Allan Hills Research Articles

Olivine in Martian meteorite Allan Hills 84001: Evidence for a high‐temperature origin and implications for signs of life

Abstract— Olivine from Martian meteorite Allan Hills (ALH) 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from ∼40 μm to submicrometer. Some of the inclusions are elongate and boudinage‐like. The olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite. The olivine exhibits a very limited range of composition from Fo65 to Fo66 (n = 25). The δ18O values of the olivine and orthopyroxene analyzed by ion microprobe range from +4.3 to +5.3‰ and are indistinguishable from each other within analytical uncertainty.The mineral chemistries, O‐isotopic data, and textural relationships indicate that the olivine inclusions were produced at a temperature >800 °C. It is unlikely that the olivines formed during the same event that gave rise to the carbonates in ALH 84001, which have more elevated and variable δ18O values, and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine. The reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel. If the olivine was formed by either reaction during a postcarbonate heating event, the implications are profound with regards to the interpretations of McKay et al. (1996). Due to the low diffusion rates in carbonates, this rapid, high‐temperature event would have resulted in the preservation of the fine‐scale carbonate zoning, while partially devolatilizing select carbonate compositions on a submicrometer scale (Brearley, 1998a). This may have resulted in the formation of the minute magnetite grains that McKay et al. (1996) attributed to biogenic activity.

Thermally metamorphosed carbonaceous chondrites from data for thermally mobile trace elements

Abstract— We report radiochemical neutron activation analysis (RNAA) data for U, Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl, and Cd (ordered by increasing ease of vaporization and loss from the Murchison CM2 chondrite during open‐system heating) in nine Antarctic C2 and C3 chondrites. These meteorites exhibit properties (obtained by reflectance spectroscopy, O isotopic mass spectrometry and/or mineralogy‐petrology) suggesting thermal metamorphism in their parent bodies.Five of these meteorites (Asuka (A) 881655, Yamato (Y) 793495, Y‐790992, Pecora Escarpment (PCA) 91008, and Y‐86789—paired with Y‐86720) exhibit significant depletion of the most thermally mobile 1–5 trace elements, which is consistent with open‐system loss during extended parent‐body heating (under conditions duplicated by week‐long heating of the Murchison C2 chondrite, heated at 500–700 °C in a low‐pressure (initially 10−5atm) H2 atmosphere). From earlier data, three other C3 chondrites—Allan Hills (ALH) 81003, ALH 85003, and Lewis Cliffs (LEW) 85332—show significant Cd depletion. Nine additional C2 and C3 chondrites show no evidence of mobile trace element depletion—including Y‐793321, which by all other criteria was mildly metamorphosed thermally. Either metamorphism of these nine meteorites occurred under closed conditions and/or alteration took place under such mild conditions that even Cd could not be lost.The RNAA data suggest that 10 of the 46 Antarctic carbonaceous chondrites (including 4 of 37 from Victoria Land and 6 of 9 from Queen Maud Land) exhibit open‐system loss of at least some thermally mobile trace elements by heating in their parent bodies, whereas none of the 25 non‐Antarctic falls experienced this. These results are consistent with the idea that the Antarctic sampling of near‐Earth material differs from that being sampled today.

Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration

Abstract— The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali‐like (CV3oxB) and Allende‐like (CV3oxA), in which chondrules, calcium‐aluminum‐rich inclusions (CAIs) and matrices show characteristic alteration features (Weisberg et al, 1997; Krot et al, 1997d; Kimura and Ikeda, 1997). The CV3oxB lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low‐Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni‐rich sulfides, fayalite (Fa>90), Ca‐Fe‐rich pyroxenes (Fs10–50Wo45–50) and andradite. Phyllosilicates replace primary Ca‐rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca‐Fe‐rich pyroxenes and minor andradite. Fayalite veins crosscut fine‐grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large Δ18Ofayalite‐magnetite fractionation (∼20%). We infer that phyllosilicates, magnetite, fayalite, Ca‐Fe‐rich pyroxenes and andradite formed at relatively low temperatures (<300 °C) by fluid‐rock interaction in an asteroidal environment.Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3oxA lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low‐Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath‐shaped grains in matrix. Secondary Ca‐Fe‐rich pyroxenes are abundant. We infer that the CV3oxA lithology experienced alteration at higher temperatures than the CV3oxB lithology. The presence of the reduced and CV3oxA lithologies in the Vigarano breccia and CV3oxA and CV3oXB lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid‐rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.

