Open- versus closed-system magmatism on Mars revealed by shergottites

The origin of depleted, intermediate, and enriched geochemical reservoirs on Mars has long been debated, with competing models invoking mantle heterogeneity versus crustal contributions. Resolving this is central to understanding the martian mantle composition, crust-mantle interactions, and magma evolution. Here we examine the role of closed- and open-system processes on shergottite magma rare earth element (REE) evolution. We present the first complete REE dataset for olivine, the earliest crystallising phase and proxy for primitive melts, and REEs for olivine-hosted melt inclusions and late-stage merrillite, enabling robust tracking of REE evolution throughout shergottite crystallisation. Depleted and intermediate shergottites exhibit closed-system behaviour, indicating derivation from distinct mantle sources. Enriched shergottites record mixed processes, some reflecting enriched mantle sources and others open-system REE evolution, likely through assimilation of enriched material by depleted magmas. Shergottite diversity thus reflects both preserved mantle heterogeneities and dynamic open-system processes, highlighting the importance of an enriched crustal reservoir.