Element variations in rhyolitic magma resulting from gas transport

Tuffisite veins are glass-filled fractures formed when magma fragments during degassing within the conduit. These veins form transient channels through which exsolved gases can escape from magma. The purpose of this study is to determine the extent to which chemical heterogeneity within the melt results from gas transport, and assess how this can be used to study magma degassing. Two tuffisite veins from contrasting rhyolitic eruptions at Torfajokull (Iceland) and Chaiten (Chile) were studied in detail. The tuffisite vein from Torfajokull is from a shallow dissected conduit (similar to 70 ka) that fed a degassed lava flow, while the sample from Chaiten was a bomb ejected during the waning phases of Plinian activity in May 2008. The results of detailed in situ chemical analyses (synchrotron XRF, FTIR, LA-ICP-MS) show that in both veins larger vesiculated fragments are enriched in volatile elements (Torfajokull: H, Li, Cl; Chaiten: Li, Cl, Cu, Zn, As, Sn, Sb) compared to the host, while the surrounding smaller particles are depleted in the Torfajokull vein (Li, Cl, Zn, Br, Rb, Pb), but enriched in the Chaiten vein (K, Cu, Zn, As, Mo, Sb, Pb). The lifespans of both veins and the fluxes of gas and particles through them can be estimated using diffusion profiles and enrichment factors. The Torfajokull vein had a longer lifespan (similar to a day) and low particle velocities (similar to cm/s), while the Chaiten vein was shorter lived (<1 h) with a high gas velocity (similar to m/s). These differences are consistent with the contrasting eruption mechanisms (effusive vs. explosive). The amount of magma that degassed through the Chaiten vein is more than ten times the volume of the vein itself, requiring the vein to tap into pre-exsolved gas pockets. This study highlights that tuffisite veins are highly efficient gas pathways and thereby impart chemical diversity in volatile elements on the melt. (c) 2013 Elsevier Ltd. All rights reserved.