ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01089.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01090.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01091.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The SNC meteorites are thought to be igneous martian rocks, based on their young crystallization ages and a close match between the composition of gases implanted in them during shock and the atmosphere of Mars. A related meteorite, ALH84001, may be older and thus may represent ancient martian crust. These petrologically diverse basalts and ultramafic rocks are mostly cumulates, but their parent magmas share geochemical and radiogenic isotopic characteristics that suggest they may have formed by remelting the same mantle source region at different times. Information and inferences about martian geology drawn from these samples include the following: Planetary differentiation occurred early at ∼4.5 Ga, probably concurrently with accretion. The martian mantle contains different abundances of moderately volatile and siderophile elements and is more Fe‐rich than that of the Earth, which has implications for its mineralogy, density, and origin. The estimated core composition has a S abundance near the threshold value for inner core solidification. The former presence of a core dynamo may be suggested by remanent magnetization in SNC meteorites, although these rocks may have been magnetized during shock. The mineralogy of martian surface units, inferred from reflectance spectra, matches that of basaltic shergottites, but SNC lithologies thought to have crystallized in the subsurface are not presently recognized. The rheological properties of martian magmas are more accurately derived from these meteorites than from observations of martian flow morphology, although the sampled range of magma compositions is limited. Estimates of planetary water abundance and the amount of outgassed water based on these meteorites are contradictory but overlap estimates based on geological observations and atmospheric measurements. Stable isotope measurements indicate that the martian hydrosphere experienced only limited exchange with the lithosphere, but it is in isotopic equilibrium with the atmosphere and has been since 1.3 Ga. The isotopically heavy atmosphere/hydrosphere composition deduced from these rocks reflects a loss process more severe than current atmospheric evolution models, and the occurrence of carbonates in SNC meteorites suggests that they, rather than scapolite or hydrous carbonates, are the major crustal sink for CO2. Weathering products in SNC meteorites support the idea of limited alteration of the lithosphere by small volumes of saline, CO2‐bearing water. Atmospheric composition and evolution are further constrained by noble gases in these meteorites, although Xe and Kr isotopes suggest different origins for the atmosphere. Planetary ejection of these rocks has promoted an advance in the understanding of impact physics, which has been accomplished by a model involving spallation during large cratering events. Ejection of all the SNC meteorites (except ALH84001) in one or two events may provide a plausible solution to most constraints imposed by chronology, geochemistry, and cosmic ray exposure, although problems remain with this scenario; ALH84001 may represent older martian crust sampled during a separate

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01092.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Four different types of calcium‐ and aluminium‐rich inclusions (CAIs) have been identified in the CM2 chondrite Murray, three of which contain alteration products. Two types of altered CAIs, spinel inclusions and spinel‐pyroxene inclusions, contain primary spinel (± perovskite ± hibonite ± diopside) and secondary Fe‐rich serpentine phyllosilicates (± tochilinite ± calcite). Original melilite in these CAIs is inferred to have been altered during aqueous activity in the parent body and Fe‐rich serpentines, tochilinite and calcite were formed in its place.The other type of altered CAI is represented by one inclusion, here called MCA‐1. This CAI contains primary spinel, perovskite, fassaite and diopside with secondary calcite, paragonite, Mg‐Al‐Fe phyllosilicates and a Mg‐Al‐Fe sulphate. Importantly, MCA‐1 is similar in both primary and secondary mineralogy to a small number of altered CAIs described from other CM2 meteorites including Essebi, Murchison and a CM2 clast from Plainview. Features that these CAIs have in common include an unusually large size, a CV3‐like primary mineralogy and the presence of secondary aluminosilicates and calcite. The Al‐rich alteration products in MCA‐1 are also reminiscent of secondary minerals in refractory inclusions from CV3 meteorites, which have previously been interpreted to form by interaction of the inclusions with solar nebula gases. In common with the other types of altered CAIs in Murray, MCA‐1 is inferred to have experienced its main phase of alteration in a parent body environment. The Mg‐Al‐Fe phyllosilicates, calcite and the Mg‐Al‐Fe sulphate formed following aqueous alteration of an Al‐rich precursor, possibly Ca dialuminate. This episode of parent body alteration may have overprinted an earlier phase of alteration in a solar nebula

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01093.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—High‐purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases.P3, released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios.HL, consisting of isotopically anomalous Xe‐HL and Kr‐H, Ar with high38Ar/36Ar, and most of the gas making up Ne‐A2 and He‐A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase.P6has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble‐gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds.We find no evidence of separability of Xe‐H and Xe‐L or other isotopic variations in the HL component. However, because ∼1010diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble‐gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble‐gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was p

