Lunar Plagioclase Research Articles

Plagioclase iron content variance: A complication for efforts to identify lunar terrains of extremely high plagioclase abundance

According to several orbital reflectance spectroscopy studies, numerous widely scattered lunar surface locales feature remarkably high (>98%) abundance of plagioclase. These “pure” anorthosite claims are based on an absorption band at ∼1.25 μm, associated with minor Fe2+ within plagioclase. But utilization of the 1.25 μm band as a direct gauge of plagioclase abundance requires an underlying assumption that plagioclase FeO is uniform across all measured materials. Available data for FeO in lunar plagioclase are in many cases suspect because electron-probe FeO measurements may be inflated by secondary fluorescence from nearby mafic phases. We studied plagioclase FeO in a set of 13 anorthositic lunar rocks, taking care to avoid proximity to mafic phases; or in cases where complete avoidance was not possible without sacrificing representativeness (i.e., fine-grained impact melt rocks with zoned silicates), correcting close-to-mafic analyses for secondary fluorescence (Sugawara, T. [2001], Japanese Mag. Mineral. Petrol. Sci. 30, 159–163). Results for rock-average plagioclase FeO range from 0.030 wt% in plutonic troctolitic anorthosite 76335, to 0.23 wt% in impact-melt rock 14310. In general, as shown by exsolution of mafic silicates from plagioclase within several samples, final plagioclase FeO in lunar plutonic anorthosites is determined by reequilibration during slow postigneous cooling and/or metamorphism. In contrast, four fast-cooled anorthositic impact melt rocks have consistently higher plagioclase FeO, averaging 1.9 times higher in comparison to average plutonic anorthositic rock. Thus, impact melted lunar anorthosite will have far more prominent 1.25 μm absorption than plutonic anorthosite of the same plagioclase abundance. In view of the inconstancy of plagioclase FeO, orbital spectral reflectance using the 1.25 μm absorption band, or the ratio between that band and one or more mafic silicate bands, cannot be precise enough to justify claims of ability to resolve “purest” (98 vol% +) anorthosite from compositions that are high-plagioclase but not that extreme.

Non-Henrian behavior of hydrogen in plagioclase – basaltic melt partitioning

Plagioclase crystals in lunar anorthosite may have formed directly from the lunar magma ocean (LMO), with their elemental and isotopic compositions providing unique probes of LMO composition, age, and evolution. Low hydrogen concentrations have been reported in lunar plagioclase, but translating this to estimates of LMO hydrogen contents is complicated by incomplete knowledge of hydrogen partitioning between plagioclase and melt at low hydrogen contents. We conducted a series of high-temperature experiments to better quantify the plagioclase-melt partition coefficient of hydrogen in the Moon at low melt H abundances. Hydrogen contents of plagioclase and coexisting melt, reported in terms of water equivalent concentrations, were analyzed using Fourier Transform Infrared Spectroscopy. Resulting plagioclase-melt partition coefficients of water, Dplag-melt water, at melt water contents below 300 ppm, are 0.10 ± 0.05 to 0.36 ± 0.19. the highest Dplag-melt water reported to date. Our results, in conjunction with literature data, indicate a strong dependence of Dplag-melt water on water concentration in the melt (x in ppm): Dplag-melt water=20±5·x−0.88±0.03. This strong non-Henrian behavior is likely related to changes in the hydrogen activity in basaltic melt due to variations in H2O/OH speciation as a function of melt hydrogen content, implying that hydrogen partitioning in other mineral-melt systems may also be non-Henrian. At the low water contents in the lunar interior, Dplag-melt water is approximately 2 orders of magnitude higher than previously assumed. If hydrogen contents measured in lunar plagioclase crystals reflect pristine plagioclase-LMO equilibrium without subsequent modification, we conclude that <14 ppm H2O equivalent would have been present in the Moon when 95% of the initial LMO had crystallized.

