Stromboli volcano (Aeolian Archipelago, Italy): An open window on the deep‐feeding system of a steady state basaltic volcano

Abstract

[1] Paroxysms at Stromboli are the most violent manifestations of the persistent activity and are related to the emission of small volumes (10 3 -10 5 m 3 ) of nearly aphyric HKbasaltic pumices. They offer the exceptional opportunity to detail the mixingcrystallization-degassing processes that occur in a steady state basaltic arc volcano. We present mineralogy, major, volatile, and trace element geochemistry of olivine-hosted melt inclusions of these pumices. In all the paroxysms, melt inclusions hosted in olivines Fo 88 -91 have recorded the parental melts rich in CaO (up to 14.5 wt %) but low in FeO (6-7 wt %). They demonstrate recurrent variations in the K 2 O content (1.6-1.3 wt %) and S/Cl ratios (1.2-0.8) of the melts that entered the deep system. Dynamic magma mixing between melts slightly distinct by their degree of evolution, rapid crystallization, and entrapment of gas-oversaturated melts during decompression are indicated by (1) the high density of irregular, clear melt inclusions, and embayments in homogeneous olivines (Fo 87±0.5 -Fo 83±0.5 ), (2) the variable ratio between melt and gas bubble, and (3) the variability of melt inclusion compositions in both major (CaO/Al 2 O 3 = 1-0.59) and volatile (3.4-1.8 wt % H 2 O, 1582-1017 ppm CO 2 ) elements. FeO-rich melt inclusions in patchy, reversely zoned olivines also demonstrate interactions between ascending melt blobs and inherited olivine crystals. We propose a model involving a vertically extended dike-like system, where magmas progress and differentiate. On the basis of olivine growth rate calculations the volatile-rich magma blobs may ascend within few hours to few tenths of hours. Finally, we propose that sulfur degassing is possibly initiated during the early stage of magma differentiation.

Radon mapping, automatic measurements and extremely high 222Rn emissions during the 2002–2007 eruptive scenarios at Stromboli volcano

“…However, gases sampled by Maurizio Ripepe along the fractures surrounding the active craters are CO 2 richer, with XCO 2 reaching 0.67 and a sampling temperature of 410°C (Martini et al, 1996). This CO 2 fraction is very similar to the ones measured in melt inclusions (e.g., Bertagnini et al, 2003;Metrich et al, 2010). Thus this type of fluids will be called high temperature, HT fluids).…”

A model of degassing for Stromboli volcano

“…The composition of the deep-rising CO 2 -rich gas bubbles will be dependent on the depth on their separation from (and thus last equilibration with) the silicate melt. In the most extreme conditions, Métrich et al (2001Métrich et al ( , 2005Métrich et al ( , 2009 and Bertagnini et al (2003). ( Comparison of natural and modelled compositions confirms that the deep (P N 100 MPa) LP magma contains a high (2-5 wt.%; model curves 2-3) fraction of gas bubbles at reservoir conditions.…”

“…Redox conditions along the decompression path were fixed by the Fe 2+ /Fe 3+ buffer, for which we adopted the value of 3.4. This choice is based on XANES determinations on a hydrous (H 2 O = 2.9 wt.%) LP magma melt inclusion (Bonnin-Mosbah et al, 2001), but is also consistent with the olivine-liquid iron and magnesium partition observed in a large set of Stromboli MIs (Bertagnini et al, 2003). The resulting logfO 2 conditions range from 0.07 to 0.82 NNO (NNO is the Nickel-Nickel Oxide buffer).…”

“…Paroxysms are impulsive events consisting of several explosions from different craters, associated with the ejection of large ballistic blocks and the emission of vertical jets of gas and pyroclasts evolving in short-lived convective columns. A peculiar feature of paroxysms is the co-emission of a nearly aphyric basaltic pumices along with the "usual" crystal-rich scoria (Bertagnini et al, 2003).…”

Lava effusion — A slow fuse for paroxysms at Stromboli volcano?

“…Thus, it is striking that both 2003 and 2007 paroxysms ensued on discharge of comparable DRE volumes of magma (~ 4.0 × 10 6 m 3 ), implying that paroxysmal events can occur after the start of an apparently gentle effusive eruption. That eruption of such a magma volume could be enough to destabilize the LP magma likely reflects the volume of HP magma stored above the LP source region ( [Bertagnini et al, 2003], [Francalanci et al, 2005] and [Métrich et al, 2005]). Applying the model for Stromboli's conduit of (Bonaccorso and Davis, 1999) and (Genco and Ripepe, 2010) estimated a conduit radius of 5 m by modelling of the tilt recorded during the volcano ordinary Strombolian activity.…”