Robert T. Dodd

Metamorphism of the ordinary chondrites: A review

Abstract Textural variations among the ordinary chondrites are paralleled by mineralogioal and chemical trends, most of which are consistent with the view that these chondrites have experienced various degrees of alteration in the solid state. On the basis of mineralogical and textural data, it is inferred that this alteration took place at temperatures of roughly 400 to greater than 820°C, under load pressures on the order of 1–2 kbar, in the absence of penetrative stress and under relatively dry, reducing conditions. The mineralogical data do not indicate whether alteration took place during reheating of cold material (progressive metamorphism) or during cooling of a hot agglomerate (autometamorphism). The former mechanism seems more likely, for the latter requires extremely rapid and deep burial, which would be difficult to achieve within existing models for the origin of chondritic material. Alternative models which attribute the textural, mineralogical and chemical variations among ordinary chondrites to crystal-liquid-vapor interactions prior to accumulation find little support in the chondrites and are regarded as less satisfactory than the metamorphic hypothesis.

The petrology of chondrules in the sharps meteorite

Correlated petrographic and microprobe studies of 96 chondrules in the Sharps (H-3) chondrite indicate that chondritic material had a highly varied pre-accumulation history. Some chondrules, chiefly excentroradial and barred types, appear to be quenched droplets. Others, including most of the metal poor microporphyritic type, appear to have crystallized more slowly and are thought to be fragments of pre-existing rock. Although chondrules of all types show various effects similar to those produced by shock, such effects are most conspicuous in metal-rich chondrules and least conspicuous in spherical chondrules. It is concluded that shock was involved in the origin of chondrules and not simply a secondary effect.It is proposed that chondrules were formed by shock processes during the accumulation of nebular dust into asteroid-sized bodies. Olivine-rich microporphyritic chondrules are thought to be due to complete melting of large masses of target material; metal-rich chondrules represent shock melting and partial vaporization; and spherical, pyroxene-rich chondrules are interpreted as condensates from shock-generated vapor.

Pyroxenes in the Shaw (L-7) chondrite

Abstract The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs 11.3 Wo 38.2 ) and calcic orthopyroxene (Fs 19.4 WO 4·5 ) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs 16·8 Wo 1·2 ) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite ( K E + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C. Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7.

Recrystallized chondrules in the Sharps (H-3) chondrite

Abstract Microprobe analyses of fifty chondrules in the Sharps (H-3) chondrite showed three to contain essentially homogeneous, calcium-poor olivines. All three show textural evidence of recrystallization, which is subtle in two and pronounced in one. It is concluded that these chondrules were recrystallized before they entered Sharps. Although Sharps is an H-group chondrite, the recrystallized chondrules have olivine and pyroxene compositions appropriate to recrystallized LL-group chondrites. They thus suggest that low-iron material, with a complex previous history, was present at the place of formation of the H-group chondrites.

An H chondrite stream - Identification and confirmation

Fall data indicate that a significant, elongate cluster of co-orbital H chondrite falls in May between 1855 and 1895 (H Cluster 1) records encounters with two or three closely spaced and probably related meteoroid stream components, each of which was met near its perihelion. Although meteorites included in the Cluster vary widely in petrographic type (3–6), shock facies (a-d), and 21Ne exposure age (<5 to 50 Ma), they have a distinct labile trace element signature that confirms a common thermal history and, thus, a common source region within an H chondrite parent body. Hence, meteorites selected by one criterion (fall parameters) as distinguishable from all other H chondrites, are distinguished from them by another completely different criterion (contents of labile trace elements).

