1. Isotope Laboratory, Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, California 92093-0220, USA
2. School of Ocean & Earth Science & Technology, University of Hawaii, Honolulu, Hawaii 96822, USA
3. Earth & Environmental Science Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

Correspondence to: D. R. Hilton¹ Correspondence and requests for materials should be addressed to D.R.H.
(e-mail: Email: drhilton@ucsd.edu).

Loihi seamount, an active submarine volcano situated about 30 km south of the island of Hawaii, is the youngest manifestation of the hotspot responsible for the Emperor–Hawaiian seamount chain and Hawaiian islands. This seamount has been the focus of numerous studies characterizing the geophysical, geochemical and biological features of an active intraplate volcano^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}. In July–August 1996, Loihi seamount experienced the most intense period of seismic activity yet recorded for any Hawaiian volcano¹. Within two months of the 'seismic crisis', summit and flank hydrothermal vent fluids were collected using a manned submersible. Here we report data from these samples that indicate large and systematic changes in the CO₂/³He ratios of the vent fluids compared to pre-seismic-crisis values²,³. These changes are consistent with an abrupt transition from alkalic to tholeiitic basaltic magma having supplied volatiles to the vents. This rapid change in magma chemistry has been discernible only through CO₂/³He monitoring, and suggests that the anticipated evolution of the Hawaiian plume to a phase of shield-building tholeiitic magmatism is highly episodic at Loihi and not yet complete.