Lunar Regolith Breccias Research Articles

Helium in Lunar Samples Analyzed by High‐Resolution Stepwise Etching: Implications for the Temporal Constancy of Solar Wind Isotopic Composition

The 3He/4He ratio in the present and past solar wind (SW) is of interest because it provides an estimate for the protosolar deuterium abundance. SW He and Ne composition has been analyzed in ilmenites of lunar soils (71501, 12001, 74241) and regolith breccias (79135, 79035, 14301) by closed system stepwise etching. The samples have SW antiquities ranging from ~0.1 to ~3.7 Gyr. The experiments have a very high depth resolution; thus, possible artifacts that may compromise the precise determination of the SW composition can be controlled. The nominal average (3He/4He)SW ratios correlate with the SW antiquity, suggesting at first glance a (3He/4He)SW increase of 6% per Gyr. In addition, also the (20Ne/22Ne)SW ratio seems to have increased with time by about 2% per Gyr. However, there are reasons indicating that the apparent temporal increase of (3He/4He)SW is an artifact. First, there is no straightforward explanation for a concurrent temporal change of (20Ne/22Ne)SW to the extent observed. Second, the average nominal (3He/4He)SW and (20Ne/22Ne)SW ratios correlate with each other, and this correlation parallels the one displayed by the single etch steps, indicating a progressive admixture of the deeper sited, isotopically heavier solar energetic particles with ongoing etching. We suggest that the nominal temporal variation of (3He/4He)SW and (20Ne/22Ne)SW is the result of secondary processes that cause erosion of the outermost layers of the regolith grains and thus a partial removal of the very surface-sited SW. Correcting this partial grain surface loss, assuming a constant SW Ne isotopic composition throughout the Sun's lifetime, leads to constant (3He/4He)SW at (4.47 ± 0.13) × 10-4 within the last ~4 Gyr. The results suggest that the present-day (3He/4He)SW can directly be used to deduce the protosolar (D+3He)/4He composition and that a possible mixing between radiative interior and convective zone must be restricted to a thin boundary layer.

Porosities of lunar meteorites: Strength, porosity, and petrologic screening during the meteorite delivery process

Porosity has been directly measured for eight lunar meteorite breccias and calculated for two more on the basis of literature density measurements. Lunar meteorite regolith breccias display systematically low porosity in comparison to otherwise analogous Apollo regolith breccias. Among seven meteoritic regolith breccias, porosity ranges from 1 to 11% and averages 7.5 ± (1‐σ) 3.2%, whereas for 44 analogous Apollo samples (porosities mostly calculated from literature density data) the average is 25 ± (1‐σ) 7%. The origin of this disparity is enigmatic, but the trend probably reflects mainly a bias in favor of strong, compact breccias among fragments that manage to survive the violent process of launch to lunar escape velocity (2.38 km/s). In addition, compaction during launch may play an important role. The population of lunar meteorites is clearly not a random, unmodified sample of lithic materials near the surface of the parent body.

Lunar meteorite Queen Alexandra Range 93069: A lunar highland regolith breccia with very low abundances of mafic components

Abstract— Lunar meteorite QUE 93069 found in Antarctica is a mature, anorthitic regolith breccia with highland affinities that was ejected from the Moon <0.3 Ma ago. The frequency distribution of mineral and lithic clasts gives information about the nature of the regolith and subregolith basement near the ejection site as well as about the abundances of rock types shocked to different degrees prior to the breccia formation.Thin section QUE 93069,37 consists of 67.5 vol% fine‐grained (<∼130 μm) constituents and 32.5 vol% mineral and lithic clasts and an impact melt vein. The most abundant types of these clasts are intragranularly recrystallized anorthosites and plagioclases (together 26.3 vol%) and feldspathic fine‐grained to microporphyritic crystalline melt breccias (21.9 vol%). Mafic crystalline melt breccias are extremely rare (1.3 vol%). Granulitic lithologies are 10.4 vol%, recrystallized feldspathic melt breccias are 15.0 vol%, and glasses are 3.5 vol%. The impact melt vein cutting across the entire thin section was probably formed subsequent to the lithification process of the bulk rock at pressures below 20 GPa, because the bulk rock never experienced a higher peak shock pressure.Lunar meteorite QUE 93069 has a higher abundance of clear glass, occurring within melt spherules, glassy fragments, and an impact melt vein than lunar meteorites ALHA81005, Y‐791197, Y‐82192/3, Y‐86032, or MAC 88104/5. The high abundance of melt spherules indicates that this lunar meteorite contains the highest content of typical regolith components. Mafic crystalline melt breccias are much rarer in QUE 93069 than in all other lunar highland regolith breccias. The extremely low abundance of mafic components may constrain possible areas of the Moon, from which the breccia was derived. The source area of QUE 93069 must be a highland terrain lacking significant mafic impact melts or mare components.