Glacier-fed(?) sandstone sheets in the Weller Coal Measures (Permian), Allan Hills, Antarctica

The upper member of the Weller Coal Measures (Permian) at Allan Hills, Victoria Land, Antarctica, consists of repetitive coal layers separated by sheetlike bodies of predominantly sandstone and subordinate siltstone and conglomerate, all of fluvial origin. Investigation of the lowermost inter-coal sheet (WC1) reveals abrupt planar basal contacts, persistent upward fining, and a strong westward palaeocurrent trend. Three lithofacies are recognized in the WC1 sheet: (A) small-scale cross-bedded pebbly sandstone, which occupies most of the basal portions of the sheet, (B) large-scale cross-bedded medium-coarse sandstone, confined to a linear NE–SW belt overlying or locally replacing facies A, and (C) siltstone and rippled fine-grained sandstone, which drapes and overlaps facies A and B and contains Glossopteris leaves and Vertebraria root traces. Facies distribution patterns indicate the WC1 body initiated as an abrupt sheetlike incursion of coarse sediment over a low-lying mire (facies A), followed by entrenchment and confinement of flow as the sheet extended distally (facies B). Facies C represents low-energy floodplain sedimentation co-existing with and later succeeding facies B as deposition evolved toward abandonment and eventual renewal of peat deposition. Relatively rapid deposition of the sandstone sheet is implied by the lack of any significant hiatus markers. Outsized clasts up to 45 cm in diameter, one observed to display prominent striations, occur sporadically throughout the WC1, including within siltstone beds of facies C. These are inferred to be ice-rafted deposits which, together with other evidence, suggest meltwater influence, possibly glacial, on sediment transport and deposition. Preliminary examination of other sandstone sheets in the upper Weller indicates attributes similar to the WC1, including abrupt bases, upward-fining character, and predominantly westward palaeocurrent patterns. The Allan Hills exposures are believed to represent a succession of low-lying mire complexes interrupted by abrupt but infrequent incursions of coarse sediment, possibly by extreme meltwater floods.

Recognition of minor constituents in reflectance spectra of Allan Hills 84001 chips and the importance for remote sensing on Mars

Abstract— Reflectance spectra of splits 92 and 271 from the Martian meteorite Allan Hills (ALH) 84001 are presented and analyzed in this paper. Although the visible and infrared spectra of both chips show that the dominant mineralogy is low‐Ca pyroxene, the focus here is on identification of the minor constituents. Infrared spectra measured at multiple spots along the surface of chips 92 and 271 show subtle spectroscopic variations due to changes in the low‐Ca pyroxene texture and composition and to the presence of secondary minerals. Absorption bands observed near 0.93 and 1.95 μm are characteristic of low‐Ca pyroxene. Strong mid‐infrared reststrahlen bands are observed near 9 and 19.5 μm in all surface spectra, and additional bands near 7, 10.5, 11.4, 17.8, 20.5 and 23 μm are variable depending on the low‐Ca pyroxene texture and the presence of secondary minerals. Selected spectra exhibit carbonate features near 4, 6.4–7.1 and 11.3 μm. Detailed analysis of these carbonate features indicates the presence of Mg‐Fe carbonate, which is consistent with petrographic studies. Many of these spectra with strong carbonate features exhibit a magnetite feature near 17.9 μm and a shoulder near 20.5 μm that cannot be uniquely ascribed to one mineral. Spectroscopic identification of the minor carbonate and magnetite minerals in this probable piece of Mars indicates that detection of small amounts of these minerals of possible biological significance will be possible using infrared hyperspectral analyses of the Martian surface. Also of importance for remote sensing on Mars is the result that Mg, Fe and Mg‐Fe carbonates in a low‐Ca pyroxene matrix should be distinguishable from one another in the spectral region measured by the thermal emmitance spectrometer (TES).

The history of Allan Hills 84001 revised: multiple shock events.

