Volatile loss from melt inclusions in pyroclasts of differing sizes

We have investigated the loss of H₂O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H₂O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H₂O contents (loss of ~1 wt% H₂O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10^?9 to 10^?10 m²/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H₂O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H₂O concentrations in olivine-hosted MIs may reflect unrecognized H₂O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent.