3He/4He, δ13C and CO2/3He Systematics of Deep Rift Zone Lavas from Lō'ihi Seamount, Hawaiian Islands

Geochemical investigations at Lō'ihi Seamount, Hawai'i's youngest volcano, have been central to understanding the nature of primitive mantle components, the pre-shield stage of ocean island volcanism, and the composition and structure of the Hawaiian mantle plume. Submarine basalt glasses at Lō'ihi are known to have ³He/⁴He ratios > 30 R_A, the highest values observed in the Hawaiian Islands. Although these ³He/⁴He ratios are much higher than MORB values (7-10 R_A), there is also a large range of variability in Lō'ihi basalts (more than 10 R_A) due to either mantle source heterogeneity, magmatic processes, or both. We have begun a study of a suite of 50 Lō'ihi basalts ranging in composition from alkali basalt to tholeiite, collected mostly from the south rift zone at water depths of 1300-5000 m. The high pressures of quenching make the deep rift zone lavas among the most important ocean island hotspot lavas for characterizing mantle volatiles such as CO₂ and He. Our analytical approach includes in vacuo crushing to extract He and CO₂ trapped in vesicles, in vacuo melting to extract dissolved He, and FTIR to measure dissolved CO₂ and H₂O. Current results (16 samples) reveal that ³He/⁴He ranges from 23.6 to 30.7 R_A, vesicle CO₂ from 1 to 300 ppm, dissolved CO₂ from 50 to 260 ppm, and CO₂/³He ratios from 6.4x10⁸ to 2.7x10⁹. The vesicle CO₂/³He ratios overlap those of vent fluids from both the summit and flank of Lō'ihi. The most gas-rich basalts also have vesicle δ¹³C_PDB = -2.0 to -2.7‰, identical to CO₂ in warm geothermal fluids, supporting the idea that the hydrothermal CO₂ is derived by direct degassing of magmatic volatiles rather than by gas stripping from the crust (Sedwick et al., 1994). Assuming equilibrium C isotope fractionation between melt and vapor, δ¹³C = -3.6 to -4.2‰ in the deep Lō'ihi magmas, identical to δ¹³C of Mid-Atlantic Ridge popping rocks (Pineau & Javoy, 1994). In light of recent studies showing high C contents for Hawaiian magmas (Anderson and Poland, 2017; Tucker et al., 2019), this suggests that both primitive and recycled carbon components coexist in the Hawaiian mantle plume source.