The multi-component mantle source of Roman province ultrapotassic magmas revealed by melt inclusions

A key effect of subduction is the upward infiltration of slab-derived melts and fluids to create a chemically and lithologically heterogeneous region in the mantle wedge. Magmatism within the post-subduction setting of peninsular Italy is derived from a sediment-metasomatised lithospheric mantle source, with primitive products recording substantial temporal and spatial variations in the nature and extent of metasomatism. The Roman magmatic province, in central Italy, is host to ultrapotassic (HKS), leucite-bearing magmas that require source modification that is not yet well-constrained in terms of mineralogy and sediment provenance. Here, we present the chemistry and Sr-Nd-Pb isotope compositions of melt inclusions in forsterite-rich olivine (Fo₈₈₋₉₂) from three key Roman volcanic centres (Vulsini, Sabatini, Alban Hills) to characterise the metasomatic imprint of their mantle sources. The melt inclusion suites (Vulsini HKS, Vulsini melilite-bearing HKS, Sabatini HKS, Alban Hills HKS) have clear differences in major-element (K, Ca, Al, P, Ti), volatile (F, Cl, B, Be), trace-element (e.g., Rb, Sr/Y, Th/Nb), and radiogenic-isotope (⁸⁷Sr/⁸⁶Sr) compositions that reflect the diversity of the primary melts beneath the region. For each suite, the extraction of melt batches from different domains in a heterogeneous (veined) source rock with different compositions and mineralogies can explain the chemical characteristics of the melts. At least three metasomatic vein types are required to form the respective parental melts and we infer phase assemblages consisting of phlogopite + Ca-amphibole + clinopyroxene ± apatite. The Sr-isotope differences between Vulsini melilite-bearing HKS and Vulsini and Sabatini HKS require at least two distinct metasomatic events to have occurred beneath the Roman province. These events need not be associated with two distinct subduction systems. Instead, we infer that different metasomatic agents could have originated under different P-T-t conditions from different domains in the same subducting slab.