In quest of lunar regolith breccias of exotic provenance: a uniquely anorthositic sample from the Fra Mauro (Apollo 14) highlands

Polymict samples can be used to establish mass-balance constraints regarding the bulk composition of the lunar crust, and to gauge the degree of regional heterogeneity in the composition of the lunar crust. The most ideally polymict type of sample is finely-mixed regolith (lunar soil), or its lithified equivalent, regolith breccia. Fortunately, lunar regolith breccias can occasionally be found at great distances from their points of origin — most of the known lunar meteorites are regolith breccias. We are searching for examples of exotic regolith samples among the Apollo regolith breccia collection. Most of the 21 Apollo regolith breccias analyzed for this study strongly resemble the local soils over which they were collected. Nine regolith breccias from Apollo 16 are surprisingly mature compared to previously-analyzed Apollo 16 regolith breccias, and six of the seven from Apollo 16 Station 5 have lower, more local-soil-like, mg ratios than previously analyzed regolith breccias from this station. Several of the Apollo 14 regolith breccias investigated show significantly higher mg, and lower Al, than the local soils. The most interesting sample we have investigated is 14076,1, from a lithology that constitutes roughly half of a 2.0-g pebble. The presence of spherules indicates a regolith derivation for 14076,1, yet its highly aluminous (30 wt.% Al 2O 3) composition is clearly exotic to the 1.6-km traverse surface over which the Apollo 14 samples were collected. This sample resembles soils from the Descartes (Apollo 16) highlands far more than it does any other polymict sample from the Fra Mauro (Apollo 14) region. The I/ sFeO maturity index is extremely low, but this may be a result of thermal annealing. A variety of siderophile elements occur in 14076,1 at typical regolith concentrations. The chemistry of the second most aluminous regolith sample from Apollo 14, 14315, can only be roughly approximated as a mixture of local regolith and 14076,1-like material. However, the low a priori statistical probability for long-distance horizontal transport by impact cratering, along with the relatively high contents of incompatible elements in 14076,1 (despite its high Al content), suggest that this regolith breccia probably originated within a few hundred kilometers of the Appollo 14 site. If so, its compositional resemblance to ferroan anorthosite tends to suggest that the regional crust is, or originally was, far richer in ferroan anorthosite than implied by the meager statistics for pristine rocks from this site. Thus, 14076,1 tends to strengthen the hypothesis that ferroan anorthosite originated as the flotation crust of a global magmasphere.

Long-term changes in solar wind elemental and isotopic ratios: A comparison of two lunar ilmenites of different antiquities

An ilmenite separate from lunar regolith breccia 79035, a sample presumed to have been exposed to solar wind more than 2 Ga ago, was analyzed for noble gas and nitrogen elemental and isotopic abundances by stepwise oxidation and pyrolysis. The gases appear to be distributed between two distinct reservoirs in the ilmenite, defined by release patterns and isotopic considerations. One of the reservoirs, near grain surfaces, yields elemental ratios that for the most part are “solar” while the other, sited at greater depths within grains, has severely fractionated elemental abundances and generally heavier isotopic ratios as well. Xenon provides an exception to the solar abundance pattern in the near-surface reservoir, being enhanced by about a factor of 2 relative to the expected value. A comparison of the 79035 separate with a previously analyzed ilmenite from soil 71501, which received its solar wind exposure much more recently, indicates that the two-fold xenon enhancement occurs in the fractionated reservoir as well as the “solar” one, and that it may therefore be attributable to a change in the solar wind elemental abundances. Other differences between the two ilmenites occur in helium and neon isotopic ratios and in He Ar elemental ratios. Since mineralogical influences on retentivities of the gases in the two samples should be the same, and possible contributions of non-solar wind components to one ilmenite in preference to the other can generally be eliminated or accounted for, all of these differences may reflect changes in the solar wind over time.

A potpourri of regolith breccias: “New” samples from the Apollo 14, 16, and 17 landing sites

Forty suspected regolith breccias from the Apollo 14, 16, and 17 landing sites were studied as part of a search for regolith samples exotic to the small traverse areas associated with these missions, as well as a general effort to constrain the nature and origins of regolith breccias. Of these 40 samples, 31 are indeed regolith breccias. Regolith breccias from Apollo 14 exhibit much greater compositional diversity than their soil (sensu stricto) counterparts: 14004,55 displays incompatible element concentrations about 1.4× those found in typical Apollo 14 regolith, while 14315 is radically different in many respects (higher Al, lower Mg and Fe, lower incompatible elements) from all other Apollo 14 regolith materials, and hence probably exotic to the Apollo 14 traverse area. Application of “mixing” models to Apollo 14 regolith materials suggests that whereas “normal” Apollo 14 regolith contains only slightly more ferroan anorthosite (FA) than alkali anorthosite (AA) and far more KREEP than FA and AA combined, 14315 contains more FA than KREEP and roughly 10 times more FA than AA. Three “new” regolith breccias from Apollo 16 bring to 20 the total number of regolith breccias that have been analyzed for Mg and Fe from the site. The molar Mg/(Mg + Fe) (or mg*) ratios of lunar regolith breccias are of great interest, because lunar‐meteorite ALHA81005 has a much higher mg* than two other lunar‐meteoritic regolith breccias (Y791197 and Y82192/3), even though these three samples are remarkably similar in all other compositional respects. A bimodality in mg* among Apollo 16 regolith breccias may result from: (1) statistical scattering among the small number of samples, (2) a tendency for older regolith breccias from this site to be compositionally distinct from their younger counterparts, or (3) differences between the Cayley and Descartes Formations. Among 10 samples from Apollo 17, all but one proved to be typical Apollo 17 regolith breccias. However, 72504,10 is >99% pure pyroclastic glass, compositionally identical to the Apollo 17 orange glass (74220), even though 72504,10 was obtained at a location over 4 km from Shorty Crater where 74220 was obtained. The nearly pure concentration of 74220‐identical glass spherules in 72504,10 attests to the widespread distribution of these distinctive deposits.

Noble component organization in Apollo 14 Breccia 14318: 129I and 244Pu regolith chronology

Noble gas, petrological, and chemical studies made on grain‐size separates from lunar regolith breccia 14318 demonstrate that the noble gases are organized into two functional components, volume‐correlated and surface‐correlated. As in regolith breccia 14301, volume‐correlated xenon in 14318 is primarily spallation‐derived and the surface‐correlated component contains not only solar wind xenon but also significant amounts of “parentless” xenon from the fission of now extinct 244Pu and the decay of now extinct 229I (“parentless” means the daughter products were incorporated onto grain surfaces following decay of the parent nuclide elsewhere). The ratio of 129Xe/136Xe in the total surface‐correlated parentless component, as identified in grain‐size analysis, is substantially higher than in the least tightly bound parentless component identified in stepwise heating analyses, confirming the trend seen in 14301. If the order of release of gases in stepwise heating is related to the order of incorporation in the simplest way (first in, last out), incorporation of these grain‐surface components was probably time‐ordered. The 129Xe/136Xe ratio in each identifiable parentless component would then be characteristic of the xenon available for surface adsorption at the particular time of acquisition. Continuous variations in this ratio further suggest that incorporation of the parentless xenon was closely coupled with production. Such observations provide the basis for a new chronometer from which we conclude that acquisition of parentless xenon was an ongoing process spanning at least 90 m.y., beginning no more than 63 + 34 m.y. after the formation of primitive meteorites and possibly predating xenon acquisition for the earth.

Petrology of Apollo 11 regolith breccias

Petrographic and mineral chemical data for 16 Apollo 11 regolith breccias show that: (1) the regolith breccias differ from soil 10084 with respect to (a) agglutinate content, (b) glass population, (c) plagioclase compositions, and (d) proportions of high‐K mare and low‐K mare basalt components; (2) the A‐11 breccias and soil have highland components that are similar both in abundance and petrology; and (3) lunar regolith breccias provide a better comparison with howardites than do lunar soils. The data and observations are consistent with formation of the regolith breccias from immature soil. It appears that little or no highland material has been added to the Tranquillitatis regolith since the formation of the breccias.

