EMPLACEMENT PROCESSES AND MARTIAN MANTLE SOURCE COMPOSITION OF NAKHLITES AND CHASSIGNITES

The martian nakhlite and chassignite meteorites, which are clinopyroxenites and dunites, respectively, have similar mineralogy, geochemical compositions, and crystallization (1.3 Ga) and ejection (11 Ma) ages. Consequently, the consensus view is that these meteorites originate from the same magmatic system. Nakhlites and chassignites show significant variation in modal abundances of olivine, pyroxene, and intercumulus material, and corresponding thermal equilibration of these phases. These samples can therefore be used in an analogous way to fractionally crystallized and thermally equilibrated suites of terrestrial rocks, to examine processes of emplacement and to determine parental melt composition and, ultimately, mantle source composition. However, these goals have been hampered by a lack of comprehensive quantitative textural, mineral chemical or whole-rock geochemical datasets for these rocks. We present a coherent study of the major and trace element bulk rock composition of a suite of nakhlites and chassignites (Northwest Africa (NWA) 998, NWA 6148, Nakhla, the four paired Miller Range nakhlites, Lafayette, Yamato 000593, Yamato 000749, Chassigny). In addition, we present textural properties of the various nakhlites and chassignites by conducting crystal size distribution (CSD) and spatial distribution pattern (SDP) analyses to better constrain textural evolution. The presented results are part of our on-going study to fully characterize the petrogenesis of nakhlites and chassignites.

Major and trace element results are broadly similar to previously published data on the meteorites, with MgO contents ranging from 7.3 to 13 wt.% and Mg-numbers of between 27 and 52. These compositions reflect the variable modal abundance of intercumulus material. Samples all show systematic but variable light rare earth element (LREE) enrichment ([La/Yb]_CI = 1.8 - 5.7). In general, a progressive increase in incompatible elements is observed in samples located from the bottom to the top of a hypothetical igneous body, also consistent with an increase in intercumulus material. The relative enrichments in the LREE from the bottom to the top of the magmatic system may be due to progressive fractional crystallization effects or to infiltration of Cl-rich fluids in these rocks, as suggested by previous studies.