HE, SR AND ND ISOTOPIC VARIATIONS IN LAVAS FROM THE JUAN-FERNANDEZ ARCHIPELAGO, SE PACIFIC

Helium. Sr, and Nd isotopic ratios and major and trace element compositions have been measured on a suite of lavas from the intra-plate volcanos of the Juan Fernandez Archipelago, Chile. Lavas from the islands of Mas Afuera and Mas a Tierra and from Monte Alpha and Friday seamounts have Sr and Nd isotopic ratios lying on the low Nd-143/Nd-144 side of the mantle array (Sr-87/Sr-86: 0.7034-0.7037; Nd-143/Nd-144:0.51281-0.51289). The homogeneity of these tracers suggests the involvement of a restricted range of mantle source compositions throughout Juan Fernandez volcanism. In marked contrast is the large range in He-3/He-4, from 7.8 to 18.0 R(A). A bimodal He-3/He-4 distribution on Mas a Tierra is associated with two distinct volcanic lineages, He-3/He-4 ratios of 14.5-18.0 R(A) (n = 15) occur in alkalic and tholeiitic shield basalts, whereas post-shield basanites range from 11.2 to 13.6 R(A) (n = 12). Elemental and isotopic systematics demonstrate a transition from an enriched (Loihi-like) plume source in the shield lavas to a more MORB-like source in the post-shield volcanics. The transition between these sources is much more pronounced in He-3/He-4 than in the other isotopic tracers. The predominantly tholeiitic basalts of Mas Afuera have exceptionally uniform isotopic and elemental characteristics; Sr and Nd ratios are similar to those of Mas a Tierra, but He-3/He-4 ratios are lower and more uniform at 8.3 +/- 0.5 R(A) (n = 17). The dramatic distinction between Mas Afuera and Mas a Tierra helium is surprising given the great similarity between the two islands in other geochemical characteristics. Both the Mas Afuera and Mas a Tierra results demonstrate that helium records systematic processes not readily apparent from other isotopic or elemental indicators. Neither magma chamber degassing nor local metasomatic events are likely to be responsible. We suggest that the observed variations may be attributed to mixing of plume and asthenospheric sources in which the plume component is characterized by a heterogeneous distribution of volatiles or has suffered extraction of small degree partial melts prior to mixing.