Rhian H. Jones

PETROLOGY AND MINERALOGY OF TYPE-II, FEO-RICH CHONDRULES IN SEMARKONA (LL3.0) - ORIGIN BY CLOSED-SYSTEM FRACTIONAL CRYSTALLIZATION, WITH EVIDENCE FOR SUPERCOOLING

Abstract The petrology of type II porphyritic olivine chondrules in Semarkona (LL3.0) has been studied in detail. Olivines in these chondrules are euhedral, Fe-richand are strongly zoned from cores to rims of grains in FeO (Fa10–30), Cr2O3 (0.2–0.6 wt%), MnO (0.2–0.7 wt%) and CaO (0.1–0.4 wt%). Interstitial mesostasis is rich in Si, Aland Ca and is glassy with abundant microcrystallites. Minor minerals include troilite, Fe,Ni metaland chromite. Some olivine grains contain euhedral, fayalite-rich cores that are probably produced during initial supercooling of the chondrule melt. Rare relict grains of forsteritic olivine have compositions very similar to olivines in type IA chondrules in Semarkona and may result from disaggregation of such chondrules. Apart from these relics, all properties of type II chondrules can be described by closed-system fractional crystallization of droplets which were essentially entirely molten. The chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks. Cooling rates of the order of 1000°C/h are indicated, from peak temperatures of around 1600°C. The chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization. There is no evidence for chondrule-nebula exchange either during or after crystallization, or any secondary processes such as aqueous alteration or thermal metamorphism. Bulk composition data suggest that precursor material included a refractory (Ca, Al, Ti-rich) component, similar to one of the precursor components of type IA chondrulesand a non-refractory (Fe and Si-rich) component. Type IA chondrules may have formed from type II chondrules by loss of Fe and volatiles. Alternatively, the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different Fe Mg ratios.

Petrology of FeO-poor, porphyritic pyroxene chondrules in the Semarkona chondrite

Abstract The mineralogy and petrology of FeO-poor, porphyritic, pyroxene- and olivine-rich chondrules in the Semarkona (LL3.0) chondrite are described in detail. In an extension of the textural classification scheme, these chondrules are designated types IAB and IB. In type IAB chondrules, the proportion of olivine phenocrysts is between 20–80% and in type IB chondrules, olivine constitutes

DISENTANGLING NEBULAR AND ASTEROIDAL FEATURES OF CO3 CARBONACEOUS CHONDRITE METEORITES

Abstract Analyses of olivines and low-Ca pyroxenes in a suite of porphyritic chondrules of types IA and II in each of ten CO3 chondrites and comparisons with similar chondrules in LL chondrites suggest that the CO3 chondrites may be divided into a metamorphic sequence of subtypes 3.0 to 3.7. Chondrules in Allan Hills A77307 and Colony, which we classify as type 3.0, show only igneous zoning, whereas in the other more metamorphosed type 3.1–3.7 CO chondrites, chondrule silicates are enriched in FeO, most markedly in olivine crystals near the rims of type IA chondrules and along cracks. The FeO enrichments in metamorphosed chondrules result largely from equilibration by solid-state diffusion between chondrules and FeO-rich matrices. Type 3.1–3.7 CO chondrites were formed from chondrites like Colony and ALH A77307 in heated asteroids or planetesimals. We find no evidence for nebular condensation of fayalite rims around these chondrules, as reported for CV3 chondrites, or nebular modification of chondrule silicates, as favored by some workers. Metallic Fe,Ni in ALH A77307 contains up to 0.1–1 wt% Cr, Si, and P, as in the least metamorphosed ordinary chondrites and CM2 chondrites, and grains of cohenite, Fe3C, and haxonite, Fe23C6. These carbides, which are the first reported in carbonaceous chondrites, are associated with pentlandite and magnetite, as in certain LL3 chondrites. We assign the following petrologic subtypes to other CO chondrites: Kainsaz, 3.1; Felix, 3.2; ALH 82101 and Ornans, 3.3; Lance and ALH A77029, 3.4; ALH A77003, 3.5; Warrenton, 3.6; and Isna, 3.7.

FeO-rich, porphyritic pyroxene chondrules in unequilibrated ordinary chondrites

Abstract A suite of FeO-rich (type II), porphyritic, olivine/pyroxene (POP) chondrules has been studied in detail. Data for ten chondrules from Semarkona (LL3.0) are emphasized, and one chondrule from Chainpur (LL3.4) and two from Parnallee (LL3.6) are included as further examples of certain properties. The chondrules contain phenocrysts of pyroxene and olivine in varying proportions, and have Fe/(Fe+Mg) > 10 mol% in the silicate minerals. Four pyroxene minerals are present: clinoenstatite, orthorhombic enstatite, pigeonite, and augite. Pyroxene phenocrysts may consist of all four minerals, with progressive overgrowths in order of increasing Wo content. Enstatite may occur independently of clinoenstatite, but Ca-rich pyroxenes (pigeonite and augite) always nucleate on low-Ca pyroxene phenocrysts. Pyroxene and olivine phenocrysts may be independent of each other, or may be intergrown, and commonly show hopper morphologies indicative of rapid growth. Pyroxenes and olivines are commonly strongly zoned as a result of fractional crystallization of the chondrules. In addition, low-Ca pyroxene may contain ghost regions of slightly more FeO- and minor element-rich material in the cores of grains. These regions are partially resorbed and appear to be derived from previous crystallization of essentially the same chondrule. Oscillatory zoning is observed in enstatite in two chondrules, and is believed to be the result of disquilibrium kinetic effects. Relict grains, including a forsterite relict and a partially reduced FeO-rich relict in the same chondrule, and clinoenstatite cores of pyroxene phenocrysts in another chondrule, are evidence for recycling and mixing of chondrule material. All of the chondrules are highly disequilibrium assemblages. They appear to have cooled from essentially molten droplets, with little volatile loss or recondensation occurring during the heating event. Peak temperatures were probably close to liquidus temperatures, 1500–1700°C, and cooling rates were probably of the order of hundreds of degrees per hour. This description will serve as a basis for future interpretation of the effects of metamorphism on porphyritic, pyroxene-rich chondrules.

Composition of chondrule silicates in LL3-5 chondrites and implications for their nebular history and parent body metamorphism

Our petrologic studies of 75 type IA and type II porphyritic olivine chondrules in nine selected LL group chondrites of type 3.3 to type 5 and comparisons with published studies of chondrules in Semarkona (LL3.0) show that compositions of silicates and bulk chondrules, but not overall chondrule textures, vary systematically with the petrologic type of the chondrite. These compositional trends are due to diffusive exchange between chondrule silicates and other phases (e.g., matrix), such as those now preserved in Semarkona, during which olivines in both chondrule types gained Fe2+ and Mn2+ and lost Mg2+, Cr3+, and Ca2+. In a given LL4-5 chondrite, the olivines from the two chondrule types are identical in composition. Enrichments of Fe2+ in olivine are particularly noticeable in type IA chondrules from type 3.3–3.6 chondrites, especially near grain edges, chondrule rims, grain boundaries, and what appear to be annealed cracks. Compositional changes in low-Ca pyroxene lag behind those in coexisting olivine, consistent with its lower diffusion rates. With increasing petrologic type, low-Ca pyroxenes in type IA chondrules become enriched in Fe2+ and Mn2+ and depleted in Mg2+, Cr3+, and A13+. These compositional changes are entirely consistent with mineral equilibration in chondritic material during metamorphism. From these compositional data alone we cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula, but we see no evidence favoring nebula over asteroidal metamorphism, nor evidence that the chondrule reacted with nebular gases after crystallization. Modelling of the equilibration of chondrule olivines suggests that heterogeneous FeO concentrations in olivine could be preserved after cooling from 600°C at rates of 1–10°C/Ma for at least tens to hundreds of millions of years. This is consistent with published estimates for the maximum metamorphic temperatures in type 3 chondrites, thermal histories derived from metallographic and fission-track cooling rates, and 4.4 Ga 40Ar-39Ar ages for ordinary chondrites. Since the lifetime of the solar nebula was not more than 106 years and there is abundant evidence that meteorite parent bodies were heated, some even above their melting point, we confidently conclude that the formation of type 3.3–5 ordinary chondrites from type 3.0 material by metamorphism occurred in parental asteroids, not the solar nebula.

On the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite ALHA77307

Abstract The origin of isolated olivine grains in carbonaceous chondrites has been suggested to be either by direct condensation or by fragmentation of chondrules. In an attempt to resolve this debate, isolated olivine grains in the carbonaceous chondrite ALHA77307 have been studied in detail. Zoning characteristics and minor element compositions of olivines from all isolated and chondrule occurrences are described. The data show that a strong relationship exists between isolated olivines and the chondrule population. Chondrule olivine compositions and zoning properties are consistent with olivine crystallization from the molten chondrules. Because properties of isolated grains are very similar to chondrule olivines, an igneous origin for most, if not all, isolated grains is proposed. Large isolated forsterite grains may be derived from fragmentation of “macroporphyritic” type I chondrules which contain one or very few large olivine grains. Relic forsterite grains in type II chondrules, and complex isolated olivine grains with forsteritic cores and fayalite-rich rims, provide evidence that fragmentation and recycling of chondrules and their components was a common and efficient process in the chondrule-forming region.

