Helium isotope variations in Juan de Fuca Ridge basalts

Abstract. We have measured 3He/4He ratios and He and Ne concentrations on a suite of 24 basalt glasses from the neovolcanic zone of the Juan de Fuca Ridge (JdFR) from 44.6'N near the Blanco Transform up to 48.O'N on the Endeavour Segment. The helium isotope ratios exhibit a clear geographic variation, with relatively constant values of 7.8 RA along the southern JdFR increasing to a maximum of 8.8 RA at 46.9N on the Cobb Segment, and then dropping to values of 8.0 RA at the Cobb Offset. Ratios along the Endeavour Segment further north are somewhat higher, averaging 8.2 RA. Basalts dredged from Axial Seamount have 3He/4He = 7.9.8.4 RA. indicating that the scamount does not have a geochemical or isotopic signature distinct from other portions of the JdFR. This confirms that while Axial Sea-mount is the locus of excess magma generation. it is devoid of any hotspot or ocean island basalt geochemical signature. For the whole sample set, 3He/4He ratios show a negative correlation with 87Sr/Sr. We attribute the geographical variations in 3He/4He to broad-scale heterogeneity in the mantle source region beneath the JdFR. Beginning at Axial Seamount and further north. 3He/4He shows a positive correlation with Fe80, suggesting that melting dynamics in the underlying mantle may exert some control on the helium isotope ratios along part of our survey area