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 composi tions have been measured on a suite of lavas from the intra-plate volc anos of the Juan Fernandez Archipelago, Chile. Lavas from the islands of Mas Afuera and Mas a Tierra and from Monte Alpha and Friday seamoun ts 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 o f a restricted range of mantle source compositions throughout Juan Fer nandez 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 Tierr a 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 basa lts, whereas post-shield basanites range from 11.2 to 13.6 R(A) (n = 1 2). Elemental and isotopic systematics demonstrate a transition from a n enriched (Loihi-like) plume source in the shield lavas to a more MOR B-like source in the post-shield volcanics. The transition between the se sources is much more pronounced in He-3/He-4 than in the other isot opic tracers. The predominantly tholeiitic basalts of Mas Afuera have exceptionally uniform isotopic and elemental characteristics; Sr and N d ratios are similar to those of Mas a Tierra, but He-3/He-4 ratios ar e lower and more uniform at 8.3 +/- 0.5 R(A) (n = 17). The dramatic di stinction between Mas Afuera and Mas a Tierra helium is surprising giv en the great similarity between the two islands in other geochemical c haracteristics. Both the Mas Afuera and Mas a Tierra results demonstra te that helium records systematic processes not readily apparent from other isotopic or elemental indicators. Neither magma chamber degassin g nor local metasomatic events are likely to be responsible. We sugges t that the observed variations may be attributed to mixing of plume an d asthenospheric sources in which the plume component is characterized by a heterogeneous distribution of volatiles or has suffered extracti on of small degree partial melts prior to mixing.