Persistent polybaric rests of calc-alkaline magmas at Stromboli volcano, Italy: pressure data from fluid inclusions in restitic quartzite nodules

Abstract

A fluid-inclusion study has been performed on quartzite nodules of stromboli volcano hosted by calc-alkaline lavas of both the Strombolicchio (200 ka) and Paleostromboli II (60 ka) periods. The nodules are mainly composed of quartz crystals with subordinate plagioclase and K-feldspar. Small interstitial minerals such as plagioclase, K-feldspar, clinopyroxene, biotite, and quartz are also found, together with glass. Muscovite, epidote and zircon occur as accessory minerals. Different quartzite nodules occur: (1) equigranular polygonal granoblastic quartzite nodules forming a polygonal texture with clear triple points; (2) inequigranular polygonal granoblastic quartzite nodules; and (3) break-up nodules with strongly resorbed quartz. These quartzites are restites from partial melting, involving felsic crustal rocks at the magma/wall rock contact. Restitic quartz re-crystallises at variable and generally high temperatures, leading to the formation of quartzites with different textures. Quartz grains contain five types of fluid inclusions distinguished on the basis of both fluid type and textural/phase relationships at room temperature. Type I are two-phase (liquid+vapour) CO2-rich fluid inclusions. They are primary and subordinately pseudosecondary in origin and have undergone re-equilibration processes. Type II mono-phase/two-phase (vapour/liquid+vapour) CO2-rich fluid inclusions are the most common and, based on their spatial distribution and shape, they can be divided into two subclasses: type IIa and type IIb. Type II inclusions are secondary or pseudosecondary and they are assumed to have formed after decrepitation of type I inclusions and cracking of the host quartz. Type III inclusions are mono-phase (vapour); they possibly contain CO2 at very low density and surround the inner rims of quartz grains. Type IV two-phase silicate-melt inclusions contain glass±CO2-rich fluid. Some of them are cogenetic with type II inclusions. Finally, type V two-phase (liquid+vapour) aqueous inclusions are both vapour-rich and liquid-rich aqueous inclusions. Microthermometric experiments were performed on both type I and II inclusions. Type I inclusions homogenise to liquid between 20 and 30.5 °C. Type IIa inclusions homogenise to vapour in the 24 to 30 °C range, with a maximum peak of frequency at 29 °C. Type IIb inclusions also homogenise to vapour between 14 and 25 °C. There appears to be no difference in homogenisation temperature distribution between the Strombolicchio and Paleostromboli II samples. The trapping pressures of the fluid inclusions have been obtained by combining the microthermometric data of the Strombolicchio and Paleostromboli II samples with the pressure–temperature–volume (i.e. density) characteristics for a pure CO2 system. The data on the early inclusions (type I) suggest an important magma rest at a pressure of about 290 MPa (i.e. about 11-km depth). Type IIa CO2 inclusions suggest that a second magma rest occurred at a pressure of about 100 MPa (i.e. about 3.5-km depth), whereas type IIb inclusions were trapped later at a shallower depth during the final magma upwelling. No pressure/depth differences seem to occur between the Strombolicchio and Paleostromboli II periods, indicating the same polybaric rests for the calc-alkaline magmas of Stromboli, despite their significantly different ages. This persistence in magma stagnation conditions from 200 to 60 ka suggests a similar plumbing system for the present-day Strombolian activity.