Using Loihi Basaltic Rocks to Understand the Hawaiian Plume

University Of Hawaii, Honolulu

Investigators

Abstract

Hotspot volcanoes are thought to tap the Earth's deep mantle, and to provide fundamental insights into understanding its composition and processes. Hawaii is an excellent venue for studying hotspot lavas because of its distant location from any continent or tectonic plate boundary (2500 km), and the high temperature (>1500 degrees C) and volume output (>100,000 cubic meters/year) of its hotspot. This proposal focuses on Loihi, the youngest and still underwater Hawaiian volcano. Loihi lavas have the potential to provide important constraints on the mantle plume beneath the islands because: 1) The volcano's youth (preshield stage) allows for the characterization of its mantle source before it has been extensively melted. 2) Loihi basalts are one of the key geochemical end members for Hawaiian basalts. Thus, analyses of its lavas (especially from previously unsampled areas) will provide a unique perspective into melting processes and source heterogeneity within the Hawaiian hotspot, the Earth's hottest mantle plume. Loihi lavas are especially noteworthy for their primordial noble gas signatures. 3) Loihi's interior and early history are exposed by major landslides on its eastern flank (>1 km thick stratigraphic section) and in ~300 m deep craters at its summit. 4) An extensive suite (196 samples) of Loihi basalts collected by submersible are available from its summit pit craters, its dissected east flank and the full extent of its two rift zones. Most of these samples have received limited geochemical characterization. These samples will be made available to the Earth science community for other geochemical studies. This project supports the training of a graduate student at the University of Hawaii. The goals of this project are: 1) Determine the lifespan of Hawaiian shield volcanoes using Loihi lavas to document the duration of the early stage, which is the major missing puzzle piece for resolving this issue. Current estimates range widely. 2) Better understand the structure of the Hawaiian plume (e.g., whether it is bilaterally asymmetrical as currently thought). Loihi is at the young end of the LOA trend but paradoxically, it shares geochemical features with KEA trend lavas. 3) Evaluate whether the Hawaiian mantle plume source varies in time and space. Lavas from Loihi deep interior and submarine flanks will provide a new perspective on the Loihi compositional end member of the Hawaiian plume. 4) Determine how crustal contamination works at a submarine Hawaiian volcano. Do rift zone magmas bypass the summit magma reservoir and more faithfully preserve mantle source signatures? Previous studies of Loihi summit lavas showed they were extensively modified by crustal contamination, whereas rare gas compositions of deep south rift lavas suggest they were uncontaminated. Loihi basalts are ripe for geochemical characterization using a new generation of geochemical instruments for determining ages, crustal and mantle processes, and source variations. These analyses will provide fundamental insights into the workings of the Hawaiian hotspot. The newly geochemically characterized Loihi samples, all precisely located and collected by submersible, will be made available to the Earth science community for other complementary future geochemical studies (e.g., Os isotopes, platinum group elements, rare gases).