• Chronology, geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age, origin, structure, and impact history of the lunar crust

      Norman, M. D.; Borg, L. E.; Nyquist, L. E.; Bogard, D. D. (The Meteoritical Society, 2003-01-01)
      The petrology, major and trace element geochemistry, and Nd-Ar-Sr isotopic compositions of a ferroan noritic anorthosite clast from lunar breccia 67215 have been studied in order to improve our understanding of the composition, age, structure, and impact history of the lunar crust. The clast (designated 67215c) has an unusually well preserved igneous texture. Mineral compositions are consistent with classification of 67215c as a member of the ferroan anorthositic suite of lunar highlands rocks, but the texture and mineralogy show that it cooled more rapidly and at shallower depths than did more typical ferroan anorthosites (FANs). Incompatible trace element concentrations are enriched in 67215c relative to typical FANs, but diagnostic signatures such as Ti/Sm, Sc/Sm, plagiophile element ratios, and the lack of Zr/Hf and Nb/Ta fractionation show that this cannot be due to the addition of KREEP. Alternatively, 67215c may contain a greater fraction of trapped liquid than is commonly present in lunar FANs. 147Sm-143Nd isotopic compositions of mineral separates from 67215c define an isochron age of 4.40 +/- 0.11 Gyr with a near-chondritic initial 143Nd of +0.85 +/- 0.53. The 40Ar-39Ar composition of plagioclase from this clast records a post-crystallization thermal event at 3.93 +/- 0.08 Gyr, with the greatest contribution to the uncertainty in this age deriving from a poorly constrained correction for lunar atmosphere 40Ar. Rb-Sr isotopic compositions are disturbed, probably by the same event recorded by the Ar isotopic compositions. Trace element compositions of FANs are consistent with crystallization from a moderately evolved magma ocean and do not support a highly depleted source composition such as that implied by the positive initial 143Nd of the ferroan noritic anorthosite 62236. Alternatively, the Nd isotopic systematics of lunar FANs may have been subject to variable degrees of modification by impact metamorphism, with the plagioclase fraction being more strongly affected than the mafic phases. 147Sm-143Nd isotopic compositions of mafic fractions from the 4 ferroan noritic anorthosites for which isotopic data exist (60025, 62236, 67016c, 67215c) define an age of 4.46 +/- 0.04 Gyr, which may provide a robust estimate for the crystallization age of lunar ferroan anorthosites.
    • Solar wind and other gases in the regoliths of the Pesyanoe parent object and the moon

      Mathew, K. J.; Marti, K. (The Meteoritical Society, 2003-01-01)
      We report new data from Pesyanoe-90,1 (dark lithology) on the isotopic signature of solar wind (SW) Xe as recorded in this enstatite achondrite which represents a soil-breccia of an asteroidal regolith. The low temperature (less than or equal to 800 degrees C) steps define the Pesyanoe-S xenon component, which is isotopically consistent with SW Xe reported for the lunar regolith. This implies that the SW Xe isotopic signature was the same at two distinct solar system locations and, importantly, also at different times of solar irradiation. Further, we compare the calculated average solar wind "SW-Xe" signature to Chass-S Xe, the indigenous Xe observed in SNC (Mars) meteorites. Again, a close agreement between these compositions is observed, which implies that a mass-dependent differential fractionation of Xe between SW-Xe and Chass-S Xe is <1.5 ppm per amu. We also observe fractionated (Pesyanoe-F) Xe and Ar components in higher temperature steps and we document a fission component due to extinct 244Pu. Interestingly, the Pesyanoe-F Xe component is revealed only at the highest temperatures (>1200 degrees C). The Pesyanoe-F gas reveals Xe isotopic signatures that are consistent with lunar solar energetic particles (SEP) data and may indicate a distinct solar energetic particle radiation as was inferred for the moon. However, we cannot rule out fractionation processes due to parent body processes. We note that ratios 36Ar/38Ar less than or equal to 5 are also consistent with SEP data. Calculated abundances of the fission component correlate well with radiogenic 40Ar concentrations, revealing rather constant 244Pu/K ratios in Pesyanoe, and separates thereof, and indicate that both components were retained. We identify a nitrogen component (delta-15N = 44 ppm) of non-solar origin with an isotopic signature distinct from indigenous N (delta-15N = -33 ppm). While large excesses at 128Xe and 129Xe are observed in the lunar regolith samples, these excesses in Pesyanoe are small. On the other hand, significant 126Xe isotopic excesses, comparable to relative excesses observed in lunar soils and breccias, are prominent in the intermediate temperature steps of Pesyanoe-90,1.
    • KREEPy lunar meteorite Dhofar 287A: A new lunar mare basalt

