The preservation of zoning patterns for major and trace elements in olivine and pyroxene reflect their differing diffusion rates. These have been applied to develop insights into the plumbing systems of olivine-phyric and poikilitic shergottite volcanoes, their eruption triggers and timescales. Here, we exploit crystal zoning to reconstruct magmatic histories recorded in martian meteorites, applying a combination of microanalytical techniques. We investigate eleven olivine-phyric and poikilitic shergottites, which have long been distinguished via the unique textural relationships between their large olivine and pyroxene phenocrysts. Major element compositions of large olivine crystals in the olivine-phyric shergottites indicate that the cores of the phenocrysts formed from earlier, less fractionated magma (Fo = 62-75) than the rims (Fo = 50-62), which in turn formed at the same time as a secondary generation of smaller olivine crystals (Fo = 48-56). Olivine crystals in the poikilitic shergottites are unzoned but have more fractionated compositions in light rare earth element-enriched samples (Fo = 59-61) than in intermediate samples (Fo = 68-71). We interpret that the olivine in olivine-phyric shergottites crystallised from an evolving magma, whereas olivine in poikilitic shergottites crystallised from two distinct magmas, either evolved with high Mn concentrations or primitive with lower Mn. Although olivine major elements show gradual zoning patterns or no zoning, slower diffusing trace elements show that multiple magma recharge events occurred within the shergottite magma reservoirs. LA-ICP-MS trace element mapping reveals oscillatory P and Sc zoning, which when coupled with Ti zoning, indicates that up to three recharge events occurred before final emplacement of the olivine-phyric shergottites within the shallow crust. Diffusion timescales for Fo and Ni zoning are comparable to those obtained in basaltic hotspot systems on Earth (such as Hawaii). The magmas that formed the poikilitic shergottites also experienced multiple recharge events, which are recorded by P and Ti zoning within olivine. Pyroxene crystals support three recharge events for the olivine-phyric shergottites, involving increasingly Cr- and Ca-rich magmas. In contrast, pyroxene oikocrysts in the poikilitic shergottites record one recharge event, which formed augite rims. While the host magma was still hot, Ca and trace elements from the augite rim diffused into the pigeonite cores of the pyroxenes. Prolonged storage at relatively high temperatures allowed significant diffusion that caused most of the olivine and pyroxene crystals to fully equilibrate, removing any previous major element zoning patterns. Through detailed examination of these phenocrysts, we build on the existing models of the two volcanic plumbing systems of the poikilitic and olivine-phyric shergottites, and construct quantitative models, revealing that the olivine-phyric shergottites experienced crystallisation and short storage at depth followed by shallow emplacement, whereas the poikilitic shergottites experienced prolonged deep storage before final shallow emplacement.
Burke et al. (Wed,) studied this question.