Explosively Erupted Gabbro From Kilauea Volcano's Summit Magmatic System

Gabbro clasts up to 13cm in diameter were blasted to the surface of Kilauea volcano by a powerful pyroclastic eruption 1500-1000 years b.p. They occur in the Kulanaokuaiki 3 tephra and as sparsely distributed lag lithics over 70 square km of the volcano's south flank where the tephra has been eroded away. These holocrystalline and glass-bearing gabbros make up about 12 per cent of the 1100 lithic clasts studied. Some are enclosed in cored bombs, but most occur as monolithologic clasts. Lag-lithic isopleths from an array of 110 south-flank sites suggest that this eruption emanated from the volcano's summit. The gabbros are remarkably fresh and consist chiefly of plagioclase + clinopyroxene + opaque oxides with varying percentages of olivine and rare orthopyroxene. Plagioclase and clinopyroxene crystals range up to 5 mm in length in the coarsest examples. Interstitial glass comprises as much as 25 modal per cent of the rock. In some samples this glass is moderately vesicular, reflecting an abrupt pressure drop accompanying eruption. The interstital glass contains 2.0 to 0.5 wt. per cent MgO and some has locally abundant apatite needles. Major element compositional trends of interstitial glass and glass melt inclusions in olivine suggest that the gabbros formed by crystallization along multiple liquid lines of descent. Whole-rock XRF compositions of 11 samples suggest that the gabbros are of two distinct types. Nine have 11 to 15 wt. per cent MgO and are samples of magma bodies that were crystallizing or had crystallized at depth. Two gabbros contain 6 wt. per cent MgO; these probably originated as segregation veins in as yet unknown parent bodies. All of the gabbros texturally resemble segregation veins from Kilauea Iki and other Kilauean lava lake deposits, but only the two low-MgO samples likely had such an origin. Most of the gabbros are fragments torn from partially to entirely crystallized bodies adjacent to the Kulanaokuaiki 3 parent magma body. We infer that the extremely energetic eruption necessary to disrupt these bodies and to project their shattered remnants as far as 15 km from the summit originated deep within the volcano, perhaps within or beneath the volcano's 2 to 6 km deep summit magmatic system.