Environments of Crystallization and Compositional Diversity of Mauna Loa Xenoliths

Abstract

Two picrite flows from the SW rift zone of Mauna Loa contain xenoliths of dunite, harzburgite, lherzolite, plagioclase-bearing lherzolite and harzburgite, troctolite, gabbro, olivine gabbro, and gabbronorite. Textures and olivine compositions preclude a mantle source for the xenoliths, and rare earth element concentrations of xenoliths and clinopyroxene indicate that the xenolith source is not old oceanic crust, but rather a Hawaiian, tholeiitic-stage magma. Pyroxene compositions, phase assemblages and textural relationships in xenoliths indicate at least two different crystallization sequences. Calculations using the pMELTS algorithm show that the two sequences result from crystallization of primitive Mauna Loa magmas at 6 kbar and 2 kbar. Independent calculations of olivine Ni–Fo compositional variability in the plagioclase-bearing xenoliths over these crystallization sequences are consistent with observed olivine compositional variability. Two parents of similar bulk composition, but which vary in Ni content, are necessary to explain the olivine compositional variability in the dunite and plagioclase-free peridotitic xenoliths. Xenoliths probably crystallized in a small magma storage area beneath the rift zone, rather than the large sub-caldera magma reservoir. Primitive, picritic magmas are introduced to isolated rift zone storage areas during periods of high magma flux. Subsequent eruptions reoccupy these areas, and entrain and transport xenoliths to the surface.

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

“…Similar evidence suggests that olivine-gabbronorites and the hornblende-gabbros also originated from tholeiites. Moreover, gabbronorite phase assemblages, textures and mineral compositions are similar to those of gabbronorites from K ı ılauea (Fodor & Moore, 1994), Mauna Loa (Gaffney, 2002), Mauna Kea (Fodor & Galar, 1997) and Hual a alai (Clague & Bohrson, 1991), which have all been interpreted as tholeiitic cumulates.…”

“…Crystalline xenoliths present in some Hawaiian lavas offer the possibility of studying crystallization environments and processes that are otherwise inaccessible (e.g. Fodor & Vandermeyden, 1988;Gaffney, 2002). Hual a alai Volcano, in particular, is noted for the abundance of gabbroic and ultramafic xenoliths transported in the $1800 AD Ka' u up u ulehu alkali basalt lavas.…”

Jurassic dikes of Vestfjella, western Dronning Maud Land, Antarctica: Geochemical tracing of ferropicrite sources

“…Fig 4), but common in ferropicrites (e.g., Hanski, 1992). (Sun and McDonough, 1989; V and Sc from Salters and Stracke, 2004), OIB (Sun and McDonough, 1989; V and Sc calculated after GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/)), and subset of Hawaiian picrites (Mauna Loa; Gaffney, 2002) are shown in gray for comparison. Normalizing values are after Sun and McDonough (1989) and, for V and Sc, after McDonough and Frey (1989).…”

Application of a textural geospeedometer to the late‐stage magmatic history of MIL 03346

“…Picrites from the 1852 and 1868 eruptions of Mauna Loa are interpreted as a combination of deep-seated crystal mush mixing with more typical shallow reservoir material (Rhodes 1995). On Mauna Loa, entrainment and suspension of high crystal concentrations may require high magma supply rate and unusually vigorous magma column dynamics (Rhodes 1995;Gaffney 2002;Mueller et al 2004). Consistent with this inference, vents on the rift zones and flanks of both Kilauea and Mauna Loa produce crystalrich magma more frequently than do summit vents (Trusdell, personal communication) Most importantly, crystal accumulation by settling occurs at the bases of picritic pahoehoe flows of Mauna Loa and Kilauea, with crystallinities commonly reaching 50 vol% (Trusdell, personal communication) Again, a'a behaves quite differently, because crystal settling is inhibited by the internal stirring associated with this flow type; phenocryst cumulates are not observed in a'a flows.…”