J. Janicke

Partitioning of Zr and Nb between coexisting opaque phases in lunar rocks—determined by quantitative proton microprobe analysis

Abstract Proton microprobe analyses were carried out on Zr and Nb in coexisting opaque phases in lunar rocks. Zr contents in armalcolite and coexisting ilmenite in Apollo 17 rocks were determined in mineral pairs of two different origins: (a) in olivine porphyritic basalts where ilmenite was formed by reaction of armalcolite with the basaltic liquid or with Fe 0 ; (b) in plagioclase poikilitic basalts where ilmenite crystallized directly from the melt either simultaneously with armalcolite or after it. In armalcolite- and ilmenite-bearing assemblages Zr partitions in favor of armalcolite in (a); in favor of ilmenite in (b). These results can be understood by extensive equilibration between armalcolite and ilmenite in (a) due to the reactions involving armalcolite. The reverse partitioning in (b) may represent initial ratios because equilibration was not accomplished. In Apollo 15 rocks coexisting Cr-ulvospinel and ilmenite occur in two distinct assemblages, (a) ulvospinel with ilmenite lamellae formed by subsolidus reactions, (b) ulvospinel and ilmenite involved in the isobarically invariant reaction ulvospinel + silica → ilmenite + fayalite. In the first case Zr partitions between ilmenite and ulvospinel, in the second case between ulvospinel and ilmenite, and between ilmenite and silica. Experimental partitioning data for Zr [6] of the ulvospinel and ilmenite pair are not applicable to assemblages containing primary ulvospinel with ilmenite formed later in the above reaction. In both assemblages, Zr shows a zonal distribution in Cr-ulvospinel as a function of Fe 2 TiO 4 content and FFM ratio. Therefore Zr contents in the natural assemblages should always be measured at the contacts of the coexisting phases. The Zr/Nb ratio in ilmenites in the groundmass of Apollo 14 breccia 14305,55 is much lower (0.18–0.25) than that in a gabbro clast from the same breccia (1.25). Limited equilibration between groundmass and clasts in the breccia during thermal metamorphism is indicated.

Trace element determination in lunar rocks — quantitative pixe analyses on thick samples

Abstract Information about the formation processes and geochemical coherence can be obtained by determination of the partitioning of trace elements in coexisting minerals. Systematic trace element analysis among coexisting mineral phases requires the measurement of the elemental concentrations in a small volume, which can be obtained by applying a proton microprobe. Different opaque minerals, like armalcolite (Fe, Mg) Ti2O5, ilmenite FeTiO3, ulvospinel Fe2TiO4 and chromite FeCr2O4 (of the spinel solid solution series Fe2TiO4FeCr2O4) in samples of three Apollo missions were investigated. Comparative analyses of the mineral pair armalcolite/ilmenite in two Apollo 17 basalts (sample 74242,19), which differ in their crystallization sequences, show that the partitioning of Zr among the two mineral phases is different in the two basalt types. Coexisting ilmenite/ulvospinel were studied in the Apollo 15 basalts (samples 15065,93; 15555,42). The trace elements Zr and Nb were found to partition in favour of ilmenite. The investigated mineral pairs of chromite/ulvospinel in Apollo 12 (samples 12051,60; 12051,62) and Apollo 15 basalts are zoned in their major element contents. The trace element Zr detected by proton microprobe analyses also shows zoning behaviour. Primarily, crystallized chromite does not incorporate Zr, whereas the Zr content in the coexisting ulvospinel rises with increasing Ti content.