by Matthew Jackson, Ricardo Ramalho, Donato Giovannelli, Barbara Kleine-Marshall

Volcanic hotspots, like Hawai'i and Samoa, are sourced by mantle plumes that upwell from the deep mantle. Excellent exposure and sampling of Hawaiian volcanoes provide data for what has been considered the prototypical model for ocean island evolution worldwide. Available data, however, suggest that Samoan volcanoes grow in a manner very different from Hawai'i. To better understand how a Samoan volcano grows and evolves, this proposal seeks funding for a workshop to discuss drilling a 2500 m borehole on the Samoan island of Savai'i. In addition to providing a critical counterpoint to the Hawaiian model, the drill core will help address long-standing problems associated with the evolution of a Samoan volcano, including: 1) evaluating the poorly-understood genesis of Samoan lavas with extreme continental signatures recovered by dredging the deep submarine flanks of Savai'i; 2) investigating the near-absence of tholeiites in the Samoan shield stage in light of their ubiquity at Hawaii; 3) testing a possible plate flexure origin for the unusual volumes of rejuvenated lava on Savai'i that takes into account Savai'i's juxtaposition with the nearby Tonga trench. This will shed light into the globally relevant problems of how different tectonic mechanisms interact with mantle plumes to modulate hotspot volcanism. A deep drill core in Samoa will allow us to probe the deep biosphere at an ocean island in a manner not undertaken at HSDP, where drilling was carried out without minimizing or tracking possible sources of contamination. Additionally, the Savai'i drill core will provide access to geothermal fluids in the deep volcanic stratigraphy that can yield key information about geochemical cycling in the crust of ocean islands, with possible outcomes that include deep carbon storage. The proposal will fund a 3-day workshop on Savai'i, Samoa. The workshop includes experts in the fields addressed by the science questions, and will bring together specialists in volcanology, petrology and hard rock geochemistry, microbiology of the deep biosphere, hydrology, geothermal exploration and hydrothermal geochemistry. We have budgeted for 35 attendees, including representatives of the Samoan government. The PIs and attendees represent 12 countries, and will include 15 Early-Career researchers. The workshop will focus on developing the science goals and questions and evaluate whether additional scientific expertise is needed by augmenting the science team. In addition to developing the science plan, an important goal of the workshop will be to develop drilling logistics, evaluate operational risks, and define responsibilities of the participants.

• Drilling is required to address several hypotheses surrounding the long-term evolution of the Samoan island of Savai'i. Existing data suggest that the shield stage of Savai'i persisted for at least 3 million years, compared to 1.5 million years of shield growth at Hawaiian volcanoes. In spite of this extraordinary difference in shield stage duration, 70% of the Samoan shield stage remains unsampled. Sampling this uncharacterized portion of the Samoan shield stage requires a 2500 m drill core. Data from the drill core will address why Samoa appears to lack abundant tholeiites present at Hawai'i, which has implications for the compositional and thermal evolution of the Samoan plume. The drill core will allow us to access lavas with extreme continental crust signatures in the deep volcanic stratigraphy, permitting evaluate their origin. Additionally, unlike Hawaii, which is located far from plate boundaries, Samoa is located near the Tonga trench, which is suggested to be the cause of the exceptional rejuvenated lava cover that has completely resurfaced the westernmost Samoan island, Savai'i. Drilling through the rejuvenated veneer at Savai'i will allow us to test whether the onset of rejuvenated volcanism relates to tectonic forces resulting from flexure of the subducting Pacific plate. Finally, a deep core will provide access to hydrothermal fluids to evaluate elemental cycling. The core may provide an opportunity to test deep carbon storage strategies, and may yield some of the deepest biology at an ocean island, providing a key biological comparison with the deep Hawaii Scientific Drilling Project borehole.

Volcanic Hotspot, Mantle Plume, Plate Flexure, Hydrothermal, Deep Biosphere

Latitude: -13.6252, Longitude: -172.45381