A Geochemical Investigation of the Role of Recycled Oceanic Crust in Hawaiian Magmatism
Amy M. Gaffney Lawrence Livermore National Laboratory
Investigations of the role of oceanic lithosphere in the geochemistry of Hawaiian magmas have generated two main families of hypotheses. One invokes ancient oceanic lithosphere that has been subducted, stored in the mantle for some length of time and recycled into the plume source where it contributes to compositional variation in the plume-generated lavas (e.g., White & Hofmann, 1982; Lassiter & Hauri, 1998; Blichert-Toft et al., 1999). Alternatively, plume-generated magmas may acquire geochemical characteristics of oceanic lithosphere by interacting with or assimilating it as they rise to the surface (e.g., Chen & Frey, 1985; Eiler et al., 1996). Singly or together, these processes may contribute to the range of compositional variability expressed through the two primary compositional endmembers of Hawaiian shield-stage magmatism: the relatively enriched Ko’olau component and the relatively depleted Kea component.
Kea-type lavas have the most depleted Sr and Nd isotopic compositions of all Hawaiian shield-stage lavas, and thus the Kea source material has been interpreted as having an association with or derivation from ambient depleted mantle or oceanic lithosphere. Identification and characterization of oceanic lithosphere associations and the resulting compositional variations within Kea-type lavas require high-density sampling as well as stratigraphic (i.e., time) control. This approach was successfully implemented for the young Kea-type Mauna Kea volcano with the multi-disciplinary work on the Hawaii Scientific Drilling Project (HSDP) cores (e.g., Stolper et al., 1996, and included papers; Blichert-Toft & Albarede, 1999; Abouchami et al., 2000; DePaolo et al., 2001; Blichert-Toft et al., 2003; Eisele et al, 2003; Huang & Frey, 2003).
From this project, broad conclusions were drawn about the compositional structure of the Hawaiian plume and the origin of the compositional variations within Mauna Kea lavas. However, in order to evaluate the role of oceanic lithosphere in the origin of geochemical heterogeneity in Kea-type volcanoes on an inter- and intra-volcano scale, analogous studies on older volcanoes are important. Thus, a study was undertaken to geochemically characterize stratigraphically-controlled sequences of shield-stage lavas from West Maui volcano, an older (~1.8 Ma) Kea-type volcano.
Questions Addressed With this Research
What is the range of chemical variability that characterizes the Kea component?
What is the role of oceanic lithosphere in generating the chemical variability among Kea-type lavas?
How, if at all, does the contribution of oceanic lithosphere to geochemical heterogeneity in Kea-type lavas vary on an intra- and inter-volcano scale?
The samples used for this study were collected from a monitoring well at the Mahinahina Water Treatment Facility near Honokawai, Maui. This study site was an exceptional location for this investigation, as the samples collected provided a stratigraphic sequence representing the late shield-stage of magmatism at West Maui volcano. This sequence of lavas is comparable to those obtained in the early stages of drilling on the HSDP core on Mauna Kea, thus enabling comparison between the two sets of data and testing of the broad applicability of models of magmatism derived from individual volcanoes. Comprehensive geochemical data set was obtained for these samples, including major and trace element and Sr-Nd-Hf-Pb-O-Os isotopic data. A variety of geochemical modeling techniques were used to assess contributions of oceanic lithosphere to these magmas, and evaluate the origin of the Kea component. This work led to several conference presentations as well as three publications (Gaffney et al., 2004; Gaffney et al., 2005a; Gaffney et al., 2005b).
This study resulted in several important conclusions that have contributed to current understanding of the origin of geochemical heterogeneity in Hawaiian magmatism. First, it was showed that although Kea-type magmatism at West Maui and other Kea-type volcanoes derives its primary geochemical characteristics from ancient recycled oceanic crust in the plume source, some lavas from these volcanoes also carry geochemical fingerprints of Pacific oceanic crust that was assimilated by the plume-derived magmas. This shallow source is identifiable only within relatively short stratigraphic sequences of lavas, indicating that oceanic crust contamination is an episodic, rather than continuous, process.
Second, it was showed that recycled oceanic crust in the Hawaiian plume controls geochemical variability in erupted magmas on an inter- and intra-volcano scale. Whereas the different parts of recycled oceanic crust (upper vs. lower crust) control the large compositional differences observed between Kea-type and Ko’olau-type Hawaiian volcanoes, the physical mechanisms of melting recycled oceanic crust in the plume control the chemical variability observed within any individual volcano. Thus, the relatively narrow range of chemical variability that characterizes the Kea component reflects the processes involved with melting of recycled lower oceanic crust (peridotite and eclogite), whereas the relatively large range of chemical variability that characterizes Ko’olau-type Hawaiian volcanoes reflects the processes involved in melting the lithologically distinct recycled upper oceanic crust (eclogite and metamorphosed sediment).
Lastly, through comparing the time-compositional relationships on an intra- and inter-volcano scale for Hawaiian magmatism over the last ~3 million years, it was concluded that the scale of oceanic crust heterogeneities in the Hawaiian plume is large enough that they are sampled over the lifespans of several volcanoes. In contrast, the chemical variability introduced by assimilation of Pacific oceanic crust reflects a process that only operates in the latest stages of the volcano’s life, once magma flux from the plume has decreased considerably from its peak flux during the main stages of volcanic shield-building.
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