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NASA NTRS1993

An iridium-rich iron micrometeorite with silicate inclusions from the Moon

We have found a 0.1 mg iron micrometeorite containing meteoritic silicate inclusions in an agglutinate from 2-2.5 cm deep in regolith core 60014. The metal is 93 percent iron, 6.5 percent nickel, 0.5 percent cobalt, approximately 150 ppm iridium, and less than 2 ppm gold. Although the Ir concentration is higher than that reported previously for any iron meteorite group, it lies on the extrapolation to low Ni and high Ir concentrations of several meteorite groups on Ni,Ir plots (groups 2C,D,E, and 3AB,E,F). Tiny, subrounded silicate inclusions comprise low-Ca pyroxene (En83), olivine (FO80), and albitic and potassic feldspars, as mixtures of minerals or glasses. Minor phases include oldhamite (CaS) and, tentatively, hercynite (FeAl2O4). The inclusions have pyroxene FeO/MnO of approximately 25 and olivine FeO/MnO of 40-60. In comparison with known iron meteorites, the inclusions are most similar to those in type 2E, e.g., Weekeroo Station, Colomera, and Kodaikanal. As far as we know, this is the first observation of an iron meteorite with silicate inclusions from a lunar sample. No metal fragments with meteoritic, nonmetallic inclusions were reported in several previous, exhaustive studies of soil particles.

Jolliff, Bradley L.↗

NASA NTRS1993

An initial perspective of S-asteroid subtypes within asteroid families

Many main belt asteroids cluster around certain values of semi-major axis (a), inclination (i), and eccentricity (e). Hirayama was the first to notice these concentrations which he interpreted as evidence of disruptions of larger parent bodies. He called these clusters 'asteroid families'. The term 'families' is increasingly reserved for genetic associations to distinguish them from clusters of unknown or purely dynamical origin (e.g. the Phocaea cluster). Members of a genetic asteroid family represent fragments derived from various depths within the original parent planetesimal. Thus, family members offer the potential for direct examination of the interiors of parent bodies which have undergone metamorphism and differentiation similar to that occurring in the inaccessible interiors of terrestrial planets. The differentiation similar to that occurring in the inaccessible interiors of terrestrial planets. The condition that genetic family members represent the fragments of a parent object provides a critical test of whether an association (cluster in proper element space) is a genetic family. Compositions (types and relative abundances of materials) of family members must permit the reconstruction of a compositionally plausible parent body. The compositions of proposed family members can be utilized to test the genetic reality of the family and to determine the type and degree of internal differentiation within the parent planetesimal. The interpretation of the S-class mineralogy provides a preliminary evaluation of family memberships. Detailed mineralogical and petrological analysis was done based on the reflectance spectra of 39 S-type asteroids. The result is a division of the S-asteroid class into seven subtypes based on compositional differences. These subtypes, designated S(I) to S(VII), correspond to surface silicate assemblages ranging from monomineralic olivine (dunites) through olivine-pyroxene mixtures to pure pyroxene or pyroxene-feldspar mixtures (basalts). The most general conclusion is that the S-asteroids cannot be treated as a single group of objects without greatly oversimplifying their properties. Each S-subtype needs to be treated as an independent group with a distinct evolutionary history.

Kelley, M. S.↗

NASA NTRS1993

Liquidus equilibria of lunar analogs at high pressure

Melting experiments have been performed in the range of 20 to 40 kbar on partially crystallized synthetic glasses in order to test the accuracy of the polybaric fractional fusion model for picritic lunar green glasses. Results show that the model predicts the position of the olivine (ol) + orthopyroxene (opx) liquidus boundary within the uncertainty of the measurements, but that details of the calculations are subject to change because of new crystal/liquid partitioning data for olivine and pyroxene.

Longhi, J.↗

NASA NTRS1993

The Nakhla parent melt: REE partition coefficients and clues to major element composition

Nakhla is one of the SNC meteorites, generally believed to be of Martian origin. It is a medium-grained augite-olivine cumulate with a variolitic groundmass of sodic plagioclase, alkali feldspar, and Fe-rich pyroxenes and olivine. One of the major tasks in deciphering Nakhla's petrogenesis is determining the composition of its parent melt. Gaining an understanding of the composition and petrogenesis of this parent melt may help unravel Nakhla's relationship to the other SNCs, and provide clues to Martian petrogenesis in general. Our experimental partitioning studies provide new information that helps constrain both the major and trace element composition of the Nakhla parent melt.