Chronology of alkali anorthosite 14304 clast “b” records basin forming impact at ∼3.95 Ga

Alkali-suite rocks constitute one of three major suites of lunar crustal rocks. As such, constraining their formation timescales and petrogenesis is important for understanding the earliest magmatic history of the Moon. However, the magmatic history of alkali-suite rocks is partly obscured by superimposed effects of major basin-forming impact events on the lunar nearside. Consequently, unambiguous crystallization ages of samples from this suite of rocks have not been determined. The aim of this study is to better understand the petrogenetic history of the alkali-suite and the potential superimposed effects of impact metamorphism by determining Sm-Nd, Rb-Sr, and 40Ar/39Ar ages for an alkali anorthosite clast from Apollo 14 lithic breccia 14304 termed clast “b”. The new chronologic measurements of clast “b” yield concordant Sm-Nd, Rb-Sr, and 40Ar/39Ar ages of 3947 ± 13 Ma, 3975 ± 34 Ma, and 3937 ± 37 Ma respectively, resulting in a weighted mean age of 3949 ± 11 Ma. This age is not interpreted to date an igneous event related to production of the lunar highlands crust and instead the chronology and petrography of clast “b” are most readily explained by an impact event at ∼3.95 Ga that caused near-complete isotopic re-equilibration of the Sm-Nd, Rb-Sr, and 40Ar/39Ar chronometers. The weighted mean age of 3949 ± 11 Ma of clast “b” is several hundred Ma younger than 4.3–4.4 Ga ages typically determined for samples of other crustal rock suites but in very good agreement with independent estimates for the formation of Imbrium basin ejecta and other marginally older impact events which are thought to have been sampled at the Apollo 14 landing site. Thus, although petrologic and geochemical examination suggest that clast “b” is a pristine igneous clast, its age likely records an impact event at the Apollo 14 landing site. Whereas the various determined ages of clast “b” do not reflect the timescales of alkali-suite magmatism, the relatively low initial Sr and Nd isotopic compositions of clast “b” indicate that its protolith evolved with very low 147Sm/144Nd and 87Rb/86Sr that are distinct from estimates for urKREEP but similar to that of lunar plagioclase. This implies that the igneous protolith of clast “b” derived from a plagioclase-dominated KREEP-rich source that must have formed after the formation of the urKREEP source at ∼4.35 Ga but well before the impact event recorded by clast “b” at ∼3.95 Ga.

Constraints on early Earth's water budget from the evolution of the lunar hydrogen cycle

During the Hadean, Earth recovered from the Moon-forming giant impact, became covered with liquid water oceans, and witnessed the onset of plate tectonics and life. Quantifying the abundances, distribution, and chemical states of water in the atmosphere, on the surface, and in the interior of the early Earth is essential to constrain the early evolution of System Earth. Assessing these parameters is hampered by the general dearth of early Earth samples, the difficulty of distinguishing primary signatures from later alteration processes in such samples, leading to large uncertainties on the influx and outflux of water to and from the early Earth. Given the close proximity of Earth and Moon, constraints on the early hydrogen cycle in the Moon may reflect coeval aspects of the water cycle on early Earth. Here, we assess constraints on the lunar water cycle from the time the Moon formed until the end of late accretion at ~3.8 Ga, and implications of these constraints for the early Earth water budget. Dynamic accretion models suggest the Moon initially contained ~455 ppm of water. Recent experimental studies of lunar magma ocean crystallization suggest similarly substantial initial lunar water contents. Hydrogen concentration measurements in lunar plagioclase crystals derived from the magma ocean illustrate that the Moon experienced significant degassing during the solidification of the lunar magma ocean (thought to have occurred between 4.5 and ~ 4.3 Ga). Hydrogen and chlorine systematics in lunar magmatic apatite grains formed between ~4.1 Ga and ~ 3 Ga indicate that lunar hydrogen reservoirs were replenished by volatile delivery during late accretion (~4.1–3.8 Ga), after which the water abundance of the Moon stabilized. Using this knowledge of the lunar water cycle to model Earth's early water budget leads to two scenarios that are consistent with the observed present-day terrestrial water content of 1000–3000 ppm: (1) Earth contained significantly more water than the Moon-forming material immediately after the giant impact, suggesting hydrogen heterogeneity in the initial Earth-Moon system; (2) Earth did not experience significant degassing in the aftermath of the giant impact, and the late accretion mass added to Earth was large and water-rich

A lunar hygrometer based on plagioclase-melt partitioning of water

The Moon was initially covered by a magma ocean. Hydrogen detected in plagioclase of ferroan anorthosites, the only available samples directly crystallised from the lunar magma ocean (LMO), can be used to quantify LMO hydrogen content. We performed experiments to determine plagioclase-melt partition coefficients of water under LMO conditions with water contents of co-existing plagioclase and melt quantified using Fourier-Transform Infrared Spectroscopy. Results indicate lunar plagioclase can incorporate approximately one order of magnitude more water than previously assumed. Using measured water contents of lunar plagioclase, this suggests that ~100 μg/g H2O equivalent was present in the residual magma when 95 % of the initial LMO had crystallised. Our results constrain initial LMO water contents to ~ 5 μg/g H2O equivalent if water was conserved throughout LMO evolution. If on the other hand the initial LMO contained >1000 μg/g water as suggested by experiments on LMO crystallisation, >99 % hydrogen degassing occurred during the evolution of the LMO.

