Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original 11B/ 10B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, 11B/10B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive 11B/10B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ11B values for the shield lavas are -4.5 to -5.4‰ for Koolau (N = 8), -3.6 to -4.6‰ for Kilauea (N = 11), and -3.0 to -3.8‰ for Mauna Loa (N = 6). Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ11B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ11B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau-decreasing δ11B coupled with decreasing εNd as well as increasing 87Sr/ 86Sr and 206Pb/204Pb-is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas.
ASJC Scopus subject areas
- Geochemistry and Petrology