Mckay, G.↗

NASA NTRS1993

Refractory precursor components in an Allende ferromagnesian chondrule

Chemical and petrological studies of chondrules revealed that they were formed through melting of pre-existing solid precursor materials, and that one of the refractory lithophile precursors was a high temperature condensate from the nebular gas and related to Ca, Al-rich inclusions (CAIs). Sheng et al. found relict spinel grains with isotopically fractionated Mg in plagioclase-olivine inclusions from CV chondrites and suggested that the major fractionation processes were common to CAIs and chondrules. We have determined the Mg isotopic compositon of five barred olivine chondrules and one coarse-grained rim from the Allende (CV3) meteorite. A reproducibility of instrumental isotope fractionation is plus or minus 2 per thousand per amu. The precision of the Mg-26/Mg-24 data after normalization for mass fractionation can be as good as 0.5 per thousand (2 sigma(mean)). The Mg analytical results are given and indicate that delta Mg-25/Mg-24 and sigma Mg-26 of the chondrules are normal within errors.

Misawa, Keiji↗

NASA NTRS1993

Fragments of ancient lunar crust: Ferroan noritic anorthosites from the descartes region of the Moon

Noritic anorthosite clasts from breccia 67016 have bulk compositions similar to that of the upper crust of the Moon and petrogenetic affinities with pristine ferroan anorthosites. Rb-Sr and Sm-Nd isotopic compositions of mineral separates from one of these clasts suggest very old (greater than or = 4.4 Ga) ages, but interpretation of these data is complicated by the multi-stage history of the clasts which involved magmatic crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. These clasts record some of the earliest events on the Moon, including early crust formation, accretionary bombardment, and degassing of the lunar interior. Modal analyses of these clasts show they are now composed of about 70 percent plagioclase, 28 percent pyroxene, 2 percent troilite, and minor amounts of ilmenite and chromite. No metallic iron, phosphates, or other trace phases were observed. Olivine is very rare, occurring only as relicts within secondary troilite+pyroxene intergrowths which may reflect reaction of olivine with sulfurous vapors. PIXE proton microprobe analyses of the sulfides show that the metasomatism was accompanied by enrichments of Cu, Zn, Ni, Se, and Sb. The clasts have been only mildly shocked since the observed texture was established. Major and minor element mineral compositions are very homogeneous and strikingly similar to those of pristine ferroan anorthosites. Pyroxene compositions indicate equilibration temperatures of 850-900 C. Except for the sulfide and chalcophile element metasomatism, these clasts appear to be essentially monomict and probably represent a noritic member of the ferroan anorthosite suite. Their low Ni contents and Ni/Co ratios are consistent with the interpretation of these clasts as igneous rocks which have escaped mixing with meteoritic material.

Norman, M. D.↗

NASA NTRS1993

Orthopyroxenes as recorders of diogenite petrogenesis: Trace element systematics

Eucrite, howardite, and diogenite members of the achondrites are considered, by many, to be genetically related. Therefore, each provide a piece of the puzzle for reconstructing magmatic processes on the eucrite parent body (EPB). The relationship between eucrites and diogenites can be viewed within the context of two distinctly different models: (1) fractional crystallization; and (2) partial melting. In fractional crystallization models, eucrites and diogenites represent a complementary continuum of planetary fractional crystallization products in which the diogenites represent crystal accumulations during the crystallization of eucritic magmas at shallow to deep levels in the EPB. Alternatively, experimental studies may be interpreted as indicating eucrites represent peritectic partial melts of a primitive, chondritic EPB mantle. Within this type of model, the diogenites are also generally considered to be cumulates; however, their petrogenetic relationship to the eucrites is less clear. Sack et al. proposed that the olivine diogenites represent residua from the partial melting events that produced eucritic liquids. Initial trace element studies of orthopyroxene (OPX) are consistent with this model. However, this trace element modeling of the olivine diogenites is nonunique. As a further test of these models, we did the following three things: (1) analyzed OPX from cumulate diogenites to compare with the olibine diogenite data; (2) improved ion microprobe analytical techniques for the analysis of elements critical to our interpretations; and (3) selected more relevant Kd's for OPX-eucritic melt.

