¹Carle M. Pieters,¹Kerri Donaldson Hanna,¹Leah Cheek,¹Deepak Dhingra,¹Tabb Prissel,¹Colin Jackson,¹Daniel Moriarty,¹Stephen Parman,²Lawrence A. Taylor

¹Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 U.S.A.
²Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee 37996 U.S.A.

A robust assessment is made of the distribution and (spatially resolved) geologic context for the newly identified rock type on the Moon, a Mg-spinel-bearing anorthosite (pink-spinel anorthosite, PSA). Essential criteria for confirmed detection of Mg-spinel using spectroscopic techniques are presented and these criteria are applied to recent data from the Moon Mineralogy Mapper. Altogether, 23 regions containing confirmed exposures of the new Mg-spinel rock type are identified. All exposures are in highly feldspathic terrain and are small—a few hundred meters—but distinct and verifiable, most resulting from multiple measurements. Each confirmed detection is classified according to geologic context along with other lithologies identified in the same locale. Confirmed locations include areas along the inner rings of four mascon basins, knobs within central peaks of a few craters, and dispersed exposures within the terraced walls of several large craters. Unexpected detections of Mg-spinel are also found at a few areas of hypothesized non-mare volcanism. The small Mg-spinel exposures are shown to be global in distribution, but generally associated with areas of thin crust. Confirmation of Mg-spinel exposures as part of the inner ring of four mascon basins indicates this PSA rock type is principally of lower crust origin and predates the basin-forming era.

Reference
Pieters CM, Donaldson Hanna K, Cheek L, Dhingra D, Prissel T, Jackson C, Moriarty D, Parman S, Taylor LA (2014)
The distribution of Mg-spinel across the Moon and constraints on crustal origin. American Mineralogist, October 99, 1893-1910
Link to Article [doi:10.2138/am-2014-4776]

Copyright: The Mineralogical Society of America

¹Leah C. Cheek, ²Carle M. Pieters
¹Department of Astronomy, University of Maryland, College Park, Maryland 20742, U.S.A.
²Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island 02906, U.S.A.

Anorthositic rocks dominate the Moon’s upper crust. As remnants of the lunar magma ocean (LMO), small variations in the mineralogy of these rocks may hold key information about the homogeneity of LMO composition and solidification processes. Orbital near-infrared (NIR) sensors are sensitive to mineralogy, but technologic advances have only recently enabled detection of the plagioclase component in crustal rocks based on an absorption band centered near 1250 nm. Anorthosites occupy a unique mineralogic range that is well suited for NIR studies: the highly transparent component, plagioclase, is present in high abundances while the spectrally dominant mafic or oxide minerals are present in only minor abundance. As a result, spectra of anorthosites are more likely than many other rock types to contain visually discernable signatures from more than one mineral component, facilitating their identification and characterization in NIR data.
In support of new NIR measurements for the Moon, we present laboratory spectral analyses of well-controlled plagioclase-dominated mineral mixtures. We focus on the spectral effects of varying mafic and oxide composition and abundance in mixtures with a common plagioclase end-member. The results demonstrate that plagioclase can be a significant contributor to reflectance spectra when strongly absorbing minerals are present in low abundance. We show that the contribution of plagioclase is more pronounced in mixtures with pyroxenes and certain spinels, but more easily masked in mixtures containing small amounts of olivine. Differences in minor mineral composition are clearly expressed in bulk spectra. Modeling of mixtures using a Hapke nonlinear approach accurately estimates mineral abundances in laboratory spectra to within 5 vol% for mixtures with ≥90 vol% plagioclase. Together, these results imply that not only should orbital NIR data sets be able to discern the presence of plagioclase in anorthositic crustal exposures, but also that detailed information about anorthosite mineral assemblages can be reliably accessed in reflectance spectra.

Reference
Cheek LC, Pieters CM (2014) Reflectance spectroscopy of plagioclase-dominated mineral mixtures: Implications for characterizing lunar anorthosites remotely. American Mineralogist, October 99, 1871-1892
Link to Article [doi:10.2138/am-2014-4785]

Copyright: The Mineralogical Society of America

¹Allan H. Treiman,²Jeremy W. Boyce,³Juliane Gross,²Yunbin Guan,²John M. Eiler,²Edward M. Stolper
¹Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058-1113, U.S.A.
²Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, U.S.A.
³Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A.

