PETROGENESIS OF MAUNA KEA HIGH-CAO LAVAS FROM HAWAII SCIENTIFIC DRILLING PROJECT

The relative proportions of peridotite and pyroxenite in the sources of Hawaiian volcanoes as well as the compositions of these lithologies is a subject of active debate in which Mauna Kea (MK) lavas recovered from the Hawaii Scientific Drilling Project (HSDP) have played and continue to play an important role. At least three distinct compositional groups of glasses have been identified in MK tholeiites from HSDP: Low- and High-SiO₂ glasses, and a special group (n=7) of Low-SiO₂ glasses from the 1765-1810 mbsl depth interval that have high CaO contents (High-CaO group) (Stolper et al. 2004). This High-CaO group of glasses has the highest CaO content at a given MgO content among all Hawaiian tholeiites (up to 13.2 wt% at 8.0 wt% MgO). Detailed resampling of the core in the 1765-1810 mbsl interval (Rhodes et al. 2012) recovered 24 whole-rock samples. Major and trace element analyses of the whole-rocks and the glasses from this depth interval (Stolper et al., 2004; Rhodes et al., 2012; this study) show that they share the same distinctive compositional characteristics. Specifically, at a given MgO content, compared to MK Low- and High-SiO₂ group tholeiites, High-CaO lavas (both glasses and whole-rocks) have higher contents of K₂O, CaO, Sc, V and Cr, and lower contents of Y and heavy rare earth elements. Herzberg (2006, 2011) has argued that at a given MgO content, peridotite partial melts will have higher CaO contents than pyroxenite partial melts and that only the HSDP High-CaO glasses have CaO contents high enough to be partial melts of peridotite—all other MK tholeiites were derived from a source that contained garnet pyroxenites. Here we report new trace element data on the HSDP glasses (High- and Low-SiO₂ groups and High-CaO group). Comparisons of major and trace element data in three HSDP tholeiite groups can help place constraints on the source of MK High-CaO lavas, a unique suite of MK samples.