Erik H. Hauri

Water in the Hawaiian Mantle Plume: Where Did It Come From and Where Did It Go?

Water plays a highly significant role in the generation and differentiation of magmas in a variety of planetary environments. Water dissolved in minerals in the deep Earth can strongly lower both the melting temperature and the viscosity of rock. At high abundances, water can also determine, in large part, many of the physical properties of magmas that influence eruption dynamics. The presence of liquid water at the Earth's surface is also thought to be a primary factor that enables the persistence of active plate tectonics.

The primordial conditions under which the Earth formed were probably very hot, perhaps hot enough to have melted much of the planet. As a result, the Earth's interior was depleted in volatile elements during accretion and has lost volatile elements throughout its history by the eruption and degassing of magmas at locations of active volcanism. On other planets, the water expelled from volcanoes is either lost to space by a variety of physical and chemical mechanisms -- such as on Mercury and Venus -- or may be partly frozen as ice and trapped in the crust, as on Mars. However, temperature conditions on the Earth's surface favor the stability of liquid water. Geophysicists have long recognized the lack of active plate tectonics on the other terrestrial planets. This observation has led to the idea that water from the Earth's surface is carried into the interior by plate subduction and serves as the lubricant that facilitates plate tectonics. Water-rich explosive volcanism occurring at subduction zones confirms that water is carried down with the descending plate. But two major questions about the water budget at subduction zones remain unanswered: How much of the water that goes down into subduction zones comes back up at subduction-zone volcanoes, and how much water is left in the subducting plate that is carried into the deep Earth? The answers to both of these questions will constrain the amount of water that is left in the upper parts of the Earth's mantle to lubricate plate tectonics.

To address the second question about water trapped in the subducting plate, I am measuring the abundance and isotopic composition of water in magmas from the Hawaiian Islands. Our previous geochemical work on Hawaii has shown that certain Hawaiian volcanoes (Koolau, Mauna Loa, Mauna Kea) contain a component made up of recycled oceanic crust and sediment that had been carried into the deep mantle during an ancient subduction episode. Oxygen isotope work on these same samples, carried out with collaborators at the California Institute of Technology and the University of Wisconsin, showed convincingly that these components had interacted with water near the Earth's surface. We therefore selected one of these volcanoes (Koolau) to initiate our study.

Magmas that erupt on the Earth's surface lose their water through degassing. Only those that erupted at ocean depths greater than 500 meters cool quickly enough to retain their original volatile contents. Another way of examining volatiles in magmas is to study tiny inclusions of melt, which are trapped in solid crystals that have grown from the magma before eruption. Because the crystal surrounding these inclusions acts as an inert capsule, melt inclusions can preserve a record of the behavior of volatiles deep within the volcanic system even when they are contained in lava samples that have erupted on the Earth's surface and lost their own volatiles. These inclusions have been measured for their volatile abundances H₂O,( CO₂, F, S, Cl) and hydrogen isotope ratios using the DTM ion microprobe.

The melt inclusion results for several Hawaiian volcanoes (Loihi, Kilauea, Mauna Loa) show moderate water contents and hydrogen isotopes, which generally agree with the data on submarine eruptions on these same volcanoes. The H₂O and Cl data on melt inclusions reveal that many magmas are contaminated by seawater-derived components prior to eruption, a process which changes their original Cl, H₂O, and hydrogen isotope compositions. The Koolau melt inclusions, however, are characterized by the lowest abundances of H₂O, S, and Cl ever observed for any undegassed Hawaiian magma, and these low abundances are coupled with the lowest hydrogen isotope ratios ever observed at Hawaii (Fig. 11). The combined volatile data sets for melt inclusions and submarine eruptions show a correlation of hydrogen isotope ratio with other isotopes measured on the same samples, including oxygen isotopes.

The lavas from the Koolau volcano contain the largest amount of the water-altered recycled oceanic crust component, yet they appear to have the lowest abundances of volatiles. This observation is best explained if the recycled plate was extensively dehydrated during the ancient subduction episode that introduced this component into the deep mantle. The observed hydrogen isotope ratios for Koolau are consistent with hydrogen isotope changes expected during dehydration of a subducting plate containing hydrous minerals. These observations lead us to conclude that subduction dehydration is very efficient, with the result that nearly all of the water that goes down into subduction zones is released from the descending plate and transported into the zone of magma generation beneath subduction-zone volcanoes. The amount of water left in the mantle after subduction-zone volcanism is probably the lubricating force for plate tectonics. How much is left? Answering this question will require similar ion-probe studies of melt inclusions from subduction-zone volcanoes.

Fig. 11. The upper crust and lower crust of the oceanic plate interact with seawater at different temperatures, resulting in changes in their average oxygen and hydrogen isotope ratios. Addition of seawater-derived sediments (down arrow at top) adds material with high oxygen isotope ratios and high water contents. During subduction, some of the water is transported into the mantle source of subduction-zone volcanoes by dehydration, while the water remaining in the subducted plate is recycled (curved arrow) and returns to the surface in mantle plumes such as Hawaii. This cycle results in correlations between oxygen and hydrogen isotopes in Hawaiian magmas (inset).