Christine Floss

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta ∼4.50 Ga ago

We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET90020. This is one of the few rare basaltic eucrites whose 39Ar-40Ar age has not been reset during impact bombardment on the HED parent body ≤4 Ga ago and, thus, should provide a unique opportunity to study the nature of the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals ( 1–5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39Ar-40Ar measurements show a relatively flat pattern and an age of 4.49±0.01 Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at 4.51 ± 0.04 Ga appears to be closely related to the remelting of Ca-phosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvospinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages ∼4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of ∼ 870°C to above the subsolidus temperature of eucrites of ∼1060°C.

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

Can you spot a speck of space dust? NASAs Stardust spacecraft has been collecting cosmic dust: Aerogel tiles and aluminum foil sat for nearly 200 days in the interstellar dust stream before returning to Earth. Citizen scientists identified most of the 71 tracks where particles were caught in the aerogel, and scanning electron microscopy revealed 25 craterlike features where particles punched through the foil. By performing trajectory and composition analysis, Westphal et al. report that seven of the particles may have an interstellar origin. These dust particles have surprisingly diverse mineral content and structure as compared with models of interstellar dust based on previous astronomical observations. Science, this issue p. 786 Analysis of seven particles captured by aerogel and foil reveals diverse characteristics not conforming to a single model. Seven particles captured by the Stardust Interstellar Dust Collector and returned to Earth for laboratory analysis have features consistent with an origin in the contemporary interstellar dust stream. More than 50 spacecraft debris particles were also identified. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfide, in some particles indicates that individual interstellar particles diverge from any one representative model of interstellar dust inferred from astronomical observations and theory.

Lunar ferroan anorthosite petrogenesis: clues from trace element distributions in FAN subgroups

The rare earth elements (REE) and selected other trace elements were measured in plagioclase and pyroxene from nine samples of the lunar ferroan anorthosite (FAN) suite of rocks. Samples were selected from each of four FAN subgroups previously defined by James et al. (1989). Plagioclase compositions are homogeneous within each sample, but high- and low-Ca pyroxenes from lithic clasts typically have different REE abundances from their counterparts in the surrounding granulated matrices. Measured plagioclase/low-Ca pyroxene concentration ratios for the REE have steeper patterns than experimentally determined plagioclase/low-Ca pyroxene partition coefficients in most samples. Textural and trace element evidence suggest that, although subsolidus equilibration may be responsible for some of the discrepancy, plagioclase compositions in most samples have been largely unaffected by intermineral redistribution of the REE. The REE systematics of plagioclase from the four subgroups are broadly consistent with their derivation through crystallization from a single evolving magma. However, samples from some of the subgroups exhibit a decoupling of plagioclase and pyroxene compositions that probably reflects the complexities inherent in crystallization from a large-scale magmatic system. For example, two anorthosites with very magnesian mafic minerals have highly evolved trace element compositions; major element compositions in plagioclase also do not reflect the evolutionary sequence recorded by their REE compositions. Finally, a noritic anorthosite breccia with relatively ferroan mafic minerals contains several clasts with high and variable REE and other trace element abundances. Although plagioclase REE compositions are consistent with their derivation from a magma with a KREEPy trace element signature, very shallow REE patterns in the pyroxenes suggest the addition of a component enriched in the light REE.

HIGH ABUNDANCES OF CIRCUMSTELLAR AND INTERSTELLAR C-ANOMALOUS PHASES IN THE PRIMITIVE CR3 CHONDRITES QUE 99177 AND MET 00426

QUE 99177 and MET 00426 are the first known CR3 chondrites and have largely escaped the aqueous alteration experienced by most of the CR chondrite group. In addition to high presolar silicate and oxide grain abundances, these meteorites also contain high abundances of phases with anomalous C isotopic compositions. Presolar SiC abundances (~65 ppm) are consistent with independent estimates for CR1 and CR2 chondrites, indicating that aqueous alteration does not destroy SiC, but are significantly higher than earlier abundance estimates based on noble gas contents. Particularly notable are the high abundances (~120 ppm) of carbonaceous matter with anomalous C isotopic compositions that probably formed in cold molecular clouds via ion-molecule reactions. The observation of both 13C-enriched and 13C-depleted compositions suggests that multiple chemical pathways contribute to the formation of this material. The abundance of such material is significantly higher in QUE 99177 and MET 00426 than in other primitive meteorites and attests to the primitive nature of the CR3 chondrites.

Ce anomalies in the LEW85300 eucrite: evidence for REE mobilization during Antarctic weathering

Abstract There is abundant evidence for widespread REE mobilization in Antarctic eucrites, resulting in Ce and Eu anomalies and lower whole-rock REE abundances relative to typical eucrites. Using secondary ion mass spectrometry (SIMS), we investigated the microdistribution of Ce anomalies in the Antarctic polymict eucrite LEW85300. Ce anomalies (positive and/or negative) are present in all three of the minerals (plagioclase, pyroxene and silica) analysed. The anomalies occur in grains from all four clasts as well as in mineral and lithic fragments from the surrounding matrix; thus there appears to be no lithologic association. Silica, which has been heavily shocked and altered, exhibits a LREE-enriched pattern with negative Ce and Eu anomalies, consistent with a derivation of the REE from Ca-phosphate dissolution: trivalent REE are leached from the phosphates, with preferential retention of tetravalent Ce, and redeposited on silica. Pyroxene is also widely affected by REE mobilization, due to an extensive, shock-induced microcrack network along cleavage planes. A tendency for the largest Ce anomalies to occur at low LREE concentrations suggests that pyroxene itself experienced REE leaching with concomitant Ce retention. Ce anomalies are rare in plagioclase because this mineral lacks an extensive network along which REE can be mobilized. The widespread REE mobilization and evidence for other forms of weathering requires that trace element data be used cautiously, especially for Antarctic eucrites.

