New Evidence for Dual-Trend Volcanism in the Pacific

Dual trend volcanism is a phenomenon first noted in Hawaii (e.g., Tatsumoto, 1978; Abouchami et al., 2005). This remarkable geographic and geochemical separation of volcanoes along a hotspot appears not be a feature unique to Hawaii. Huang et al. (2011) identified corresponding isotopic and geographic divisions in two other Pacific hotspots: Samoa and the Marquesas. Like Hawaii, both the Marquesas and Samoa exhibit a North-South isotopic division of en echelon volcanic trends. The southern trends at Samoa and Marquesas are enriched relative to the northern trends, mimicking the geographic distribution of isotopic enrichment observed in Hawaii. The isotopic enrichment of the southern volcanic trend in all three hotspot tracks can be explained by the location of their respective plume conduits relative to the DUPAL anomaly, a globe encircling feature of isotopic enrichment in the southern hemisphere that is associated with a low velocity anomaly (Hart, 1984; Castillo, 1988). Huang et al. (2010) suggested that the isotopic enrichment in the southern side of the hotspots results from the south side of the plume conduits interacting with and entraining the deep mantle DUPAL belt, while the northern side of the plumes extends north of (and away from) the DUPAL region. Since the DUPAL anomaly is a global feature, it might be expected that other hotspots in the Pacific exhibit similar isotopic zoning to that observed in Hawaii, Samoa, and the Marquesas. In keeping with this hypothesis, we have identified an additional Pacific hotspot track composed of two distinct subparallel trends with distinct isotopic signatures. However, in this particular hotspot the northern trend is enriched relative to the southern trend. Thus, this geographic pattern is a reversal relative to Hawaii, Samoa, and the Marquesas. However, unlike these three hotspots, which are located on the northern side of the DUPAL belt, the volcanic chain with the newly discovered dual trends lies on the southern margin of the plume. This geographic difference may account for the reversal in the isotopic trends, and is based on the following conceptual model. The base of a mantle plume entrains material over great distances from the plume conduit. Farnetani and Hofmann (2009; 2010) suggest that large-scale geographic patterns of geochemical enrichment at the base of the mantle can be preserved in plume conduits, and plume-fed volcanoes erupted at the surface reflect patterns of geochemical enrichment in the plume conduit. If the enriched DUPAL belt is concentrated near the core-mantle boundary in a global-encircling band centered south of the equator, the Farnetani and Hofmann model predicts that the side of the plume conduit nearest the DUPAL belt will exhibit significantly greater isotopic enrichment, a pattern of enrichment that is then expressed in volcanoes erupted at the surface. In the case of Hawaii, Samoa, and the Marquesas, the south side of the plume conduit faces the DUPAL belt, consistent with the DUPAL signature being expressed in the southern volcanic trends at the surface. This model predicts that a plumed located south of the DUPAL anomaly should exhibit greater enrichment on the north side of the hotspot, and this model describes the pattern of isotopic enrichment observed in the 4th dual-trend hotspot discovered in the Pacific.