      Anand, M.; Taylor, L. A.; Misra, K. C.; Demidova, S. I.; Nazarov, M. A. (The Meteoritical Society, 2003-01-01)
      Dhofar 287 (Dho 287) is a new lunar meteorite, found in Oman on January 14, 2001. The main portion of this meteorite (Dho 287A) consists of a mare basalt, while a smaller portion of breccia (Dho 287B) is attached on the side. Dho 287A is only the fourth crystalline mare basalt meteorite found on Earth to date and is the subject of the present study. The basalt consists mainly of phenocrysts of olivine and pyroxene set in a finer-grained matrix, which is composed of elongated pyroxene and plagioclase crystals radiating from a common nucleii. The majority of olivine and pyroxene grains are zoned, from core to rim, in terms of Fe and Mg. Accessory minerals include ilmenite, chromite, ulvaspinel, troilite, and FeNi metal. Chromite is invariably mantled by ulvaspinel. This rock is unusually rich in late-stage mesostasis, composed largely of fayalite, Si-K-Ba-rich glass, fluorapatite, and whitlockite. In texture and mineralogy, Dho 287A is a low-Ti mare basalt, with similarities to Apollo 12 (A-12) and Apollo 15 (A-15) basalts. However, all plagioclase is now present as maskelynite, and its composition is atypical for known low-Ti mare basalts. The Fe to Mn ratios of olivine and pyroxene, the presence of FeNi metal, and the bulk-rock oxygen isotopic ratios, along with several other petrological features, are evidence for the lunar origin for this meteorite. Whole-rock composition further confirms the similarity of Dho 287A with A-12 and A-15 sam- ples but requires possible KREEP assimilation to account for its rare-earth-element (REE) contents. Cooling-rate estimates, based on Fo zonation in olivine, yield values of 0.2-0.8 degrees C/hr for the lava, typical for the center of a 10-20 m thick flow. The recalculated major-element concentrations, after removing 10-15% modal olivine, are comparable to typical A-15 mare basalts. Crystallization mod- eling of the recalculated Dho 287A bulk-composition yields a reasonable fit between predicted and observed mineral abundances and compositions.
    • Northwest Africa 773: Lunar origin and iron-enrichment trend

      Fagan, T. J.; Taylor, G. J.; Keil, K.; Hicks, T. L.; Killgore, M.; Bunch, T. E.; Wittke, J. H.; Mittlefehldt, D. W.; Clayton, R. N.; Mayeda, T. K.; et al. (The Meteoritical Society, 2003-01-01)
      The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole-rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP-like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologies: a two-pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO-rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe-enrichment trend that culminated in crystallization of fayalite + silica + hedenbergitebearing symplectites. The Fe-enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow-level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE-enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE-depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements.
    • 40Ar-39Ar chronology of lunar meteorites Northwest Africa 032 and 773

      Fernandes, V. A.; Burgess, R.; Turner, G. (The Meteoritical Society, 2003-01-01)
      The 40Ar-39Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 +/- 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of 39Ar, or the presence of pre-shock 40Ar incorporated into shock-melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61-2.86 Gyr and a mean of 2.73 +/- 0.03 Gyr, identical to the total age of ~2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 +/- 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.
    • Magnetic fields of lunar multi-ring impact basins