The geologic history of Martian meteorite Allan Hills (ALH) 84001 is more complex than previously recognized, with evidence for four or five crater-forming impacts onto Mars. This history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient Martian life in ALH 84001. Allan Hills 84001 formed originally from basaltic magma. Its first impact event (I1) is inferred from the deformation (D1) that produced the granular-textured bands ("crush zones") that transect the original igneous fabric. Deformation D1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater. An intense thermal metamorphism followed D1 and may be related to it. The next impact (I2) produced fractures, (Fr2) in which carbonate "pancakes" were deposited and produced feldspathic glass from some of the igneous feldspars and silica. After I2, carbonate pancakes and globules were deposited in Fr2 fractures and replaced feldspathic glass and possibly crystalline silicates. Next, feldspars, feldspathic glass, and possibly some carbonates were mobilized and melted in the third impact (I3). Microfaulting, intense fracturing, and shear are also associated with I3. In the fourth impact (I4), the rock was fractured and deformed without significant heating, which permitted remnant magnetization directions to vary across fracture surfaces. Finally, ALH 84001 was ejected from Mars in event I5, which could be identical to I4. This history of multiple impacts is consistent with the photogeology of the Martian highlands and may help resolve some apparent contradictions among recent results on ALH 84001. For example, the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition, whereas the elongate magnetite grains, epitaxial on carbonates, could be ascribed to vapor-phase deposition during I3.

Epitaxial growth of nanophase magnetite in Martian meteorite Allan Hills 84001: Implications for biogenic mineralization

Crystallographic relationships between magnetite, sulfides, and carbonate rosettes in fracture zones of the Allan Hills (ALH) 84001 Martian meteorite have been studied using analytical electron microscopy. We have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties. Individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in "stacks" of single-domain crystals. Other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates. The observed magnetite/carbonate (hkl) Miller indices orientation relationships are (1, 1,3)m ¿¿ (1, 1 ,0)c and (1, 1 ,1)m ¿¿ (0,0, 3)c with lattice mismatches of approximately 13% and approximately 11%, respectively. Epitaxy is a common mode of vapor-phase growth of refractory oxides like magnetite, as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in ALH 84001. Epitaxy rules out intracellular precipitation of these magnetites by (Martian) organisms, provides further evidence of the high-temperature (> 120 degrees C) inorganic origins of magnetite in ALH 84001, and indicates that the carbonates also have been exposed to elevated temperatures.

Cation diffusion in calcite: determining closure temperatures and the thermal history for the Allan Hills 84001 meteorite.

The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases associated with the cracks and inclusions of the Allan Hills (ALH) 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer-diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may be examined, therefore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550-800 degrees C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (Ea(Ca) = 271 +/- 80 kJ/mol) and for Mg (Ea(Mg) = 284 +/- 74 kJ/mol) were used then to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 micrometers compositional zone would be completely homogenized at a temperature of 300 degrees C for cooling rates <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, they indicate that the carbonate globules were not exposed to a high-temperature environment for long time scales following formation.

Bulk and stable isotopic compositions of carbonate minerals in Martian meteorite Allan Hills 84001: no proof of high formation temperature.

Understanding the origin of carbonate minerals in the Martian meteorite Allan Hills (ALH) 84001 is crucial to evaluating the hypothesis that they contain traces of ancient Martian life. Using arguments based on chemical equilibria among carbonates and fluids, an origin at >650 degrees C (inimical to life) has been proposed. However, the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high-temperature origin. Other methods (possibly less direct) will have to be used to determine the formation temperature of the carbonates in ALH84001.

Isotopic composition of trapped and cosmogenic noble gases in several Martian meteorites

Abstract— Isotopic abundances of the noble gases were measured in the following Martian meteorites: two shock glass inclusions from Elephant Moraine (EET) 79001, shock vein glass from Shergotty and Yamato (Y) 793605, and whole‐rock samples of Allan Hills (ALH) 84001 and Queen Alexandra Range (QUE) 94201. These glass samples, when combined with literature data on a separate single glass inclusion from EET 79001 and a glass vein from Zagami, permit examination in greater detail of the isotopic composition of Ne, Ar, Kr, and Xe trapped from the Martian atmosphere. The isotopic composition of Martian Ne, if actually present in these glasses, remains poorly defined. The 40Ar/36Ar ratio of trapped Martian atmospheric Ar is probably considerably lower than the nominal ratio of 3000 measured by Viking, and data on impact glasses suggest a value of ∼1900. The atmospheric 36Ar/38Ar ratio is ≤4.0. Martian atmospheric Kr may be enriched in lighter isotopes by ∼0.5%/amu compared to both solar‐wind Kr and to the Martian composition previously reported. The isotopic composition of Xe in these glasses agrees with that previously reported in the literature. The Martian atmospheric 36Ar/132Xe and 84Kr/132Xe elemental ratios are higher than those reported by Viking by factors of ∼2.5–1.6 (depending on the 40Ar/36Ar ratio adopted) and ∼1.8, respectively, and are discussed in a separate paper. Cosmogenic gases indicate space exposure ages of 2.7 ± 0.6 Ma for QUE 94201 and Shergotty and 14 ± 1 Ma for ALH 84001. Small amounts of 21Ne produced by energetic solar protons may be present in QUE 94201 but are not present in ALH 84001 or Y‐793605. The space exposure age for Y‐793605 is 4.9 ± 0.6 Ma and appears to be distinctly older than the ages for basaltic shergottites. However, uncertainties in cosmogenic production rates still makes somewhat uncertain the number of Martian impact events required to produce the exposure ages of Martian meteorites.

Carbonates in fractures of Martian meteorite Allan Hills 84001: petrologic evidence for impact origin.

Carbonates in Martian meteorite Allan Hills 84001 occur as grains on pyroxene grain boundaries, in crushed zones, and as disks, veins, and irregularly shaped grains in healed pyroxene fractures. Some carbonate disks have tapered Mg-rich edges and are accompanied by smaller, thinner and relatively homogeneous, magnesite microdisks. Except for the microdisks, all types of carbonate grains show the same unique chemical zoning pattern on MgCO3-FeCO3-CaCO3 plots. This chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process. The heterogeneous distribution of carbonates in fractures, tapered shapes of some disks, and the localized occurrence of Mg-rich microdisks appear to be incompatible with growth from an externally derived CO2-rich fluid that changed in composition over time. These features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual Mg-rich fluid that was squeezed along fractures. Carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate. We infer that carbonates in fractures formed from grain boundary carbonates associated with plagiociase that were melted by impact and dispersed into the surrounding fractured pyroxene. Carbonates in fractures, which include those studied by McKay et al. (1996), could not have formed at low temperatures and preserved mineralogical evidence for Martian organisms.

Exposure of Allan Hills 84001 and other achondrites on the Antarctic ice.

The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions 19F(p, alpha gamma)16O and 19F(p,p gamma)19F, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.

Spectroscopic analysis of Martian meteorite Allan Hills 84001 powder and applications for spectral identification of minerals and other soil components on Mars

Abstract— Spectroscopic measurement and analysis of Martian meteorites provide important information about the mineralogy of Mars, as well as necessary ground‐truths for deconvolving remote sensing spectra of the Martian surface rocks. The spectroscopic properties of particulate ALH 84001 from 0.3 to 25 μm correctly identify low‐Ca pyroxene as the dominant mineralogy. Absorption bands due to electronic transitions of ferrous iron are observed at 0.94 and 1.97 μm that are typical for low‐Ca pyroxene. A strong, broad water band is observed near 3 μm that is characteristic of the water band typically associated with pyroxenes. Weaker features near 4.8, 5.2 and 6.2 μm are characteristic of particulate low‐Ca pyroxene and can be distinguished readily from the features due to high‐Ca pyroxene and other silicate minerals. The reflectance minimum occurs near 8.6 μm for the ALH 84001 powder, which is more consistent with high‐Ca pyroxene and augite than low‐Ca pyroxene. The dominant mid‐infrared (IR) spectral features for the ALH 84001 powder are observed near 9 and 19.5 μm; however, there are multiple features in this region. These mid‐IR features are generally characteristic of low‐Ca pyroxene but cannot be explained by low‐Ca pyroxene alone.Spectral features from 2.5–5 μm are typically associated with water, organics and carbonates and have been studied in spectra of the ALH 84001, split 92 powder and ALH 84001, splits 92 and 271 chip surfaces. Weak features have been identified near 3.5 and 4 μm that are assigned to organic material and carbonates. Another feature is observed at 4.27 μm in many surface spots and in the powder but has not yet been uniquely identified. Spectroscopic identification of minor organic and carbonate components in this probable piece of Mars suggests that detection of small amounts of organics and carbonates in the Martian surface regolith would also be possible using visible‐infrared hyperspectral analyses. Laboratory spectroscopic analysis of Martian meteorites provides a unique opportunity to identify the spectral features of minerals and other components while they are embedded in their natural medium.

A nitrogen and argon stable isotope study of Allan Hills 84001: implications for the evolution of the Martian atmosphere.

The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] approximately 0.34 ppm; delta 15N approximately +23%); although during stepped combustion, 15N-enriched N (delta 15N approximately +143%) is released in a narrow temperature interval between 700 degrees C and 800 degrees C (along with 13C-enriched C (delta 13C approximately +19%) and 40Ar). Cosmogenic species are found to be negligible at this temperature; thus, the isotopically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH84001. The N and Ar data show that ALH84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess 40Ar from K decay. Deconvolution of radiogenic 40Ar from other Ar components, on the basis of end-member 36Ar/14N and 40Ar/36Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5-4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmosphere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: delta 15N > or = +200%, 40Ar/36Ar < or = 3000 and 36Ar/14N > or = 17 x 10(-5). In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.

Petrology of magmatic silicate inclusions in the Allan Hills 77005 lherzolitic shergottite

Abstract— Magmatic inclusions occur in both chadacrystic olivine and oikocrystic pigeonite in ALH 77005 but are different from each other. Magmatic inclusions in olivine consist mainly of aluminous pyroxenes, intergrowths of plagioclase and silica, silica‐predominant glass, and rhyodacitic glass, with minor amounts of chromite, spinel, pyrrhotite, and whitlockite. Those in pigeonite consist mainly of aluminous pyroxenes, nonaluminous ferroan pyroxenes, kaersutite, spinel, and K‐rich trachytic glass, with minor amounts of pyrrhotite and whitlockite. The magmatic inclusions in chadacrystic olivine formed from trapped melts that were basaltic, apparently dry and crystallized additional olivine metastably. The basaltic magma, with entrained olivine, experienced magma mixing with K‐rich and wet magmas, or assimilation of such crustal rocks, in the early to middle stages of the crystallization sequence of ALH 77005 during crystallization of chadacrystic olivine prior to precipitation of oikocrystic pigeonite. However the amount of mixed magmas or assimilated rocks was minor in comparison to the basaltic magma. Crystallization of pigeonite, augite, and plagioclase in the host lithologies took place in a shallow magma reservoir under an open‐system condition, and the pigeonite trapped basaltic andesite to trachyandesitic melts, which resulted in magmatic inclusions in oikocrystic pigeonite. The magmatic inclusions in both olivine and pigeonite were formed under a rapid‐cooling condition, resulting in a variety of inclusions. Kaersutite in magmatic inclusions in oikocrystic pigeonite crystallized under a closed‐system wet condition during the late‐stage crystallization of the inclusions.

OSMUNDA (OSMUNDACEAE) FROM THE TRIASSIC OF Antarctica: an example of evolutionary stasis

Compressed specimens of the fern Osmunda are described from the Triassic of the Allan Hills, Antarctica. The specimens consist of a once pinnate, deeply pinnatifid fertile frond as well as several sterile specimens. Six pinnae are present on the partial fertile rachis, with two sterile pinnae above four fertile pinnae. Both sterile and fertile specimens are virtually identical to the modern species Osmunda claytoniana. Entire fronds are fragmentary; the longest is 21 cm in length. Sterile pinnae are alternate and deeply pinnatifid, with slightly toothed pinnules and dichotomous venation. Fertile pinnae are 1-1.3 cm long, once pinnate, and lack vegetative lamina. Sporangia are clustered, each 300-375 um in diameter, and possess a transverse annulus 6-8 cells long; dehiscence is by a vertical slit. Fronds arise from a rhizome 4 cm long by 1 cm wide; two croziers are present on the rhizome. Two frond segments up to 6 cm long and three deeply pinnatifid pinnae are present on the uppermost part of one rachis. Pinnules are ~4 mm long and 2-3 mm wide. The presence of this Osmunda species in the Triassic demonstrates stasis of frond morphology, both fertile and vegetative, for the genus.

Isotopic evidence for a terrestrial source of organic compounds found in martian meteorites Allan Hills 84001 and Elephant Moraine 79001.

