Do secondary magma chambers exist in Mauna Loa’s Southwest Rift Zone? - A Petrologic and Geochemical Analysis of Xenoliths

The Hapaimamo flow contains xenoliths of gabbronorites, troctolites, dunites, websterites and periodotites. Composition of cumulate olivines ranges from Fo78-86, clinopyroxenes ranges from En47-53Wo37-40Fs8-12, and orthopyroxenes ranges from En71-83Wo2-5Fs13-25. Most xenoliths display an intergranular, subpoikilitic or poikilitic texture. Petrologic and compositional information point towards at least two olivine-controlled magmatic sources in order to produce the observed mineral compositions – one at 1437C with 18.5 wt% MgO and another at 1311C with 15 wt% MgO. Clinopyroxene thermobarometry indicates clinopyroxene equilibrating in chambers at 13 km and <3.3 km depth during periods free of physical disturbances. The majority of the Hapaimamo flow does not contain magmas that are characteristic of the summit magma chamber, thus magma pulses from deep chambers travelled via pathways other than the primary conduit and directly contributed to the southwest rift zone secondary magma chamber. The shallow chamber is inferred to be underneath the SWRZ located at ~3.3km depth and is isolated from the primary conduit to the summit magma chamber. Cycles of magma recharge, mixing and crystallization related to periods of high magma flux occurred to produce the observed range of xenolith compositions.