Light and Heavy: Fe-Isotopes in Shield And Rejuvenated Hawaiian Lavas Point to Mineralogically Distinct Sources

The mineralogical and lithological variability in the source of oceanic magmas and the convecting mantle is key to understand volcanism and the Earth's chemical differentiation. The typical "endmember" sources of magmas are peridotite ("depleted"), and pyroxene-rich (pyroxenite or eclogite; "enriched") lithologies. However, their respective compositions are unlikely to be fixed, while volatiles and magma chamber processes further change the erupted melt compositions, thereby creating challenges to unequivocally deduce the source lithology of magmas.

Here we test the use of stable Fe-isotopes (as δ56Fe, using the double spike method) as a tracer of lithological variability in the Hawaiian plume. Shield lavas from Ko'olau, Kaua'i and Ni'ihau vary little in δ56Fe (0.05 - 0.11 ‰) despite a range in TiO2, MgO and radiogenic isotopes, and fall on the "light" side of MORB in δ56Fe. Rejuvenated lavas from Kaua'i and North Arch have heavier δ56Fe (0.11 to 0.18 ‰) for similar MgO and TiO2 contents as the shield lavas. Pyroxenites from Ko'olau overlap the rejuvenated magmas and extend to much heavier δ56Fe = 0.28‰.

The relatively light δ56Fe of the shield lavas implies a peridotite source, one that may be more depleted than the average MORB source with δ56Fe~0.11‰. The heavy δ56Fe of the rejuvenated magmas and the pyroxenites are difficult to generate by melts from a peridotite based on existing models, particularly as Ti, Fe and Mg concentrations overlap the shield lavas. We suggest that the heavy δ56Fe of the rejuvenated lavas require contribution from a pyroxene-rich source, similar to the Hawaiian pyroxenites. In this model the pyroxenites represent a transient mineralogy formed possibly at the base of the lithosphere from earlier plume melts, that subsequently contribute to the generation of rejuvenated magmas. The view of the Hawaiian plume from the δ56Fe perspective contrasts the view derived primarily from the radiogenic isotopes, where shield magmas require a "crustal" component while rejuvenated magmas a depleted mantle source. Instead, the Fe isotopes imply decoupling of major elements (and therefore the source mineralogy) from trace elements in the plume.