Origin of prismatine from the Sondalo granulites (Central Alps, Northern Italy)

A granulite from the Sondalo femic complex, Italian Central Alps, contains prismatine, the boron-rich member of the kornerupine group. This is the first report of prismatine in the Alps. The granulite consists of albite-rich plagioclase + cordierite + sillimanite + rutile + hercynite + corundum + quartz and is interpreted as a restite formed after partial melting of amphibolitefacies tourmaline-bearing metasediments at about 900°C and 0.8 GPa. Prismatine grains, which form a coarse aggregate with tourmaline, biotite and albitic plagioclase, have inclusions of sapphirine, hercynite and corundum. Secondary-ion mass spectrometry (SIMS) analyses on prismatine gives B2O3 = 2.30-2.89 wt. %, Li2O = 0.067-0.125 wt. %, BeO = 0.005-0.007 wt. %, F = 0.32-0.49 wt. % and H2O = 0.90-1.02 wt. %. The cell parameters a and c and V of the Sondalo prismatine fit with the B2O3 co-variation reported in literature. We propose a mechanism of prismatine formation involving the breakdown of tourmaline during the anatexis of the amphibolite-facies metasediments and the development of prismatine as a refractory phase. No intervention of a metasomatic boron-rich fluid is required. The possible tourmaline breakdown process is the reaction 7.82 tourmaline + 1.03 biotite + 2.41 sapphirine + 3.22 quartz = 8.00 prismatine + 5.93 melt + 1 B2O3.