The role of oceanic lithosphere in inter- and intra-volcano geochemical heterogeneity at Maui Nui, Hawaii
Gaffney, Amy Michelle
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The volcanoes of Maui Nui (West Molokai, East Molokai, Lanai, West Maui, Haleakala and Kahoolawe) record Hawaiian magmatism at ∼1--2 Ma. These volcanoes nearly span the compositional range erupted from all the Hawaiian volcanoes over the past 5 My, and erupt lavas representing both the Kea and Koolau compositional endmembers of Hawaiian lavas. Using lavas from these volcanoes, we evaluate the role that oceanic lithosphere, both ancient recycled lithosphere in the Hawaiian plume and modern Pacific lithosphere, plays in the generation of geochemical variability on inter- and intea-volcano scales. We also present trace element models that explicitly address the petrologic complexities of melting eclogite (derived from ancient oceanic lithosphere) in the plume. Trace element, major element and isotope compositions of Lanai are consistent with the origin of these lavas in large degree (∼70%) melts of ancient upper oceanic crust (basalt + sediment) that mix with plume-derived Haleakala-type melts. Trace element and isotope compositions of West Maui and East Molokai are consistent with an origin in ancient depleted oceanic lithosphere that has been re-fertilized with moderate-degree melts (20--40%) of associated crustal gabbro. The physical mechanisms through which the oceanic lithospheric components melt and mix within the plume lead to the generation of isotopically homogeneous Kea-type lavas and isotopically heterogeneous Koolau-type lavas.Stratigraphically-controlled sequences of late shield-building stage lavas from West Maui volcano show age-dependent compositional variability distinct from that seen in shield-stage lavas from any other Hawaiian volcano. These distinctions are defined by 206Pb/204Pb 207Pb/204Pb variation as well as 87Sr/ 86Sr correlation with 206Pb/204Pb, 187Os/188Os and trace element compositions. The 87Sr/86Sr-206Pb/204Pb and 87Sr/86Sr-187Os/188 Os variation in the deep lavas is orthogonal to the all-Hawaii variation, indicating that it is not the result of mixing between components normally sampled by Hawaiian shield-stage magmas. We compare our West Maui data to observed compositions of Pacific oceanic basaltic and gabbroic crust, and predicted compositions for 2 Ga basaltic and gabbroic oceanic crust. The observed fine-scale compositional variability in the stratigraphically deep West Maui lavas is consistent with 10--15% mixing of small degree (2%) partial melts of the Pacific gabbroic oceanic crust with plume-derived, Kea-type magmas.
- Geology