Shearer, C. K.↗

NASA NTRS1993

Implantation of nitrogen: Effects of hydrogen and implantation energy

To solve the question on solar nitrogen in lunar soils, i.e. variation in isotopic composition and apparently high retentivity compared with rare gases, nitrogen implantation experiments were conducted. At the Meteoritical Society Meeting in Copenhagen, the results of stepped combustion of implanted nitrogen in ilmenite and olivine were presented. The degassing behavior of nitrogen (and also Ar) was quite different from that observed in the case of lunar soils. Extraction temperatures are higher (greater than 1100 C for ilmenite and 1500 C for olivine) than that for lunar soils. Both nitrogen and Ar seem to be retained at the same efficiency. Therefore, additional experiments were conducted to make degassing behavior of nitrogen more close to that observed in the case of lunar soils.

Sugiura, Naoji↗

NASA NTRS1993

The parent magma of the Nakhla (SNC) meteorite: Reconciliation of composition estimates from magmatic inclusions and element partitioning

The composition of the parent magma of the Nakhla meteorite was difficult to determine, because it is accumulate rock, enriched in olivine and augite relative to a basalt magma. A parent magma composition is estimated from electron microprobe area analyses of magmatic inclusions in olivine. This composition is consistent with an independent estimate based on the same inclusions, and with chemical equilibria with the cores of Nakhla's augites. This composition reconciles most of the previous estimates of Nakhla's magma composition, and obviates the need for complex magmatic processes. Inconsistency between this composition and those calculated previously suggests that magma flowed through and crystallized into Nakhla as it cooled.

Treiman, A. H.↗

NASA NTRS1993

Origin of high-Ti lunar ultramafic glasses

A series of high pressure melting experiments were performed on a synthetic analog of the Apollo 14 Black Glass. At 15 kb and 1400 C, olivine is the liquidus phase. At 25 kb and 1450 C, orthopyroxene and Cr-spinel crystallize. A multiple saturation point with olivine + orthopyroxene + spinel on the liquidus occurs at 20 kb and 1440 C. It was proposed that the high-Ti ultramafic glass evolved from a low-Ti Green Glass generated deep in the lunar interior that assimilated shallow-level, late stage cumulates of magma ocean differentiation. Heat for assimilation is provided by superheat of the low-Ti magma and melts high-Ca pyroxene and ilmenite cumulates at a depth of 100 km. This model is a variant of that proposed by Hubbard and Minear and rejected by Ringwood and Kesson.

Wagner, T. P.↗

NASA NTRS1993

Morphology correlation of craters formed by hypervelocity impacts

Dust-sized olivine particles were fired at a copper plate using the Space Power Institute hypervelocity facility, simulating micrometeoroid damage from natural debris to spacecraft in low-Earth orbit (LEO). Techniques were developed for measuring crater volume, particle volume, and particle velocity, with the particle velocities ranging from 5.6 to 8.7 km/s. A roughly linear correlation was found between crater volume and particle energy which suggested that micrometeoroids follow standard hypervelocity relationships. The residual debris analysis showed that for olivine impacts of up to 8.7 km/s, particle residue is found in the crater. By using the Space Power Institute hypervelocity facility, micrometeoroid damage to satellites can be accurately modeled.

Crawford, Gary D.↗

NASA NTRS1993

Carbonate formation on Mars: Latest experiments

Laboratory simulations of Martian CO2 storage address whether carbonate formation could have reduced CO2 pressure from a hypothetical greater than 1 bar to the present 7 mbar in less than or equal to 3 to 4 billion years. This problem is addressed with experiments and analysis designed to verify and improve previous kinetic measurements, reaction mechanisms, and product characterizations, with the goal of improving existing models of Martian CO2 history. A sensitive manometer monitored the pressure drop of CO2 due to uptake by powdered silicate for periods of 3 to 100+ days. Pressure drops for diopside 1 and basalt show rapid short-term (approximately one day) CO2 uptake and considerably slower long-term pressure drops. Curves for diopside 2, olivine 1, and olivine 2 are qualitatively similar to those for diopside 1, whereas quartz and plagioclase show near-zero short-term pressure drops and very slow long-term signals, indistinguishable from a leak (less than 10(exp 11) mol/sq m/s).

Stephens, S. K.↗

NASA NTRS1994

Reduction of lunar mare soil and pyroclastic glass

Mare soil and orange and black pyroclastic glass were reduced in hydrogen gas at temperatures of 900 to 1100 C. The experiments support studies of regolith maturation, lunar volcanism, and the production of oxygen on the moon. The most reactive component in the high-Ti soil was FeO in ilmenite, which was completely reduced to iron metal at all temperatures. Vitreous orange glass crystallized and was partially reduced to iron metal, pyroxene, and minor olivine. Initially devitrified black glass was similarly reduced and converted to iron metal, pyroxene, and minor olivine. The degree of reaction in both glasses increased with temperature.

