Abundant Melt Inclusions of Andesite-Dacite Composition (59-68 wt% SiO2) in Basaltic Andesite and Andesite Lavas from the Mexican Arc

The vast majority of published major-element analyses of melt inclusions (MI) fall within three categories: (1) olivine-hosted MI in whole-rock basalts, (2) quartz-hosted MI in whole-rock rhyolites, and (3) plagioclase-hosted MI in phenocryst-rich (25-45%) whole-rock andesites and dacites (typically from large central volcanoes at arcs). Histograms of these published MI analyses as a function of bulk-rock composition (wt% SiO2) show that the olivine-hosted MIs in whole-rock basalts tend to be basaltic in composition, whereas both the quartz-hosted MIs in rhyolites and the plagioclase-hosted MIs in the phenocryst-rich andesites and dacites tend to be rhyolitic (>70 wt% SiO2) in composition. This has led to the observation that there is a paucity of published MI analyses with andesitic-dacitic (59-68 wt% SiO2) composition (e.g., Reubi and Blundy, 2009). This paucity may reflect: (1) a relative rarity of intermediate MI compositions in arc lavas, or (2) a sampling bias. To test sampling bias, the following three lava types were targeted for a systematic study of their phenocryst-hosted MIs: (1) phenocryst-poor to phenocryst-rich basaltic andesites; (2) phenocryst-poor to phenocryst-rich andesites, and (3) phenocryst-poor dacites. These three types of lavas permit a study of how MI composition varies as a function of both whole-rock composition and extent of phenocryst+microphenocryst crystallization. MIs were analyzed in 15 lavas from the Mexican volcanic arc. The most abundant MI mineral host was olivine and plagioclase, respectively, in lavas with 54-59 wt% and 60-69 wt% SiO2 (whole rock). The results show a systematic variation in the composition of MIs as a function of the whole-rock composition. Most MIs in the basaltic andesites range from 59-68 wt% SiO2, showing that intermediate MI compositions are common in this lava type; a few MIs extend down to 55 wt% SiO2. In all samples, there is a wide and continuous range of SiO2 concentration (5-15 wt%) recorded by the MIs. Lavas with MIs that record the largest range in wt% SiO2 also have MIs that extend down to the composition of the whole rock; typically they are phenocryst-poor (<10 vol%). Thus, andesite-dacite MIs are also found in phenocryst-poor (<5 vol%) andesite and dacite lavas. Not one example of a MI with a lower wt% SiO2 concentration than the whole rock was found. A comparison of MI compositions with phase-equilibrium experiments (e.g., Moore and Carmichael, 1998) shows that the continuous range in MI compositions in the lavas from this study can be explained by progressive silica enrichment of interstitial melt as a function of degree of crystallization. Complexities in MI compositions arise from: (1) rapid growth of the host phase, which strips the MI of chemical components in the host mineral, thus producing chemical gradients between the MI and the matrix liquid, (2) different diffusion rates for different elements when the MI retains communication with the matrix liquid for variable time intervals prior to complete entrapment, and/or (3) post-entrapment crystallization of the host phase.