U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The ²⁰⁶Pb/²⁰⁴Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the ²⁰⁶Pb/²⁰⁴Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁴Pb/²⁰⁴Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed ⁸⁷Sr/⁸⁶Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.     

Oxygen isotopic distribution in an amoeboid olivine aggregate from the Allende CV chondrite: Primary and secondary processes

The oxygen isotopic distribution in an amoeboid olivine aggregate (AOA), TTA1-02, from the Allende CV3 chondrite has been determined by secondary ion mass spectrometry. The irregular shaped TTA1- 02 (5×3mm) consists mostly of olivine grains of ca. 5μm in diameter. Olivine grains of Mg-rich (Fo₉₅) and Fe-rich (Fo₆₀) composition are in direct contact with each other, with a sharp compositional boundary. Oxygen isotopic compositions of Fe-rich olivine grains are ¹⁶O-poor (Δ¹⁷O ≅ −5‰), whereas Mg-rich olivine is ¹⁶O-rich (Δ¹⁷O ≅ −25‰). Several Al-rich inclusions (<ca. 500 μm in diameter) are enclosed by olivine grains in the AOA. Oxygen isotopic compositions of spinel and fassaite in Al-rich inclusions are ¹⁶O-rich (Δ¹⁷O ≅ −20‰), whereas those of anorthite, nepheline and phyllosilicate are ¹⁶O-poor (Δ¹⁷O ≅ −5‰). We propose the following sequence of events during the formation of AOAs in the Allende meteorite: 1) Formation of Al-rich inclusions with ¹⁶O-rich oxygen isotopic composition; 2) Accretion of Mg-rich olivine grains with ¹⁶O-rich oxygen isotopic composition around Al-rich inclusions; 3) Accretion into parent body; and 4) Aqueous alteration in the parent body, which led to crystallization of ¹⁶O-poor minerals, Fe-rich olivine, anorthite, nepheline, and phyllosilicate. This is reflecting reactions among primary ¹⁶O-rich AOA minerals and aqueous fluid having ¹⁶O-poor oxygen isotopic composition. Fe-rich olivine grains precipitated from aqueous fluids, which partially dissolved pre-existing Mg-rich olivine grains. Sintering and Mg-Fe diffusion occurred during thermal metamorphism. Anorthite, nepheline and phyllosilicate in Al-rich inclusions replaced primary anorthite or melilite during the aqueous alteration stage.     

Matrices of type 3 ordinary chondrites—primitive nebular records

Petrologic studies were made on the fine-grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype. The matrix minerals, in order of abundance, are olivine (Fo₉₉ to Fo₉), enstatite or bronzite, augite or subcalcic augite, albite, Fe-Ni metal, troilite, magnetite, spinel (MgAl₂O₄), chromite, and calcite. Fe- and Mg-rich fluffy particles and albite-like particles are also major constituents. The chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium, showing that the matrix materials were hardly heated after their formation. Textural relationships indicate that magnesian olivine was formed after Ca-pyroxene, followed by intermediate to iron-rich olivine. Intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions. The observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules. They were mostly the products of condensation and reaction among solids and/or between solids and the ambient gas mostly at low temperatures, and thus they contain records of primitive processes in the nebula. In order to explain the presence of olivines more iron-rich than Fo₅₀, the presence of free SiO₂ or a high activity of SiO₂ in the gas is necessary, which was not shown in previous thermochemical calculations. Mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites, indicating that they were formed at different oxygen fugacities. The rims of chondrules, and surrounding matrix materials, must have accreted onto chondrules during turbulent movements of the nebula.     

