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Oxidation states and ascent history of the Apollo 17 volcanic beads as inferred from metal-glass equilibria

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/s0016-7037(97)00119-1


Abstract The composition, mode of occurrence, and relationships of metal blebs in the Apollo 17 74001/2 core have been examined to provide insight into variations in oxidation states during ascent and eruption of the lunar volcanic glasses. The 74001/2 core is composed of ∼99% orange volcanic glasses and their crystallized equivalents (black beads), with olivine phenocrysts, metal blebs, and, in the upper 8 cm of the core, lithic fragments composing the remaining 1%. In an analysis of this core, twenty-nine metal blebs larger than 4 μm in size were discovered in thin sections taken along the length of the 68 cm deep core. These blebs are classified into three types on the basis of their shape, composition, and distribution. Fourteen of the metal blebs were found trapped inside or between olivine phenocrysts (Type I blebs). Microprobe analyses showed that these spherical blebs have a homogeneous composition of 85% Fe, 14% Ni, and 1.1% Co from core to rim. Six Type II rounded blebs were discovered trapped inside volcanic beads. Their core compositions are similar to the Type I but near the rim these blebs have a sharp decrease in Fe and increase in Ni content. In the lower half of the 74001/2 core where the beads are predominantly black, many contain ≤5 μm amoeba-shaped Type III metal blebs. Type III blebs are composed of Fe with trace amounts of Ni. Two angular metal fragments could not be classified into any of the three types. We propose that the Type I blebs formed initially at 4.0 km depth and 1.3 log units below IW at a pressure of 200 bars and a temperature of 1320°C by the formation of CO gas during graphite oxidation. Once trapped by olivine phenocrysts, the blebs were buffered from further reactions with the melt. The Type II blebs remained in contact with the melt during ascent and experienced loss of Fe to the surrounding melt phase by oxidation reactions which range from 1.3 to 0.9 log below IW. Finally, Type III blebs formed either by exsolution of Fe during crystallization in the fire fountain or by loss of volatiles originally coordinated with Fe to the volcanic gas phase under the reduced conditions in the fountain.

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