Hypersthene Chondrites Research Articles

Mass Spectrometrical Study of Rare Gas Compositions and Neutron Capture Effects in Yamato-74191 (L 3) Chondrite

Abstract The unequilibrated hypersthene chondrite Yamato-74191 was studied mass spectrometrically for rare gases released at various temperatures. Cosmogenic gases dominate in He and Ne. The meteorite contains large amounts of trapped Ar, Kr and Xe, and radiogenic 40Ar and 129Xe. Cosmic-ray irradiation and K-Ar ages were determined. In addition to spallogenic components of Kr and Xe, isotopic excesses of 80Kr, 82Kr, 128Xe and 126Xe relative to AVCC-Kr and -Xe were found. The ratio of 80Kr-excess to 82Kr-excess is 2.66 after correction for spallogenic Kr. A correlation between 128Xe/132Xe and 129Xe/132Xe was found The 129Xe/132Xe ratio for trapped Xe in Yamato-74191 was determined as 1.12 ± 0.29 with the correlation plot. The excesses found in Yamato-74191 are best explained by epithermal neutron capture on Br and I, and by the 127I (n, 2nβ) 126 Xe reaction. Using neutron-produced 80Kr, the neutron slowing -down density was estimated to be 0.14 ± 0.03 cm-3sec-1. A minimum mass and a preatmospheric radius was estimated to be 470 kg and 32 cm, respectively.

THE MINERALOGY AND CHEMISTRY OF THE ALTA'AMEEM METEORITE

The Alta'ameem hypersthene chondrite is a light gray brecciated and metamorphosed meteorite composed mainly of olivine (27% Fa), orthopyroxene (24.5% Fs) and plagioclase (An10). Other minerals include troilite, kamacite, taenite, chromite, ilmenite, clinopyroxene, chalcopyrite, and apatite or merrillite.The mineralogical and chemical analyses suggest that the Alta'ameem meteorite belongs to the amphoterite group of chondrites. The chemical composition includes the following: Fe 3.39, Ni 1.13, Co 0.05, Cu 0.01, FeS 6.48, SiO2 39.48, TiO2 0.28, Al2O3 2.25, FeO 16.46, MnO 0.40, MgO 25.66, CaO 1.47, Na2O 1.05, K2O 0.15, P2O5 0.47, Cr2O3 0.45; total 99.18.

RARE GASES IN BANSUR, UDAIPUR AND MADHIPURA CHONDRITES

We have analysed three Indian meteorites — Bansur, Udaipur and Madhipura — for the elemental and isotopic composition of Ne, Ar, Kr and Xe and determined their radiation and gas‐retention ages. It is found that Udaipur belongs to the group of unequilibrated ordinary chondrites and Madhipura probably belongs to the group of shocked hypersthene chondrites.

The unique cosmic-ray history of the Malakal chondrite

Cosmogenic radionuclides, including 22Na, 26Al and 54Mn, were measured in a sample of the recently-fallen Malakal hypersthene chondrite. The high 26Al activity, 79 ± 2 dpm/kg, greatly exceeds the levels expected from elemental production rates, shielding considerations, or comparisons with other ordinary chondrites, and can only be explained by exposure to a uniquely high cosmic-ray flux. Calculations including noble gas, 3H, and 53Mn data from other laboratories require a two-stage irradiation. Malakal's most probable history is: exposure in excess of 4 m.y. to an effective cosmic-ray flux approximately three times that experienced by other chondrites, an orbit change (very possibly caused by a collision), and a final period of about 2 m.y. during which it was exposed to a “normal” cosmic-ray flux.

Minor and trace elements in some meteoritic minerals

The mineral phases including olivine, orthopyroxene, clinopyroxene, troilite, nickel-iron, plagioclase, chromite and the phosphates were separated from several meteorites. These were a hypersthene chondrite (Modoc), a bronzite chondrite (Guareña), an enstatite chondrite (Khairpur), and two eucrites (Haraiya and Moore County); diopside was separated from the Nakhla achondrite. The purified minerals were analyzed for trace and minor elements by spark source mass spectrometry and instrumental neutron activation analysis. On the meteorites examined our results show that Co, Ni, Cu, Ge, As, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt and Au are entirely or almost entirely siderophile; Na, Rb, Sr, Y, Ba and the rare earth elements lithophile; Se chalcophile. The transition elements So, Ti, V, Cr and Mn are lithophile in most stony meteorites, but show chalcophile affinities in the enstatite chondrites (and enstatite achondrites), as do Zn, Zr and Nb. In the ordinary chondrites Ga shows both lithophile and siderophile affinities, but becomes entirely siderophile in the enstatite chondrites. Molybdenum and tellurium show strong siderophile and weaker chalcophile affinity. The lithophile elements are distributed among the minerals according to the crystallochemical factors, the most effective controlling factor being ionic size.

