Occurrences and Origins of Hawaiian Intermediate- to Silicic-composition Magmas

Magmas and liquids of intermediate- to high-SiO2 compositions -- namely, dioritic, dacitic, and rhyolitic rock types of SiO2 ~54 to 76 wt% -- are represented in various occurrences on the Hawaiian Islands. In view of the Puna dacite, it is meaningful to examine these occurrences and evaluate their significance and origins in the context of basaltic parentages. The manifestations of intermediate to silicic melts amidst Hawaiian basalts can be assessed as having small, medium, or large scale presence. (i) At small scales are rhyolitic melts, ~71-76 wt% SiO2 that formed interstitially in groundmasses and as vesicle linings. These represent last liquids from crystallization of basaltic magmas as lavas, lava lakes, and gabbros. MgO in these 'rhyolites' is generally depleted to <0.5 wt%, demonstrating the extreme differentiation that basalt can achieve. However, these occurrences, even though common, are at microscopic scales and have not meaningfully contributed to evolving oceanic crust. (ii) Medium-scale evolved melts are diorite (~54-59% SiO&2) in lava lakes and as quartz-bearing cumulate tonalite (67% SiO2; Mauna Kea xenolith). The diorites have segregation vein origins, and one example models by mass balance and by MELTS to represent 67% crystallization of largely ol, cpx, and pl from parental basalt (MgO 8.3%) at slightly below FMQ buffer over 1210-1060°C at 250 bars P. The tonalite may represent SiO2-rich interstitial liquids in a shield solidification zone that accumulated as pools or sills in fractures resulting from roof collapse. These medium-scale differentiates, while seemingly local, may be indications of how ocean-island bimodality (basalt-rhyolite) can be achieved by segregation and accumulative processes. (iii) Large-scale evolved magma is represented by Waianae Range rhyodacite lava, ~66.5 wt% SiO2, 0.01 km3, 0.1 km2, and avg. 100 m thick. This manifestation of intermediate magma is most appropriate to compare with Puna dacite. Mass balancing the rhyodacite with 'parental' Waianae tholeiitic basalt suggests that it represents liquid after ~67% crystallization of ol (4%), cpx (20%), pl (32%), FeTi ox (11%). MELTS modeling only generally confirms this origin, suggesting rather high P (~7 kb) equilibrium crystallization, but perhaps eclogite melting had a role.