Hot spot plume dynamics and tectonic migration
The Hawaiian Hot Spot is now located beneath the Island of Hawaii, at the extreme southeast end of the Hawaiian-Emperor Chain. Each of the volcanoes of the chain was once located over the hot spot, and migration of the Pacific tectonic plate over the hot spot pulled the volcanoes away from the hot spot's magmatic source. As a result, the islands and seamounts are progressively older to the northwest. Changes in the direction of the plate motion led to bends in the chain, the largest of which is between the Hawaiian Ridge and Emperor Seamounts.
- Maps of the Hawaiian-Emporer seamount chain
The origin of the Hawaiian Hot Spot plume magmas is the Earth's deep mantle, in contrast to the mid-ocean ridge magmas that come from the shallow mantle. Analysis of the chemical composition of the lavas gives important clues about the source and dynamics of the hot spot plume.
The supply and composition of magma to the volcanoes changes with time as the volcanoes grow over the hot spot and migrate away. Tectonic migration of the Pacific Plate, and growth and subsidence of the islands have influenced the biogeography of the islands of the chain. For our research on these related topics, see:
Our research on hot spot dynamics
Ancient carbonate sedimentary signature in Hawaiian plume
Lavas from Mahukona, a small Hawaiian volcano on the Loa trend, exhibit major and trace element abundance variations exceeding those in lavas from large Hawaiian shields, such as Mauna Loa and Mauna Kea. Mahukona lavas define three geochemically distinct groups of tholeiitic shield basalt and a transitional group of postsshield basalt. At 10% MgO the tholeiitic groups range from 9 to 12% CaO; such differences in CaO can reflect partial melts derived from garnet pyroxenite (low CaO) and peridotite (high CaO), but the negative CaO-Yb (both at 10% MgO) trend formed by Mahukona lavas is inconsistent with this explanation. Within Mahukona lavas, radiogenic Nd-Hf-Pb isotopic ratios are highly correlated with each other; however, 87Sr/86Sr is decoupled from these radiogenic isotopic ratios. Rather, 87Sr/86Sr is correlated with trace element abundance ratios involving Sr, and importantly, Mahukona lavas define a negative Rb/Sr-87Sr/86Sr trend, implying that a Sr-rich source component characterized by high 87Sr/86Sr is important in the petrogenesis of Mahukona lavas. We infer that this Sr-rich source component is recycled ancient carbonate-rich sediments. Intershield heterogeneity among Hawaiian shields also shows a negative Rb/Sr-87Sr/86Sr trend. For example, Makapuu-stage Koolau lavas have higher 87Sr/86Sr but lower Rb/Sr than Mauna Kea lavas. Consequently, we infer that a recycled ancient carbonate-rich sedimentary source component is important in the Hawaiian plume. Although most lavas from Loa and Kea trend volcanoes define distinct fields in isotopic ratios of Sr, Nd, Hf, and Pb, the majority of Mahukona lavas have isotopic ratios at the boundary between the fields defined by Loa and Kea trend lavas. However, a subgroup of Mahukona shield lavas have Kea-like isotopic and trace element signatures, an observation that can be explained by vertical heterogeneity in a bilaterally asymmetrical plume.
Reference: Huang, S., W. Abouchami, J. Blichert-Toft, D. A. Clague, B. L. Cousens, F. A. Frey, and M. Humayun (2009), Ancient carbonate sedimentary signature in the Hawaiian plume: Evidence from Mahukona volcano, Hawaii, Geochem. Geophys. Geosyst., 10, Q08002, doi:10.1029/2009GC002418. [Article]
Rejuvenated stage lavas: evidence for recycled subducted oceanic crust in the deep mantle
NIIHAU - We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated-stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated-stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated-stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated-stage lavas, with enrichments in Ba, Sr, and light-rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica-undersaturated of the rejuvenated-stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated-stage lavas by small degrees (~1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated-stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica-undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is best preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift. Formation of carbonatite melts may provide a mechanism to transfer plume He to the margins of the plume.
Reference: Dixon, J., D. A. Clague, B. Cousens, M. L. Monsalve, and J. Uhl (2008), Carbonatite and silicate melt metasomatism of the mantle surrounding the Hawaiian plume: Evidence from volatiles, trace elements, and radiogenic isotopes in rejuvenated-stage lavas from Niihau, Hawaii, Geochem. Geophys. Geosyst., 9, Q09005, doi:10.1029/2008GC002076. [Article]
Hawaiian plume models constrained by West Molokai lavas
WEST MOLOKAI - There are systematic geochemical differences between the < 2 Myr Hawaiian shields forming the double-chain of volcanoes along subparallel spatial trends, known as Loa and Kea. These spatial and temporal geochemical changes provide insight into the spatial distribution of geochemical heterogeneities within the source of Hawaiian lavas, and the processes that create the Hawaiian plume. Lavas forming the ~1.9 Ma West Molokai volcano are important for evaluating alternative models proposed for the spatial distribution of geochemical heterogeneities because (1) the geochemical distinction between Loa and Kea trends may end at the Molokai Fracture Zone and (2) West Molokai is a Loa-trend volcano that has exposures of shield and postshield lavas. This geochemical study (major and trace element abundances and isotopic ratios of Sr, Nd, Hf, and Pb) shows that the West Molokai shield includes lavas with Loa- and Kea-like geochemical characteristics; a mixed Loa-Kea source is required. In contrast, West Molokai postshield lavas are exclusively Kea-like. This change in source geochemistry can be explained by the observed change in strike of the Pacific plate near Molokai Island so that as West Molokai volcano moved away from a mixed Loa-Kea source it sampled only the Kea side of a bilaterally zoned plume.
