Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

by Sears, D. W. G.; Saxton, J. M.; Lyon, I. C.

A large chondrule from Semarkona, the most primitive ordinary chondrite known, has been discovered to contain a record of mass transport during its formation. In most respects, it is a normal Type 1, group Al, low-FeO chondrule that was produced by reduction and mass-loss during the unidentified flash-heating event that produced the chondrules, the most abundant structural component in primitive meteorites. We have previously measured elemental abundances and abundance profiles in this chondrule. We here report oxygen isotope ratio abundances and ratio abundance profiles. We have found that the mesostasis is zoned in oxygen isotope ratio, with the center of the chondrule containing isotopically heavier oxygen than the outer regions, the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling. The delta O-17 values range from -2.0 parts per thousand to 9.9 parts per thousand, while delta O-18 range from -1.9 parts per thousand to 9.6 parts per thousand. More importantly, a plot of delta O-17 against delta O-18 has a slope of 1.1 +/- 0.2 (1 sigma) and 0.88 +/- 0.10 (1 sigma) when measured by two independent methods. Co-variation of delta O-17 with delta O-18 that does not follow mass-dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs. We argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived. Furthermore, there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation. Although it is possible to formulate a series of exchanges between the chondrule and external O-16-rich and O-16-poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule, such a sequence of events looks very contrived. We therefore hypothesize that reduction, devolatilization, and crystallization of the chondrule melt may have produced O-16-rich olivines and O-16-poor mesostasis plotting on a slope-one line as part of the chondrule-forming process in an analogous fashion to known chemical mass-independent isotopic fractionation mechanisms. During cooling, volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and O-16-rich mesostasis. The chondrule therefore not only retains a record of considerable mass transport accompanying formation, but also may indicate that the isotopes of oxygen underwent mass-independent fractionation during the process.

Geochimica Et Cosmochimica Acta
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1872-9533; 0016-7037