MINERALOGICAL AND CHEMICAL RESEARCHES ON L‐CHONDRITES: GIRGENTI

A mineralogical and chemical analysis has been performed on the largest mass (8750 g) of the Girgenti, Italy, meteorite, from the collection of the Smithsonian Institution, Washington, D.C. The mineralogical composition is olivine, Fa24–25; hypersthene, Fs21 –22; plagioclase, An12; maskelynite; whitlockite; nickel‐iron; troilite; chromite and ilmenite. Girgenti is a severly metamorphosed stone, whose total iron content (23.5%) is somewhat higher than the average for hypersthene chondrites.

THE TIMMERSOI†, NIGER, HYPERSTHENE CHONDRITE

The Timmersoi meteorite, a new type L5 hypersthene chondrite from the Niger Republic is described and microprobe analyses of its olivine, orthopyroxene, clinopyroxene, plagioclase, kamacite, taenite, troilite, chromite, whitlockite, chlorapatite and limonite presented. Veins and patches of black “glassy” material are regarded as products of shock melting. In places this material contains immiscible droplets of troilite each with one or more well‐formed crystals of taenite. Calculations indicate equilibrium between olivine and orthopyroxene with a temperature of equilibration of about 850 °C.

Distribution of uranium and thorium among components of some chondrites

The distribution of uranium and thorium was studied in the fractionated components of several chondritic meteorites. A wet fractionation method was applied to separate the component mineral groups of powdered samples of two olivine-- hypersthene chondrites, one olivine -bronzite chondrite, and three enstatite chondrites. Uranium was determined in four samples and thorium was determined in all six samples. Both element determinations were by neutron activation. Uranium was found to be enriched in the HCl soluble fraction of the hypersthene chondrite Bruderheim, and of the enstattte chondrites, Abee and St. Marks, whereas it was enriched in the HCl insoluble fraction of the bronzite chondrite, Richardton. On the other hand, thorium was found to be relatively concentrated in the HCl soluble fractions of enstatite chondrites and in the HCl insoluble fractions of all the ordinary chondrites examined. In ordinary chondrites thorium behaves as a lithophile element, whereas in enstatite chondrites it seems to be partly chalcophile. (auth)

Rubidium-strontium studies on black hypersthene chondrites: Effects of shock and reheating

Whole-rock Rb-Sr data have been measured for 15 hypersthene chondrites including 10 black chondrites. The latter appear to have been the most heavily shocked and reheated. Several density fractions for two of the black chondrites, Orvinio and Farmington, were separated, and Rb-Sr analyses were performed. Whole-rock data for observed black hypersthene falls conform (within experimental error) to the 4.5–4.6×109-year isochron defined by other chondrites. Although black ‘finds’ plot uniformly to the left of the isochron, this behavior is also shown by unshocked finds. Therefore the total-rock data do not reveal any significant shock or reheating effects. The density separates, particularly those of Orvinio, depart significantly from the 4.5-b.y. isochron. Although their 87Rb/86Sr ratios vary from 0.5 to 1.1, their 87Sr/86Sr ratios are the same as that of the total meteorite to within 0.002. This similarity results in a nearly horizontal distribution of points on a Sr evolution diagram. To a lesser extent, data from Farmington also show the same effect. It is concluded that this behavior is due to partial homogenization of the Sr isotopes within these two meteorites very recently in their history. The scatter in the data precludes any precise determination of the time of the event.

Assam: A gas rich hypersthene chondrite

Concentrations and isotopic compositions of He, Ne and Ar as well as track densities due to galactic and solar flare irradiations were determined in light and dark fractions of the L-chondrite Assam. The investigation showed Assam to be a gas rich dark-light structured hypersthene chondrite. Two percent of the L-group chondrites are now known to contain solar light noble gases (H-group: 12%, LL-group: 8%).

