Helium isotope variations in peridotite, gabbro and basalt from the Kane Oceanic Core Complex

Graduation date: 2013The Kane Oceanic Core Complex (OCC) is a valuable window into crustal architecture and chemical composition of the lithosphere beneath a slow-spreading ocean ridge. A suite of > 30 samples (comprised of whole rocks, mineral separates and basalt glasses) has been analyzed for ³He/⁴He isotope ratios and He concentrations. Gas extraction experiments included crushing in vacuum, step heating and fusion in a high-vacuum furnace. We found ³He/⁴He in the two freshest peridotites (harzburgite and olivine websterite) to be identical to ratios measured in basalt glasses collected from the Kane fracture zone-ridge axis intersection (8.4–8.7 R[subscript A]). Notably, the freshest and least deformed peridotite (a porphyroclastic harzburgite) has the highest helium content of any of the OCC ultramafic rocks (170 ncc STP/g), while the majority of its helium (70%) is released only upon melting. Lower helium contents (1-45 ncc STP/g) and more variable\ud ³He/⁴He (2.0-6.3 R[subscript A]) are found in other more altered/deformed peridotite samples (whole rocks, clinopyroxene and orthopyroxene separates). For example, three mylonitized peridotites have lower helium contents compared to the less deformed peridotites. Troctolite intrusions from the Adam and Eve Domes appear to record very late-stage melt impregnation fed through dunite conduits near the MOHO. ³He/⁴He in these troctolites overlaps with values in the fresh peridotites and axial basalts, but extends to slightly higher values (8.6-9.0 RA). Collectively, fresh Kane peridotites and troctolites having He concentrations above 10 ncc STP/g show ³He/⁴He ratios of 8.4-9.0 RA that are higher than the median value (8.0-8.2 R[subscript A]) for mid-ocean ridge basalts. This suggests that domains of depleted upper mantle in the Kane region tend to have ³He/⁴He ratios of 9 RA or higher, similar to what is observed in the most trace-element depleted MORBs globally. Kane gabbroic rocks are more variable in 3He/⁴He. A subset of gabbros show systematically lower ³He/4He ratios (0.9-7.2 R[subscript A]), with He concentrations of 1-24 ncc STP/g, reflecting the increased importance of atmospheric and radiogenic components in several cases. Nonetheless, two whole rock gabbroic samples plus an amphibole separate have ³He/⁴He ratios of 7.6-8.0 R[subscript A] and He contents between 13 and 57 ncc STP/g. Variation in the sampling of a lithologically heterogeneous mantle source by the partial melting process may account for the observed ³He/⁴He variability of 7.6-9.0 R[subscript A] in peridotite, gabbro and basalt in the Kane area