Textural and morphological characterization of the ca. 4 ka Astroni eruptive products (Campi Flegrei, Italy)

In the last 15 ky, Campi Flegrei caldera was characterized by numerous phreatomagmatic and magmatic eruptions from several intra-caldera vents. Understanding the eruption dynamics and fragmentations styles is fundamental to constrain the impact related to ash depositions over densely populated areas. The Astroni volcano is located in the centraleastern sector of Campi Flegrei and grew during at least seven eruptions, which occurred in a relatively short time interval (4.1-3.8 ka). The Astroni eruptive activity was dominated by explosive eruptions, characterized by the deposition of pyroclastic density currents (PDCs) and subordinate fallout deposits. In this context, the Astroni deposits offer a valuable case study to gain insights into the role played by magma-water interaction during typical eruptions of Campi Flegrei. To provide a better understanding of conduit dynamics and fragmentation styles, we present a characterization of the Astroni eruptive products via density measurements and morphological and textural analyses of juvenile clasts from two selected units of the Astroni sequence (Units 3 and 5). Samples from Units 3 and 5 are retrieved from key sections both inside and outside the crater. Density of juvenile fragments was measured for seven layers from Unit 3 and two layers from Unit 5, on subsamples of 50 clasts in the -4 phi to -3 phi range; density variation with grain-size (from -4phi to +1phi) was also measured on selected samples. Vesicle textures were studied through image analysis for five pumice clasts (-3 phi and -2 phi in size) of different samples along the stratigraphic sequence of Units 3 and 5. Finally, the ash fraction (below 0.250 mm) of Unit 3 and 5, retrieved from inner and extra crater samples, was analyzed using the static particle analyzer Malvern Morphologi G3SE to quantify the whole particles’ morphology. Juvenile clast densities show mostly unimodal trends with similar modes across both units (around 700 kg/m3 and 600 kg/m3 for Units 3 and 5). Density variations with grain-size reveal the classical sigmoidal variation, reflecting the presence of vesicles with homogeneous size distributions and the scarcity of large (over 1mm) vesicles. Scanning electron microscopy observations reveal heterogeneous vesicle textures in both units, with clasts displaying both circular and elongated vesicle shapes, including highly stretched, tube-like vesicles. In samples from Unit 3, vesicularity clusters between 53 and 73%, with few clasts having lower vesicularity (39–47%). Samples from Unit 5 show vesicularity values consistent with those of Unit 3, varying between 55 and 71%. Vesicle size distribution and vesicle volume distribution plots suggest for both Unit 3 and Unit 5 processes of multiple nucleation and growth events, however possibly dominated by a final event of delayed vesiculation. The external surfaces of the ash fragments was observed at SEM. 3D ash shape was quantified using elongation, circularity, convexity and solidity as descriptors. Particles between 30 and 70 microns show two different trends which mainly reflect differences in vesicle shape and size. Inner-crater samples follow one trend , while extra-crater samples follow a different trend. In Unit 5, differences in ash morphology can also be noted between fallout and PDC deposits. The collected textural, morphological and density data are interpreted as related to the fragmentation of moderate to highly vesicular magma during the Astroni eruptions, with only limited interactions with external water. Although the Astroni deposits are generally considered as the result of phreatomagmatic activity, the data collected in this work raises some questions on the nature and extent of the role of magma-water interaction on the dynamics of the Astroni and other Campi Flegrei eruptions.