Ti valence in refractory phases is an important recorder of redox conditions in the early solar nebula. We report the valence of Ti in pyroxene, spinel and hibonite in spinel-hibonite and spinel-pyroxene inclusions and in a coarse hibonite grain. A system of solar composition is so reducing that Ti{sup 3+} and Ti{sup 4+} can coexist, making the valence of Ti a valuable indicator of f{sub O2} conditions during formation of nebular materials. The Ti{sup 3+}/Ti{sup 4+} ratios observed in the Ti-rich phases fassaite and rhoenite in coarse-grained refractory inclusions from CV3 chondrites have been shown to be quantitatively consistent with formation in a gas of solar composition (log f{sub O2} = IW-6.8), but these are the only objects in chondrites for which this is the case. Here, we report the valence of Ti in various phases in refractory inclusions from the Murchison CM2 chondrite. The second-highest temperature, major-element-bearing phase predicted to condense from a gas of solar composition, hibonite (ideally CaAl{sub 12}O{sub 19}), can contain significant amounts of Ti, but the hibonite structure can have oxygen vacancies, so calculation of Ti valence from stoichiometry of electron probe analyses is not recommended for hibonite. To date, the only reported measurement ofmore » Ti valence in meteoritic hibonite was done by electron spin resonance, on coarse crystals from a Murchison hibonite-perovskite-melilite inclusion. Spinel and most of the pyroxene in CM inclusions contain too little Ti for derivation of Ti{sup 3+}/Ti{sup 4+} ratios from electron probe analyses. X-ray absorption near edge spectroscopy (XANES), however, allows determination of Ti valence in relatively Ti-poor phases. In the present work, we apply synchrotron microXANES to a large hibonite grain from Murchison and to spinel-hibonite (sp-hib) and spinel-pyroxene (sp-pyx) inclusions from Murchison, refractory materials whose Ti{sup 3+}/Ti{sup 4+} ratios have not been previously measured. Analysis of these samples allows comparison of Ti valence of (1) pyroxene in sp-pyx inclusions with that of fassaite; (2) spinel in hibonite-bearing with that of hibonite-free inclusions; and (3) hibonite in sp-hib inclusions with that of large, single grains and the previously analyzed sample.« less

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First Ti-XANES analyses of refractory inclusions from Murchison

We report the mineralogy, petrology, major, minor and trace element geochemistry, O and Si isotopes of a complex compound chondrule from the Allende meteorite. The chondrule contains zones of refractory (Ca, Al-rich) regions along with regions more similar to ferromagnesian chondrules. The bulk silicon isotopic composition of the object is δ30Si = –0.71 ± 0.03‰. The oxygen isotopic composition of the different phases within the compound chondrules fall along the Allende chondrule line and range from Δ17O – 12.5 to –2‰. Rare earth element abundances are enriched compared to chondritic levels by up to 15× CI, and show a nebular condensate signature with depletions in Eu and Yb. Our data show that the object evolved in oxygen isotopes in a nebular environment, most likely due to formation from a mixture of diverse components combined with interaction with nebular gas. In addition, differences in olivine composition across the inclusion suggest isotopically distinct oxygen regions also existed in dust in the protoplanetary disk. This object and other compound objects demonstrate that CAIs were present in the chondrule forming region, however they are not found in the majority of chondrules. We speculate that the bulk of the CAIs may have been added into the CV parent body after initial accretion of the body.

Relationship between CAIs and chondrules: A case study of a compound chondrule from the Allende (CV3) meteorite

A nebula setting as the origin for bulk chondrule Fe isotope variations in CV chondrites

Introduction: Chondrules have highly variable chemical compositions [e.g. 1]. One of the major questions is how chondrules obtained this large variability. The answer to this will potentially reveal important details of chondrule evolution. Popular explanations for the chemical variations of chondrules comprise (i) heterogeneities among chondrule precursor grains, (ii) elemental exchange between chondrule and surrounding gas during chondrule formation and (iii) elemental exchange on the meteorite parent body. Knowledge of the chondrule isotopic composition help to discriminate between these possibilities. However, different elements and hence, isotope systems have different cosmochemical and geochemical properties and are therefore affected to different degrees by the above mentioned processes to produce chemical variations among chondrules. The elements Fe and Si have similar condensation temperatures, but the diffusion and mobility of Fe is much higher than that of Si. In addition, Fe occurs in three different valence states: Fe0, Fe2+ and Fe3+. It has been shown that redox reactions fractionate Fe isotopes [2]. The presence of metal and/or sulfide in many chondrules also raises the question of which chondrule phase governs the bulk Fe isotopic composition of the chondrule. In order to answer this, the modal abundance of metal/sulfide in chondrules must be known. There is so far only one recent study of 8 chondrules in CR chondrites showing that modal abundances of metal in these have an extremely large range between 1 and 37 vol% [3]. We recently measured Fe-isotopic compositions and studied the petrology of a large set of chondrules and CAIs in CV chondrites [4]. We now measured the Si-isotopic composition of some of these chondrules and CAIs. There are so far only few studies of Siisotopic compositions of chondrules [5-7], but no systematic study. In addition we measured the modal abundance of opaque phases (combined sulfides and metal) in Allende, in order to get an idea of the spread of opaque abundances in CV chondrites. Techniques: The Si-isotopes have been measured using the Nu Plasma MC-ICP-MS at the University of Oxford. The column chemistry was done using the procedure of [8]. The modal abundances of opaques have been determined using an X-Tek HMX-ST CT 3D scanner. The edge length of each voxel was 8.1 μm. Results: We measured Si-isotopic compositions of 7 chondrules and 3 CAIs from the CV chondrites Mokoia and Grosnaja. The results of the Fe-isotope measurements and the petrologic studies are published in [4]. Figure 1 compares the Siand Fe-isotopic compositions of the 7 chondrules and 3 CAIs.

Combined Fe- and Si-isotope measurements of CV chondrite chondrules and CAIs

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Combined Fe- and Si-isotope measurements of CV chondrite chondrules and CAIs