Geochemistry, petrology, and cooling history of 14161,7373; a plutonic lunar sample with textural evidence of granitic-fraction separation by silicate-liquid immiscibility

Alkali suite anorthosites and norites are the second most common plutonic rock association in the western lunar highlands (after the magnesian suite), but their origin poses an enigma for most petrologic models of lunar crustal evolution. Some models suggest that the alkali and magnesian suites formed from distinct, unrelated parental magmas, whereas other models propose that both suites formed from the same parental magma. The contrast in major element chemistry of the cumulus phases in each suite is difficult to reconcile with their similar incompatible element chemistry. We present herein a detailed ion microprobe (SIMS) and electron microprobe study of seven alkali suite rocks. Our data show that most alkali suite anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks, and that these characteristics can be used to reconstruct the parental-magma composition. The data indicate that cumulates of the alkali suite crystallized from magmas with rare-earth element (REE) contents ~0.6–2.0× high-K KREEP, and small but consistently positive Eu anomalies (Eu/Eu* ~2) relative to KREEP. Snyder and others (1995a) have proposed that the alkali suite parental magma is similar to Apollo 15 pristine KREEP basalt. Our model suggests that the major element composition of cumulus plagioclase in most alkali suite rocks is too sodic for the calculated crystal line-of-descent of pristine KREEP basalt, and that assimilation of pre-existing calcic anorthosite is required. This conclusion is supported by the REE patterns of th alkali-suite parental magma determined here. We propose that alkali suite anorthosites formed as flotation cumulates in KREEPy plutons that may have formed norites as complementary bottom cumulates. The alkali flotation cumulates reflect fractional crystallization of their parental pluton, local equilibrium crystallization, assimilation of plagioclase-rich roof rock, and episodic magma-mixing during convective overturn of the crystallizing magma bodies. Texturally pristine alkali anorthosites exhibit petrographic characteristics that are consistent with their origin as cumulates in a KREEPy pluton, including abundant modal plagioclase, post-cumulus pyroxenes (both augite and pigeonite) that generally lack exsolution lamellae and that have equilibration temperatures of 950–1100 °C, relict igneous textures and, in some cases, igneous lamination. The lack of cumulus mafic phases in rocks that should be pyroxene-saturated suggests separation of the plagioclase by flotation, not sinking. Assimilation of plagioclase from older, anorthositic highlands crust is indicated by the high Eu contents of the cumulates and by the positive Eu anomalies in their calculated parental melts relative to high-K KREEP. Mixing of the evolved alkali-suite parental magma with primitive melt occured episodically, as shown by reverse zoning profiles in some cumulus plagioclase. Injection of this primitive, hot magma into the crystallizing pluton may have induced convective overturn of the magma chamber.

REE geochemistry of oldhamite-dominated clasts from the Norton County aubrite: Igneous origin of oldhamite

Abstract Oldhamite-dominated lithic clasts represent a new igneous lithology of the aubrite parent body. They contain single crystals of oldhamite up to 2 cm in size, with inclusions of ferromagnesian alabandite, troilite, daubreelite, caswellsilverite, and Fe,Ni metal; they are usually in intimate contact with a silicate portion consisting of enstatite, forsterite, and/or plagioclase. Textural evidence for igneous origin includes apparent primary igneous grain boundaries between oldhamite and forsterite, coarse grain size, and the presence of round, droplet-like Mn-Fe-Mg-Cr-Na sulfide inclusions within oldhamite which appear to represent an immiscible sulfide liquid. We propose that the oldhamite-dominated lithology formed during the melting and fractionation of enstatite chondrite-like precursor material and represents a locally CaS-rich facies. During melting, two mutually immiscible sulfide liquids—a Ca sulfide and a Mg-Fe-Mn-Cr-Na sulfide—formed in the silicate magma. Upon cooling, the immiscible Sulfides crystallized, forming large oldhamite crystals containing inclusions of Mn-Fe-Mg-Cr-Na-bearing sulfides; forsterite, enstatite, and plagioclase crystallized from the surrounding silicate melt. At subsolidus temperatures, tiny ferromagnesian alabandite crystals exsolved from oldhamite. REE abundances in oldhamite are high (about 200 × CI ), but REE patterns are nearly identical within single crystals and from clast to clast, indicating equilibrium conditions. High REE abundances have been cited as evidence that oldhamite grains in aubrites are nebular relics. However, we find it difficult to imagine that the rather homogeneous REE patterns of oldhamite in the oldhamite-dominated lithology of Norton County are not the result of equilibration of the REEs with a silicate melt during formation of the igneous aubrites through parent body melting, differentiation, fractionation, and cooling, where peak temperatures of around 1450–1500°C must have been reached. We conclude that oldhamite in the oldhamite-dominated lithology of Norton County is of igneous origin and that its REE abundances were established by equilibration with the aubrite silicate melt.