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Datowanie meteorytów kamiennych metodą potasowo-argonową

Mackiewicz P., Hałas S.

Przegląd Geologiczny

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2003

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Vol. 51, nr 7

594-596

PL

W niniejszej pracy, całkowicie wykonanej w Pracowni Spektrometrii Mas Instytutu Fizyki UMCS, przedstawiamy wyniki datowania sześciu meteorytów kamiennych: Baszkówka, Pułtusk, Norton County, Chico Hills, Point of Rocks i Dimmitt. Koncentrację potasu wyznaczyliśmy precyzyjnie metodą rozcieńczenia izotopowego - maksymalny błąd względny wynosi 2% w przypadku Norton County przy zawartości potasu 0,0101%. Analiza argonu została przeprowadzona za pomocą spektrometru mas z próżnią statyczną. Cały wydzielony argon potraktowaliśmy jako radiogeniczny, ponieważ atmosferyczny argon został usunięty podczas kilkugodzinnego wygrzewania w temperaturze 150°C i pompowania. Otrzymane wieki meteorytów zawierają się pomiędzy 1,28 a 3,91 miliardów lat.

EN

We have determined the K/Ar dates of the six stone meteorites: Baszkówka, Norton County, Dimmitt, Pułtusk, Chico Hills and Point of Rocks. We have obtained very precisely K content by isotope dilution mass spectrometry - max. relative error was 2% in the case of Norton County, in which the lowest %K was encountered. We have determined content of40Ar by static vacuum mass spectrometry. The total argon released was treated as radiogenic argon, because the atmospheric argon was released by heating samples in 150°C and pumping off over a few hours. The obtained dates of stone meteorites varied from 1.28 to 3.91 Ga. The oldest dates are younger than the age of the Solar System by about 0.5 Ga, likely because we did not release all the radiogenic argon.

2

Two spinel-bearing compound chondrules from the Baszkówka meteorite

Borucki J., Stępniewski M.

Geological Quarterly

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2001

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Vol. 45, No 3

281-288

EN

The present paper describes two compound chondrules labelled panda and chevron, discovered in the Baszkówka meteorite. Optical microscope, electron microscope and electron microprobe studies revealed an unusual mineral composition and fabric. Both chondrules include "primary" and "secondary" components. The panda and chevron chondrules have apparent diameters of ~0.86 and ~0.54 mm respectively with spinel dominant in primary components and olivine-plagioclase in secondary components overlain by thin microcrystalline rims. Euhedral or subhedral Mg, Fe2+/Al, Cr-spinel crystals from the panda's primary component (zoned, cavernous or homogeneous) were not found in Mg-Fe chondrules in Baszkówka, where Cr occurs exclusively as chromite. Olivine crystals from panda's primary (Fa27.1) and secondary (Fa26.5) component have similar compositions to those from panda's rim (Fa26.4) and comparable to olivine from other Baszkówka Mg-Fe chondrules (average ~Fa25.7). The Ca-plagioclase laths from the groundmass of panda's primary component has an extremely variable composition (~An49 to ~An60), evidently more calcic than those in other chondrules of the meteorite (mean ~An13). The primary components of the two spinel-bearing chondrules may have been formed in a fireball generated by powerful impact during early accretion of a protoplanet. After crystallisation the spinel crystals were abraded and polished, probably during rapid flight through a dense cloud of interplanetary dust. The moderately reducing conditions of the hypothetical fire ball were replaced by a more reducing environment during the condensation of the rims of both chondrules.

3

Radioactivity of the Baszkówka meteorite

Biernacka M., Isajenko K., Lipiński P., Żak A.

Geological Quarterly

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2001

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Vol. 45, No 3

327-330

EN

In January 1996 a fragment, and in May 1996 a cut slab of the Baszkówka meteorite, were delivered for the radioactivity analysis to the Dosimetry Department of the Central Laboratory for Radiological Protection (CLOR) in Warsaw. The measurement method was gamma spectrometry using HPGe (High Purity Germanium) detectors. The CLOR prepared volume calibration source identical to the cut slab of the meteorite delivered for analyses. The presence of 40K and the cosmogenic radionuclides such as 54Mn and 22Na was detected in the Baszkówka meteorite. The results and measurement method are discussed.

4

Petrography of the Baszkówka chondrite

Siemiątkowski J.

Geological Quarterly

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2001

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Vol. 45, No 3

263-280

EN

The Baszkówka chondrite comprises chondrules, individual crystals of olivine, xenomorphic concentrations of kamacite and troilite, matrix and pores up to 3.0 mm in diameter; porosity reaches 20% by volume. Measurements of 697 chondrules in an area of 250 m2 enabled distinction of three populations. The chondrules and olivine crystals have been sintered at high temperatures. Six structural types showing transitions between porphyritic and granular structures have been discriminated. Rims showing a similar diversity surround about 10% of chondrules. The composition and structure of the chondrules suggest a source from the nebular dust disk around the Sun. The abudance of voids and their relatively large size and a lack of fissures, indicate a small size of the host body, which must have been derived from an early phase of protoplanet formation.