Stepped-heating experiments on martian meteorites Allan Hills 84001 (ALH84001) and Elephant Moraine 79001 (EETA79001) revealed low-temperature (200 to 430 degrees Celsius) fractions with a carbon isotopic composition delta13C between -22 and -33 per mil and a carbon-14 content that is 40 to 60 percent of that of modern terrestrial carbon, consistent with a terrestrial origin for most of the organic material. Intermediate-temperature (400 to 600 degrees Celsius) carbonate-rich fractions of ALH84001 have delta13C of +32 to +40 per mil with a low carbon-14 content, consistent with an extraterrestrial origin, whereas some of the carbonate fraction of EETA79001 is terrestrial. In addition, ALH84001 contains a small preterrestrial carbon component of unknown origin that combusts at intermediate temperatures. This component is likely a residual acid-insoluble carbonate or a more refractory organic phase.

A search for endogenous amino acids in martian meteorite ALH84001.

Trace amounts of glycine, serine, and alanine were detected in the carbonate component of the martian meteorite ALH84001 by high-performance liquid chromatography. The detected amino acids were not uniformly distributed in the carbonate component and ranged in concentration from 0.1 to 7 parts per million. Although the detected alanine consists primarily of the L enantiomer, low concentrations (<0.1 parts per million) of endogenous D-alanine may be present in the ALH84001 carbonates. The amino acids present in this sample of ALH84001 appear to be terrestrial in origin and similar to those in Allan Hills ice, although the possibility cannot be ruled out that minute amounts of some amino acids such as D-alanine are preserved in the meteorite.

Calculating flux from meteorite decay rates: a discussion of problems encountered in deciphering a 10 5 –10 6 year integrated meteorite flux at Allan Hills and a new approach to pairing

Abstract Meteorite accumulation sites offer the prospect of observing changes in flux over time; however, two main problems must be overcome: calculating a decay constant for samples in an area, and providing accurate pairing data, such that the number of true meteorites (not fragments) can be ascertained. We have used a comparison of meteorite terrestrial age and weathering data to constrain decay constants for hot desert meteorite populations. Meteorites resident in these sites typically have terrestrial ages &lt;50 ka. Our estimates of the flux of meteorites to the Earth are within a factor of 2–3 of independent estimates made by the Meteorite Observation and Recovery Project network, and suggest no change in the flux over the last 50 ka. A similar approach applied to meteorites from Allan Hills, Antarctica, finds much lower levels of weathering, although terrestrial ages for these samples are much longer. We also observe a correlation between terrestrial age and degree of weathering in the Allan Hills meteorites, suggesting a weathering rate 2–3 orders of magnitude slower than values typical of hot desert sites. Given that blue ice regions are subject to some horizontal flow, we propose a model in which the observed oxidation-terrestrial age distribution is largely a result of ice movement, rather than weathering. Comparing oxidation and terrestrial age data in the Antarctic and hot desert accumulations, we estimate a lifetime for the Allan Hills population of 200–300 ka. Given the distance to the nearest ice divide (200 km), this suggests an average horizontal flow rate of 1 m a −1 . This approach may allow an estimate of the catchment area for these samples to be made. In addition, we suggest an alternative method for pairing samples which makes use of automated image processing and data analysis of reflected light photomicrographs to acquire a ‘textural fingerprint’ of a sample, and a genetic algorithm to compare the numerous data variables.

14 C terrestrial ages of meteorites from Victoria Land, Antarctica, and the infall rates of meteorites

Abstract The results of 14 C measurements of 95 meteorites from the Allan Hills region in Antarctica are reported, and terrestrial residence ages calculated. This includes meteorites from the different icefields at Allan Hills and the adjacent Elephant Moraine meteorite stranding area. We determined that terrestrial ages of these Antarctic meteorites can range from recent falls to &gt;40 ka, which is the practical limit for these 14 C measurements. The terrestrial age determinations on meteorites from these sites can vary dramatically; the differences between the ages observed from these sites and some of the factors influencing them are discussed. Weathering products found on these meteorites show 14 C ages younger than the terrestrial age of the meteorites studied. Calculation of infall rates based on meteorites recovered and their age distributions suggests a minimum infall rate of 40–60 meteorites (&gt;10 g) per 10 6 km 2 per year, in reasonable agreement with the infall rates estimated by Halliday’s group based on meteoroid fluxes.