The role of supercritical fluids in the potassium enrichment of magmas at Mount Etna volcano (Italy)

Abstract

The enrichment in potassium shown by the basic lavas erupted at Mount Etna volcano after 1971 (K 2O max ~ 2.2 wt.%) has been considered by previous researchers to be too high to be related to simple crystal fractionation and instead linked this high K 2O content to either crustal assimilation or changes in the magma source. Unfortunately all existing models for the post-1971 K 2O enrichment fail to explain the phenomenon satisfactorily leaving the question still open. We present a critical re-examination of published data for major elements (633 whole rock analyses), trace elements (376 whole rock analyses) and isotopic ratios (136 87Sr/ 86Sr analyses), for historical and pre-historical lavas. Potassium enrichment is not limited to the products of the last 35 years. A comparable increase in potassium is noticed in lavas erupted during the pre-historic phase of the recent Mongibello (K 2O max ~ 2.5 wt.%) and in lavas related to the early phase of the ancient Mongibello (K 2O max ~ 3 wt.%). Moreover, data from melt inclusions in olivines from the 2001 and 2002 eruptions, reveal that potassium contents remain constant for melts with entrapment pressure between 490 to 100 MPa and increase significantly in melts entrapped at pressures below ~ 100 MPa. We propose that supercritical fluids coming from deeper magmas and carrying alkali Cl-complexes migrate through basic to intermediate magmas residing in the shallow feeding system. As chlorine exsolves and leaves the system alkalis are released contributing to the observed potassium enrichment of the shallow magma. Fluctuations of the volatiles influx throughout time are likely related to the magma supply rate. Considering that the amount of magma entering a plumbing system is determined by the rate of regional extension, the flux of alkali Cl-complexes entering the melt might be related to an extensional regime acting in the Etnean area.