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: impact-induced 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.

Reference
Treiman AH, Boyce JW, Gross J, Guan Y, Eiler JM, Stolper EM (2014) Phosphate-halogen metasomatism of lunar granulite 79215: Impact-induced fractionation of volatiles and incompatible elements. American Mineralogist October 99, 1860-1870
Link to Article [doi:10.2138/am-2014-4822]

Copyright: The Mineralogical Society of America

^1,3Juliane Gross,²Peter J. Isaacson,³Allan H. Treiman,⁴Loan Le,³Julia K. Gorman
¹Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, U.S.A.
²Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology (SOEST), 1680 East-West Road, Post 508B, Honolulu, Hawaii 96822, U.S.A.
³Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
⁴Jacobs Technology, JETS, NASA Johnson Space Center Engineering, Technology and Science, 2101 NASA Parkway, Code KR111, Houston, Texas 77058, U.S.A.

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.

Reference
Gross J, Isaacson PJ, Treiman AH, Le L, Gorman JK (2014) Spinel-rich lithologies in the lunar highland crust: Linking lunar samples with crystallization experiments and remote sensing. American Mineralogist, October 99, 1849-1859
Link to Article [doi:10.2138/am-2014-4780]

Copyright: The Mineralogical Society of America

¹Cristian Carli,²Giovanni Serventi,²Maria Sgavetti
¹Istituto di Astrofisica e Planetologia Spaziali-INAF Roma, Via fosso del cavaliere 100, 00133, Rome, Italy
²Dipartimento di Fisica e Scienze della Terra, Macedonio Meloni, Università degli studi di Parma, via Usberti 157/A, 43100, Parma, Italy

Lunar highlands are plagioclase-rich terrains produced by crystal floating in a Magma Ocean system. Lunar samples revealed the presence of anorthositic (plagioclase > 90%) samples from the Highlands, associated to more mafic rocks. Recently, remote sensing data permit mapping those terrains with high spatial and spectral resolution allowing detection of plagioclase and mafic crystal field (C.F.) absorptions.
In this paper we have studied bidirectional spectral characteristics in the visible near-infrared (VNIR) of rocks from the Stillwater Complex, a cumulitic igneous stratified complex, with composition varying from mafic to sialic (e.g., pyroxenite, anorthosite). We investigated both slabs and powders of these rocks to give indication of the spectral variability of rock analogs of lunar crust, from a mineralogical point of view. Samples have been spectrally separated in four main groups considering the different C.F. absorption association, reflectance and spectral shape for both slab and powder spectra. More spectral details can be obtained from the analysis of powder spectra than from the slab spectra.
The composition of rocks can be addressed by studying spectral parameters, such as the position and the intensity of the absorption (e.g., band center and band depth). The analysis of our plagioclase-pyroxene-bearing samples indicates that mafic composition can be clearly obtained for samples characterized by one pyroxene phase, even for few amounts of pyroxene, from powder spectra. On the other hand, slab spectra show clear pyroxene absorptions only for rocks with mafic abundance at least >20%. The intensity of the mafic absorptions of these samples shows a linear trend with respect to the abundance of pyroxenes (orthopyroxene + clinopyroxene, for samples with ferrosilite amount less than ca. 25%). Considering all pyroxene-bearing samples, the band depth of slab spectra are linearly related to the volumetric distribution of ferrous iron in pyroxenes.

