FY 1993

Helium isotope variations in Juan de Fuca Ridge basalts

Lupton, J.E., D.W. Graham, J.R. Delaney, and H.P. Johnson

Geophys. Res. Lett., 20(17), doi: 10.1029/93GL01271, 1851–1854 (1993)

We have measured³He/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 48.0°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°N on the Cobb Segment, and then dropping to values of ~8.0 R_A at the Cobb Offset. Ratios along the Endeavour Segment further north are somewhat higher, averaging ~8.2 R_A. Basalts dredged from Axial Seamount have ³He/⁴He = 7.9-8.4 R_A, indicating that the seamount does not have a geochemical or isotopic signature distinct from other portions of the JdFR. This confirms that while Axial Seamount 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/⁸⁶Sr. 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 Fe_8.0, suggesting that melting dynamics in the underlying mantle may exert some control on the helium isotope ratios along part of our survey area.