HECK AND HECKLE SEAMOUNTS, NORTHEAST PACIFIC-OCEAN - HIGH EXTRUSION RATES OF PRIMITIVE AND HIGHLY DEPLETED MIDOCEAN RIDGE BASALT ON OFF-RIDGE SEAMOUNTS

We analyzed, petrographically and chemically, basalt from eight dredge hauls from the Heck and Heckle seamounts, northeast Pacific Ocean. Major elements were determined for mineral, glass, and whole rock samples, and trace and rare earth elements were determined for glass and whole rock samples. The dredge hauls included hyaloclastites and fragments from sheet flows and pillows. The clinkery fragments are interpreted to be deformed sheet flow tops, characteristic of high effusion rates. The hyaloclastites recovered are reworked deposits, as indicated by the wide compositional range of the glass shards, abundance of clay and calcite matrix, and bedding. Most rocks are aphyric, but the analyzed plagioclase and olivine phenocrysts and microcrysts are equilibrium compositions and show minor compositional zonation (up to 7.5% An, < 1% Fo). Calculated quenching temperatures range from 1228-degrees to 1216-degrees-C, decrease with decreasing Mg #, and are consistent with phase equilibria. The seamount lavas are primitive (Mg # 68.7-59.5) and restricted in compositional range ([La/Ce]n = 0.65-0.77). The basalts are equivalent to highly depleted mid-ocean ridge basalt ([La/Sm]n = 0.29-0.41) and are at the depleted end of the range in composition of basalts from East Pacific seamounts. They are more depleted than basalts from the adjacent West Valley spreading axis. The lavas can be produced by partial melting of a depleted source. Mineralogical and geochemical evidence is consistent with rapid ascent of the Heck and Heckle lavas through oceanic crust with little residence time in magma chambers. Lavas of the Heck and Heckle Seamount chains have a limited range of incompatible element ratios, whereas the adjacent West Valley Segment of the Juan de Fuca Ridge is highly heterogeneous. In contrast, lavas from the East Pacific near-ridge seamounts exhibit a wider range of incompatible element ratios than do the adjacent East Pacific Rise basalts. On the West Valley Segment, magma supply is less robust associated with lower spreading rates compared to the East Pacific Rise at 10-degrees-N. In contrast, at fast spreading centers robust melting produces a mixed mantle signature in axial lavas, while suppressed melting at the seamounts reveals the heterogeneities. We suggest that at some spreading ridges, more fertile portions of the mantle are preferentially melted such that the outwelled portions of the mantle tapped by the seamounts are more depleted.