Titanium isotopic anomalies in meteorites

High-precision analyses of Ti are reported for samples from a variety of meteorite classes. The expanded data base for Allende inclusions still shows Ti isotopic anomalies in every inclusion. All the coarse-grained inclusions give quite similar patterns, but fine-grained inclusions show more variable, and sometimes larger, anomalies. One inclusion, 3675A, was analyzed because others identified it as a possible “FUN” inclusion due to its mass-fractionated Mg. This designation is supported by the significantly more complex Ti isotopic pattern for 3675A compared to all our other Allende inclusions. Available data fail to suggest that any particular Allende mineral phase, including a chromite-carbon fraction from an acid residue, is especially rich in anomalous Ti. We also find anomalous Ti in a bulk sample of a Cl chondrite and in matrix separates from C2 chondrites. The excesses of ⁵⁰Ti are smaller than for Allende inclusions, and subtle differences in Ti isotopic patterns tentatively suggest that parent materials for C1-C2 matrix and Allende inclusions are not directly related. Analyses of chondrules from unequilibrated ordinary chondrites did not yield clear evidence for anomalous Ti, but some “larger than usual” deficits at $\text{50}\text{46}$ give encouragement for future work in this direction. Comparing the magnitude of isotopic shifts at ⁵⁰Ti and ¹⁶O for all these meteorite samples indicates that they are not correlated, but the data do not preclude a correlation between concentrations of “exotic” ⁵⁰Ti and ¹⁶O atoms.Whether or not Allende “FUN” inclusions are considered, at least 4 distinct isotopic components of Ti are required to account for the observed isotopic variations. The Ti data cannot be plausibly explained in terms of an early solar-system particle irradiation; instead, neutron-rich hydrostatic burning within a star is probably responsible for the dominant ⁵⁰Ti anomalies, while s-process mechanisms are viable sources for some of the more subtle Ti variations. We suggest that the Ti anomalies are linked to a diversity of nucleosynthetic sources and the highly refractory behavior of Ti. Therefore, some form of “chemical memory” from the ISM, rather than “late stage supernova injection”, is most likely responsible for the preservation of observed isotopic heterogeneities.     

Refractory inclusions in the Murchison meteorite

Mineralogical and petrographic studies of a wide variety of refractory objects from the Murchison C2 chondrite have revealed for the first time melilite-rich and feldspathoid-bearing inclusions in this meteorite, but none of these is identical to any inclusion yet found in Allende. Blue spinel-hibonite spherules have textures indicating that they were once molten, and thus their SiO₂-poor bulk composition requires that they were exposed to higher temperatures (>1550°C) than those deduced so far from any Allende inclusion. Melilite-rich inclusions are similar to Allende compact Type A's, but are more Al-, Ti-rich. One inclusion (MUCH-1) consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products, and probably originated by direct condensation of hibonite from the solar nebular vapor. The sinuous, nodular and layered structures of another group of inclusions, spinel-pyroxene aggregates, suggest that these also originated by direct condensation from the solar nebular gas. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physico-chemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases. These could not have formed by reaction of the inclusions with fluids in the Murchison parent body because the latter would presumably have been very rich in oxidized iron. Other rim phases and alteration products could have formed at relatively low temperatures in the parent body, but some inclusions were not in the locations in which they were discovered when this took place. Some of these inclusions are too fragile to have been transported from one region to another in the parent body, indicating that low temperature alteration of these may have occurred in the solar nebula.     

On the nature and origin of isolated olivine grains in carbonaceous chondrites

Many carbonaceous chondrites contain discrete olivine fragments that have been considered to be primitive material, i.e. direct condensates from the solar nebula or pre-solar system material. Olivine occurring in chondrules and as isolated grains in C3(0) chondrites has been characterized chemically and petrographically. Type I chondrules contain homogeneous forsterite grains that exhibit a negative correlation between FeO and CaO. Type II chondrules contain zoned fayalite olivines in which FeO is positively correlated with CaO and MnO. The isolated olivines in C3(0) chondrites form two compositional populations identical to olivines in the two types of porphyritic olivine chondrules in the same meteorites. Isolated olivines contain trapped melt inclusions similar in composition to glassy mesostasis between olivines in chondrules. Such glasses can be produced by fractional crystallization of olivine and minor spinel in the parent chondrule melts if plagioclase does not nucleate. The isolated olivine grains are apparently clastic fragments of chondrules. Some similarities between olivines in C3(0), C2, and Cl chondrites may suggest that olivine grains in all these meteorites crystallized from chondrule melts.     

