Marilyn M. Lindstrom

Acapulco- and Lodran-like achondrites: Petrology, geochemistry, chronology, and origin

We have performed petrologic and geochemical studies of five primitive achondrites: ALHA81187 and ALHA81261 (Acapulco-like), EET 84302 (transitional, but Acapulco-like), and LEW 88280 and MAC 88177 (Lodran-like). We have also performed 39Ar-40Ar chronology on ALHA81187, ALHA81261, and EET 84302. LEW 88280 and MAC 88177 contain more ferroan olivines, orthopyroxenes, clinopyroxenes, and chromites than do the Acapulco-like achondrites. Plagioclase is present in only ALHA81187, ALHA81261, and EET 84302. Its composition does not track that of the mafic silicates; EET 84302, with the most calcic plagioclase, contains olivine and pyroxene with an mg# intermediate between ALHA81187 and ALHA81261. Similarly, spinels in EET 84302 have the same mg# as those in ALHA81187, while pyroxenes in the former are more ferroan than in the latter. Acapulco-like achondrites have Sm/Sc ratios between 0.8–1.5 times H chondrites, and Na/Sc ratios are between 0.81.0 times H chondrites, indicating that silicate partial melts have not been lost from these rocks. Siderophile and chalcophile elements are fractionated and show a trend of increasing Ir/Ni ratios with decreasing Se/Co ratios. This variation is consistent with fractionation by partial melting in the Fe-Ni-S system. EET 84302 is very depleted in Se and troilite is a accessory phase, indicating essentially total loss of the low temperature Fe-Ni-S melt. In contrast, the Lodran-like achondrites are depleted in the highly incompatible lithophile elements relative to more compatible elements: Sm/Sc ratios are 0.2−0.7 times and Na/Sc ratios are 0.04–0.18 times H chondrites. These data are consistent with a model for formation of the Lodran-like achondrites as partial melting residues. However, the low Ir/Ni and high Se/Co ratios in two Lodran-like achondrites suggest the metal + troilite in these rocks is dominated by the low melting fraction in the Fe-Ni-S system. This metal + troilite was added to the silicates after partial melting depleted them in a basaltic fraction. The Ar-Ar release spectra yield good plateau ages of 4.507 ± 0.024 Ga (ALHA81187), 4.511 ± 0.007 Ga (ALHA81261), and 4.519 ± 0.017 Ga (EET 84302), in good agreement with previously determined Ar-Ar ages for other Acapulco-like achondrites. Together, the Ar-Ar ages demonstrate that metamorphism on the Acapulco-Lodran parent body occurred 4.51 ± 0.02 Ga ago, roughly 50 Ma after its formation. The geochemical and petrologic evidence on the Acapulco- and Lodran-like achondrites suggest that heating on the parent body was localized and heterogeneous in space and time. The parent body was probably not subjected to parent body-wide magmatism, in contrast to the case for 4 Vesta and the HED meteorites.

Generation of abnormal trace element abundances in Antarctic eucrites by weathering processes

Abstract Based on REEs, Antarctic eucrites can be divided into two groups: those showing normal trace element characteristics (e.g., similar to Juvinas) and those showing abnormal trace element abundances. Many Antarctic eucrite, polymict eucrite, and basaltic clast samples show the abnormal trace element abundances with REE patterns exhibiting positive Ce anomalies (sometimes negative Ce anomalies), positive Eu anomalies, and low abundances of the remainder of the REEs, with the LREEs generally being at lower relative abundances than the HREEs. Most samples of crystalline clasts from the polymict eucrites LEW85300, LEW85302, and LEW85303 show the abnormal patterns, while the glassy matrixes of these meteorites show normal patterns. Exterior samples generally show more abnormal patterns (larger anomalies, greater depletions) than interior samples from the same meteorites. Comparison of all basaltic eucrite literature data combined with our data shows that positive and negative Ce anomalies and positive Eu anomalies are found in about 61% of Antarctic eucrite analyses and are virtually unknown in non-Antarctic eucrite analyses. Further, positive Ce anomalies and positive Eu anomalies are commonly associated with Antarctic eucrites having low REE concentrations. Consideration of mineral/melt partition coefficients shows that it is unlikely that Ce anomalies are magmatic features from the HED parent body. Cerium anomalies on earth are generally restricted to the weathering zone where the relative ease of oxidizing Ce to the +4 state allows for fractionation of Ce from the +3 REEs. We believe the unusual REE patterns of abnormal Antarctic eucrites arise from weathering effects generated in or on the Antarctic ice. Our suggested scenario involves formation of melt water and its equilibration with the atmosphere which promotes dissolution of REE-rich phosphates and oxidation of Ce. Tetravalent Ce can then be fractionated from the trivalent REE in solution. The details of the weathering process are unclear and will require detailed chemical and SEM investigations of eucrites for their elucidation. We predict that rapidly chilled eucrites with glassy, rather than crystalline, mesostases will be more likely to survive the Antarctic environment without alteration of their REE patterns. Occasional S, Se, and K enrichments are likely due to weathering in the Antarctic environment as well, but these enrichments are not well correlated with Ce anomalies.