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Oxygen-isotopic compositions of minerals from the Baszkówka chondrite

Maruyama S.

Geological Quarterly

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2001

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Vol. 45, No 3

303-312

EN

Oxygen isotopic compositions of two picotite-bearing chondrules from Baszkówka L5(S1) equilibrated ordinary chondrite have been determined by secondary ion mass spectrometry. The O-isotopic compositions of the minerals in the picotite-bearing chondrules are slightly more 16O-rich than the bulk O-isotopic composition of Baszkówka. The petrological properties and the O-isotopic compositions of these chondrules suggest that they have closer relationship to ferromagnesian chondrules than CAIs. The 16O-enrichment of the picotite-bearing chondrules may result from incomplete O exchange reaction between 16O-rich chondrule precursors and the 16O-poor solar nebular gas. This implies that the picotite-bearing chondrules are more primitive than ordinary ferromagnesian chondrules in Baszkówka. The O-isotopic compositions of the minerals in one of these chondrules relate to the crystallization sequence. These O-isotopic variations of minerals within the chondrule may result from incomplete O exchange between the chondrule melt droplet and the nebular gas. The heterogeneity of the O-isotopic composition in Baszkówka indicates that Baszkówka has not been homogenized completely during thermal metamorphism and/or secondary alteration.

6

Noble gases in Baszkówka and Mt. Tazerzait

Scherer P., Schultz L.

Geological Quarterly

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2001

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Vol. 45, No 3

313-314

EN

Concentration and isotopic composition of noble gases have been measured in bulk samples of the L5 chondrites Baszkówka and Mt. Tazerzait. Compared to other chondrites both meteorites have very large cosmic-ray exposure ages of 74 and 60 million years, respectively. During their history both meteorites were not involved in major thermal events.

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Mineralogy of the Baszkówka chondrite (L5 S1): new data on silicates, opaques and minor minerals

Borucki J., Stępniewski M.

Geological Quarterly

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2001

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Vol. 45, No 3

229-256

EN

The mineral composition of the Baszkówka meteorite comprises: olivine, pyroxenes, plagioclase, Fe,Ni metal, troilite and chromite with minor chlorapatite, whitlockite, magnetite, haematite, spinel, idaite, calcite and native Cu. A rare variety of spinel (picotite), probably the oldest among the minerals of Baszkówka, was identified in the only two chondrules named: panda and chevron. The composition of the olivine (Fa 26.2š1.8 wt.%) and the high degree of chondrite homogenisation, a result of thermal metamorphism, are consistent with earlier results and indicate the L5 group. No distinct shock effects were observed in Baszkówka, classified as S1. Troilite-Fe,Ni and Fe,Ni metal lumps are defined and interpreted as molten planetesimals impact splashes.

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K/Ar dating and stable isotope analysis of the Baszkówka and Mt. Tazerzait L5 chondrites

Hałas S., Wójtowicz A.

Geological Quarterly

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2001

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Vol. 45, No 3

315-318

EN

We have determined the content of 40Ar* (1.49 nmol/g) and 4He (1.75 nmol/g) in the Baszkówka meteorite by static vacuum mass spectrometry. The radiogenic argon content was calculated from the measured argon spectrum using the equation: 40Ar* = 40Ar - 295.536Ar assuming a potassium content of 680 ppm wgt. We have obtained K/Ar ages of 3.78 Ga for the chondrules and 3.47 Ga for a bulk sample of Baszkówka. A similar 4He content (1.47 nmol/g) but larger 40Ar* (2.32 nmol/g) content was found for the Mt. Tazerzait bulk sample. The K/Ar age of this meteorite on the basis of the K content (732 ppm) is 4.34 Ga. Sulphur isotope analysis of troilite specimens from both meteorites reveals essentially negative delta 34S values: -1.25š0.06o/oo for Baszkówka and -1.18š0.06o/oo for Mt. Tazerzait expressed on the V-CDT scale. This indicates that both chondrites may have a common origin. The oxygen isotope data (delta 18O = 4.88š0.03o/oo and delta 17O = 3.66š0.10o/oo) suggests that Baszkówka belongs to the L chondrites.

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Euhedral crystals in interstitial pores of the Baszkówka and Mt. Tazerzait L5 chondrites

Wlotzka F., Otto J.

Geological Quarterly

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2001

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Vol. 45, No 3

257-262

EN

Euhedral to anhedral crystals of the main meteoritic minerals are found in interstitial pores of the unshocked L5 chondrites Baszkówka and Mt. Tazerzait. The composition of these pore minerals is the same as elsewhere in the meteorite. They must have grown from a vapour phase during metamorphic equilibration of the meteorite minerals. The pores are primary features, i.e. open space remaining after compaction of the constituents of the meteorite.

10

A study on chemical composition of Baszkówka and Mt. Tazerzait chondrites

Dybczyński R., Chwastowska J., Danko B., Kulisa K., Polkowska-Motrenko H., Samczyński Z., Sterlińska E., Szopa Z.

