摘要:

Abstract—We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microanalyser and scanning electron microscope. Although originally described as having fine‐grained texture, the new specimens show diversity of texture. The compositions (Mg/Fe ratios) of the host pigeonites and augite lamellae are homogeneous, respectively, in spite of the textural variation. In addition to their chemical homogeneity, pyroxenes in coarse and fine‐grained clasts are partly inverted to orthopyroxene. Chemical zoning of plagioclase during crystal growth is preserved. This eucrite includes areas of granulitic breccias and impact melts. Large scale textures show a subparallel layering suggesting incomplete mixing and deposition of impact melt and lithic fragments. An39Ar‐40Ar age determination for a coarse‐grained clast indicates a strong degassing event at 3.55 ± 0.02 Ga. We conclude that Millbillillie is among the most equilibrated eucrites produced by thermal annealing after impact brecciation. According to the classification of impact breccias, Millbillillie can be classified as a mixture of granulitic breccias and impact melts. The last significant thermal event is characterized by network‐like glassy veins that run through clasts and matrices. Consideration of textural observations and requirements for Ar‐degassing suggests that the39Ar‐40Ar age could in principle date either the earilier brecciation and annealing event or the event which pr

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00677.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—On 1992 August 14 at 12:40 UTC, an ordinary chondrite of type L5/6 entered the atmosphere over Mbale, Uganda, broke up, and caused a strewn field of size 3 × 7 km. Shortly after the fall, an expedition gathered eye witness accounts and located the position of 48 impacts of masses between 0.1 g and 27.4 kg. Short‐lived radionuclide data were measured for two specimens, one of which was only 12 days after the fall. Subsequent recoveries of fragements has resulted in a total of 863 mass estimates by 1993 October. The surfaces of all fragments contain fusion crust. The meteorite shower caused some minor inconveniences. Most remarkably, a young boy was hit on the head by a small specimen.The data are interpreted as to indicate that the meteorite had an initial mass between 400–1000 kg (most likely ∼1000 kg) and approached Mbale from Az = 185 ± 15, H = 55 ± 15, and V∞= 13.5 ± 1.5/s. Orbital elements are given. Fragmentation of the initial mass started probably above 25 km altitude, but the final catastrophic breakup occurred at an altitude of 10–14 km. An estimated 190 ± 40 kg reached the Earth's surface minutes after the final breakup of which 150 kg of material ha

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00678.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Pecora Escarpment (PCA)91002 is a light/dark‐structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains ∼10–20 vol% equilibrated (type −5 and −6) clasts within a clastic groundmass, much of which was metamorphosed to type‐3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa38–40; by contrast, low‐Ca pyroxene compositions are highly variable. Opaque phases identified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3‐S4; there are numerous sulfide‐rich shock veins and 50‐μm plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration.The Rumuruti‐like (formerly Carlisle‐Lakes‐like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 ± 0.05x CI), high bulk Δ17O, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 ± 100 μm range, abundant NiO‐bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe‐Ni. We propose that this group be called R chondrites after Rumuruti, the only fall.The abundant NiO‐bearing ferroan olivine grains, the occurrence of Cu‐bearing sulfide, and the paucity of metallic Fe‐Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type‐I porphyritic olivine chondrules containing olivine grains with cores of Fa3–4composition occur in PCA91002; these chondrules probably formed initially as metallic‐Fe‐Ni‐bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the>5‐μm olivine grains in the R chondrites reflect equilibration with fine‐graine

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00679.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) −75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes‐type” chondrites (Weisberget al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y‐793575, Y‐82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulzeet al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y‐75302, Y‐793575, Y‐82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R).Acfer 217 is a regolith breccia consisting of up to cm‐sized clasts (∼33 vol%) embedded in a fine‐grained, well‐lithified clastic matrix. The most abundant mineral is olivine (∼72 vol%), which has a high Fa‐content of 37–39 mol%. The major minerals (olivine, low‐Ca pyroxene, Ca‐pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa37.8 ± 5.7, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2).The bulk composition of Acfer 217 and other R‐meteorites show that the R‐meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y‐75302. In a δ17Ovs. δ18O‐diagram, the R‐meteorites form a group well resolved from other chondrite groups.Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. B

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00680.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine‐rich chondritic breccia with light‐dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa∼39) with about 70 vol%. Feldspar (∼14 vol%; mainly plagioclase), Ca‐pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low‐Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni‐rich and Ge‐containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE,etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ17O (5.52 ‰) and δ18O (5.07 ‰).Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes‐type chondrites and they comprise: Carlisle Lakes, ALH85151, Y‐75302, Y‐793575, Y‐82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni‐metals, occurrence of chalcopyrite); (c) exceptionally high Δ17O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (∼70 vol%); (e) Ti‐Fe3+−rich chromite (∼5.5 wt% TiO2); (f) occurrence of various noble metal‐rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies.With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R chondrites and ordinary chondrites exclude formation of R chondrites from ordinary chondrites by oxidation. This implies

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00681.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Book reviewed in this article:Rendezvous in Space: The Science of Cometsby John C. Brandt and Robert D. ChapmanThe Care and Conservation of Geological Material: Minerals, Rocks, Meteorites and Lunar Findsedited by Frank M. HowieRemote Geochemical Analysis: Elemental and Mineralogical Compositionedited by Carlé M. Pieters and Peter A. J. EnglertTransuranium Elements: A Half Centuryedited by L. R. Morss and J. Fuge

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00682.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.tb00683.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.tb00684.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.tb00685.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.tb00686.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY