Activity-composition relations in the fayalite-forsterite solid solution between 900° and 1300°C at low pressures

Williams, Richard J.

doi:10.1016/0012-821x(72)90176-8

Cited by 40 publications

(6 citation statements)

References 10 publications

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“…In JaH 422, chromite crystallizes at f O 2 conditions in the impact melt corresponding to Fo 83–86 . Assuming an average X Fa of 0.15 for JaH 422 matrix olivine, −log f O 2 change as a function of temperature can be estimated (e.g., Williams 1972): it evolves from 13.2 at 1250 °C to 9.7 at 1650 °C, close to melting temperature of pure olivine with Fo 80 . Thermal increase, required to melt JaH 422, likely changed the Cr valence as well as the Cr solubility (Hanson and Jones 1998).…”

Section: Discussionmentioning

confidence: 99%

Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

Janots

Gnos

Hofmann

et al. 2011

Meteorit & Planetary Scien

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Abstract-The Jiddat al Harasis (JaH) 422 ureilite was found in the Sultanate of Oman; it is classified as a ureilitic impact melt breccia. The meteorite consists of rounded polycrystalline olivine clasts (35%), pores (8%), and microcrystalline matrix (57%). Clasts and matrix have oxygen isotopic values and chemical compositions (major and trace elements) characteristic of the ureilite group. The matrix contains olivine (Fo 83-90 ), low-Ca pyroxene (En 84-92 Wo 0-5 ), augite (En 71-56 Wo 20-31 ), graphite, diamond, Fe-metal, sulfides, chromite, and felsic glass. Pores are partly filled by secondary Fe-oxihydroxide and desert alteration products. Pores are surrounded by strongly reduced silicates. Clasts consist of fine-grained aggregates of polygonal olivine. These clasts have an approximately 250 lm wide reaction rim, in which olivine composition evolves progressively from the core composition (Fo 79-81 ) to the matrix composition ). Veins crossing the clasts comprise pyroxene, Fe-oxihydroxide, C-phases, and chromite. Clasts contain Ca-, Al-, and Cr-rich glass along olivine grain boundaries (<1 lm wide). We suggest that a significant portion of JaH 422, including olivine and all the pyroxenes, was molten as a result of an impact. In comparison with other impact-melted ureilites, JaH 422 shows the highest melt portion. Based on textural and compositional considerations, clasts and matrix probably originated from the same protolith, with the clasts representing relict olivine that survived, but was recrystallized in the impact melt. During the melt stage, the high availability of FeO and elevated temperatures controlled oxygen fugacity at values high enough to stabilize olivine with Fo 83-87 and chromite. Along pores, high Mg# compositions of silicates indicate that in a late stage or after melt crystallization FeO became less available and fO 2 conditions were controlled by C)CO + CO 2 .

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Section: Discussionmentioning

confidence: 99%

Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

Janots

Gnos

Hofmann

et al. 2011

Meteorit & Planetary Scien

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“…3. This figure incorporates thermodynamic calculations for both the graphite‐CO equilibrium (French and Eugster 1965) and for the partitioning of FeO into olivine in the presence of Fe‐metal (Williams 1972). The olivine Fo ratio is insensitive to P , but strongly linked to T and f O 2 .…”

Section: Implications For Models Of Ureilite Formationmentioning

confidence: 99%

“… Curves for the C–CO buffer (light blue curves) and olivine‐reduction (dark red curves) shown on a plot of temperature versus log( f O 2 ) for the region of parameter‐space relevant to ureilite genesis; based, as discussed in Warren and Kallemeyn (1992), on thermodynamic treatments of French and Eugster (1965) and Williams (1972). With pressure‐controlled smelting, any specified combination of pressure and olivine‐core Fo implies a unique temperature.…”

Section: Implications For Models Of Ureilite Formationmentioning

confidence: 99%

Parent body depth–pressure–temperature relationships and the style of the ureilite anatexis

Warren

2012

Meteorit & Planetary Scien

104

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Abstract-New analyses of mafic silicates from 14 ureilite meteorites further constrain a strong correlation (Singletary and Grove 2003) between olivine-core Fo ratio and the temperature of equilibration (T E ) recorded by the composition of pigeonite. This correlation may be compared with relationships implied by various postulated combinations of Fo and pressure P in models for ureilite genesis by a putative process of anatectic (depth-linked, P-controlled) smelting. In such models, any combination of Fo and P together fixes the temperature of smelting. Agreement between the observed correlation and these models is poor. The anatectic smelting model also carries implausible implications for the depth range at which ureilites of a given composition (Fo) form. Actual ureilites (and polymict ureilite clasts: Downes et al. 2008) show a distribution strongly skewed toward the low-Fo end of the compositional range, with approximately 58% in the range Fo 76-81 . In contrast, the P-controlled smelting model implies that the Fo 76-81 region is a small fraction of the volume of the parent body: not more than 3.2%, in a model consistent with the Fo-T E observations; and even ignoring the Fo-T E evidence not more than 11% (percentages cited require optimal assumptions concerning the size of the parent body). This region also must occur deep within the body, where no straightforward model would imply a strong bias in the impact-driven sampling process. The ureilites did not derive preponderantly from one atypical ''largest offspring'' disruption survivor, because cooling history evidence shows that after the disruption (whose efficiency was increased by gas jetting), all of the known ureilites cooled in bodies that were tiny (mass of order 10 )9 ) in comparison with the precursor body. The Ca ⁄ Al ratio of the ureilite starting matter cannot be 2.5 times chondritic, as has been suggested, unless the part of the body from which ureilites come is at most 50% of the whole body. Published variants of the anatectic, P-controlled smelting model have the ureilites coming from a region that is >50 vol% of their parent body; and to invoke a larger body would have the drawback of implying that the Fo 76-81 spike represents an even smaller fraction of the parent body's interior. The ureilites' moderate depletions in incompatible elements are difficult to reconcile with a fractional fusion model. It is not plausible that melt formed grossly out of equilibrium with the medium-sized ureilite crystals. The alternative to pressurecontrolled smelting, i.e., a model of gasless or near-gasless anatexis, has very different implications for the size and evolution of the original parent body. To yield internal pressures prohibitive of smelting in even the shallowest and most ferroan portion of its anatectic mantle, the body would have to be larger than roughly 690 km in diameter. A 400 km body would have approximately 12 vol% of the interior (or 13 vol% of the interior apart from the thermal ''skin'' that never undergoes anatexis) prone, if both...

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“…Graphite-CO + CO 2 equilibria calculated using the precise thermodynamical treatment of French and Eugster (1965). Olivine-silica-metal (OSI) equilibria calculated from the equations of Nitsan (1974) and Williams (1972).…”

Section: Introductionmentioning

confidence: 99%

Fractional melting and smelting on the ureilite parent body

Goodrich

Orman

Wilson

2007

Geochimica et Cosmochimica Acta

101

170

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Activity-composition relations in the fayalite-forsterite solid solution between 900° and 1300°C at low pressures

Cited by 40 publications

References 10 publications

Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

Parent body depth–pressure–temperature relationships and the style of the ureilite anatexis

Fractional melting and smelting on the ureilite parent body

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