Reference
Carli C, Serventi G, Sgavetti M (2014) VNIR spectral variability of the igneous stratified Stillwater Complex: A tool to map lunar highlands. American Mineralogist October 99 1834-1848
Link to Article [doi:10.2138/am-2014-4808]

Copyright: The Mineralogical Society of America

¹Colin R.M. Jackson et al. (>10)*
¹Geological Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, U.S.A.
*Find the extensive, full author and affiliation list on the publishers website

Remote sensing observations have identified aluminate spinel, in the absence of measureable olivine and pyroxene, as a globally distributed component of the lunar crust. Earlier remote sensing observations and returned samples did not indicate the presence of this component, leaving its geologic significance unclear. Here, we report visible to mid-infrared (V-IR) reflectance (300–25 000 nm) and Mössbauer spectra of aluminate spinels, synthesized at lunar-like oxygen fugacity (fO2), that vary systematically in Fe abundance. Reflectance spectra of particulate (6 Fe#. Although the 2000 and 2800 nm bands are assigned to Fe2+IV electronic transitions, spectra of aluminate spinels with excess Al2O3 demonstrate that the strengths of the 1000 nm bands are related to the abundance of Fe2+VI. The abundance of Fe2+VI depends on bulk Fe content as well as factors that control the degree of structural order-disorder, such as cooling rate. Consequently the strength of the 1000 nm bands are useful for constraining the Fe content and cooling rate of remotely sensed spinel. Controlling for cooling rate, particle size, and fO2, we conclude that spinels with >12 Fe# (

Reference
Jackson CRM et al. (2014) Visible-infrared spectral properties of iron-bearing aluminate spinel under lunar-like redox conditions. American Mineralogist October 99, 1893-1910
Link to Article [doi:10.2138/am-2014-4793]

Copyright: The Mineralogical Society of America

¹Alexander Goldmann,^2,3Gregory Brennecka,⁴Janine Noordmann,¹Stefan Weyer,²Meenakshi Wadhwa
¹Leibniz Universität Hannover, Institut für Mineralogie, Callinstr. 3, 30167 Hannover, Germany
²Arizona State University, School of Earth & Space Exploration, Tempe, AZ 85287, USA
³Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁴Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany

Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the 238U/235U ratio, which was previously assumed to be invariant (=137.88). In this study, we have investigated 27 bulk meteorite samples from different meteorite groups and types, including carbonaceous (CM1 and CV3), enstatite (EH4) and ordinary (H-, L-, and LL-) chondrites, as well as a variety of achondrites (angrites, eucrites, and ungrouped) to constrain the distribution of U isotopic heterogeneities and to determine the average 238U/235U for the Solar System.
The investigated bulk meteorites show a range in 238U/235U between 137.711 and 137.891 (1.3 ‰) with the largest variations among ordinary chondrites (OCs). However, the U isotope compositions of 20 of the 27 meteorites analyzed here overlap within analytical uncertainties with the narrow range defined by terrestrial basalts (137.778 – 137.803), which are likely the best representatives for the U isotope composition of the bulk silicate Earth. Furthermore, the average 238U/235U of all investigated meteorite groups overlaps with that of terrestrial basalts (137.795 ± 0.013). The bulk meteorite samples studied here do not show a negative correlation of 238U/235U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic 235U was generated by the decay of extant 247Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of 238U/235U with Nd/U and with 1/U.
The following conclusions can be drawn from this study: (1) The Solar System has a broadly homogeneous U isotope composition, and bulk samples of only a limited number of meteorites display detectable U isotope variations; (2) Bulk planetary differentiation has no significant effect on the 238U/235U ratio since the Earth, achondrites, and chondrites have indistinguishable U isotope compositions in average. (3) The cause of U isotopic variation in Solar System materials remains enigmatic; however, both the decay of 247Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively.
The average 238U/235U of the investigated meteorite groups (including data compiled from the literature) and terrestrial basalts is 137.794 ± 0.027 (at a 95% student’s t confidence level, including all propagated uncertainties) and represents the best estimate for the U isotope composition of the Earth and the Solar System. This value may be used for U-Pb and Pb-Pb dating of Solar System materials, provided the precise U isotope composition of the sample is unknown. Compared to Pb-Pb ages that were determined with the previously assumed value for 238U/235U (137.88), this new value results in an age adjustment of -0.9 Ma.

Reference
Goldmann A, Brennecka G, Noordman J, Weyer S, Wadhwa M (2014) The Uranium Isotopic Composition of the Earth and the Solar System. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.008]

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