Thermal histories of CO3 chondrites: Application of olivine diffusion modelling to parent body metamorphism

Abstract The petrologic sequence observed in the CO3 chondrite group has been suggested to be the result of thermal metamorphism on a parent body. We have developed a model to examine the possibility that chondrule and matrix olivines equilibrated in situ, during parent body metamorphism. The model considers Fe Mg interdiffusion between chondrule and matrix olivines. Zoning profiles comparable to those observed in chondrule olivines from partially equilibrated members of the series are reproduced successfully. Metamorphism of CO3 chondrites on a parent body is therefore a viable model for the observed equilibration. Results indicate that peak metamorphic temperatures experienced by the CO3 chondrites were around 500°C, and that the range of peak temperatures between unequilibrated and equilibrated subtypes was relatively narrow, around 100°C.

Minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules

Abstract We present minor and trace element (REE, Sr, Y, and Zr) data for pyroxenes and mesostases in four porphyritic chondrules from the Semarkona ordinary chondrite. Apparent partition coefficients for clinoenstatite, orthoenstatite, pigeonite, and augite are compared with experimental and petrologic data from the literature, and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated. For most elements, the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition. However, for LREE apparent partition coefficients are significantly higher than comparable equilibrium data, and the ratio of HREE/LREE partition coefficients is lower, particularly for Ca-poor pyroxene. We attribute this flattening of REE patterns to the effect of rapid cooling. Apparent partition coefficients of all REE and Y in augite are higher than equilibrium data, particularly in one chondrule with a high Al2O3 content. We suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge-balanced substitutions with Al3+ cations.

CLASSIFICATION, METAMORPHIC HISTORY, AND PRE-METAMORPHIC COMPOSITION OF CHONDRULES

Abstract Sears et al. (1992) and DeHart et al. (1992) have proposed that the existing scheme for chondrule classification, which is based on modal mineralogy, texture, and mineral composition, should be replaced by a new scheme that is based primarily on the compositions of olivine phenocrysts and mesostases, but also on their cathodoluminescence. We have used published and unpublished compositional data for chondrules in ordinary and carbonaceous chondrites to compare the two classification schemes. We find that the new scheme does not preserve the identity of the existing chondrule groups, even in the least metamorphosed chondrites. Because of difficulties in assigning chondrules to the proposed groups and the lack of olivine grains or mesostasis areas of sufficient size, 30% of chondrules in some type 3 ordinary chondrites cannot be classified in the new scheme using compositional data. For type 4–6 chondrites, the existing scheme provides more information about the history of individual chondrules than the new scheme. We conclude that textural and modal criteria should be retained for the classification of chondrules.

Effect of metamorphism on isolated olivine grains in CO3 chondrites

Abstract The presence of a metamorphic sequence in the CO3 chondrite group has been shown previously to result in changes in properties of chondrule silicates. However, the role of isolated olivine grains during metamorphism of these chondrites has not been addressed. Isolated olivine grains in two metamorphosed CO3 chondrites, Lance and Isna, have been investigated in this study in order to assess the compositional properties of isolated olivine grains that may be attributable to metamorphism. Compositional changes in isolated olivines with increasing petrologic subtype are very similar to changes in chondrule olivines in the same chondrites. Olivine compositions from all occurrences (chondrules, isolated grains, and matrix) converge with increasing petrologic subtype. The degree of equilibration of minor elements is qualitatively related to the diffusion rate of each element in olivine, suggesting that diffusion-controlled processes are the most important processes responsible for compositional changes within the metamorphic sequence. The data are consistent with metamorphism taking place in a closed system on the CO3 chondrite parent body. Fe-poor olivine grains in metamorphosed chondrites are characterized by an Fe-rich rim, which is the result of diffusion of Fe into the grains from Fe-rich matrix. In some instances, “complex,” Fe-rich rims have been identified, which appear to have originated as igneous overgrowths and subsequently to have been overprinted by diffusion processes during metamorphism. Processes experienced by CO3 chondrites are more similar to those experienced by the ordinary chondrites than to those encountered by other carbonaceous chondrites, such as the CV3 group.