Isotopic studies of ferroan anorthosite 62236: a young lunar crustal rock from a light rare-earth-element-depleted source

Isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a Sm-Nd isochron with an age of 4.29 ± 0.06 Ga and an initial eNd143 value of +3.1 ± 0.9. We have also measured eNd142 anomalies of +0.25 on two fractions of 62236. These values are higher than the value of −0.1 predicted if 62236 was derived from a chondritic source at 4.29 Ga, but are consistent with the positive initial eNd143 value. The Sm-Nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the 147Sm/144Nd, 143Nd/144Nd, and 142Nd/144Nd ratios measured on the mineral fractions and whole rocks must be corrected. The corrections do not significantly alter the Sm-Nd isotopic results determined on 62236. Despite the fact that the Ar-Ar and Rb-Sr isotopic systematics of 62236 have been reset by impact metamorphism at 3.93 ± 0.04 Ga, the Sm-Nd systematics appear to have been unaffected. The Sm-Nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization. First, the relatively young age of 62236, as well as the old ages determined on several crustal plutonic rocks of the Mg-suite, require multiple sources of magmas on the Moon very early in its history. Second, positive eNd143 values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements. And third, models based on initial eNd143 and present-day eNd142 values suggest that the source of 62236 was depleted in light rare earth elements at ∼4.46 Ga. In order to reconcile these observations with the lunar magma ocean model (1) the magma ocean must have existed for a very short period of time, and may have had a sub-chondritic Nd/Sm ratio, and (2) the youngest ferroan anorthosites, such as 62236, cannot be cumulates from the magma ocean, but must form by other processes.

Origin and history of impact-melt rocks of enstatite chondrite parentage

We have conducted petrologic, chemical, and isotopic studies of two impact-produced rocks of enstatite chondrite parentage. Ilafegh 009 is a total impact-melt rock with no residual lithic clasts. Formation on the EL chondrite parent body is suggested by its mineralogy and mineral compositions. Cooling of the impact melt was rapid at melt temperatures and decreased at subsolidus temperatures. In contrast to previous studies, we show that Happy Canyon is not a new enstatite achondrite but an impactmelt breccia of enstatite chondrite (and not aubrite) parentage. This rock formed by impact melting and incorporation into the melt of clastic material (which resulted in relatively rapid cooling at all temperatures). Mineralogical and bulk compositional data (probably biased by the heterogeneous nature of this rock) do not allow unequivocal determination of its parent body (i.e., EL vs. EH), although some data such as bulk total Fe content seem to favor EL parentage. Both rocks were subjected to post-solidification shock, which was more severe for Ilafegh 009 than for Happy Canyon. It appears that both impact melt rocks could have formed by impact melting ∼4.57 Ga ago, as is indicated by the nearly identical IXe closure ages of 1.6 and 1.4 Ma before Bjurbole for Ilafegh 009 and Happy Canyon, respectively. An apparently younger 39Ar40Ar age of 4.53 Ga for Happy Canyon may be due to small biases in the intercalibration of the IXe and 39Ar40Ar chronometers, whereas the much younger 4.34–4.44 Ga age for Ilafegh 009 reflects thermal resetting during shock metamorphism. Shallowater, which was impact-derived from a different enstatite achondrite parent body, has an IXe closure age 0.4 Ma younger than that for Ilafegh 009 and an 39Ar40Ar age of 4.53 Ga. The ancient ages of these three rocks attest to the intense, early bombardment in this region of the solar system.