Allen, C. C.↗

NASA NTRS1994

The parent magma of xenoliths in shergottite EETA79001: Bulk and trace element composition inferred from magmatic inclusions

The SNC meteorites are samples of the Martian crust, so inferences about their origins and parent magmas are of wide planetologic significance. The EETA79001 shergottite, a basalt, contains xenoliths of pyroxene-olivine cumulate rocks which are possibly related to the ALHA77005 and LEW88516 SNC lherzolites. Olivines in the xenoliths contain magmatic inclusions, relics of magma trapped within the growing crystals. The magmatic inclusions allow a parent magma composition to be retrieved; it is similar to the composition reconstructed from xenolith pyroxenes by element distribution coefficients. The xenolith parent magma is similar but not identical to parent magmas for the shergottite lherzolites.

Treiman, Allan H.↗

NASA NTRS1994

What does the fine-scale petrography of IDPs reveal about grain formation and evolution in the early solar system?

The 'pyroxene' interplanetary dust particles (IDP's) may be the best samples for investigation of primordial grain-forming reactions because they appear to have experienced negligible post-accretional alteration. They are likely to continue to yield information about gas-to-solid condensation and other grain-forming reactions that may have occurred either in the solar nebular or presolar interstellar environments. An immediate challenge lies in understanding the nanometer-scale petrography of the ultrafine-grained aggregates in 'pyroxene' IDP's. Whether these aggregates contain components from diverse grain-forming environments may ultimately be answered by systematic petrographic studies using electron microscopes capable of high spatial resolution microanalysis. It may be more difficult to decipher evidence of grain formation and evolution in 'olivine' and 'layer silicate' IDP's because they appear to have experienced post-accretional alteration. Most of the studied 'olivine' IDPs have been subjected to heating and equilibration, perhaps during atmospheric entry, while the 'layer silicate' IDP's have experienced aqueous alteration.

Bradley, John↗

NASA NTRS1994

Relict grains in chondrules: Evidence for chondrule recycling

The presence of relict grains in chondrules, which offers some insight into the degree to which chondrule material was recycled in the chondrule-forming region, is discussed in this report. Relics are grains that clearly did not crystallize in situ in the host chondrule. They represent coarse-grained precursor material that did not melt during chondrule formation, and provide the only tangible record of chondrule precursor grains. Relics are commonly identified by a large difference in size, textural differences, and/or significant compositional differences compared with normal grains in the host chondrule. Two important types of relics are: (1) 'dusty,' metal-bearing grains of olivine and pyroxene; and (2) forsterite (Mg-rich olivine) grains present in FeO-rich chondrules.

Jones, R. H.↗

NASA NTRS1994

Collision of cometlike and slightly differentiated bodies as an origin for ordinary chondrites

One of the keys to understanding the origins of chondrites is given by the characteristics of relict minerals in chondrules that survived chondrule formation. Some of the relic materials show shock deformation textures, such as high dislocation densities in relict olivine and stacking faults in relict pyroxene. The textures suggest that shock deformation was the cause of melting of the chondrules. Some of the olivine and pyroxene fragments in the matrix of the chondrites have been shown to have a composition closer to the relict minerals than minerals crystallized during the chondrule formation. This suggests that some of the fragments originated from the precursor materials of the chondrules. A proposed origin for chondrites is discussed here along with supporting observations and conclusions.

Kitamura, M.↗

NASA NTRS1994

Porphyritic versus nonporphyritic chondrules

Chondrules can be divided into two broad textural types: porphyritic and nonporphyritic. Porphyritic chondrules are the most common in most chondrites and range texturally from olivine-rich (PO) to intermediate (POP) to pyroxene-rich (PP). Barred olivine (BO) chondrules can be considered a special case of porphyritic. Compositionally they can be divided into type I and II. Nonporphyritic are less abundant than porphyritic chondrules in most chondrites -- they make up approximately 125% of the chondrules in ordinary chondrites -- and range texturally from glassy (g) to cryptocrystalline (C) to radial pyroxene (RP). Compositionally nonporphyritic differ from porphyritic chondrules and within this group they are very similar to one another. Here we (1) review and contrast the characteristics of the nonporphyritic and porphyritic chondrules; (2) specify some of the problems associated with the origins of the textural and compositional differences between them; and (3) suggest a possible scenario for their origin, which may have important implications for the evolution of chondrules.

Nehru, C. E.↗