New Noble Gas and Volatile Measurements in Basalt Glasses from the Mid-Atlantic Ridge Near 14° North

In an effort to constrain the origin and distribution of gas-rich popping rocks, and mantle CO₂ abundances, we used HOV Alvin and AUV Sentry to study the Mid Atlantic ridge axis near 14° N (on R/V Atlantis). Popping rocks were recovered from the east side of the rift valley, roughly 8 km southwest of an oceanic core complex. We report new helium, neon, argon and CO₂ measurements from popping rock glasses, along with samples from the adjacent ridge axis, all collected from in situ lava flows with Alvin. The data provide a new geological context for volatile abundance variability at a slow spreading ridge. The ³He/⁴He varies from 6.9 to 8.2 times atmospheric (Ra), within the range of extant data from Atlantic MORB. The lowest ³He/⁴He values are obtained from lava flows closest to the extinct oceanic core complex at 13.8° N, demonstrating short length-scale isotopic variability. Total ⁴He abundances (crushing plus melting) vary by a factor of 4000 (0.02 to 83 μcc STP/gram), and the fraction of total helium released by crushing varies between 0.025 to 0.99, reflecting the importance of degassing processes and vesicle/melt partitioning. Several of the samples were step-crushed in vacuum to obtain noble gas abundances, abundance ratios, and isotopic compositions. Neon and argon isotopic data define mixing relationships between atmospheric and mantle values that are consistent with variable incorporation of air and/or seawater gases into the glasses, syn- and post-emplacement. The lower concentration glass samples (non-popping) have systematically higher abundance ratios (³He/²²Ne, ⁴He/²²Ne, and ⁴He/⁴⁰Ar*) than the popping rocks. Since solubility in basaltic melts decreases from helium to neon to argon, the abundance ratio systematics suggest that lava flows from the adjacent ridge axis are more extensively degassed during magma storage and transport. Individual lava flows apparently have distinct abundance ratios, which may provide a new diagnostic for single magmatic emplacement events; the preliminary data suggests that popping rocks were emplaced in two stages. Both the popping rocks and adjacent ridge samples have CO₂/³He ratios close to the canonical MORB value of 2 ± 1 x 10^{^9}. Further comparisons between the popping rocks and lavas from the adjacent ridge axis will be presented at the meeting.