Pb, Sr, and Nd isotopes in basalts and sulfides from the Juan de Fuca Ridge

Abstract

Pb, Sr, and Nd isotopes of seven basalt glasses collected by the submersible Alvin from the southern Juan de Fuca Ridge (SJFR) are almost identical (206Pb/204Pb ∼ 18.45, 207Pb/204Pb ∼ 15.47, 208pb/204pb ∼ 37.81 87Sr/86Sr ∼ 0.70249, 143Nd/144Nd ∼ 0.51315). Whereas all basalts appear cogenetic, four of the samples have uniform abundances of U, Th, Rb, Nd, Sm, Pb, and Sr, indicating that they are also comagmatic. Two basalt glasses dredged previously at the SJFR have similiar isotopic compositions but higher concentrations of U, Th, and Pb. The 206Pb/204Pb ratios are intermediate between generally less radiogenic ridge basalts from south of the Juan de Fuca Ridge (JFR) and often more radiogenic basalts from the northern JFR and NE Pacific seamounts. Sr and Nd isotopic compositions closely resemble data of other ridge basalts from the northernmost East Pacific Rise and are intermediate between isotopically more diverse seamount basalts produced nearby. Eight sulfides collected from three hydrothermal vent sites at SJFR have very similar Pb isotopic compositions to that of the associated basalts. Sr and Nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data, indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater.