Antarctic glaciology and meteorites

The state of knowledge of meteorites and glaciology is summarized, and directions for research are suggested.by Colin Bull and Michael E. Lipschutz.Contributions from oxygen isotope studies to paleoclimatology and the knowledge of ice flow conditions / Grootes, P.M. -- Weathering effects in antarctic meteorites / Lipschutz, Michael E. -- Mineralogy and petrology of unique and rare meteorites recovered in Antarctica / Keil, Klaus -- Radar sounding of ice sheet inland of Transantarctic Mountains / Drewry, D.J. -- Transantarctic Mountains glacial history-general problems / Mayewski, Paul

Consortium study of labile trace elements in some Antarctic carbonaceous chondrites: Antarctic and non-Antarctic meteorite comparisons

We report data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn determined by radiochemical neutron activation analysis in consortium samples of Belgica (B)-7904,Yamato (Y)-82042,Yamato-82162 and Yamato-86720 carbonaceous chondrites. These trace elements cover a wide volatility/mobility range and give unique information on thermal histories of meteorites. The results indicate the unique nature of these carbonaceous chondrites. Y-82042 proves to have the volatile element pattern of a C2 (≡CM) chondrite and the petrologic characteristics of a C1 (≡CI) chondrite. These must be primary nebular condensation/accretion features, unaffected by post-accretionary processes. The other three meteorites were thermally metamorphosed in &ge;2 parent regions over the 600-700℃ range, at relative temperatures B-7904<Y-82162<Y-86720. Before heating, B-7904 and Y-86720 had C2-levels of volatile elements : Y-82162 had uniquely high volatile element concentrations, at about C1-levels. The data require a new classification scheme for such chondrites. Belgica-7904 and Y-82162 and -86720 seem to be derived from one or more thermally altered carbonaceous asteroids and their spectral characteristics should be compared with those of B-, F-, G-, or T-asteroids. These results indicate substantial differences in the thermal histories of Antarctic and non-Antarctic C1 and C2 chondrite populations. In reviewing all that is known about the Antarctic and non-Antarctic meteorite populations, the overwhelming weight of evidence supports the view that these populations sample different extraterrestrial source materials, differing in thermal histories. It may be that over the extended collecting period of the Antarctic ice sheet, it has sampled a considerable greater proportion of near-Earth asteroids than do current falls

Labile trace elements in lunar meteorite Yamato-86032

Contents of siderophile Au, Co and Sb, mobile Ag, Bi, Cd, In, Se, Te, Tl and Zn and lithophile Cs, Ga, Rb and U were determined by RNAA in samples of the lunar meteorite (anorthositic breccia) Yamato (Y)-86032. Contents of the 4 lithophiles in matrix (, 75AM) and anorthositic clast (, 101AC) are similar to those in samples of the other 3 lunar meteorites. This is consistent with all these anorthositic breccias being from the lunar highlands. Contents of the other 11 elements indicate a micrometeorite component of 2.5±1.1% (Cl-equivalent) in the parent regolith of Y-86032,75AM. This value is unusual for lunar samples and is virtually identical to the value for the paired samples Y-82192/3 found in the same bare ice region, suggesting that these 3 specimens derive from the same lunar region in the same impact. No micrometeorite component is detectable in Y-86032,101AC. Slight compositional differences between Y-82192/3 and Y-86032 indicate that they did not travel Earthward as a single rock. Allan Hills-81005 and Y-791197 each exhibit characteristic siderophile/mobile element patterns indicating deviation from different parent regions in separate events. Hence, the 5 lunar meteorites studied thus far derive from 3 distinct impacts

New petrographic and trace element data on thermally metamorphosed carbonaceous chondrites

Mineral grains and matrix of heated chondrites Yamato (Y)-82162,Belgica (B)-7904,Y-86720,and Asuka (A)-881655 were examined for major elements and, where appropriate, phyllosilicate and matrix samples were characterized by TEM. CM chondrites A-881655 and B-7904 were only partially aqueously altered before thermal metamorphism initiated dehydration and recrystallization. Tochilinite is absent in both A-881655 and B-7904 probably due to mild thermal metamorphism. Phyllosilicates in B-7904 are dehydrated but not completely recrystallized to olivine and pyroxene. Y-86720 experienced a history very different from other CM chondrites : its chondrules and other coarse-grained components were completely altered by aqueous fluids. Fine-grained olivine was subsequently replaced during an episode of thermal metamorphism producing compositions near Fo_ and complete destruction of serpentine occurred. Y-82162 comes from a CI parent and is characterized by complete destruction of preexisting anhydrous silicates during aqueous alteration. Subsequent heating at 600-700℃ resulted in dehydration of phyllosilicates. The matrix contains abundant fine-grained olivine which would not likely survive the original aqueous alteration and therefore must have been formed by phyllosilicate metamorphism. Alternating episodes of oxidation and sulfidization following aqueous alteration are evident in all but Y-86720. Either Y-86720 escaped an oxidizing event or effects of such an event were obliterated by later sulfidization. Based on RNAA data for thermally mobile trace elements in them and in Murchison CM2 samples heated for one week at 500°, 600°, and 700℃ under conditions reasonable for interiors of primitive parent bodies, the four thermally metamorphosed carbonaceous chondrites can be ordered by the severity of open-system heating as 500°&le;A-881655<B-7904<Y-82162<Y-86720&le;700℃. Petrographic studies of these meteorites indicate that each bears the signature of a unique sequence of oxidation/reduction/hydration episodes followed by a late stage thermal metamorphic event which occurred in an asteroidal setting, on at least two separate asteroids (the CM and CI parent bodies). Thermal alterations in these meteorites can be used to identify other carbonaceous chondrites that experienced thermal metamorphism in their parent bodies. Interestingly, all thermally metamorphosed carbonaceous chondrites identified to date were found in Antarctica, mainly in Queen Maud Land, and none are observed falls. Apparently, carbonaceous chondrite parent sources sampled in near-Earth space in the past, differ from those being sampled by the Earth today

Possible thermal metamorphism on the C, G, B, and F asteroids detected from their reflectance spectra in comparison with carbonaceous chondrites

Reflectance spectra (0.3-2.6μm) of the C, G, B, and F asteroids and carbonaceous chondrites are compared in detail. Mixing calculations of 13 carbonaceous chondrite powders including three unusual CI/CM meteorites are done to characterize reflectance spectra of the 23 C, G, B, and F asteroids. Similar calculations are done with Murchison (CM2) samples heated at 400-1000℃. Both of two sets of calculations show that the C, G, B, and F asteroids may contain a significant amount of thermally metamorphosed materials. Comparison of ultraviolet absorption strengths between 160 C, G, B, and F asteroids and 21 carbonaceous chondrite powders suggests that surface minerals of most of those asteroids are thermally metamorphosed at temperatures around 600-1000℃

Labile trace elements and cosmogenic radionuclides in chondritic hosts of three consortium igneous inclusions

Three ordinary chondrites from the Yamato Mountains (Y) region of Antarctica contain cm-sized igneous inclusions being studied by a consortium. RNAA data for the labile elements Ag, Bi, Cd, Cs, Ga, In, Rb, Sb, Se, Te, Tl and Zn, and refractory Au and U in the L6 hosts of Y-75097 and Y-793241 indicate that each experienced at least one preterrestrial, high-temperature episode. This heating occurred during formation of the igneous inclusions and/or as a result of the severe shock that affected most equilibrated L chondrites. Cosmogenic 720ka ^Al, and 301ka^Cl (which are determined by AMS) in metal from these two meteorites and nominal terrestrial ages (based on ^Cl) hint that the L6 chondrites are not paired but are inconclusive in this regard. RNAA data for the H chondrite host of Y-794046 generally resemble those of other H4-6 chondrites : its contents of cosmogenic radionuclides in general, and its ^Cl content, in particular, correspond to a nominal terrestrial age of 70±60ka

Geochemistry and origin of achondritic inclusions in Yamato-75097, -793241 and -794046 chondrites

