Occurrence and origin of H2-bearing melt inclusions in the two-mica granite of the Jiajika lithium deposit

• Unique H 2 -bearing melt inclusions were found in quartz of the Jiajika two-mica granite. • H 2 was sourced from the re-speciation of C–H–O fluids in the late-magmatic stage. • H 2 fluids may promote the melt–melt immiscibility of Jiajika granitic melts. In quartz of the Jiajika two-mica granite, H 2 -bearing melt inclusions were recognized. The origin of these unique inclusions, however, remains enigmatic. Here, we conduct petrographic, Raman spectroscopic and mineralogical studies on quartz-hosted melt inclusions and the two-mica granite rocks. The results obtained were further discussed in context of genetic models proposed for the origin of H 2 -bearing inclusions (i.e., mantle source, radiolysis of water, late-magmatic re-speciation of C–H–O fluids, post-magmatic water–rock interactions). The mantle and radiogenic source origins were precluded because Jiajika granite has a crustal genesis without the addition of mantle materials, and no uranium-rich minerals is found within or adjacent to H 2 -bearing melt inclusions. The post-magmatic water–rock interaction model was also eliminated, since the two-mica granite rocks are fresh, unmetamorphosed and rock-forming minerals are petrographically unaltered. The H 2 -bearing melt inclusions in Jiajika granite were likely originated from the re-speciation of C–H–O fluids under reduced, graphite-saturated conditions in the late-magmatic stage. Evidence supporting this conclusion includes: (1) vapor phase of melt inclusions in quartz of Jiajika granite rocks consist either of H 2 –CH 4 –H 2 O or CO 2 –CH 4 –H 2 O, which are typical products of the re-speciation of C–H–O fluids; and (2) the presence of graphite in H 2 -bearing melt inclusions and biotite compositions record local reduced conditions (near fayalite–magnetite–quartz (FMQ) buffer) in the late-magmatic stage, under which C–H–O fluids would exist as H 2 –CH 4 –H 2 O. Dissolution of H 2 in granitic melts would increase H 2 O solubility, reduce polymerization degree and enhance mass exchange in the felsic melts, which may facilitate the formation of mineralized pegmatite melts through melt–melt immiscibility.