White Pumice vs. Black-Grey Scoria in the Tufo Giallo della Via Tiberina (Sabatini Volcanic District, Roman Province): Evidence of Temperature- and H2O-Zoning in Phonolitic Magma Chambers

Changes in colour and texture of the juvenile component are often observed in the deposits of explosive eruptions. The transition from white aphyric juvenile clasts (white pumice) towards highly crystalline ones (black-grey scoria) is usually explained by means of compositional layering of the magma chamber or by syn-eruptive magma mixing. However, these textural variations may even occur without accompanying compositional changes, as is the case of the Tufo Giallo della Via Tiberina (Roman Province, central Italy). This pyroclastic succession comprises two eruptive units, emplaced at ca. 561 and 548 ka, during the early explosive activity of the Sabatini Volcanic District. White pumice and black-grey scoria, occurring respectively in the lower and intermediate-upper portions of each eruptive unit, are characterized by remarkably different textures, in spite of common phonolitic bulk compositions: white pumice is subaphiric (P.I.<10%), vitrophyric and highly vesicular, while black-grey scoria is highly crystallized (P.I.>60%, with dominant leucite phenocrysts) and poorly vesicular. Phase relationships (MELTS simulations) and cpx-liquid geothermometry yield temperature estimates at 890-920°C and 920-940°C, respectively for white pumice and black-grey scoria feeder-magmas, and P=150-200 MPa. The diffuse occurrence of leucite phenocrysts in black-grey scoria records crystallization at initially H2O-undersaturated conditions, while the vitrophyric texture of white pumice indicates H2O-saturated conditions. In our model, the pre-eruptive H2O over-saturation was achieved by water diffusion from the cooler and highly crystallized, peripheral portions (black-grey scoria) toward the inner, subaphyric, portions (white pumice) of the magma chambers, as a consequence of radial H2O concentration gradients. The withdrawal of the white pumice feeder-magma in the early eruptive stages caused decompression and extensive crystallization in the peripheral magma portions, triggering fragmentation and eruption of the black-grey scoria magma in the late eruptive stages.