Research: Juan Fernandez Islands

• Read the full article in Chemical Geology: Publisher’s website or pdf copy
• 2018 Presentation slides (pdf)

Motivation and Context:

• By the late 1980s, mantle geochemists constructed various flavors of mantle endmember compositions within oceanic island lavas ¹.
• Explorations of the historically uninhabited Juan Fernandez Islands² near Chile revealed basalts with high volumes of olivine crystals³. The coarse olivines were promising candidates for Helium isotope analyses.
• A range of ³He/⁴He=12-18 R_A from the older, larger island, was consistent with a deeply sourced mantle plume. However, lavas from the younger island exhibited a narrow range similar to MORB = 8-11 R_A. Sr-Nd isotopes measured in lavas were homogeneous, yet inconsistent with MORB-OIB mixing⁴.
• Another study of global oxygen isotopes grouped Juan Fernandez with other high-³He/⁴He localities, for their similar ratios ⁵.
• Skeptics of deep mantle plume sources proposed a xenocryst origin of the olivines, where the Helium isotope ratios were from volatiles pervading shallow magma chambers ⁶.

Research Questions:

• Are the olivines from the same mantle source as the lavas?
• Ae Pb isotopes similar to other SE Pacific Islands, showing enriched mantle input?
• Do trace elements reveal differences in magmatic evolution between the older and younger island?
• Are isotopic ratios, overall, a product of mantle plume evolution or variation, or mixing with MORB mantle?

Summary: Olivine crystals were separated and analyzed for Sr-Nd isotopes. Their ratios were within error to the mafic lavas (published values). Assuming similar D_He and D_Nd, this is consistent with olivines forming in equilibrium with the lava suite. Pb isotopes show a heterogeneous but well-mixed mantle source that can be characterized as predominantly FOZO component. The Pb isotope signature of the younger island, reflect a possible EM2 source, and need not derive from depleted mantle. The evolution of the younger island can be described as late shield stage, where diminsihing plume influence results in low ³He/⁴He. Overall, the trace element compositions, Sr-Nd-Pb isotopes of lavas and Sr-Nd of olivines indicate variable sampling of a dominantly FOZO-like source during shield, late stage shield, and post-shield construction among the two islands.