Meteorites

At present, meteorites collected in Antarctica dominate the total number of the world's known meteorites. We focus here on the scientific advances in cosmochemistry and planetary science that have been enabled by access to, and investigations of, these Antarctic meteorites. A meteorite recovered during one of the earliest field seasons of systematic searches, Elephant Moraine (EET) A79001, was identified as having originated on Mars based on the composition of gases released from shock melt pockets in this rock. Subsequently, the first lunar meteorite, Allan Hills (ALH) 81005, was also recovered from the Antarctic. Since then, many more meteorites belonging to these two classes of planetary meteorites, as well as other previously rare or unknown classes of meteorites (particularly primitive chondrites and achondrites), have been recovered from Antarctica. Studies of these samples are providing unique insights into the origin and evolution of the Solar System and planetary bodies. ▪ Antarctic meteorites dominate the inventory of the world's known meteorites and provide access to new types of planetary and asteroidal materials. ▪ The first meteorites recognized to be of lunar and martian origin were collected from Antarctica and provided unique constraints on the evolution of the Moon and Mars. ▪ Previously rare or unknown classes of meteorites have been recovered from Antarctica and provide new insights into the origin and evolution of the Solar System.