CRYSTAL SIZE DISTRIBUTIONS AS A POTENTIAL NON-DESTRUCTIVE MEANS FOR DETERMINING SEDIMENT CONTAMINATION IN BASALTIC METEORITES

The textural analysis technique of crystal size distributions (CSDs) may have the potential to determine whether a basaltic meteorite originated from a lava which incorporated desert pavement or ash sediments into its mineralogy as it flowed over a planetary surface, or if the meteorite is a pristine representation of the original melt. Recent CSD-generating techniques using microtomography require very small sample masses and are 100% non-destructive. Such analyses may be preferred in cases of meteorites that would be completely destroyed by geochemical techniques, or when preservation of the precious sample mass is of upmost importance. In this study we explore the robustness of this technique by searching for evidence of desert pavement influence in the CSD profiles of a set of terrestrial basalts.

We have analyzed stratigraphically continuous samples from several basaltic lava flows (3 – 4 samples per flow) from the Cima Volcanic Field in the Mojave Desert, CA – a terrain texturally analogous to volcanic areas of the Martian surface. Using x-ray microtomography, CSDs were calculated for plagioclase crystals. From this preliminary data, one flow shows a predictable textural coarsening, with the CSD slopes becoming increasingly shallow as the samples become further from the quenching contact surface of the desert floor. A second flow shows similar CSD slopes among all samples in the column. This suggests that this flow may have absorbed desert pavement sediments which helped to quench or provide additional nucleation sites for crystals in the cooling basaltic magma, and that this influence is dominating and masking the usual textural coarsening.

The ability to see sediment influence in our CSD measurements is subject to several factors, such as the viscosity and turbulence of the lava, the amount and type of sediments that the lava flowed over, and the execution of the microtomography scanning and processing. Preliminary data so far suggests that this method has potential for our proposed use.