Cathodoluminescence microscopy and spectroscopy of plagioclases from lunar soil

Cathodoluminescence (CL) microscopy and spectroscopy of single plagioclase grains from lunar soil show that plagioclases from Luna 20 (highland) have more or less homogeneous CL with both blue or green colors, whereas plagioclase grains sampled by Luna 24 (mare) luminesce dominantly green with partially distinct oscillatory zoning. The three main emission bands in the blue (~450 nm), green (~560 nm), and red-IR (~690 nm), mimic the most common emission bands in terrestrial feldspars. Mn 2+ is the most important activator element in lunar plagioclases. Variations in the amount of structurally incorporated Mn 2+ cause variations in the intensity of the green emission band at 560 nm, in some cases resulting in zoning of the CL intensity within single crystals. Calculations by a combination of quantitative spectral analysis of CL emission and PIXE measurements yield Mn concentrations of 7-47 ppm. The intense intrinsic emission band at 450 nm (probably an Al-O - -Al center), which was especially prominent in Luna 20 plagioclases, causes their blue CL color. The occurrence of a CL emission band at ~690 nm in plagioclases from Luna 24 samples confirms that Fe 3+ -activated CL is common in these grains. The results indicate that at least some of the Fe in Luna 24 plagioclases is Fe 3+ , whereas all Luna 20 plagioclases have Fe 3+ -near the CL detection limit of about 0.1 ppm.

He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes

He, Ne, and Ar in ilmenite and pyroxene separates from two lunar regolith samples exposed to the solar corpuscular radiation at widely different epochs were analyzed by the closed system stepped etching technique. This method avoids element or isotope fractionation due to diffusion during the experiment and thus depth profiles of element and isotopic composition of implanted solar noble gases can be determined. Ilmenites from a relatively recently irradiated soil release in the first steps He and Ne with isotopic compositions identical to modern solar wind values. We conclude that ilmenite retains isotopically unfractionated He, Ne, and Ar from the solar wind (SW) in the outermost grain layers. The isotopic composition of SW noble gases has remained constant over the last 100 Ma but there is some evidence for a slight change over a Ga timescale. The ratio 36Ar/38Ar in the solar wind is 2–3% larger than that in the terrestrial atmosphere. We give further evidence for the identification of a solar energetic particle component (SEP), which was implanted with energies exceeding those in the solar wind. All ilmenites and pyroxenes contain SEP‐Ne with 20Ne/22Ne = 11.2±0.2, in agreement with SEP‐Ne found earlier in lunar plagioclase. (21Ne/22Ne)SEP is consistent with SEP‐Ne being derived from SW‐Ne by mass dependent fractionation. In addition, the new data reveal SEP‐He and SEP‐Ar, both isotopically heavier than the SW components. The isotopic fractionation factors required to relate SW and SEP gases are approximately equal to the square of the mass ratios. Element ratios in the SEP dominated steps are similar (He: Ar) or identical (Ne: Ar) to present‐day solar wind values, indicating little or no element fractionation in the SEP reservoir.

Reply to “Solar wind effects on the surface chemistry of lunar grains” by E. Bilson

Bilson (1978) has criticized our interpretation of ion probe measurements on lunar plagioclase grains by contending that the sputter action of solar wind H and He can account for observed surface enhancements of Mg and Fe. We think that an inappropriate analogy between lunar bulk soils and plagioclase grains led Bilson to this conclusion. While it is true that ion implantation is unlikely to have a dominant effect on the major element composition of the surface of lunar grains of average bulk soil composition, we show that chemical alteration by solar wind sputtering cannot explain our ion probe result on plagioclase grains. Glass splashes on shielded grains and solar wind implanted ions on some selected exposed crystals are the most likely source of Mg and Fe enhancements.

