UC San Diego

The origin of the Juan Fernandez Islands in the South Pacific is of geologic importance due to high-3He/4He ratios that span a considerable range (7.8-18.0 Ra). To constrain the petrogenesis and mantle source of the islands, bulk trace element abundances and Pb isotopic compositions were obtained for mafic lavas from islands Robinson Crusoe and Alexander Selkirk. Trace element data confirm grouping where on Robinson Crusoe, group I represents the shield-building phase and group II represents post-shield building. Group III represents shield-building on Alexander Selkirk. Sub-parallel incompatible trace elements patterns among all samples suggest a common mantle origin. Shield-building groups I and III have nearly identical trace element concentrations, and lower Nb/Zr, Ba/Zr and La/YbN than the more enriched group II. Incompatible trace element modeling indicate group III was produced by the highest degree of partial melting, while low degrees of partial melting plus fractional crystallization account for group II. Robinson Crusoe lavas exhibit more radiogenic Pb isotopes (206Pb/204Pb: 19.163-19.292) than Alexander Selkirk (206Pb/204Pb: 18.939-19.228). The range of Sr-Nd-Pb and He isotopes result from variable degrees of partial melting of a slightly heterogeneous mantle plume source. The source contains an EM1 mantle component, and lies near the focus zone (FOZO). Group I represent young HIMU in the source of ocean island basalts while group III represent EM1. Juan Fernandez is unlike other high-3He/4He volcanic chains for lack of binary mixing between its low and high-3He/4He sources, showing no correlation between Sr-Nd-Pb and He isotopes.