(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Photomicrograph of olivine crystal encompassing a melt inclusion. The dark spot within the melt inclusion is a gas bubble.

Understanding how the chemical composition of magma changes before an eruption occurs is not an easy task. A sample of lava erupted at the surface provides a look at the final composition of magma; however, lava samples do not provide a complete picture of how the magma changed on the way to the eruption site.

During its ascent beneath the summit and storage within the volcano, magma often undergoes compositional changes associated with degassing, cooling, and crystallization prior to eruption. For example, when olivine crystallizes from magma, the remaining magma is depleted in magnesium and said to be more “evolved.” As shallow magmatic pathways change and develop during a prolonged eruption, the chemical composition of magma can also be changed as it courses through the volcano.

Magma mixing occurs when a new batch of magma flows into and mixes with a pre-existing molten magma body. The old, cold rock around the magma conduit can also be incorporated into the magma as it moves through the system. Such changes are not always easy or straightforward to identify from erupted lava alone; however, there are a few tools volcanologists can employ.

Melt inclusions — small pockets of magma (including dissolved gases) preserved inside a growing crystal — provide a means of directly sampling the composition of the magma when the host crystal started to grow. Melt inclusions are time capsules similar to the way insects can be preserved within amber.

Upon eruption, melt inclusions quench into glass and can be analyzed separately from the host crystal and the rest of the rock sample. The original composition of magma (in the melt inclusions), compared with the composition of the olivine crystals in the erupted lava, can provide unique information, like the depth(s) of crystallization and crystallization history.

Kilauea is an excellent setting to study melt inclusions at an active volcano. Lava flows are easily accessed, and HVO already collects fresh lava samples on a regular basis. Moreover Kilauea’s most common mineral, olivine, proves additional information for understanding early magma compositions. Olivine’s chemical composition is dependent on the magma composition from which it crystallizes and provides another means of observing changing magma compositions. By analyzing melt inclusions (magma) and olivine crystals, we should be able to deduce how the magma moving within the volcano changed prior to eruption.

With a magma history through the volcano, HVO geologists can test ideas on how the magmatic system of Halema`uma`u and Pu`u `O`o are connected. One test would be to compare the olivine and melt inclusion compositions from the summit and those from the rift zone. If olivine and melt inclusion compositions from Pu`u `O`o are both more evolved than those from Halema`uma`u, then the compositional change could be due to the transit time from the summit underground to Pu`u `O`o, degassing, and/or heat loss. If the olivine and melt inclusion compositions from Pu`u `O`o are less evolved than those from Halema`uma`u, then Pu`u `O`o draws its magma from depths deeper than the Overlook vent. If the compositions from Pu`u `O`o and Halem`auma`u prove to be similar, our understanding of how fast the magma moves between two vents or how quickly minerals crystallize needs refining.

Recent Kilauea melt inclusion research for this eruption revealed that magma transport from the summit to the East Rift Zone alters the magma to a more evolved composition; however, more work in this area would serve to reinforce our rudimentary understanding and possibly depict how the Overlook vent influences magmatic changes or how the summit-East Rift Zone relationship has changed over the years.