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01094.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Using the isotopic compositions derived in Huss and Lewis, 1994a (Paper I), abundances of the P3, HL, and P6 noble‐gas components were determined for 15 diamond separates from primitive chondrites of 8 chondrite classes. Within a meteorite class, the relative abundances of these components correlate with the petrologic subtype of the host meteorite, indicating that metamorphism is primarily responsible for the variations. Relative abundances of P3, HL, and P6 among diamond samples can be understood in terms of thermal processing of a single mixture of diamonds like those now found in CI and CM2 chondrites. With relatively gentle heating, primitive diamonds first lose their low‐temperature P3 gases and a “labile” fraction of the HL component. Mass loss associated with release of these components produces an increase in the HL and P6 content of the remaining diamond relative to unprocessed diamond. Higher temperatures initiate destruction of the main HL carrier, while the HL content of the surviving diamonds remains essentially constant. At the same time, the P6 carrier begins to preferentially lose light noble gases. Meteorites that have experienced metamorphic temperatures ≳650 °C have lost essentially all of their presolar diamond through chemical reactions with surrounding minerals.The P3 abundance seems to be a function only of the maximum temperature experienced by the diamonds and thus is independent of the nature of the surrounding environment. If all classes inherited the same mixture of primitive diamonds, then P3 abundances would tie together the metamorphic scales in different meteorite classes. However, if the P3 abundance indicates a higher temperature than do other thermometers applicable to the host meteorite, then the P3 abundance may contain information about heating prior to accretion. Diamonds in the least metamorphosed EH, CV, and CO chondrites seem to carry a record of pre‐accretionary ther

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01095.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—We studied five new Antarctic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony‐iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse‐grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco‐like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg‐rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse‐grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase‐rich silicate inclusions in IAB iron meteorites and may have been derived by a similar process but in a d

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01096.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The Nova 001 [= Nuevo Mercurio (b)] and Nullarbor 010 meteorites are ureilites, both of which contain euhedral graphite crystals. The bulk of the meteorites are olivine (Fo79) and pyroxenes (Wo9En73Fs18, Wo3En77Fs20), with a few percent graphite and minor amounts of troilite, Ni‐Fe metal, and possibly diamond. The rims of olivine grains are reduced (to Fo91) and contain abundant blebs of Fe metal. Silicate mineral grains are equant, anhedral, up to 2 mm across, and lack obvious preferred orientations. Euhedral graphite crystals (to 1 mm x 0.3 mm) are present at silicate grain boundaries, along boundaries and protruding into the silicates, and entirely within silicate mineral grains. Graphite euhedra are also present as radiating clusters and groups of parallel plates grains embedded in olivine; no other ureilite has comparable graphite textures. Minute lumps within graphite grains are possibly diamond, inferred to be a result of shock. Other shock effects are limited to undulatory extinction and fracturing. Both ureilites have been weathered significantly. Considering their similar mineralogies, identical mineral compositions, and identical unusual textures, Nova 001 and Nullarbor 010 are probably paired. Based on olivine compositions, Nova 001 and Nullarbor 010 are in Group 1 (FeO‐rich) of Berkleyet al.(1980). Silicate mineral compositions are consistent with those of other known ureilites. The presence of euhedral graphite crystals within the silicate minerals is consistent with an igneous origin, and suggests that large proportions of silicate magma were present locally and crystallizedin

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01097.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Infrared diffuse reflectance spectra (2.53–25 μm) of some carbonaceous (C) chondrites were measured. The integrated intensity of the absorption bands near 3 μm caused by hydrous minerals were compared with the modal content of hydrous minerals for the meteorites. The CM and CI chondrites show larger values of the integrated intensity than those of the unique C chondrites Y82162, Y86720 and B7904, suggesting that the amount of hydrous minerals in the CM and CI chondrites is larger, which supports the contention that hydrous minerals were dehydrated by thermal metamorphism in the unique chondrites. Orgueil (CI) has the largest value of the integrated intensity among the C chondrites we measured and shows a sharp absorption band at 3685 cm−1(2.71 μm) that is not seen in the spectra of the CM chondrites. There is an excellent correlation between the observed hydrogen content in C chondrites and the integrated intensity. The CM chondrites show a wide variation in the strength of absorption bands at 1470 cm−1(6.8 μm), despite the similarity in absorption features near 3 μm for all CM chondrites. The 1470 cm−1band could be due to the presence of some hydrocarbons but may also be a result of terrestrial altera

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb01098.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY