Iron/manganese ratio and manganese content in shield lavas from Ko’olau Volcano, Hawai’i

Abstract

Precise Fe/Mn ratios and MnO contents have been determined for basalts from the Hawaiian shields of Ko’olau and Kilauea by inductively coupled plasma mass spectrometry. It is well known that the youngest Ko’olau (Makapu’u-stage) shield lavas define a geochemical endmember for Hawaiian lavas in terms of CaO and SiO 2 contents and isotopic ratios of O, Sr, Nd, Hf, Pb, and Os. We find that their MnO content is also distinct. Despite the small range in MnO, 0.146 to 0.176 wt%, the precision of our data is sufficient to show that among unaltered Ko’olau lavas MnO content is correlated with Nd–Hf–Pb isotopic ratios, La/Nb and Al 2O 3/CaO elemental ratios, and contents of SiO 2, MgO and Na 2O + K 2O adjusted for olivine fractionation. These trends are consistent with two-component mixing; one endmember is a SiO 2-rich, MnO-, and MgO-poor dacite or andesite melt, generated by low degree (10–20%) partial melting of eclogite. Since this low-MgO endmember (dacite or andesite melt) has very low FeO and MnO contents, mixing of high Fe/Mn dacite or andesite melt with a MgO-rich picritic melt, the other endmember, does not significantly increase the Fe/Mn in mixed magmas; consequently, Ko’olau and Kilauea lavas have similar Fe/Mn. We conclude that the high Fe/Mn in Hawaiian lavas relative to mid-ocean ridge basalt originates from the high MgO endmember in Hawaiian lavas.