Mineralogy and petrology of chondrules and inclusions in the Mokoia CV3 chondrite

All objects >100 μm in apparent diameter in five polished thin sections of the Mokoia CV3 chondrite were studied and classified. Number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined. Three major types of olivine chondrules were observed: igneous chondrules, recrystallized chondrules, and chondrules that appear to be accretional aggregates. Coarse-grained CAI's have igneous textures and mineral parageneses, while fine-grained CAI's are aggregates containing varying proportions of Al-rich concentric objects, Ca-rich chaotic material, and inclusion matrix. Chondrules and refractory inclusions in Mokoia and Allende are broadly similar in texture and mineral chemistry, but Mokoia refractory inclusions contain phyllosilicates rather than feldspathoids, and melilite-rich CAI's are more abundant in Allende.We think that most CAI's formed during the metamorphism, partial melting, and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula. In the process, Ca-rich melt appears to have been physically separated from Al-rich residues, producing the observed fractionation of Ca from Al into distinct constituents of CAI's. Some CAI's may be aggregates of devitrified, amorphous metastable condensates. Inclusion matrix may have condensed from silicate-rich vapors produced during distillation. Mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material.     

Trace elements in the Allende meteorite—IV. Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.     

Carbonaceous chondrites of the Ornans type: A metamorphic sequence

C3(O) chondrites comprise a metamorphic sequence. The following order reflects increasing grade: Kainsaz. Felix. Ornans. Lance. Isna, Warrenton. Assignment of Karoonda to the Ornans subtype is uncertain, but it is certainly of higher petrologic type than C3. Average olivine and pyroxene compositions in the metamorphic sequence change progressively from Fo₁₂. Fs₃ to Fo₃₄, Fs₁₁, respectively, and per cent mean deviation decreases. Kamacite and taenite change composition with increasing grade, reflecting higher equilibration temperatures. Blurring of textural features and Fe/Mg exchange between matrix and inclusions are also evident. As in the ordinary chondrites. contents of rare gases and possibly volatiles correlate with degree of metamorphism, but the effects are small. The meteorite Ornans presents an intriguing paradox. Observed chemical enrichment and depletion patterns reflect a higher metamorphic grade than do petrographic properties. The data suggest that abundance patterns of volatile components were not generated by metamorphism, but may represent primary differences. Strong correlations present in other C3(O) chondrites indicate some genetic link between metamorphism and composition, although the relationship is probably not causal. The autometamorphism model of Larimer and Anders (1967) appears to be the most straightforward explanation, but an observed negative correlation between the amount of matrix and content of volatiles suggests a re-examination of the two-component model. The decoupling mechanism required for Ornans is uncertain.     

Ca-Al-rich inclusions from the Ningqiang meteorite: continuous assemblages of nebular condensates and genetic link to Type B inclusions

The petrography and mineral chemistry of 110 Ca-, Al-rich inclusions (CAIs) and 9 Ca- and/or Al-rich amoeboid olivine aggregates (AOAs) from the Ningqiang carbonaceous chondrite are reported. These CAIs are referred to as hibonite-bearing and hibonite-free melilite-spinel-rich (Type A), and spinel-pyroxene inclusions. Melilite is more gehlenitic in the hibonite-bearing Type As than in the other two types, and all of them vary within a range of Åk_0-30. Modal compositions of the three types of CAIs overlap with each other, and make up a continuum with wide ranges of melilite: spinel: diopside. The diopside occurs as rims on the CAIs or their individual concentric objects. The 9 AOAs contain spinel ± diopside ± anorthite in the centers of the aggregates; the spinel grains rimmed by diopside in the centers are similar to the spinel-pyroxene inclusions. Bulk compositions of these CAIs vary along the condensation trajectory, with the hibonite-bearing Type As plotting at the beginning followed by hibonite-free Type As then by spinel-pyroxene inclusions as temperature decreases. Bulk compositions of the AOAs are close to the lowest temperature condensation trajectory. Except for a few with compact textures, most of the Type As and spinel-pyroxene inclusions are fluffy aggregates, probably pristine vapor-solid condensates of the nebula.The bulk compositions of the Type As appear to overlap with the range of most melilite-Ti-Al-clinopyroxene-rich (Type B) inclusions. Hence, crystallization of liquids produced by melting the Type As can form Type B inclusions, without significant evaporative loss of MgO or SiO₂. A few Type Bs have bulk compositions deviating from the range of their proposed precursors, and may have suffered significant evaporation, as suggested in previous studies.     