Chemical studies of L chondrites. IV. Antarctic/non-Antarctic comparisons

Abstract We report shock facies and RNAA data for 13 siderophile, lithophile and chalcophile volatile/mobile trace elements (Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl, Zn) in interior portions of 19 different L3-6 chondrites from Victoria Land, Antartica. Comparison of these data for essentially unweathered Antarctic samples with those previously published for 39 L4-6 chondrite falls indicate substantial differences. The Victoria Land population lacks the most heavily shocked (> 35 GPa) representatives found in the non-Antarctic population. While contents of nearly all trace elements studied are lower at statistically significant levels in strongly shocked (> 22 GPa) falls than in mildly shocked (> 22 GPa) ones, this is not true for the Victoria Land population. Contents of most trace elements vary with petrologic type of heavily shocked (facies d, 22–35 GPa) L chondrites from Victoria Land: such a variation is not observed for non-Antarctic falls or mildly shocked samples of either population. L4-6 chondrites from Victoria Land contain significantly lesser amounts of nearly every trace element than do falls. These differences are reflected in correlations between elements, including the putative cosmothermometric trace elements Bi, In and Tl. While Antarctic weathering could account for the compositional differences between Antarctic and non-Antarctic L chondrites, such processes cannot be responsible for the shock differences observed. Physical and chemical differences between other sorts of Antarctic and non-Antarctic meteorites suggest, rather, that the L chondrite differences are preterrestrial in origin. If so, the parent region(s) for the Victoria Land samples—that fell to earth 0.1 to 1 Myr ago—generally formed and/or evolved under higher temperature conditions than those regions yielding contemporary falls. The dominant influence of late shock that altered mobile trace element contents of L chondrites falls is not evident in samples from Victoria Land.

Crater frequency and the interpretation of lunar history

Abstract The significance of lunar crater frequencies as a function of crater diameter has been investigated. We conclude that the absolute ages of isochronous surfaces previously determined by this technique are not valid. Relative ages can be obtained from crater frequencies when account is taken of complications caused by secondary and, perhaps, igneous craters, and obliteration of old craters by younger ones. Studies made at AFCRL indicate that the maria are of similar, but not identical ages, and that some maria may contain surfaces of different ages. They also indicate that the Apenninian debris sheet is older, but not much older, than the maria. The present surface of the crater Ptolemaeus is evidently composed of either Apenninian debris, or Procellarian material. We conclude that crater frequency is a useful stratigraphic tool.

Preferred orientation of olivine crystals in porphyritic chondrules

Abstract Ten porphyritic chondrules from the Hallingeberg and Krymka chondrites were examined on the universal stage to determine whether their olivine phenocrysts show preferred shape orientation. From 50 to 150 olivine crystals were examined in each chondrule. In nine of the ten chondrules studied, the largest crystals (larger than about 0.1 mm) show a distinct preferred orientation of the longest (β) axes indicative of a planar orientation and a linear alignment in the plane of foliation; the small crystals in each chondrule show a weak preferred orientation or none. The olivine fabrics in these chondrules resemble mineral fabrics found in the Moore County eucrite and in terrestrial basic intrusive rocks. They are readily explained by motion within a partly crystalline silicate melt, and they suggest that chondrules were drawn from one or more extended magma bodies and did not form and crystallize as separate entities in space.

Distribution and significance of cordierite in paragneisses of the Hudson Highlands, southeastern New York

AbstractCordierite occurs locally and sporadically in biotite-quartz-two feldspar paragneisses of the Precambrian Highlands complex in southeastern New York. Cordieritic and associated non-cordieritic gneisses were compared to determine the significance of cordierite for the metamorphic history of the complex.Microprobe analyses of the ferromagnesian phases show the following ranges in Fe/Mg (mol.): cordierite 0.19–0.43; biotite 0.33–0.73; garnet 1.98–3.56. Feldspar compositional ranges are: plagioclase An25–53; K-feldspar in microperthite Or62–87Ab12–37 An0–1. Garnet and plagioclase associated with cordierite are depleted in Ca relative to those in cordierite-free assemblages.Textural evidence, phase rule considerations and consistent distribution coefficients for FeO and MgO in coexisting garnet, cordierite and biotite from each locality examined suggest that all phases formed in at least local equilibrium during the hornblende-granulite subfacies metamorphism. The assemblages studied limit the conditions of metamorphism to between 700 and 750° C and 3.0 to 5.5. Kb. total pressure, with PT greater than