THE VALDINIZZA METEORITE: MINERALOGY, CHEMISTRY AND MICROTEXTURES

The principal data are collected about the fall and the distribution of the fragments of the Valdinizza, Italy, meteorite. A complete individual, weighing 872 g, preserved in the United States National Museum, Washington, D. C., is described in some detail. The mineralogical composition is olivine, Fa25; hypersthene, Fs23; plagioclase, An10; maskelynite; nickel‐iron; troilite; chromite; ilmenite and possibly a phosphate mineral. Valdinizza is a fairly typical hypersthene chondrite, belonging to the type 6 chondrites of Van Schmus and Wood (1967); its structure shows evidence of a period of high‐temperature recrystallization; interesting features of shock‐metamorphism are notable, the microtexture deformations suggesting a high level of stress

The formation of clear taenite in ordinary chondrites

The metallic phases in six bronzite and six hypersthene chondrites were studied metallographically and by electron microprobe. All of the chondrites studied contain zoned taenite. In bronzite chondrites, only about 5 per cent of the zoned taenite abuts on kamacite (the rest being apparently isolated from it) whereas in hypersthene chondrites an average of over 20 per cent abuts on kamacite. The compositions of the centers of zoned taenite can be used to obtain cooling rates by Wood's method. Including Wood's results, 14 out of 18 ordinary chondrites have cooling rates between 1 and 10°C/m.y. In addition to zoned taenite, all of the bronzite chondrites contain clear taenite which nearly always borders on kamacite and contains 50–57 wt. % Ni. There is a slight Ni gradient in the clear taenite such that the Ni content increases on the average by 2 % Ni from silicate to kamacite borders. Clear taenite is also found in three of six hypersthene chondrites, but it is significantly less abundant in them than in the bronzites. The composition suggests that clear taenite formed in the temperature range 390 - 340°C. Apparently it nucleated on pre-existing kamacite when the diffusion of Ni from kamacite to zoned taenite had become too sluggish to maintain equilibrium. This explanation may account for the fact that clear taenite is much more abundant in bronzites than in the hypersthenes where zoned taenite frequently borders on kamacite.

Neutron-activation potassium-argon dating of meteorites

Argon analyses are presented for stepwise heating of nine specimens of seven neutron-irradiated meteorites, and potassium-argon ages calculated by the Ar 40-Ar 39 method. Within the limitations of the data, it is found that the chondrites Karoonda, St. Severin and Allegan, and the aubrites Peña Bianca Spring and Bishopville, all have identical potassium-argon gas retention ages, assumed to be 4.6 × 10 9 yr. In particular, Bishopville, for which conflicting age results have been reported, has the same K-Ar age as the chondrites and Peña Bianca Spring. For these five meteorites, there is no evidence for significant loss of radiogenic Ar 40, such as has been observed in hypersthene chondrites. Gas released at low extraction temperatures indicates that a silicate inclusion from the hexahedrite El Taco experienced a heating event 3.4 × 10 9 yr ago. Gas released from El Taco silicate at higher extraction temperatures yields a K-Ar age identical to that of the chondrites, large amounts of trapped Ar 38, and indicates a ratio ( Ar 40 Ar 36 ) $ ̆ 2 in the trapped argon. The results for separated chondrules and matrix of the unequilibrated chondrite Chainpur differ qualitively from those observed for other meteorites in a manner for which no tenable explanation is offered.

On preferred disorder and the shock history of chemical group IVA meteorites

We report the results of a metallographic and X-ray diffraction study of the shock history of 16 meteorites of chemical group IVA. Of these, 12 contain evidence for preterrestrial shock-loading to pressures of 130 kb or more. These results, together with previous ones, indicate that about 60 per cent of the group IVA meteorites have been strongly shocked and thus that the collision which produced this group was particularly violent. The physical and chemical properties of these meteorites are consistent with their origin from a single Marscrossing asteroid that was disrupted by collision with another object, tentatively identified with the parent body of a few hypersthene chondrites. In this event the shock-induced velocity impulse may have been helpful but not essential in causing these meteorites to be eventually captured by earth.

Serra de magé: A meteorite with an unusual history

Serra de Magé has a fission-track age of 540±90 Myr, and may hence be genetically related to three other meteorite classes that suffered major collisions at about that time: hypersthene chondrites, Group III irons, and shergottites. Its cosmogenic track density is very low, ranging from 170 cm −2 to 3500 cm −2 in 4 stones. Coupled with previous measurements of 26Al, 3He, 21Ne and 38Ar, this suggests a two-stage irradiation history: first for ⩽540 Myr at a depth of several meters below the surface of the parent body, and then for ≈6×10 4 yr as a small object of <10 3 kg mass.

Shock, reheating, and the gas retention ages of chondrites

In order to investigate shock and reheating effects in ordinary chondrites, 71 bronzite, 26 hypersthene and 6 amphoterite chondrites were examined by X-ray diffraction and 39 bronzite and 19 hypersthene chondrites were studied metallographically. In addition, the inert gas contents of 11 bronzites and 4 hypersthene chondrites were measured mass spectrometrically. We have found that virtually all chondrites with short gas retention ages (<2 b.y.) are substantially shocked and reheated, whereas those with ages >3 b.y. are not reheated to any appreciable extent, although they may be strongly shocked. We have concluded that the reheating observed in chondrites is generally related to shock and that these shock-induced reheating events took place before chondrites were ejected from their parent objects.