Geochemistry of eucrites: genesis of basaltic eucrites, and Hf and Ta as petrogenetic indicators for altered antarctic eucrites

Abstract We performed instrumental neutron activation analysis on a large suite of antarctic and nonantarctic eucrites, including unbrecciated, brecciated, and polymict eucrites and cumulate and noncumulate eucrites. We evaluate the use of Hf and Ta, two highly incompatible elements, as sensitive indicators of partial melting or fractional crystallization processes. Comparison with rare earth element (REE) data from nonantarctic and antarctic eucrites shows that Hf and Ta are unaffected by the terrestrial alteration that has modified the REE contents and patterns of some antarctic eucrites. The major host phases for Hf and Ta—zircon, baddeleyite, ilmenite, and titanite—are much less susceptible to terrestrial alteration than the phosphate hosts of REEs. The host phases for Hf and Ta are minor or trace phases, so sample heterogeneity is a serious concern for obtaining representative compositions. The trace lithophile and siderophile element contents of noncumulate eucrites do not allow for a single, simple model for the petrogenesis of the howardite-eucrite-diogenite suite. Fractional crystallization models cannot reproduce the compositional relationship between eucrites of the main group–Nuevo Laredo trend and those of the Stannern trend. Equilibrium crystallization models cannot explain the trace element diversity observed among diogenites. Partial melting models cannot explain the W variations among eucrites, unless source regions had different metal contents. We suggest that slight variations in oxygen fugacity of eucrite source regions during partial melting can explain the W variations without requiring different metal contents. This hypothesis may fail to account for eucrite Co contents, however.

JSC MARS-1: A Martian Soil Simulant

We have developed a simulant for the oxidized soil of Mars to support scientific research, engineering studies and education. JSC Mars-1 is the <1 mm size fraction of altered volcanic ash from a Hawaiian cinder cone. The simulant closely matches the reflectance spectrum and approximates the mineralogy, chemical composition, grain size, density, porosity and magnetic properties of Martian soil. JSC Mars-1 is currently available to qualified investigators and educators.

Geochemistry and genesis of the angrites

The angrites Angra dos Reis, LEW86010, and LEW87051 are petrologically and compositionally similar achondrites. All angrites have high FeOMnO ratios of 80–94 and very low CI normalized Na/Sm ratios of 0.001–0.003. High abundances of oxidized Fe and low abundances of moderately volatile Na most likely resulted from parent body processes, such as magmatic outgassing, rather than nebular processes. All angrites have fractionated Ca/Al ratios, with Angra dos Reis exhibiting the most extreme ratio (3.1 × CI). This Ca/Al fractionation may be due to small amounts of residual spinel in the source region after removal of the angrite melts or to crystallization of spinel from precursor melts. For LEW86010 and LEW87051, the fractionated Ca/Al ratios are not caused by the presence of significant cumulus material. For Angra dos Reis, cumulus material may be the cause of the high Ca/Al ratio. Refractory element (Ca, Eu, Al, Sc, La, Ce, Sm, Tb, Yb, Lu, Ti, Hf, Th, and Ta) abundances of LEW86010 and LEW87051 show similar patterns, while Angra dos Reis has both greater enrichments in these elements and more fractionated patterns. Compositional and petrologic constraints indicate that LEW86010 and LEW87051 are related via olivine control. The refractory element abundances and mg# of LEW86010 can be approximated by removal of olivine from LEW87051, suggesting that LEW86010 may be a residual melt from a LEW87051-like precursor. Alternatively, LEW87051 may have formed via olivine accumulation from a LEW86010-like precursor. The origin of the unusual refractory element pattern in Angra dos Reis remains obscure. The differences between the LEW86010-LEW87051 duo and Angra dos Reis suggest that either the angrite parent body was heterogeneous or that Angra dos Reis was formed on a separate parent body. Based on FeOMnO ratios and normative mineralogies, the angrite parent body(ies) may be similar in bulk composition to one of the carbonaceous chondrite groups, particularly CI-CM-CO.

Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

Abstract We have performed petrographie, instrumental neutron activation analysis (INAA) and noble gas studies on samples of 18 chondrules and matrix from the Chainpur (LL3) meteorite to study variations in R0, the ratio of 129I to 129I at the time of xenon isotopic closure. R0 varies by more than a factor of 10 among the chondrules, corresponding to a span of more than 50 Ma in apparent I-Xe ages, including the latest apparent I-Xe ages ever observed for a chondritic sample. Variations are not closely related to any petrographie properties, although low values of R0 (late apparent ages) may be associated with high sulfide abundances and/or non-porphyritic textures. Similarly, R0 is not closely related to any chemical components, but does seem to correlate with abundance of refractory lithophile elements. Also, R0is correlated with 244 Pu 238 U and anticorrelated with trapped 129 Xe 132 Xe , as might be expected if the variations in iodine isotopic composition are dominated by decay of 129I. We have not found a completely satisfying explanation of the variations in R0. Models involving gas-dust mixing or nebular heterogeneity cannot satisfactorily explain the Chainpur data. However, there are also difficulties with explanations attributing the variations to differences in formation age, metamorphic age, or time of aqueous alteration. We believe the most plausible explanation is that the variations represent times of low-grade shock events.

Geology, petrography, and petrology of Pinzon Island, galapagos archipelago

Three stratigraphic units based on geologic relationships and paleomagnetic observations may be distinguished on Pinzon Island. The oldest unit is a broad shield which forms the main body of the island and was erupted during a period of reversed magnetic polarity from an area now occupied by a caldera. Subsequent activity was centered about 1.5 km to the north-northwest from vents later engulfed by the collapse of a younger caldera. The lower portion of this sequence was erupted during a period of transitional pole positions and is overlain by flows of normal polarity. Pinzon has the most diverse suite of differentiated tholeiitic rocks found in the Galapagos Archipelago. Products of eruptive cycles are preserved as sequences of tuffs and flows that have decreasing degrees of differentiation and increasing phenocryst abundance upsection. The sequences may be a consequence of tapping successively deeper levels of compositionally zoned magma chambers. Such a model is consistent with computer calculations utilizing major and trace element data for Pinzon rocks, which suggest that lavas of the island may be related by shallow-level crystal fractionation of observed phenocryst minerals.

I-Xe studies of individual Allende chondrules

Abstract Iodine-xenon studies have been performed on nine Allende chondrules and a sample of oxidized Allende matrix material. The chondrules are all very rich in radiogenic xenon relative to trapped xenon, making it possible to determine a relatively precise model initial iodine composition for each temperature extraction. These model compositions show a total range in variation of about 20 percent, spanning the compositions seen in Bjurbole chondrules. One of the chondrules (chondrule 6) gives a well-defined isochron, with an apparent age .53 ±. 15 m.y. later than Bjurbole whole rock. The rest of the chondrules show a pattern of increasing apparent antiquity with increasing extraction temperature, which could be interpreted as relatively slow cooling (100–200°C/m.y.). Alternatively, poorly-defined plateaus in composition can be seen, perhaps indicative of a few phases with distinct initial iodine compositions (but also temperature-ordered), as has been previously suggested for Allende inclusions. Possible consequences of these interpretations are discussed. Elemental abundances were determined for some elements several months after the irradiation by INAA, and suggest that all the chondrules except chondrule 6 might be pyroxene- or mesostasis-rich. The oxidized matrix sample gives a well-defined isochron with an initial 129 I 127 I ratio higher than any plateaus seen in the chondrules, suggesting that, if this sample is representative of matrix, the matrix pre-dates the chondrules. The initial 244 Pu 238 U ratios of the Allende chondrules and 10 Bjurbole chondrules irradiated earlier appear to be consistent with. 004–.007 values quoted for unfractionated material in the early solar nebula.