Chemical evolution of basalts from 23°N along the mid-Atlantic ridge: evidence from melt inclusions

The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe²⁺), 1.01–0.68 wt% TiO₂) and are comparable to other proposed parental magmas except in having higher Al₂O₃ and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An_78.3-An_83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo₈₉-Fo₉₁) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo₈₉ but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine, plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts and in the cores of zoned plagioclases, any model must account for all phases present.     

Amoeboid olivine aggregates in the Allende meteorite

Greyish-brown, irregularly-shaped aggregates composed predominantly of olivine make up ~2% of the Allende meteorite by volume. Many of the aggregates are constructed of subspherical lumps of micron-sized crystals of olivine, pyroxene, nepheline and sodalite surrounded by coarsergrained olivine. Rarely, anorthite, spinel and perovskite are also present. The olivine ranges in composition from Fo64 to Fo99. Pyroxenes range from aluminous diopside to hedenbergite to very Al-rich and Ti-Al-rich varieties. The nepheline contains 1.6–2.4% K₂O and 1.6–5.2% CaO but the sodalite is significantly poorer in these elements. The spinel contains 2.1–13.4% FeO. Textural information and oxygen isotopic data suggest that the aggregates are composed of primary, solid condensates from the solar nebula. The perovskite. spinel and Ti-Al-rich pyroxenes are the remains of high-temperature condensates but the olivine compositions and the presence of feldspathoids indicate that some of the grains continued to react with the solar nebular vapor in the temperature range 500–900°K.     

The Kenna ureilite: an ultramafic rock with evidence for igneous,metamorphic, and shock origin

The Kenna ureilite was found in February, 1972 near the town of Kenna, Roosevelt County, New Mexico U.S.A., weighed 10.9 kg, and measured 26.7 × 14.7 × 14.2 cm; it is the seventh known ureilite. The meteorite is composed of xenoblastic olivine (Fo_79.2), commonly rimmed by forsterite (Fo₉₉), and pigeonite (En₇₃Wo₉Fs₁₈), in a volumetric ratio of 3:1, set in a matrix of three carbon polymorphs (graphite, lonsdaleite, and diamond) plus nickel-iron metal and troilite. Some thin metalliferous veins penetrating silicate grains contain secondary inclusions of melt with high-calcium clinopyroxene (high-Ca, Mg-rich augite to augite), andesine, K-feldspar, chromite, and siliceous CaO- and alkali-rich glasses of variable compositions.Textural, mineralogical and fabric information suggest a complex history for Kenna, involving igneous, metamorphic and shock processes. The rock appears to have originated as an ultramafic cumulate whose texture and structure was modified by adcumulus processes and by solution and redeposition in a weak deviatoric stress field. A strong mineral elongation lineation was produced during this high-temperature phase accompanied by mild plastic deformation of olivine on the system 0kl[100]. Superimposed on this original texture and fabric are processes resulting from light to moderate (50–250 kbar) shock deformation, as manifested by fracturing of the silicates, slip parallel to (001) in olivine, and twin and translation gliding parallel to (100) in the clinopyroxene. Lonsdaleite and diamond probably formed during this shock phase, which may be associated with the break-up of the parent body, but the relative time of introduction of the carbon-rich matrix is still unresolved.     

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca-Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and ¹⁶O-rich olivine (Fa_1-40 and Δ¹⁷O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and ¹⁶O-depleted (Fa_15-50 and Δ¹⁷O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and ¹⁶O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and ¹⁶O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ¹⁷O values. However, not all fine-grained olivines are FeO-rich and ¹⁶O-poor; some small (<1 μm in Leoville and 5-10 μm in Vigarano and Allende) ferrous (Fa_>20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in ¹⁶O (Δ¹⁷O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an ¹⁶O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as ¹⁶O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an ¹⁶O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of ¹⁶O-rich forsterites with grain overgrowths or newly formed grains of ¹⁶O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of ¹⁶O-rich forsterite and subsequently exchanged Fe-Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).     

