Insights Into the Origins and Compositions of Mantle Plumes: A Comparison of Galápagos and Hawai'i

Abstract

The Galápagos and Hawai'i archipelagos are formed by mantle plumes originating at the large low shear velocity province (LLSVP) boundary. We report new high‐precision Pb, Sr, Nd, and Hf isotopic analyses on 83 Galápagos samples and compare them with those of Hawai'i. The data confirm that like Hawai'i, Galápagos is a bilaterally asymmetric plume whose compositional boundary trends NW‐SE. On their northeast sides, the plumes share a common source, Pacific lower mantle, whose intermediate isotopic signature may be common to many plumes. The Hawaiian and Galápagos plumes' southwestern sides are anchored in the Pacific LLSVP and are compositionally distinct; in Hawai'i, Loa trend lavas reflect contributions from the EM1 mantle end‐member, whereas in Galápagos, HIMU is dominant, suggesting that the Pacific LLSVP is compositionally heterogeneous and includes different types of recycled material. Furthermore, the surficial expression of a bilaterally asymmetric plume is strongly influenced by its tectonic setting: (a) Thick Hawaiian lithosphere supports a volcano evolution process, including rejuvenated volcanism, whereas the thin Galápagos lithosphere inhibits Hawai'i‐style rejuvenated‐stage eruptive activity, instead causing extended, widespread volcanism; (b) the proximity of the Galápagos to a mid‐ocean ridge causes entrainment of the depleted upper mantle, overwhelming depleted material intrinsic to the plume and affecting volcanoes' magmatic architecture; and (c) the geometric relationship between the LLSVP boundary and plate motion influences geochemical patterns at the surface. Thus, despite striking differences in surficial expression of the Galápagos and Hawai'i plumes, they share a common generation mechanism, supplied by the Pacific LLSVP and the lower mantle.

Fine‐Scale Structure of Earth's Deep Mantle Resolved Through Statistical Analysis of Hawaiian Basalt Geochemistry

“…Statistically, the Kea trend volcanoes are classified by LDA into three separate groups with 90%–91% accuracy (90% with all isotopic systems, 91% with only Pb, supported by logistic regression results when sample sizes are n > 50, Table ). These groupings document the presence of wider heterogeneities in the Kea trend source, which was previously thought to be dominantly homogeneous and representative of the ambient Pacific mantle (e.g., Harpp and Weis, 2020; Nobre Silva et al., 2013). At the other end of the compositional spectrum, samples with the lowest 206 Pb/ 204 Pb and 208 Pb/ 204 Pb define a group, “Enriched Loa,” which represents a short‐lived heterogeneity (<1 Ma) resolvable in the Makapu‘u section of Ko‘olau, Kaho‘olawe, and Lāna‘i (e.g., Huang et al., 2005).…”

“…On the Loa side, Enriched Loa, Lō'ihi, and Average Loa regional-scale heterogeneities are likely sourced from within the Pacific LLSVP ( Figure 5), as these three groups have more "enriched" geochemical compositions (e.g., higher 208 Pb*/ 206 Pb*) than all of the Kea groups. If correct, this indicates that the LLSVP is heterogeneous, which is supported by recent seismic modeling (Cottaar & Romanowicz, 2012;Kim et al, 2020), recent detailed geochemical investigations using exhaustive high-precision isotopic datasets (Harpp and Weis, 2020), short-lived isotopic system anomalies in oceanic island basalts (Delavault et al, 2016;Mundl et al, 2017;Peters et al, 2018;Parai et al, 2019), and the spatial analysis of global geochemical data (Jackson et al, 2018).…”

“…Conversely, Pb‐Pb mixing lines from Loa trend volcanoes intersect at a value that corresponds to the averages for both Mauna Loa and all Loa trend volcanoes (i.e., the “Average Loa” composition). The Kea end‐member composition sampled by recent Kea volcanoes, as well as volcanoes from the youngest Emperor Seamounts and from along the NWHR (Harrison & Weis, 2018), is also observed in lavas on the northeast side of the Galápagos mantle plume (Harpp & Weis, 2020), whereas the “Average Loa” composition sampled by Loa volcanoes is more isotopically enriched and distinct (Figure 3a insets). The different geometries in Pb isotope systematics (i.e., individual Pb‐Pb lines for each volcano and their relative distribution in a Pb‐Pb diagram; Figure 3b) confirms that Loa and Kea trends are sampling deep (at the core‐mantle boundary [CMB]) mantle reservoirs with distinct compositions and with different time‐integrated Th/U ratios on the southwest and northeast sides of the archipelago (Harpp & Weis, 2020; Harrison & Weis, 2018; Weis et al., 2011).…”

“…This origin within the LLSVP would account for the greater geochemical variation and enriched geochemical signatures of Loa trend volcanoes, as the LLSVP is thought to contain primordial material and/or recycled, surface‐derived materials (Jackson et al., 2018; Mundl et al., 2017; White, 2015). A comparable model was recently put forward to account for the bilateral asymmetry of the Galápagos mantle plume on the basis of a geochemical study of 83 lavas – the southwestern side of the Galápagos mantle plume is also anchored in the Pacific LLSVP and is dominated by HIMU compositions (Harpp & Weis, 2020). Other Pacific hotspots including Samoa, the Marquesas, and Cook‐Austral Islands have been investigated using stable isotope, noble gas, and high‐precision radiogenic isotope systems that support long‐lived and undegassed mantle reservoirs in the sources of some Pacific islands (Chauvel et al., 2012; Dottin et al., 2020; Jackson et al., 2017; Parai et al., 2009).…”

“…the bilateral asymmetry of the Galápagos mantle plume on the basis of a geochemical study of 83 lavas -the southwestern side of the Galápagos mantle plume is also anchored in the Pacific LLSVP and is dominated by HIMU compositions (Harpp & Weis, 2020). Other Pacific hotspots including Samoa, the Marquesas, and Cook-Austral Islands have been investigated using stable isotope, noble gas, and high-precision radiogenic isotope systems that support long-lived and undegassed mantle reservoirs in the sources of some Pacific islands (Chauvel et al, 2012;Dottin et al, 2020;Jackson et al, 2017;Parai et al, 2009).…”