Evolution of water and hydrogen isotopes during magma differentiation: Melt inclusion data

Melt inclusions (Mis) hosted in Mg-rich olivines are powerful tools to retrieve pre-eruption volatile
concentrations and their pressure-related evolution during magma decompression and ascent toward
the surface. However the significance of water measurements in these inclusions is widely debated
because of their ability to loose H2O, through proton diffusion, during magma ascent and degassing.
dD values were measured in several series of basaltic melt inclusions, having from 4 wt% to < 1 wt%
of H2O, using the Cameca IMS 1270 ion microprobe (CRPG-Nancy). Water was also measured by
Raman spectroscopy.
In each sample set, hydrogen diffusion out of Mis is required to explain negative trends between the
H2O concentrations and ?D.
We specifically focussed on arc-related basaltic Mis which, after data filtering for secondary process
of proton diffusion, indicate a significant range of dD values. These latter are comprise between -53 ‰
and -25 ‰ and average at -35.9 ± 2.9‰ as reported (-34 ± 13‰) for Mariana arc Mis (Shaw et al.,
2008).
The question that arises is to what extent these dD values are representative of the last stage of melt
equilibration with their surroundings at time of their entrapment and olivine crystallization or of the
magma source. Combining dD data with measurements of Cl, trace elements and d11B, we discuss the
different processes able to reproduce the trends registered by melt inclusions representative of arcbasalts.