HELIUM ISOTOPE VARIATIONS IN JUAN-DE-FUCA RIDGE BASALTS

We have measured He-3/He-4 ratios and He and Ne concentrations on a su ite of 24 basalt glasses from the neovolcanic zone of the Juan de Fuca Ridge (JdFR) from 44.6-degrees-N near the Blanco Transform up to 48.0 -degrees-N on the Endeavour Segment. The helium isotope ratios exhibit a clear geographic variation, with relatively constant values of 7.8 R(A) along the southern JdFR increasing to a maximum of 8.8 R(A) at 46 .9-degrees-N on the Cobb Segment, and then dropping to values of appro ximately 8.0 R(A) at the Cobb Offset. Ratios along the Endeavour Segme nt further north are somewhat higher, averaging approximately 8.2 R(A) . Basalts dredged from Axial Seamount have He-3/He-4 = 7.9-8.4 R(A), i ndicating that the seamount does not have a geochemical or isotopic si gnature distinct from other portions of the JdFR. This confirms that w hile Axial Seamount is the locus of excess magma generation, it is dev oid of any hotspot or ocean island basalt geochemical signature. For t he whole sample set, He-3/He-4 ratios show a negative correlation with Sr-87/Sr-86. We attribute the geographical variations in He-3/He-4 to broad-scale heterogeneity in the mantle source region beneath the JdF R. Beginning at Axial Seamount and further north, He-3/He-4 shows a po sitive correlation with Fe8.0, suggesting that melting dynamics in the underlying mantle may exert some control on the helium isotope ratios along part of our survey area.