We have investigated the loss of H 2 O 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 H 2 O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H 2 O contents (loss of *1 wt% H 2 O) 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 m 2 /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 H 2 O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H 2 O concentrations in olivine-hosted MIs may reflect unrecognized H 2 O 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.

The explosivity of volcanic eruptions is governed in part by the rate at which magma ascends and degasses. Because the time scales of eruptive processes can be exceptionally fast relative to standard geochronometers, magma ascent rate remains difficult to quantify.

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