Petrogenesis of Planetary Basalts Using Crystal Stratigraphy

The petrogenesis of lunar and Martian basalts and lunar troctolites is explored with crystal stratigraphy, which combines textural and in situ geochemical analyses. Is situ analyses via electron microprobe and laser ablation are conducted on zoned pyroxene in basalts, and olivine and plagioclase in the single lunar troctolite. Zoned pyroxene is an excellent recorder of geochemical changes in the melts that produced these basalts because it comes on the liquidus early and continues to stay on until late in the crystallization sequence.

Combining in situ geochemical analyses with published partition coefficients allows for parental liquids to be calculated for each individual sample, which then allows a crystallization sequence and petrogenetic model to be constructed for each sample. This study finds that parental liquids determined for Apollo 12 basalts using crystal stratigraphy are more primitive than liquids calculated using a corresponding quenched sample as was done in all previous studies of Apollo 12 basalt petrogenesis. These primitive liquids are a better representation of the true nature of the parental liquids, and are therefore better suited for accurate petrogenetic modeling.

Using the crystal stratigraphy approach, the complete crystallization history from the onset of pyroxene crystallization to the final dregs of crystallization are determined for the Martian meteorite Shergotty. This study finds that Shergotty formed from 2 discrete batches of magma, both with pyroxene on the liquidus, which then mixed with continued pyroxene crystallization.

The origin of lunar troctolites has remained an enigmatic problem ever since they were described 40 years ago because they contain both primitive and evolved signatures. Using crystal stratigraphy of olivine and plagioclase crystal, we determined the nature of the parental liquids and constrain the formation mechanisms of lunar troctolites.