Determination of oxygen self-diffusion in akermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions

Journal Article · Thu Sep 01 00:00:00 EDT 1994 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(94)90161-9· OSTI ID:6766408

Ryerson, F J ^[1]; McKeegan, K D ^[2]

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1. Lawrence Livermore National Lab., CA (United States)
2. Univ. of California, Los Angeles, CA (United States)

Oxygen self-diffusion coefficients have been measured for three natural diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel, and a synthetic akermanite for oxygen fugacities ranging from the NNO to IW buffers. The oxygen diffusion data are used to evaluate the effects of three different types of thermal histories upon the oxygen isotopic compositions of minerals found in Type B Ca-Al-rich inclusions (CAIBs) in carbonaceous chondrites: (1) gas-solid exchange during isothermal heating, (2) gas-solid exchange as a function of cooling rate subsequent to instantaneous heating, and (3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. With the assumptions that the mineral compositions within a CAIB were uniformly enriched in [sup 16]O prior to any thermal processing, that effective diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyroxene, none of the above scenarios can reproduce the relative oxygen isotopic anomalies observed in CAIBs without improbably long or unrealistically intense thermal histories relative to current theoretical models of nebular evolution. The failure of these simple models, coupled with recent observations of disturbed magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicative of alteration and recrystallization, suggests that the oxygen isotopic compositions of these phases may have also been modified by alteration and recrystallization possibly interspersed with multiple melting events. Because the modal abundance of spinel remains relatively constant for plausible melting scenarios, and its relatively sluggish diffusion kinetics prevent substantial equilibration, Mg-Al spinel is the most reliable indicator of the oxygen isotopic composition of precursor material which formed Type B CAIs.

• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 6766408
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 58:17; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CHONDRITES
ISOTOPE RATIO
INCLUSIONS
OXYGEN
SELF-DIFFUSION
OXYGEN 16
OXYGEN 18
DIFFUSION
ELEMENTS
EVEN-EVEN NUCLEI
ISOTOPES
LIGHT NUCLEI
METEORITES
NONMETALS
NUCLEI
OXYGEN ISOTOPES
STABLE ISOTOPES
STONE METEORITES
400201* - Chemical & Physicochemical Properties