Shock induced thermal metamorphism and mechanical deformations in the Ramsdorf chondrite

The grey hypersthene chondrite Ramsdorf—which has short and concordant 4He-U/Th- and 40Ar- 40K-gas retention ages—shows unambiguous evidence for severe thermal and mechanical alterations. 1. (1) The distribution of metal and troilite is very inhomogeneous. Both are found together in rather large, millimeter-sized patches near slickenside “veins”. 2. (2) The normal separation of the metal into individual grains of Ni-poor kamacite and Ni-rich taenite is lacking. Instead, it occurs as rounded globules ( d ≲ 1 mm) with an interior Ni-content of about 11 per cent and a Ni-rich rim. We believe these rims not to be due to solid state lattice diffusion but to crystallization of metal from the melt. 3. (3) The silicate portion is extensively “recrystallized”: only occasionally chondrule remnants can be discovered; the feldspar (12 per cent An) has been transformed into a clear isotropic glass, which must have been a liquid melt proper. 4. (4) The pyroxenes are clinopyroxenes and are high (up to 16·3 per cent) and variable in CaO, the highest values being found at the pyroxene-glass interfaces. In about 30 per cent of all cases the CaO-content falls into the immiscibility gap of the enstatite-ferrosilite-diopside-hedenbergite system. 5. (5) Chromite occurs as small euhedral crystals in the glassy silicate matrix and as larger badly “corroded” crystals at the silicate-troilite boundaries. 6. (6) Some pyroxene crystals have been broken and the fragments displaced over short distances. In some metal globules shearing forces have shifted portions of the globules over distances up to 300 microns. At these shearing planes the Ni-rich rim is missing indicating the mechanical deformation to be a late feature acquired after the thermal alterations. It is proposed that the observed features came about by the following sequence of events: Shock heating with a mean residual temperature of about 1200°C, homogenization and redistribution of a metal + troilite melt by acceleration forces, rapid cooling to about 900°C within less than one month, a secondary collision causing the mechanical deformations. The rapid cooling indicates Ramsdorf to have had a radius of less than 2 m—more probably less than about 30 cm—at that time. At present it cannot be decided unambiguously whether this sequence of events occured 4 Ma ago-the cosmic ray exposure age-or 400 Ma ago—the radiogenic gas retention age.

Rb87-Sr87age of bronzite (H Group) chondrites

Thirteen olivine bronzite (H group) chondrite falls and two bronzite finds have been analyzed for K, Rb, and Sr elemental concentrations and Sr isotopic composition. The mean of the most extreme isochrons drawn from the bronzite chondrite data defines an age of 4.69±0.14×109 years, assuming λRbsτ = 1.39 × 10−11 year−1, and a Sr87/Sr86 initial ratio of 0.6983±0.0024. The bronzite finds do not fall on the isochron. Blanks averaged 7×10−8 gram for K, 1.7×10−10 for Rb, and 1.6×10−9 for Sr and make a negligible contribution to the analyses. The K/Rb ratio varies from 87 for Beardsley to 448 for Beaver Creek. The bronzite chondrite age is possibly higher than the recently measured Rb87-Sr87 age for hypersthene (L group) chondrites but is in excellent agreement with the measured Rb87-Sr87 ages of amphoterite (LL group) chondrites, achondrites, and iron meteorites.

THE LADDER CREEK, HORACE, AND TRIBUNE METEORITES (GREELEY COUNTY, KANSAS)

The Ladder Creek, Horace No. 1, Horace No. 2, and Tribune meteorites were found within a comparatively small area of Greeley County, Kansas, between 1937 and 1958. Microscopic examination and analyses of the principal minerals show that Ladder Creek, Horace No. 2, and Tribune are essentially identical in composition and structure; it seems reasonable to conclude they are fragments of a single meteorite, a hypersthene chondrite. Horace No. 1 is essentially different, being a bronzite chondrite, and is evidently a distinct fall. The name Ladder Creek should be used for specimens previously called Horace No. 2 and Tribune, and the name Horace should be used for specimens previously called Horace No. 1.

Uranium measurements in hypersthene chondrites and their relation to the 600–700 million year “event”

Uranium measurements in hypersthene chondrites and their relation to the 600–700 million year “event”