LARGE VARIATIONS IN VENT FLUID CO2 HE-3 RATIOS SIGNAL RAPID CHANGES IN MAGMA CHEMISTRY AT LOIHI SEAMOUNT, HAWAII/

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 isla nds. This seamount has been the focus of numerous studies characterizi ng the geophysical, geochemical and biological features of an active i ntraplate volcano(1-14). In July-August 1996, Loihi seamount experienc ed the most intense period of seismic activity yet recorded for any Ha waiian volcano(1). Within two months of the 'seismic crisis: summit an d flank hydrothermal vent fluids were collected using a manned submers ible. Here we report data from these samples that indicate large and s ystematic changes in the CO2/He-3 ratios of the vent fluids compared t o pre-seismic-crisis values(2,3). These changes are consistent with an abrupt transition from alkalic to tholeiitic basaltic magma having su pplied volatiles to the vents. This rapid change in magma Chemistry ha s been discernible only through CO2/He-3 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.