Extreme magma differentiation in a Hawaiian magma chamber: An analysis of gabbro and syenite xenoliths from Hualalai Volcano.

Patrick J Shamberger

Julia E Hammer

Large volumes of highly differentiated trachyte magma erupted from Hualalai Volcano 105±10 k.y.a. as blocks in a maar deposit, lavas at the NW tip of the main rift zone, a large pumice and obsidian cone, and a flow >275m thick on the north flank. The latter flow is recognized as the largest single eruptive event on the island of Hawaii (Moore, et al., 1987). Within the generalized “life cycle” of Hawaiian volcanos, highly evolved magmas are usually associated with the conclusion of the post-shield alkalic stage (e.g., West Maui volcano). The episode at Hualalai differed from this pattern in that the evolved magmas appeared at the beginning of the post-shield alkalic stage and were erupted over a relatively narrow time interval. These distinctions have significant implications for our understanding of Hawaiian volcanos’ magmatic plumbing systems, including the depths, magma replenishment/ extraction frequency, and longevity of the reservoirs that feed eruptions.

Gabbro and syenite nodules erupted <1000 y.a. from Hualalai’s summit vents may represent cumulates, residual liquids, and/or crystallized magmas associated with trachyte differentiation, and thus provide insight into the processes leading to extreme fractionation. Because they contain a large number of phases and preserve reaction relationships in the form of mineral textures, the crystalline nodules may more information about magma differentiation conditions than can be extracted from the relatively homogeneous and micro-crystalline trachyte itself.

The nodules were transported to the surface in alkali basalt at numerous vents and distributed near the summit of the volcano over a region several km in diameter. They span a broad compositional and textural spectrum. Modes range from 58- 93 v.% felsic minerals, dominantly plagioclase, with augite and Ca-poor pyroxene comprising most of the balance. Several nodules contain the hydrous minerals biotite and calcic amphibole. Accessory phases include ilmenite, magnetite, apatite, zircon, ± olivine or quartz. In many instances, the major minerals are clearly in reaction relationship with their surroundings. E.g., blebs of alkali feldspar dispersed throughout large plagioclase phenocrysts suggest the existence of precursor ternary feldspar; crystallographically-controlled oxide stringers along pyroxene planes suggest oxidation. We are considering deposit characteristics, mineral assemblages, and phase compositions of the basalt-hosted xenoliths to infer the depth and H2O content of magma fractionation.