Thermodynamic Model for the Effect of Post-entrapment Crystallization on the H₂O–CO₂ Systematics of Vapor-saturated, Silicate Melt Inclusions

摘要

Melt inclusions (MI) represent the best source of information concerning the pre-eruptive volatile contents of magmas. If the trapped melt is enriched in volatile species, following trapping the MI may generate a vapor bubble containing volatiles that have exsolved from the melt. Thermodynamic modeling of vapor-saturated albitic composition (NaAlSi₃O₈) MI shows that the CO₂ content of the melt phase in the MI is sensitive to small amounts of post-entrapment crystallization (PEC), whereas the H₂O content of the melt is less sensitive to PEC. During PEC, CO₂ is transferred from the melt to the vapor phase and the vapor bubble may contain a significant amount, if not most, of the CO₂ in the MI. The contrasting behaviors of H₂O and CO₂ during PEC lead to H₂O–CO₂ trends that are similar to those predicted for open-system degassing during magma ascent and decompression. Thus, similar H₂O–CO₂ trends may be produced if (1) vapor-saturated MI are trapped at various depths along a magmatic ascent path, or (2) MI having the same volatile content are all trapped at the same depth, but undergo different amounts of PEC following trapping. It is not possible to distinguish between these two contrasting interpretations based on MI volatile data alone. However, by examining the volatile trends within the context of other geochemical monitors of crystallization or magma evolution progress, it may be possible to determine whether the volatile trends were generated along a degassing path or if they reflect various amounts of PEC in an originally homogeneous melt inclusion assemblage. The volatile trends resulting from PEC of MI described in this study are directly applicable to silica-rich (granitic) MI trapped in non-ferromagnesian host phases, and are only qualitatively applicable to more mafic melt compositions and/or host phases owing to modifications resulting from Fe exchange with the host and to post-entrapment re-equilibration processes.