      Halekas, J. S.; Lin, R. P.; Mitchell, D. L. (The Meteoritical Society, 2003-01-01)
      We survey the magnetic fields of lunar multi-ring impact basins using data from the electron reflectometer instrument on the Lunar Prospector spacecraft. As for smaller lunar craters, the primary signature is a magnetic low that extends to ~1.5-2 basin radii, suggesting shock demagnetization of relatively soft crustal magnetization. A secondary signature, as for large terrestrial basins, is the presence of central magnetic anomalies, which may be due to thermal remanence in impact melt rocks and/or shock remanence in the central uplift. The radial extent of the anomalies may argue for the former possibility, but the latter or a combination of the two are also possible. Central anomaly fields are absent for the oldest pre-Nectarian basins, increase to a peak in early Nectarian basins, and decrease to a low level for Imbrian basins. If basin-associated anomalies provide a good indication of ambient magnetic fields when the basins formed, this suggests the existence of a "magnetic era" (possibly due to a lunar core dynamo) similar to that implied by paleointensity results from returned lunar samples. However, the central basin anomalies suggest that the fields peaked in early Nectarian times and were low in Imbrian times, while samples provide evidence for high fields in Nectarian and early Imbrian times.
    • Chemistry of the Calcalong Creek lunar meteorite and its relationship to lunar terranes

      Hill, D. H.; Boynton, W. V. (The Meteoritical Society, 2003-01-01)
      The Calcalong Creek lunar meteorite is a polymict breccia that contains clasts of both highlands and mare affinity. Reported here is a compilation of major, minor, and trace element data for bulk, clast, and matrix samples determined by instrumental neutron activation analysis (INAA). Petrographic information and results of electron microprobe analyses are included. The relationship of Calcalong Creek to lunar terranes, especially the Procellarum KREEP Terrane and Feldspathic Highlands Terrane, is established by the abundance of thorium, incompatible elements and their KREEP-like CI chondrite normalized pattern, FeO, and TiO2. The highlands component is associated with Apollo 15 KREEP basalt but represents a variant of the KREEP-derived material widely found on the moon. Sources of Calcalong Creek's mare basalt components may be related to low-titanium (LT) and very low-titanium (VLT) basalts seen in other lunar meteorites but do not sample the same source. The content of some components of Calcalong Creek are found to display similarities to the composition of the South Pole-Aitken Terrane. What appear to be VLT relationships could represent new high aluminum, low titanium basalt types.
    • Lunar regolith breccia Dhofar 287B: A record of lunar volcanism

      Demidova, S. I.; Nazarov, M. A.; Anand, M.; Taylor, L. A. (The Meteoritical Society, 2003-01-01)
      Dhofar 287 (Dho 287), a recently found lunar meteorite, consists in large part (95%) of low-Ti mare basalt (Dho 287A) and a minor, attached portion (~5%) of regolith breccia (Dho 287B). The present study is directed mainly at the breccia portion of this meteorite. This breccia consists of a variety of lithic clasts and mineral fragments set in a fine-grained matrix and minor impact melt. The majority of clasts and minerals appear to have been mainly derived from the low-Ti basalt suite, similar to that of Dho 287A. Very low-Ti (VLT) basalts are a minor lithology of the breccia. These are significantly lower in Mg# and slightly higher in Ti compared to Luna 24 and Apollo 17 VLT basalts. Picritic glasses constitute another minor component of the breccia and are compositionally similar to Apollo 15 green glasses. Dho 287B also contains abundant fragments of Mg-rich pyroxene and anorthite-rich plagioclase grains that are absent in the lithic clasts. Such fragments appear to have been derived from a coarse-grained, Mg#-rich, Na-poor lithology. A KREEP component is apparent in chemistry, but no highlands lithologies were identified. The Dho 287 basaltic lithologies cannot be explained by near-surface fractionation of a single parental magma. Instead, magma compositions are represented by a picritic glass; a low-Ti, Na-poor glass; and a low-Ti, Na-enriched source (similar to the Dho 287A parental melt). Compositional differences among parent melts could reflect inhomogeneity of the lunar mantle. Alternatively, the low-Ti, Na-poor, and Dho 287A parent melts could be of hybrid compositions, resulting from assimilation of KREEP by picritic magma. Thus, the Dho 287B breccia contains lithologies from multiple magmatic eruptions, which differed in composition, formational conditions, and cooling histories. Based on this study, the Dho 287 is inferred to have been ejected from a region located distal to highlands terrains, possibly from the western limb of the lunar nearside, dominated by mare basalts and KREEP-rich lithologies.
    • From the Editor

      Jull, A. J. T. (The Meteoritical Society, 2003-01-01)
    • Erratum

      The Meteoritical Society, 2003-01-01
    • Book Review: The Robert Haag Collection of Meteorites, Robert Haag