We have performed instrumental and radiochemical neutron activation analyses of the host L6 chondrite and five splits of the Yamato (Y)-75097 achondritic inclusion, and achondritic inclusions from the Y-793241 (L6) and Y-794046 (H5) chondrites. The troctolitic inclusions from Y-75097 and Y-793241 show numerous compositional similarities, including low Sc contents, fractionated siderophile element patterns at low abundances, and commonly (though not always) depleted trivalent REE abundances but with Eu at near chondritic levels. The REE, however, are highly variable in the Y-75097 inclusion due to variations in modal merrillite. The lack of pyroxene in these inclusions is best explained by assuming the protolith was non-chondritic in bulk composition. However, it is not clear that nebular processes could produce a pyroxene-poor protolith. The siderophile element patterns in these inclusions can be modeled as residual metal left after partial melting of the Fe-FeS system. The Y-794046 inclusion is compositionally and mineralogically different from the other inclusions. Abundances of refractory through moderately volatile lithophile elements are unfractionated relative to mean H chondrites. Similarly, abundances of refractory through moderately volatile siderophile elements are in H chondritic relative proportions, but at &acd;10% the abundance of mean H chondrites. This cosmochemical signature is most simply modeled as a result of impact melting of an H chondrite target, with loss of &acd;90% of the immiscible Fe-FeS melt

Geochemistry of Yamato-82192, -86032 and -793274 lunar meteorites

The major and trace element compositions of lunar meteorites Yamato (Y)-82192,Y-86032 and Y-793274 were determined by neutron activation analysis. Y-82192 and Y-86032 are anorthositic lunar meteorites rich in Al_2O_3 and CaO and poor in FeO, MgO and incompatible elements. Although these meteorites are similar in composition to each other and other anorthositic lunar meteorites, they are distinct in several key compositional characteristics. Y-793274 is a basaltic lunar meteorite rich in FeO, MgO, Sc, Cr, Co, and incompatible elements and poor in Al_2O_3 and CaO compared to anorthositic lunar meteorites. It is similar in many ways to lunar meteorite EET87521 which is also a basaltic breccia. It is distinct from EET87521 in its higher proportion of highland material, its meteoritic contamination and regolith glass, and in the composition of its dominant basalt component. Y-793274 contains 65-75% magnesian VLT basalt, while EET87521 consists of ferroan VLT basalt. The eleven lunar meteorites probably represent eight distinct falls. Four are anorthositic and four are basaltic. This 50-50 proportion of highlands-mare material contrasts strongly with the 83-17 proportion derived from photogeologic mapping. The dominance of VLT basalt among lunar meteorites contrasts with its scarcity among Apollo samples. The resolution of these discrepancies awaits further studies of basaltic lunar meteorites and further discoveries of new lunar meteorites

Geochemistry of and alteration phases in martian lherzolite Y-793605

We have done preliminary SEM characterization of alteration phases on an exterior and an interior chip of martian lherzolite Yamato-793605,and have performed instrumental and radiochemical neutron activation analyses of a glass-poor and a glass-rich interior sample of the rock for a suite of 31 major and trace elements. To date, we have identified silica (containing minor amounts of S, K, Fe, Al), K-Fe-sulfate (probably jarosite) and Fe-phosphate as alteration phases in Y-793605. Of these, the silica and K-Fe-sulfate are likely terrestrial weathering products. Other evidence of alteration consists of what appear to be partly decomposed Ca-phosphate grains, which were probably originally igneous grains. No carbonates or Ca-sulfates have been identified as yet, and none of the alteration phases we have identified are unambiguously of martian origin. Compositionally, Y-793605 is very similar to the other two martian lherzolites ALHA77005 and LEW 88516. Our sample of Y-793605 is lower in the incompatible lithophile trace elements, such as the REE, than the average of either ALHA77005 or LEW 88516,but is within the ranges of individual analyses for ALHA77005. Y-793605 is a partial cumulate like the other lherzolites, but our sample contained less of a trapped melt component

Compton Scattering from the Deuteron Above Pion-Production Threshold

The electromagnetic polarizabilities of the nucleon are fundamental nucleon-structure observables that characterize its response to external electromagnetic fields. The neutron polarizabilities can be accessed from Compton-scattering data on light nuclear targets. Recent measurements of the differential cross section for Compton scattering on the deuteron below the pion-production threshold have decreased the uncertainties in the neutron polarizabilities, yet the proton polarizabilities remain known substantially more accurately. As the sensitivity of the cross section to the polarizabilities increases with incident photon energy, measurements above the pion threshold may offer a way for an improved determination of the neutron polarizabilities. In this Rapid Communiciation, the first measurement of the cross section for coherent Compton scattering on the deuteron above the pion-production threshold is presented