Silicate inclusions in group IAB irons and a relation to the anomalous stones Winona and Mt Morris (Wis)

Silicates are found in many group IAB irons; in some cases as abundant angular cm-sized inclusions and in other cases as smaller fragments or single grains in troilite or graphite nodules. The mineralogy of the silicates is chondritic—olivine, pyroxene, albitic plagioclase—as is the bulk composition. The degree of oxidation of the olivine and pyroxene is intermediate between E and H chondrites (Fa 1–8, Fs 4–9). IAB inclusions have ages of about 4.5 Gyr, I¹²⁹-Xe¹²⁹ formation intervals in the ranges of chondrites and contain planetary-type rare gases.Samples of San Cristobal, Campo del Cielo, Mundrabilla and Woodbine were examined by microprobe and bulk inclusions from Campo del Cielo, Copiapo, Landes and Woodbine were analyzed by instrumental and radiochemical neutron activation analysis. Nonvolatile lithophilic and siderophih'c elements in Copiapo, Landes and Woodbine have approximately chondritic abundances. The chondritic level of lithophiles indicates the inclusions have not undergone igneous differentiation while the chondritic levels of siderophiles is evidence the metal is native to the inclusions and not matrix metal injected into the silicates. The two Campo del Cielo inclusions analyzed have roughly chondritic abundances of lithophiles but have fractionated rare earth patterns and widely varying amounts and abundances (relative to Ni) of siderophiles. These inclusions appear to have experienced some partial melting. Siderophile ratios for the inclusions have some differences when compared to matrix metal. One Campo del Cielo inclusion contains kamacite (5.5% Ni) with over 1000 μg Ge.Three-isotope O analyses by Clayton and coworkers of parts of the same or neighboring inclusions to those analyzed chemically place the inclusions slightly below the terrestrial fractionation line of clayton et al. (1976) and rule out the possibility of the inclusions being trapped fragments of one of the ordinary chondrite groups.The IAB silicates formed probably in a similar manner as chondrite groups but in a different region of the nebula and they record the O₂ fugacity and O isotopic composition of that location. They later became trapped in the metal-rich matrix probably as the result of collisions producing the breccialike texture. The relationship of the silicates to the kamacite-taenite structure of the metal requires that the metal-silicate mix have been heated to over 1000 K for an extended period.Two anomalous stony meteorites, Winona and Mt. Morris (Wis), are similar to IAB inclusions in mineralogy, bulk composition, $\text{FeO}\text{(FeO + Mg)}$ ratio of the silicates, and chromite composition and are possibly related to the IAB silicates. Winona also has an age of 4.6 Gyr and contains planetary-type rare gases. Microprobe data are reported for the major minerals of these anomalous meteorites. Although attempts to detect IAB levels of Ge in the metal phases were not successful, the weight of the evidence favors a relationship between these meteorites and IAB     

Petrology and magnetic properties of the Chiang Khan,Thailand, meteorite

Summary The Chiang Khan meteorite fell on 18th November, 1981 at Chiang Khan, Thailand. It consists of olivine, orthopyroxene, clinopyroxene, Fe-Ni metal, troilite, chromite, plagioclase, glass, and phosphate in order of abundance. Olivine forms barred or porphyritic chondrules, and its composition is uniform (average Fo_80.2), close to the average composition of olivine in equilibrated H chondrites. Orthopyroxene and clinopyroxene also have compositions similar to those in equilibrated H chondrites. Both well-defined chondrules and their broken fragments are present in the recrystallized matrix. Microcrystalline plagioclase and clinopyroxene often occur in the groundmass of chondrules, but clear interstitial plagioclase is absent. Chemical composition of chromite plots in the field of chromites in H chondrites. Chiang Khan meteorite is thus classified as an equilibrated H 5 type chondrite. The equilibrium temperatures estimated by using mineral pairs are as follows: Opx-Cpx 800–900°C; Ol-Chromite 510°C.Water content is 0.24 wt %, and the hydrogen isotopic composition (D) is –89.5In the thermal demagnetization experiment magnetization steadily decreased from 0 to 500°C, whereas the remanent magnetization obtained in the A.C. demagnetization experiment is very unstable, probably owing to the large grain size of the Fe-Ni metal.With 9 Figures     

Contact metamorphosed dunite-harzburgite complex in the Chugoku district,western Japan

