GEOCHEMISTRY OF NEW SAMPLES FROM ISLA COOK, AUSTRAL VOLCANIC ZONE, CHILE

Isla Cook, the southernmost recent volcanic center of the modern Andes, is a unique volcano, erupting lavas thought to represent near-primary slab melts. Nine new samples representing each of the volcanic domes on Isla Cook, Isla Kelvin, and Isla Londonderry were analyzed for major and trace element concentrations, as well as Sr, Nd, Hf, and Pb isotopic ratios. Osmium isotopic ratios have also been measured in a subset of these samples.

At all but one sample location, the lava contains obvious xenolithic fragments petrographically similar to surrounding granitoid country rocks. Xenoliths size ranges from 1-2 cm up to ≈5 cm. In thin section, these xenoliths often comprise partially disaggregated crystals surrounded by glass, presumably resulting from their partial fusion. Xenocrystic plagioclase can also be found in most samples.

One of the most striking features of these samples is a correlation between geochemistry and geography. SiO₂ contents in domes progressively decrease towards the south. Fe₂O₃, MgO, CaO, Ni, Cr, and Co increase from N to S, while alkalis and incompatible elements follow a variety of trends ranging from monotonic increases from N to S, or inflected trends. Commonly, elemental contents increase from the northern domes to the central domes, followed by a decrease from the central domes to the southern domes. Isotopic ratios also vary in their geographic trends. ¹⁷⁷Hf/¹⁷⁶Hf decreases from N to S, while ⁸⁷Sr/⁸⁶Sr increases. Curiously, because SiO₂ also decreases to S, these isotopic trends mean that Hf isotopic ratios correlate positively with SiO₂, while Sr isotopic ratios display an inverse relationship with SiO₂ content. ¹⁴³Nd/¹⁴⁴Nd ratios define a concave upwards pattern, with the lowest ratios in the central domes. Pb isotopic ratios display significant scatter when plotted against latitude, but show vague decreases from N to S.

Our data are broadly consistent with a slab melting origin, with subsequent minor crustal contamination. The very high Sr/Y and La/Yb ratios suggest an eclogitic source while the very low LILE/HFSE, in particular Rb/Hf and Ba/Zr, constrain a highly-depleted, N-MORB source type for Cook adakites, similar to the basalts from northern Chile Ridge segments, rather than to the highly heterogeneous and often enriched lavas of the southern segments, particularly south of 44°S.