      Rubin, A. E. (The Meteoritical Society, 2003-01-01)
    • Experimental and petrological constraints on lunar differentiation from the Apollo 15 green picritic glasses

      Elkins-Tanton, L. T.; Chatterjee, N.; Grove, T. L. (The Meteoritical Society, 2003-01-01)
      Phase equilibrium experiments on the most magnesian Apollo 15C green picritic glass composition indicate a multiple saturation point with olivine and orthopyroxene at 1520 degrees C and 1.3 GPa (about 260 km depth in the moon). This composition has the highest Mg# of any lunar picritic glass and the shallowest multiple saturation point. Experiments on an Apollo 15A composition indicate a multiple saturation point with olivine and orthopyroxene at 1520 degrees C and 2.2 GPa (about 440 km depth in the moon). The importance of the distinctive compositional trends of the Apollo 15 groups A, B, and C picritic glasses merits the reanalysis of NASA slide 15426,72 with modern electron microprobe techniques. We confirm the compositional trends reported by Delano (1979, 1986) in the major element oxides SiO2, TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, and CaO, and we also obtained data for the trace elements P2O5, K2O, Na2O, NiO, S, Cu, Cl, Zn, and F. Petrogenetic modeling demonstrates that the Apollo 15 A-B-C glass trends could not have been formed by fractionalcrystallization or any continuous assimilation/fractional crystallization (AFC) process. The B and C glass compositional trends could not have been formed by batch or incremental melting of an olivine + orthopyroxene source or any other homogeneous source, though the A glasses may have been formed by congruent melting over a small pressure range at depth. The B compositional trend is well modeled by starting with an intermediate A composition and assimilating a shallower, melted cumulate, and the C compositional trend is well modeled by a second assimilation event. The assimilation process envisioned is one in which heat and mass transfer were separated in space and time. In an initial intrusive event, a picritic magma crystallized and provided heat to melt magma ocean cumulates. In a later replenishment event, the picritic magma incrementally mixed with the melted cumulate (creating the compositional trends in the green glass data set), ascended to the lunar surface, and erupted as a fire fountain. A barometer created from multiple saturation points provides a depth estimate of other glasses in the A-B-C trend and of the depths of assimilation. This barometer demonstrates that the Apollo 15 A-B-C trend originated over a depth range of ~460 km to ~260 km within the moon.
    • Megaregolith evolution and cratering cataclysm models-—Lunar cataclysm as a misconception (28 years later)

      Hartmann, W. K. (The Meteoritical Society, 2003-01-01)
      The hypothesis of a lunar cataclysmic cratering episode between 3.8 and 3.9 Gyr ago lacks proof. Its strongest form proposes no cratering before about 4.0 Gyr, followed by catastrophic formation of most lunar craters and basins in 200 Myr. The premise that "zero impact melts implies zero impacts" is disproved by data from asteroids, on which early collisions clearly occurred, but from which early impact melts are scarce. Plausible cataclysm models imply that any cataclysm should have affected the whole inner solar system, but among available lunar and asteroid impact melt and impact age resetting data, a narrow, strong 3.8-3.9 Gyr spike in ages is seen only in the region sampled by Apollo/Luna. Reported lunar meteorite data do not show the spike. Asteroid data show a broader, milder peak, spreading from about 4.2 to 3.5 Gyr. These data suggest either that the spike in Apollo impact melt ages is associated with unique lunar front side events, or that the lunar meteorites data represent different kinds of events than the Apollo/Luna data. Here, we develop an alternate "megaregolith evolution" hypothesis to explain these data. In this hypothesis, early impact melts are absent not because there were no impacts, but because the high rate of early impacts led to their pulverization. The model estimates survival halflives of most lunar impact melts prior to 4.1 Gyr at <100 Myr. After a certain time, Tcritical ~4.0 Gyr, impact melts began to survive to the present. The age distribution differences among impact melts and plutonic rocks are controlled by, and hold clues to, the history of regolith evolution and the relative depths of sequestration of impact melts versus plutonic rocks, both among lunar and asteroidal samples. Both the "zero cratering, then cataclysm" hypothesis and the "megaregolith evolution" hypothesis require further testing, especially with lunar meteorite impact melt studies.