A micro-scale insight into a back-arc trans-crustal plumbing system: The case of Marsili volcano, Southern Tyrrhenian Sea

The Marsili seamount is a submarine volcano in the Tyrrhenian Sea that originated in a back-arc setting. Aiming to define the complexity of its trans-crustal plumbing system, we explored the compositional and textural variations of crystal cargoes in basaltic to andesitic lavas collected from three different sectors of the volcano (northern, axial, and lateral). Lavas collected from the northern sector are basaltic in composition and contain minerals with a narrow and more primitive composition compared to basalt and basaltic andesitic lavas sampled at the lateral and axial sectors, hosting a crystal cargo characterized by a broader chemical variability. Crystal-poor andesitic lavas were only collected at the axial summit sector of the volcano. Glomerocrysts with diverse mineralogy are ubiquitous in lavas erupted from all sectors and testify to the presence of crystal mush domains in the whole trans-crustal system. Thermobarometric calculations performed on clinopyroxene coupled to mass-balance and thermodynamic modeling collectively point to a polybaric and spatially heterogeneous plumbing system. The relatively less differentiated basaltic magmas erupted in the northern and axial sectors reside at depth corresponding to the lower crust-mantle boundary (300-450 MPa, 1040-1080 degrees C). Basaltic and basaltic andesitic magmas extracted from this deep storage zone formed, over time, scattered magma storage zones in the 10-12 km-thick oceanic crust beneath the Marsili volcano. The shallower magma storage zones sourced the andesitic magmas (<250 MPa, 920-980 degrees C) erupted in the axial summit sector. In turn, basaltic and basaltic andesitic magmas erupted in the lateral sector testify to intermediate storage conditions (200-400 MPa, 980-1060 degrees C) and variable degree of evolution. The variable content of incompatible (Al-T and Ti) and REE in clinopyroxene contained in basaltic lavas from the three sectors relates to different degrees of undercooling (triangle T), with magmas erupted from the northern sector recording higher undercooling (triangle T = 90 +/- 39 degrees C) compared to those erupted from the lateral (triangle T = 52 +/- 27 degrees C) and axial (triangle T = 30 +/- 25 degrees C) sectors. The emerging scenario is that basaltic magmas erupted from the northern sector experienced a more rapid ascent (also testified by the occurrence of high-Fo olivine and dendritic clinopyroxene in the groundmass) compared to magmas erupted at the lateral and axial sectors, otherwise experiencing prolonged residence within the crust.