Magma degassing during the April 2007 collapse of Piton de la Fournaise: The record of semi-volatile trace elements (Li, B, Cu, In, Sn, Cd, Re, Tl, Bi)

This study reports the concentrations of trace elements, including fluid mobile and semi-volatile species (e.g., Li, B, Cu, Re, In, Sn, Cd, Tl, Bi) in lavas erupted before and during the April 2007 collapse of the Piton de la Fournaise summit. Lavas erupted just prior to the collapse (April 5) display anomalous abundances in semi-volatile elements, with both depletion in Li, Cu and Tl (mostly on April 2) and enrichment in Cd, Bi, In and Sn (on April 3 and 4). These transient anomalies are thought to record unusual degassing conditions. Between March 30 and April 2, static decompression caused by magma withdrawal from the shallow magma reservoir might have triggered massive exsolution of a H2O- and S-rich phase in which Li and Cu might have partitioned. Alternatively, the Li-Cu depletion could record the degassing of a magma body that intruded at shallow depth during the same period. The Bi-Cd-In-Sn enrichment observed in the April 3-4 magmas requires conditions that prevent magma outgassing. In the absence of evidence for the pressurisation of the reservoir or the onset of collapse before April 5, it is suggested that the occurrence of less degassed lavas on April 3 and 4 reflects a high rate of magma transfer from the shallow magma reservoir to the eruption site just prior to summit collapse. The kinetic (diffusive) fractionation of elements accounts for the observed anomalies. The short time-scales required to fractionate Li from Cd diffusively (minutes to hours) and Cd from Bi (few hours to two days) support the idea that the magmas underwent rapid pressure variations a few days before the summit collapse.