抄録
Boron isotopic composition of the three representative Hawaiian shield lavas - Kilauea, Mauna Loa, and Koolau - are analyzed by thermal ionization mass spectrometry (TIMS). To eliminate the possibility of the δ11B modification by surface contamination, only fresh basalts, without iddingsite rim on the olivine, were selected for analysis. Further, boron isotopic composition of each sample was analyzed twice, with and without acid-leaching. The δ11B of subaerial lavas is not affected by acid-leaching. However, δ11B of leached submarine samples give significantly lower values than those of the unleached samples, indicating effective elimination of sea-water derived absorbed boron. Careful evaluation of the post-magmatic contamination effect indicates the pre-eruptive δ11B values for the shield lavas are -5.1 ± 0.6 ‰ (2σ) for Koolau, -4.0 ± 0.6 ‰ for Kilauea, and -3.3 ± 0.6 ‰ for Mauna Loa. Historical Kilauea lavas show systematic temporal trend for B content and Nb/B ratio coupled with the other radiogenic isotopic ratios and trace element ratios, with constant δ11B, indicating little or no assimilation of crustal materials. Uncorrelated B content and δ11B in the Koolau and Mauna Loa lavas may also indicates little or no effect of crustal assimilation in these lavas. δ11B of the Kilauea and Mauna Loa source mantle are within the range of MORB and bulk earth compositions. However, the systematic trend from Kilauea to Koolau - decreasing of δ11B coupled with the decreasing of 143Nd/144Nd and the increasing of 87Sr/86Sr and 206Pb/204Pb - indicates the involvement of subducted sediment components in the Koolau source mantle. The homogeneous δ11B in each shield was resulted from the range of δ11B between the endmember components relative to the analytical error.This study reveals that the precise analysis of δ11B by TIMS gives notable information (a) to constrain the source of the OIB magma and (b) to evaluate the assimilation and post-magmatic contamination processes of the lavas.