The Tari-Misaka ultramafic complex, which is emplaced into the Paleozoic sediments and thermally metamorphosed by two younger granitic masses, is divided into four zones on the basis of the mineral assemblage. They are, in order of increasing metamorphic grade: Zone I antigorite-olivine-orthopyroxene-clinopyroxene. Zone II olivine-talc. Zone III olivine-anthophyllite. Zone IV olivine-orthopyroxene. Strongly serpentinized clinopyroxene-bearing harzburgite in Zone I is similar to ordinary Alpine-type harzburgite. In Zonne II, two kinds of olivine are recognized. One is Mg-rich olivine (Fo₉₃ to Fo₉₇) with opaque inclusions and is probably a recrystallization product of serpentine with talc. The other is Fe-rich olivine (Fo₈₈ to Fo₉₃) free of opaque inclusions and is probably a relic of the primary peridotite. Olivine in Zone III and Zone IV is also relatively Mg-rich (Fo₉₁ to Fo₉₅). Chromitite in Zone IV commonly has an assemblage, olivine+cordierite+Mg-Al spinel (Mg/Mg+Fe²⁺, more than 0.9). Enstatite is rare and coexists with less magnesian Mg-Al spinel (Mg/Mg+Fe²⁺, less than 0.9). Petrological and mineralogical characters of the ultramafic rocks can be well explained by thermal metamorphism of strongly serpentinized peridotite by granitic intrusion. Metamorphic zones are consistent with the experimental results in the system MgO-SiO₂-H₂O. The assemblage olivine+cordierite indicates that the metamorphism occurred at relatively low pressures (<3kb).     

Evidence for heterogeneous enriched shergottite mantle sources in Mars from olivine-hosted melt inclusions in Larkman Nunatak 06319

Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo_77-73); Type-II inclusions occur in phenocryst mantles (Fo_71-66); Type-III inclusions occur in phenocryst rims (Fo_61-51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150-250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25-75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo₇₇ composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo_77-73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo₇₇. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine.The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites.     

The Chassigny meteorite: a cumulate dunite with hydrous amphibole-bearing melt inclusions

The Chassigny meteorite is a moderately shocked olivine achondrite or chassignite with features indicative of a cumulate origin with some subsolidus annealing. Chassigny is an iron-rich dunite (Fo₆₈) with minor amounts of Ca-rich and Ca-poor pyroxene, alkalic feldspar, chromite, and melt inclusions in olivine. Accessory phases include chlorapatite, troilite, marcasite, kaersutite amphibole, pentlandite, ilmenite, rutile and baddeleyite. The meteorite experienced shock pressures of ~150–200 kbar as evidenced by planar and irregular fractures in olivine, local recrystallization in pyroxene and reduced birefringence and rare deformation lamallae in feldspar. Kaersutitic amphibole (K_0.05 Na_0.45)_0.50 (Ca_1.71 Na_0.29)_2.00 (Mg_2.73 ‘Fe’_1.19 Ti_0.73 A1_0.23 Cr_0.08 Mn_0.03)_4.99 (Si_6.05Al_1.95)_8.00 O₂₂ (OH, F)₂ containing hydrogen and lesser amounts of fluorine represents the first extraterrestrial occurrence of hydrous amphibole and the first meteoritic amphibole type other than fluorichterite. Kaersutite is found only in melt inclusions.Melt inclusion bulk compositional data suggest crystallization from a low-Ca melt that may have been similar in major element abundances to the silicate portion of LL group chondrites. However, Chassigny has a fractionated pattern for REE and the lack of metallic iron, possible presence of minor Ni in the olivine and Fe³⁺ in the chromites indicates that Chassigny formed under relatively more oxidizing conditions than most other achondrites. Therefore its parental melt could not have been directly derived from a chondritic composition in a simple single-stage process. The iron-rich bulk composition, cumulate texture and abundance as well as alkalic nature of the interstitial feldspar indicate that Chassigny could not have generated eucritic magmas. This places further constraints on its relationship to other meteorites and the parent body from which it is derived. The Brachina meteorite is similar to Chassigny except that it is finer grained, more feldspathic and is unshocked. It extends the fractionation range of this group which now represents two unusual meteorites.