Aspects of a glassy meteorite from the moon bearing on some problems in extraterrestrial glass-making

Antartic meteorite ALHA 81005 is a glassy piece of lunar highland regolith breccia. Not only is it unique meteorite but the glasses contained therein probably come close to spanning the PT range that lunar impact-generated glasses have experienced. The glasses that formed near the upper end of this range are particularly instructive. They are optically and chemically homogeneous, contain no bubbles or relict minerals and are perfectly colorless (though there is no obvious chemical reason why this should be so). Color aside, these are features that have been previously attributed to lunar volcanic glasses. They are also characteristic of most tektites and have been offered as evidence against an impact origin for these important glasses and, by default, for a lunar volcanic origin. Both these positions are no longer tenable. All the criteria used to distinguish lunar volcanic from lunar impact glasses deserve re-evaluation. And the tenaceously held theory that tektites are products of recent, hyper-explosive lunar volcanism should (but will not) be abandoned.

The production curve for agglutinates in planetary regoliths

Models for the production of agglutinates are developed that can be applied to the lunar surface or to any planetary or asteroidal body lacking an atmosphere. Models are developed using rate equations for progressively more complex situations and range from Model 1, which is a simple linear increase of agglutinate content with time, to Model 4, which includes provision for recycling of existing agglutinates and replenishment and burial of exposed soil. Model 4 has some aspects of a steady state because, depending on the rate constants, agglutinate content may be limited to an intermediate value, even for long exposure times. In an extreme case, agglutinate content may be limited to a value near zero. These models predict that agglutinates should be low in abundance in areas of thin regolith, such as the Lunokhod‐2 site on the moon, and on asteroids. The models may also help explain the apparent low agglutinate abundances of lunar regolith breccias and meteorite regolith breccias.

Meteorite ALHA81005: Petrology of a new lunar highland sample

Meteorite ALHA81005 is a shock‐compacted lunar highland regolith breccia. Besides glasses (fragments, spherules, matrix glass), chondrules (of ANT composition), and mineral fragments this new lunar sample contains principally three lithologies: 1) A pristine ferroan anorthosite suite consisting of anorthosites, norites, and one plagioclase peridotite. 2) A high‐Mg granulitic breccia suite representing variable mixtures of troctolitic and noritic precursor rocks. 3) A melt‐rock suite consisting of feld‐spathic basalts and metabasalts of a composition intermediate between (1) and (2). Most glass compositions and the chondrule compositions also project into this group. All lithologies and their derivatives are characteristically poor in alkalis. Neither a KREEP nor a mare basalt composition could be identified with certainty. ALHA81005 apparently comes from a highland area far from any mare basalt and KREEP source.

Petrology of ALHA81005, the first lunar meteorite

Petrography and mineral compositions show that ALHA81005 is a lunar highland regolith breccia and is the first known lunar meteorite. Lithic clasts are abundant and most are members of the anorthosite‐norite‐troctolite highland suite. Plagioclase compositions in lithic clasts and single grains are calcic (An94‐98). Pyroxene compositions, Al‐Ti and MnO‐FeO relationships support a lunar highland origin. A non‐KREEPy source region is indicated, and the unexplored far side of the moon cannot be ruled out as a potential source.

Regolith Breccia Allan Hills A81005: Evidence of lunar origin, and petrography of pristine and nonpristine clasts

MnO/FeO ratios in pyroxene, texture (abundant brown, swirly glass, in general typical of lunar regolith breccias), and overall composition (∼75% plagioclase) all indicate that Allan Hills A81005 is of lunar origin. It is presumably from a heretofore unsampled region of the Moon. It differs in detail from other regolith samples, e.g., it has exceptionally low contents of Na and KREEP. A pristine clast contains exceptionally coarse augite, compared to similar Apollo samples. ALHA81005 is not perceptibly more shocked than typical Apollo regolith breccias. Therefore, its discovery on Earth lends further credence to suggestions that SNC achondrites were derived by impact ejection from Mars.