Petrogenesis of basalts from the Blanco Trough, northeast Pacific: Inferences for off‐axis melt generation

Abstract

Basaltic lavas recovered in a single dredge haul from the Blanco Trough, in the western portion of the Blanco Fracture Zone, demonstrate that off‐axis volcanism can produce significant compositional variability in lavas that are closely related spatially. Glass compositions range from depleted in incompatible elements (0.07 wt % K2O; 35 ppm Zr) to strongly enriched (0.43 wt % K2O; 214 ppm Zr) with SiO2 contents from 48. 1 to 50.9 wt % over a limited range of MgO (9.22 to 6.82 wt %). These basalts are distinct from mid‐ocean ridge basalts sampled globally at spreading centers in that they are characterized by high FeO* and Na2O at a given MgO content and low normative SiO2, a compositional signature consistent with relatively low extents of partial melting at high pressures. This results from the presence of thick lithosphere that forces melting to stop at a significantly greater depth than is the case at ridge axes. Forward models of low‐pressure liquid lines of descent demonstrate that the major and trace element variations observed in these lavas cannot be explained by crystal fractionation from a single parent magma. After correcting the glass compositions to 8 wt % MgO to eliminate the effects of low‐pressure crystallization, the Fe8.0‐Na8.0 and Si8.0‐Na8.0 variations display characteristics of both Atlantic‐ and Pacific‐type local trends. Variable extents of partial melting and interaction with the mantle as the magmas ascended through the lithosphere can explain these systematics. A subset of the samples shows evidence for crystallization of olivine + plagioclase + high‐Ca clinopyroxene at upper mantle pressures (∼400 MPa). Modeling of incompatible element variations suggests that two distinct mantle sources were involved in producing the Blanco Trough lavas. The first resembles the primitive upper mantle of Hart and Zindler (1986) with a small initial depletion (∼0.1 wt % melt extraction). The second is characterized by a significant depletion in incompatible elements (K2O, TiO2, Zr, Sr).