Geological Quarterly

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2001

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Vol. 45, No 3

289-302

EN

The elemental contents of two new meteorites, Baszkówka (Poland, 1994) and Mt.Tazerzait (Niger, 1991), were studied by instrumental and radiochemical neutron activation analysis (NAA), supplemented in the case of some elements by gravimetric, titrimetric, spectrophotometric and atomic absorption spectrometric measurements. In total 28 elements were determined for each meteorite. A striking similarity in the bulk chemical composition of the two meteorites was observed. In the case of Baszkówka some study of the elemental contents of the magnetic fraction and the distribution of elements between kamacite and taenite phases was also made. This study confirms that Baszkówka and Mt. Tazerzait are generally more similar to L chondrites than to any other class of chondrites. However both meteorites show also some distinctive features (e.g very low Hg content), distinguishing them from all other L chondrites known. Both meteorites may derive from a common parent body, which may be different from that typical for other chondrites.

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Fall, recovery and preliminary study of the Baszkówka meteorite (Poland)

Stępniewski M., Siemiątkowski J., Borucki J., Radlicz K.

Archiwum Mineralogiczne

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1998

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T. 51, z. 1-2

131-152

EN

The meteorite fell on August 25, 1994, near the small village of Baszkówka, ca. 25 km south-west from the centre of Warszawa (Poland). The fall was observed by people living there, therefore the stone was found at once 25 cm under the surface of the soft cultivated soil. It was the not damaged, oriented stone of 15.5 kg, covered with a fusion crust decorated by numerous, radial regmaglypts. The meteorite is classified as L5 ordinary, unshocked (Sl) chondrite. The main silicate components of the meteorite are olivine (chrysolite; 20-25% Fa) and pyroxene (bronzite ca. 20% Fs). In addition to kamacite (alpha-Fe with ca. 5.5% Ni) and troilite, some rare grains of chromite and very rare native Cu grains were found.

PL

Pełna lista polskich meteorytów jest uboga, liczy niespełna 20 okazów, w tym ponad połowa to znaleziska. Meteoryt Baszkówka spadł 25 sierpnia 1994 r., po południu, w pobliżu Warszawy, na świeżo zaorane pole. Dzięki bardzo szczęśliwemu zbiegowi okoliczności meteoryt nie rozpadł się, upadek jego został zauważony. Meteoryt został szybko odnaleziony i zabezpieczony. jest to pojedynczy okaz o masie 15,5 kg i kształcie kapelusza grzyba o średnicy 25-30 cm i grubości około 18 cm. Na ponad 90% powierzchni pokryty jest czarną polewą. Bardziej wypukła strona okazu jest stożkowata, pokryta głębokimi bruzdkami (regmagliptami) rozchodzącymi się promieniście od wierzchołka ku krawędziom, co świadczy o orientowanym spadku meteorytu. W miejscach pozbawionych polewy widoczna jest ziarnista skała koloru szarooliwkowego z rdzawymi plamkami. Gołym okiem można zaobserwować chropowate kuleczki-chondry, które mają do kilku mm średnicy i stanowią ponad 30% objętości okazu. Badaniami mikroskopowymi stwierdzono, że przestrzeń między chondrami wypełnina jest fragmentami chondr, okruchami skalnymi, pojedynczymi kryształami, a także skupieniami minerałów nieprzezroczystych, które stanowią około 10% objętości, i charakteryzuje się licznymi pustkami, co tłumaczy kruchość meteorytu i jego mały ciężar objętościowy - 2.9 g/cm3. Obserwowane pod mikroskopem obrazy chondr z meteorytu Baszkówka są bardzo zróżnicowane, w większości jednak podobne do opisywanych wcześniej w innych chondrytach; są to chondry porfirowe z kryształami oliwinów oraz piroksenów, ekscentryczno-promieniste chondry piroksenowe, belkowe chondry oliwinowe. Osobliwością Baszkówki jest "kwiecista" porfirowa chondra oliwinowa. Głównymi składnikami mineralnymi meteorytu Baszkówka są: oliwiny (chryzolit z 20-25% Fa) i pirokseny (bronzyt z około 20%Fs), następnie plagioklazy, żelazo z zawartością około 5,5% Ni, troilit oraz niewielkie ilości chromitu i śladowe ilości miedzi rodzimej. Wykonane badania wykazały, że meteoryt Baszkówki jest chondrytem zwyczajnym, który sklasyfikowano jako chondryt L5 oraz jako nie zmieniony zderzeniowo meteoryt S1. Ze wzgledu na niejednoznaczność niektórych cech (zawartość żelaza metalicznego i siarczkowego, skład chemiczny oliwinów), lokujących go na pograniczu grup L i H, niezbędne są dalsze badania. Meteoryt został zgłoszony do światowego rejestru w Instytucie Maxa Plancka w Moguncji. Okaz jest własnością Państwowego Instytutu Geologicznego i znajduje się w Muzeum Geologicznym PIG w Warszawie.