Exsolution in terrestrial and lunar plagioclases revealed by chemical etching

A new chemical etching technique reveals fine scale exsolution structures on polished surfaces of plagioclases in the bytownite and labradorite range. A very obvious blue and locally brown stain is produced (by the action of citric acid) in growth zones of a restricted range of compositions on etched surfaces. The blue-stained zones contain very fine grooves, corresponding to the traces of thin exsolution lamellae. Exsolution features have been revealed in this way in plagioclases in a terrestrial diabase, lunar basalts and an amphibolite. Compositions of the exsolved areas found by microprobe analysis represent averages of the matrix and exsolution lamellae. Analytical methods with higher spatial resolution are required to determine the composition of the lamellae. This etching method permits easy optical identification of compositional zoning and of fine scale exsolution over relatively large areas and is therefore complementary to other techniques.

The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415

A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg 1.14Fe 0.80Mn 0.02Ca 0.04)(Si 1.97Al 0.03)O 6, has a unit cell in space group Pbca with a = 18.310(15)Å, b = 8.904(10)Å, c = 5.214(7)Å, containing 2 formula units. A set of 742 counter-measured intensity data made with MoK α radiation has been used to refine the crystal structure in isotropic thermal mode to R = 0.116. Anisotropic refinement led to R = 0.092, but thermal parameters are distorted by non-random errors resulting from poor crystal texture. The resulting structure is in close agreement with that obtained by Ghose [9] for a hypersthene from Greenland. A parameter q, which gives ( Mg qFe 1−q) for cation site M(1) and (Mg 1.14− q Fe q−0.18 Ca 0.04) for site M(2), was included in the least-squares analysis, yielding q = 0.90(1). This orthopyroxene has the high degree of cation order expected of pyroxenes subjected to Apollonian metamorphism at lower than 500–600°C. No evidence exists for a subsequent thermal event of sufficient intensity to disorder the pyroxene. On the basis of previous laboratory studies of argon-release patterns of lunar plagioclase and order-disorder kinetics of terrestrial pyroxenes, we attribute the reported isotopic age (3.9–4.1 AE) to cessation of metamorphism, perhaps caused by impact excavation.

Elastic properties of anorthite and the nature of the lunar crust

A polycrystalline specimen of anorthite has been hot-pressed at P = 15kbar and T = 1000°C in a piston-cylinder apparatus. Compressional (ν p)and shear(ν s) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The specimen is less than 0.5% porous and is elastically isotropic within 1%. The velocities at 7.5 kbar are ν p = 7.29km/secandν s = 3.85km/sec . These data are consistent with those for most terrestrial and lunar plagioclase rocks but not for certain anisotropic rocks and single crystals. The measured velocities demonstrate, moreover, that it is impossible to distinguish between rocks of gabbro, anorthositic gabbro, or anorthosite compositions for the 20–55 km layer of the lunar crust on the basis of seismic data alone. The mean composition of the crust could well be that of a gabbro (17% Al 2O 3) rather than of an anorthositic gabbro (∼25%Al 2O 3) as assumed in some current models.

Rare earth element abundances in rocks and minerals from the Fiskenaesset Complex, West Greenland

Rare earth element (REE) abundances determined by activation analysis in rocks, plagioclase and mafic separates from the Fiskenaesset Complex are presented together with data on major and trace elements in the minerals. The REE data for the rocks and plagioclases are distinct from those of many other anorthositic complexes and the abundances are some of the lowest recorded for plagioclase from terrestrial anorthosites. The bulk and trace element compositions of the Fiskenaesset plagioclases show a number of similarities to those of lunar plagioclases. The plagioclases show a positive Eu anomaly of about 10 and a depletion in the heavy REE relative to the light ones. The mafic separates are enriched in the heavy REE relative to the light ones, and show no Eu anomaly except in one sample with a positive anomaly not attributable to plagioclase contamination. It is estimated, from experimental partition coefficient data, that the REE pattern in the magma at an early stage of fractionation was La (17×) to Lu (0.7× chondrites) with a possible positive Eu anomaly. This highly fractionated REE pattern may be attributed to partial melting of a garnet-bearing source.

Crystal-chemical interpretation of crystallographic anomalies in lunar plagioclases

Unit-cell dimensions of lunar plagioclases, γ in particular, notably differ from those of terrestrial plagioclases with the same An-content inferred from Ca/Ca + Na + K or Ca/8O. It is shown that this crystallographic anomaly is a consequence of the chemical anomalies affecting lunar plagioclases. In particular it is a consequence of the presence of □ [Si₄O₈] in solid solution. This molecule influences the composition of the framework, to which the γ angle is strongly related. On the basis of these results some data on the thermal state of lunar plagioclases are briefly discussed.

Trace element composition of anorthosite plagioclase

Plagioclases from massif anorthosites are strongly depleted in Rb and Cs relative to those from anorthosites associated with volcanic rocks, but are enriched in K and REE. Anorthosite plagioclases generally show strong enrichment of the lighter REE. The inclusions in Icelandic basalts are unique in having low K and K/Rb and relatively less-fractionated REE patterns; they may be the closest terrestrial analogues to the lunar highlands anorthosites. Regular variations in the trace element concentrations of terrestrial plagioclases are produced by the systematic relations between bulk composition and trace element composition of magma, on the one hand, and between plagioclase composition and element partitioning on the other. The failure of lunar plagioclases to follow these regular trends reflects differences in the relative abundances of K, Ca, Ba and Sr between terrestrial and lunar magmas of similar bulk composition.

Anomalous lunar plagioclase composition and the composition of coexisting melt

THE anomalous composition of lunar plagioclases may be accounted for by various substitutions, principally the replacement by Fe and Mg, with some Ca and Na, of Al in tetrahedral sites in the lattice1.

Unit cell parameters, compositions and 2V measurements of selected lunar plagioclases

Unit cell parameters of 12 selected lunar plagioclase specimens have been obtained using least-squares refinement of X-ray powder diffraction data. Compositions of splits of 11 of these plagioclase specimens were determined using the electron microprobe. Optical axial angles were measured on analyzed spots for nine of the probed specimens. Analyses cluster about An 96.5 ± 1.0 and unit cell parameters and 2Vx fit the values for this composition obtained in other studies.

Chemical anomalies of lunar plagioclase, described by substitution vectors and their relation to optical and structural properties

Lunar plagioclase posed an enigma because optical properties indicate slow cooling while structural properties suggest rapid quenching when compared directly to terrestrial plagioclase. If, however, the An-content is inferred from the Al/Si ratio rather than from the Ca/Na ratio, lunar plagioclase corresponds more closely to normal volcanic plagioclase. The differences between An=4(Al/(Al+Si))-1 and An=Ca/(Ca+Na+K) is caused by substitutions which appear so far to be unique to lunar feldspars. A vector representation of chemical composition makes it possible to single out several substitutions which account for the chemical peculiarities of lunar plagioclase. It permits to assign sites in the structure to various minor elements simply from the chemical analysis without having to rely on sophisticated spectroscopic techniques or an X-ray structure determination. 1. The deficiency in Al+Si (up to 0.06 per formula unit) is compensated for by Fe, Mg, and small amounts of Na or Ca occupying tetrahedral sites. The Ca/Na ratio is increased accordingly to balance the electric charge. This substitution accounts for most of the differences. 2. The vacancycoupled substitution Ca+□ → 2 Na (Weill et al., 1970) is another cause for the increased Ca/Na ratio. 3. The amount of alkali evaporation from plagioclase, if it occurred, is probably small. These unusual substitutions are an expression of special conditions on the moon during the crystallization of plagioclase. Crystallization at high temperatures, rapid cooling, and peculiar magma composition seem to be important factors, but experimental work and detailed study of terrestrial volcanic plagioclase are necessary to interpret the chemical composition quantitatively.

Paramagnetic resonance spectra of Ti3+, Fe3+, and Mn2+in lunar plagioclases

At least three paramagnetic and one ferromagnetic components have been resolved in the electron magnetic resonance spectra of plagioclase fractions of two lunar samples, 14053-47 and 14321-166. Measurements were made at 9 and 35 GHz and at 346° 300°, and 130°K. One of the paramagnetic components is attributed to Fe3+ in crystal sites of low symmetries for which the axial terms D are 0.4 ≤ D ≤ 0.6 cm−1 and the ratio of rhombic E to axial terms is 0.24 30 and 3, respectively. The absolute concentration η(Ti3+) in 14053-47 and 14321–166 is ∼3 × 1017 and ∼6 × 1017 g−1, respectively. The concentrations of Fe3+ and Ti3+ are inversely correlated. The ferromagnetic resonance is attributed to single-domain iron particles on the basis of comparison with many lunar samples. A plagioclase fraction from Stillwater specimens was measured for purposes of comparison. The spectrum of Fe3+ in this material was quite similar to the two lunar plagioclases in shape, geff values, and relative proportions in various crystal sites. The concentration was approximately five times greater than that in the sample from 14053-47. A spectral component attributable to Ti3+ was not observed, nor was a ferromagnetic component detected. A Mn2+ spectrum was observed.