Classification of Six Ordinary Chondrites from Texas

Abstract— Based on optical microscopy, modal and electron microprobe analyses, six ordinary chondrites from Texas preserved in the Monnig Meteorite Collection at Texas Christian University, Fort Worth, Texas, were classified in compositional groups, petrologic types, and shock facies. These meteorites are Comanche (stone), L5c; Haskell, L5c; Deport (a), H4b; Naruna (a), H4b; Naruna (b), H4b; and Clarendon (b), H5d.     

CLASSIFICATION OF A SECOND GROUP OF ORDINARY CHONDRITES FROM TEXAS

Based on optical microscopy and electron microprobe analyses of mafic minerals, six previously undescribed or poorly known ordinary chondrites from Texas were classified into compositional groups, petrologic types, and shock facies. These meteorites are Junction, L5d; Anton, H4b; Venus, H4d; Dalhart, H5a; Rosebud, H5c; and Cranfills Gap, H6c.     

Further classification of ordinary chondrites from the Monnig Collection

Abstract— Four ordinary chondrites from the Oscar Monnig Meteorite Collection were classified into compositional groups, petrologic types, and shock stages: Wray (b), Colorado, L5S2; Round Top (a), Texas, L5S3; Round Top (b), Texas, H4S3; Hassayampa, Arizona, H4S3.     

Determination of the equilibrium fO2 in bulk samples of H,L, and LL ordinary chondrites by solid‐state electrochemistry

High‐temperature solid‐state electrochemistry techniques (EMF method) were used to measure the oxygen fugacity (fO₂) of the ordinary chondrites Ochansk (H4), Savtschenskoje (LL4), Elenovka (L5), Vengerovo (H5), and Kharkov (L6). The fO₂ results are presented in the form of the following equations: It was found that fO₂ regularly increases from H chondrites to LL chondrites. Measured fO₂ are ~1.5 higher than those previously calculated from mineral assemblages. Kharkov (L6) is a little more oxidized than Elenovka (L5) in agreement with the progressive oxidation model. At the same time, Ochansk (H4) is more oxidized than Vengerovo (H5) and exhibits a slightly different slope compared to other chondrites and at T > 1200 K, becomes more reduced than Kharkov (L6) or Elenovka (L5). Measured oxygen fugacity values of meteorites fall within (0.1–1.0)·log fO₂ of one another. The possible explanation of discrepancies between measured and calculated values is discussed.     

The Roosevelt County 079–090 meteorites

Abstract— We have classified 12 new, moderately to severely weathered meteorites from Roosevelt County, New Mexico (RC 079–090) that were recovered between 1969 and 1993. They include nine H chondrites and three L chondrites of petrologic types 4 to 6 and shock classification S1 to S4. Among these are a flight-oriented specimen of an H5 chondrite, an L4 chondrite with a porphyritic impact-melt rock clast, an H5 fragmental breccia with an unusual weathering assemblage (probably a Ca sulfate), and an H4 chondrite with unequilibrated pyroxenes.     

Parent bodies of L and H chondrites: Times of catastrophic events

Abstract— An analysis of the distribution of ³He and ⁴He in L and H chondrites has shown that the parent body of L chondrites underwent a catastrophic collision in space 340 ± 50 Ma ago. This age differs considerably from the collision age of 520 ± 60 Ma given previously (Heymann, 1967). The parent body of H chondrites may also have undergone local heating and degassing ～200 Ma ago. Data for L chondrites argue in favour of Antarctic and non-Antarctic meteorites having originated from a common parent body.     

The Meteoritical Bulletin,No. 80, 1996 July

Abstract— The Meteoritical Bulletin No. 80 lists data for 178 meteorites. Noteworthy are 3 HED meteorites (ALH 88102, Hammadah al Hamra (HaH) 059, and Monticello); 3 ureilites (HaH 064, HaH 126, and Dar al Gani (DaG) 084); 4 irons (Baygorria (IAB), Ste. Croix (IIIAB), Sargiin Gobi (IAB), and Tarahumara (IIE)); an unusual metal-rich meteorite (Vermillion); 8 carbonaceous chondrites (HaH 043 (C03), HaH 073 (C4), DaG 055 (C3) and 5 C03 chondrites (probably paired) from DaG); an R chondrite (DaG 013); and 6 unequilibrated ordinary chondrites (ALH 88105 (L3), Camel Donga 016 (L3), HaH 093 (LL3.9), HaH 096 (LL(L)3), Richfield (LL3.7), and Sarir Quattusah (LL(L)3)). Three recent falls of ordinary chondrites (Coleman (LL5), St. Robert (H5), and Tsukuba (H5-6)) are described.     

