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

Oxygen self-diffusion coefficients have been measured for three natura l diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel. and a synthetic akermanite for oxygen fugacities ra nging from the NNO to IW buffers. The experiments employed a gas-solid isotopic exchange technique utilizing 99% O-18-enriched CO-CO2 gas mi xtures to control both the oxygen fugacity and the isotopic compositio n of the exchange reservoir. Diffusion profiles of the O-18 tracer wer e obtained by in-depth analysis with an ion microprobe. The experiment al results, fit to the Arrhenius relation D = D(0)e((-Q/RT)), yield th e following: D-degrees(m(2) s(-1)) Q(kJ mol(-1)) diopside 4.3[-3.8 +32 .6] x 10(-4) 457 +/- 26 akermanite 4.7[-4.4 +83.5] x 10(-7) 278 +/- 33 spinel 2.2[-1.8 +8.7] x 10(-7) 404 +/- 21 anorthite 8.4[-8.0 +174] x 10(-13) 162 +/- 36 At a given temperature, oxygen diffuses about 100 t imes more slowly in diopside than indicated by previous bulk-exchange experiments (CONNOLLY and MUEHLENBACHS, 1988). Our data for anorthite, spinel, and akermanite agree well with prior results obtained by gas- solid exchange and depth profiling methods (ELPHICK et al., 1988; REDD Y and COOPER, 1981; YURIMOTO et al., 1989, respectively). Since these other experiments were conducted at different oxygen fugacities, this agreement indicates that diffusion of oxygen in these nominally Fe-fre e minerals is not greatly affected by f(o2) in the range between pure oxygen and the iron-wustite buffer. However, our diffusion coefficient s for anorthite, melilite, and spinel are also uniformly lower than th ose obtained by bulk analysis of crushed powders at similar temperatur es (MUEHLENBACHS and KUSHIRO, 1974; HAYASHI and MUEHLENBACHS, 1986; AN DO and OISHI, 1974). The oxygen diffusion data are used to evaluate th e effects of three different types of thermal histories upon the oxyge n isotopic compositions of minerals found in Type B Ca-Al-rich inclusi ons (CAIBs) in carbonaceous chondrites: (1) gas-solid exchange during isothermal heating, (2) gas-solid exchange as a function of cooling ra te subsequent to instantaneous heating, and (3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. Wit h the assumptions that the mineral compositions within a CAIB were uni formly enriched in O-16 prior to any thermal processing, that effectiv e diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyrox ene, none of the above scenarios can reproduce the relative oxygen iso topic anomalies observed in CAIBs without improbably long or unrealist ically intense thermal histories relative to current theoretical model s of nebular evolution. The failure of these simple models, coupled wi th recent observations of ''disturbed'' magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicat ive of alteration and recrystallization, suggests that the oxygen isot opic compositions of these phases may have also been modified by alter ation and recrystallization possibly interspersed with multiple meltin g events. Because the modal abundance of spinel remains relatively con stant for plausible melting scenarios, and its relatively sluggish dif fusion kinetics prevent substantial equilibration, Mg-AI spinel is the most reliable indicator of the oxygen isotopic composition of precurs or material which formed Type B CAIs.