The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez lavas reveal variable contributions from a high-3He/4He mantle plume

The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in He-3/He-4 ratios (8 to 18 RA, where RA is the He-3/He-4 ratio of air), but limited ranges of Sr-87/Sr-86 and Nd-143/Nd-144. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their He-3/He-4 and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (He-3/He-4 = 7.8-9.5 R-A) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (> 10%) of a common mantle source, followed by Group I basalts (13.6-18.0 RA) and Group II basanites (11.2-12.5 RA) from older Robinson Crusoe Island. The Pb-206/Pb-204 of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable He-3/He-4 as their parental magmas originated mainly from the FOZO component with high He-3/He-4 (> 9 R-A) and variably polluted with a depleted component with lower He-3/He-4 (ca. 8 R-A). Contributions from high-He-3/He-4 mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to millionyear time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.