FRAGMENTAL BRECCIAS AND THE COLLISIONAL EVOLUTION OF ORDINARY CHONDRITE PARENT BODIES

Our survey of type 4–6 ordinary chondrites indicates that gas-poor, melt-rock and/or exotic clast-bearing fragmental breccias constitute 5%, 22% and 23%, respectively, of H, L and LL chondrites. These abundances contrast with the percentages of solar-gas-rich regolith breccias among ordinary chondrites: H (14%), L (3%) and LL (8%) (Crabb and Schultz, 1981). Petrologic study of several melt-rock-clast-bearing fragmental breccias indicates that some acquired their clasts prior to breccia metamorphism and others acquired them after metamorphism of host material. In general, the melt-rock clasts in gas-poor H chondrite fragmental breccias were acquired after breccia metamorphism and were probably formed by impacts into boulders or exposed outcrops of H4-6 material in the H chondrite parent body regolith. In contrast, most of the melt-rock clasts in gas-poor L and LL fragmental breccias were acquired prior to breccia metamorphism. The low abundance of regolith breccias among L chondrites and evidence that at least two-thirds of the L chondrites suffered a major shock event 0.5 Gyr ago, suggest that the L parent body may have been disrupted by a major collision at that time and that the remaining parent body fragments were too small to develop substantial regoliths (e.g., Heymann, 1967; Crabb and Schultz, 1981). Such a disruption would have exposed a large amount of L chondrite bedrock, some of which would consist of fragmental breccias that acquired melt-rock clasts very early in solar system history, prior to metamorphism. The exposed bedrock would serve as a potential target for sporadic meteoroid impacts to produce a few fragmental breccias with unmetamorphosed melt-rock clasts. The high proportion of genomict brecciated LL chondrites reflects a complex collisional history, probably including several episodes of parent body disruption and gravitational reassembly. Differences in the abundances of different kinds of breccias among the ordinary chondrite groups are probably due to the stochastic nature of major asteroidal collisions.     

Mass distribution of Antarctic ordinary chondrites and the estimation of the fall-to-specimen ratios

Abstract— The cumulative mass distributions (mass range > 100 g) of each type of Japanese and U.S. Antarctic ordinary chondrites are compared with those of non-Antarctic falls and those obtained from the present-day flux of meteorites. The steeper slope of the mass distribution of Antarctic chondrites is indicative of the presence of several chondrite showers. The fall-to-specimen ratio of Antarctic ordinary chondrites larger than 100 g is about 1:2, indicating that half of them are shower components. The fall-to-specimen ratios of each group range from 1:1 to 1:6; those of the Japanese and U.S. Antarctic meteorite collections are 1:1 to 1:2 and 1:4 for H chondrites, 1:1 to 1:2 and 1:2 for L chondrites, and 1:2 and 1:6 for LL chondrites, indicating that the Japanese collection includes less abundant shower components than the U.S. collection. The fall-to-specimen ratios of each H4-6 and L4-6 type range from 1:1 to 1:4, and U.S. H6 and Japanese H4 have the low ratios of 1:4.     

Infrared transmission spectra from 2.5 to 25 μm of various meteorite classes

Infrared transmission spectra from 53 meteorites in the spectral range from 2.5 to 25 μm were measured to permit comparisons with data of astronomical objects that are potential meteorite sources. Data were taken for 14 carbonaceous chondrites, 5 LL ordinary chondrites, 6 L ordinary chondrites, 10 H ordinary chondrites, 1 enstatite chondrite, 4 aubrites, 3 eucrites, 4 howardites, 1 diogenite, 1 mesosiderite, 2 nakhlites, 1 shergottite, and the anomalous achondrite Angra dos Reis. The CO and CV carbonaceous chondrites have spectra similar to each other, with 10-μm features characteristic of olivine. The CM carbonaceous chondrites have distinctive 10-μm features that are attributed to layer lattice silicates. Members of both the CI and CR classes have spectra distinct from those of other carbonaceous chondrites. The LL, L, and H ordinary chondrites have spectra that match those of olivine and pyroxene mixtures. The enstatite chondrites and enstatite achondrites (aubrites) all exhibit spectra diagnostic of the pyroxene enstatite. The angrite, howardites, aucrites, nakhlites, shergottite, and diogenite all have similar spectra also dominated by pyroxene. The single mesosiderite examined had a spectrum distinct from all the other meteorites.     

Classification of four new Roosevelt County meteorites

Abstract— We have classified four new ordinary chondrites from Roosevelt County, New Mexico (RC 091–094) that were found in June and July of 1994 by I. Wilson. They include one H4(S2) and three L5(S4) chondrites. Degree of weathering is variable and ranges from W2–6.     

Recovery of three ordinary chondrites,Rooikop 001–003, from the Namib Desert in Western Namibia

Abstract— Reconnaissance searches for meteorites were made in five selected areas of western Namibia in 1991. Three new ordinary chondrites (H5, L5, L4/5) were recovered from one of the areas searched.     

Light noble gases in 12 meteorites from the Omani desert,Australia, Mauritania,Canada, and Sweden

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for ³He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess ⁴He, either of planetary or solar origin. However, very high ⁴He/²⁰Ne ratios occur at relatively low ²⁰Ne/²²Ne ratios, which is unexpected and needs further study. The measured ⁴⁰Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped ³⁶Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).     

Phosphate and feldspar mineralogy of equilibrated L chondrites: The record of metasomatism during metamorphism in ordinary chondrite parent bodies

In ordinary chondrites (OCs), phosphates and feldspar are secondary minerals known to be the products of parent‐body metamorphism. Both minerals provide evidence that metasomatic fluids played a role during metamorphism. We studied the petrology and chemistry of phosphates and feldspar in petrologic type 4–6 L chondrites, to examine the role of metasomatic fluids, and to compare metamorphic conditions across all three OC groups. Apatite in L chondrites is Cl‐rich, similar to H chondrites, whereas apatite in LL chondrites has lower Cl/F ratios. Merrillite has similar compositions among the three chondrite groups. Feldspar in L chondrites shows a similar equilibration trend to LL chondrites, from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6. This contrasts with H chondrites which have homogeneous albitic plagioclase in petrologic types 4–6. Alkali‐ and halogen‐rich and likely hydrous metasomatic fluids acted during prograde metamorphism on OC parent bodies, resulting in albitization reactions and development of phosphate minerals. Fluid compositions transitioned to a more anhydrous, Cl‐rich composition after the asteroid began to cool. Differences in secondary minerals between H and L, LL chondrites can be explained by differences in fluid abundance, duration, or timing of fluid release. Phosphate minerals in the regolith breccia, Kendleton, show lithology‐dependent apatite compositions. Bulk Cl/F ratios for OCs inferred from apatite compositions are higher than measured bulk chondrite values, suggesting that bulk F abundances are overestimated and that bulk Cl/F ratios in OCs are similar to CI.     

Classification of five new ordinary chondrites (RC 073, 074, 076–078) from Roosevelt County,New Mexico

Abstract— Five new ordinary chondrites from Roosevelt County, New Mexico, USA, have been assigned to a chemical group, petrologic type and shock stage. All five are moderately to heavily weathered. They are: RC 073, H5(S2); RC 074, L5(S3); RC 076, L4(S3); RC 077 , L4(S2); RC 078, L4(S2).     

PETROLOGY OF NINE ORDINARY CHONDRITE FALLS FROM CHINA

Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fel(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shock facies that is lower than that present immediately after the shock event.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

The Meteoritical Bulletin,No. 83, 1999 July

Abstract— Meteoritical Bulletin No. 83 lists information for 898 newly described meteorites. These include 473 from Antarctica, 341 from the Sahara, and 22 from dry lakes in the southwestern United States. Seven of the meteorites are falls: Kunya-Urgench (H5), Lohawat (howardite), Ourique (H4), Portales Valley (H6), San Pedro de Quiles (L6), Talampaya (eucrite), and Zag (H3-6). Also included are a dozen new iron meteorites; several mesosiderites; a pallasite; several HED meteorites; several ureilites; a variety of CM, CO, CV, CR, and R chondrites; and numerous unequilibrated ordinary chondrites. All shock classifications are after Stöffler et al. (1991) and weathering grades are after Wlotzka (1993), except as noted. All italicized abbreviations refer to addresses tabulated at the end of this document.     

Classification of four ordinary chondrites from Spain

Abstract— Based on optical microscopy and electron microprobe analysis of mafic minerals, four previously poorly described ordinary chondrites from Spain are classified. The classifications of Guareña (H6), Olmedilla de Alarcón (H5) and Reliegos (L5) are confirmed. Molina is reclassified as H5, based on new data.     

40Ar-39Ar Ages of Types 3 and 4, L and H Chondrites from Antarctica

Abstract— This is a report on ⁴⁰Ar-³⁹Ar studies of 7 low petrographic type L and H chondrites from Antarctica. From petrographic similarities it has been argued that the L3 chondrites ALHA77015, ?77167, ?77249, and ?77260 are pieces from a common fall (McKinley et al., 1981). Our results now confirm this supposition: The four meteorites have identical characteristic Ar-degassing patterns, very similar K, Ca, Cl, and ³⁶Ar_trapped contents, and similar ⁴⁰Ar-³⁹Ar ages of <4 Ga which are rather unusual for ordinary chondrites and might be due to shock. The undulating age patterns could be due to weathering or to ³⁹Ar recoil. The L4 chondrite ALHA77230 shows no age plateau and only a lower limit for the time of a severe degassing, 4.0 Ga, can be given. ALHA77226 and RKPA78002, two H4 chondrites, exhibit reasonably well defined age plateaus at about 4.3 and 4.4 Ga. Two individual chondrules from RKPA78002 have the same age as the whole rock sample.