Reference: Xu, G., F.A. Frey, D.A. Clague, W. Abouchami, J. Blichert-Toft, B. Cousens, M. Weisler (2007) Geochemical characteristics of West Molokai shield- and postshield-stage lavas: Constraints on Hawaiian plume models, Geochem., Geophys., Geosystems, 8(8): doi:10.1029/2006GC001554. [Article]
Age of the Hawaiian-Emperor Bend
HAWAIIAN-EMPEROR SEAMOUNT CHAIN - The Hawaiian-Emperor bend has played a prominent yet controversial role in deciphering past Pacific plate motions and the tempo of plate motion change. New ages for volcanoes of the central and southern Emperor chain define large changes in volcanic migration rate with little associated change in the chain's trend, which suggests that the bend did not form by slowing of the Hawaiian hot spot. Initiation of the bend near Kimmei seamount about 50 million years ago (MA) was coincident with realignment of Pacific spreading centers and early magmatism in western Pacific arcs, consistent with formation of the bend by changed Pacific plate motion.
Noble gas distributions relative to hot spot plume
Differences in noble gas isotopic characteristics between alkalic volcanism downstream and upstream of the hotspot imply that upstream volcanism contains incipient melts from an upwelling mantle plume, having primitive 3He/4He. In combination with lithophile element isotopic data, we conclude that the most likely source of the upstream magmatism is depleted asthenospheric mantle that has been metasomatised (metamorphosed so that both chemistry and texture was changed) by incipient melt from a mantle plume. After major melt extraction from the mantle plume during production of magmas for the shield stage, the plume material is highly depleted in noble gases and moderately depleted in lithophile elements. Partial melting of the depleted mantle impregnated by melts derived from this volatile depleted plume source may explain the isotopic characteristics of the downstream alkalic magmatism. Lavas from the Southwest Oahu volcanic field have intermediate 3He/4He ratios of about 10 Ra, which suggests that some melt was fed to the source region of the volcano from a mantle plume, in contrast with downstream side alkalic volcanism.
Reference: T. Hanyu, D.A. Clague, I. Kaneoka, T.J. Dunai, and G.R. Davies (2005) Noble gas systematics or submarine alkalic lavas near the Hawaiian hotspot, Chemical Geology, 214: 135-155. [Abstract] [Article]
Evolution of magma supply during migration from hot spot
EAST MOLOKAI - There are geochemical differences between shield lavas from the two parallel trends, Kea and Loa, defined by young Hawaiian volcanoes. The shield of East Molokai volcano, at greater than 1.5 million years old (Ma), is the oldest volcano on the Kea trend. Sequences of older tholeiitic to younger alkalic basalt that erupted as this volcano evolved from the shield to postshield stage of volcanism are well exposed. Much younger, ~0.34–0.57 Ma, alkalic basalt and basanite erupted during rejuvenated stage volcanism. Like rejuvenated stage lavas erupted at other Hawaiian volcanoes, rejuvenated stage East Molokai lavas have relatively low 87Sr/86Sr and high 143Nd/144Nd. Such ratios reflect a source component with a long-term depletion in abundance of incompatible elements. On the basis of positive correlations of 87Sr/86Sr versus 206Pb/204Pb and negative correlations of these isotopic ratios with Nb/Zr, a smaller proportion of this depleted component also contributed to the late shield/postshield lavas erupted at East Molokai and the other Kea-trend volcanoes, Haleakala and Mauna Kea.
At each of these Kea-trend volcanoes, as the volcano moved away from the hot spot, the extent of melting and magma supply from the mantle decreased, the depth of melt segregation increased, and there was an increasing role for a component with long-term relative depletion in incompatible elements. This depleted component has Kea-trend Pb isotopic characteristics and relatively low 208Pb/204Pb at a given 206Pb/204Pb, and it is probably not related to oceanic lithosphere or the source of mid-ocean ridge basalt. The overlap in Sr, Nd, and Pb isotope ratios of recent Kilauea shield lavas and 550,000 year old Mauna Kea shield lavas has been used to argue that Kea-trend shield volcanism samples a vertically continuous, geochemically distinct stripe which persisted in the hot spot source for 550,000 years (Eisele et al., 2003; Abouchami et al., 2005). As Kea-trend volcanoes migrate away from the hot spot and evolve from the shield to postshield stage, there are systematic changes in Sr, Nd, and Pb isotope ratios. However, the overlap of Sr, Nd, and Pb isotope ratios in late shield/postshield lavas from Mauna Kea (<350,000 years) and East Molokai (~1.5 Ma) show that the periphery of the hot spot sampled by Kea-trend postshield lavas also had long-term geochemical homogeneity.
Reference: Xu, G., F. A. Frey, D.A. Clague, D. Weis, and M. H. Beeson (2005), East Molokai and other Kea-trend volcanoes: Magmatic processes and sources as they migrate away from the Hawaiian hot spot, Geochem. Geophys. Geosyst., 6, Q05008, doi:10.1029/2004GC000830 [Abstract] [Article]
Origin of volatiles in magma plumes
LOIHI - The high volatile content of ocean island basalts compared to mid-ocean ridge basalts has been suggested to drive the bouyancy of the mantle plumes under hot-spot volcanoes, rather than being a thermal phenomenon alone. Water, carbon-dioxide and sulfur concentration data for basaltic glasses from Loihi seamount allow modeling of degassing, assimilation and distribution of major volatiles within and around the Hawaiian plume. With these samples, it is possible to accurately account for the shallow processes in the magma chamber and during eruption, allowing characterization of deeper processes of thermal and geochemical plume structure.
Degassing and brine assimilation (both shallow processes) have affected CO2 and Cl but not H20 in most Loihi glass samples. However, the water content relative to non-volatile, similarly crystal-incompatible trace elements is dryer as compared to Kilauea, North Arch, and Kauai-Oahu, which are equivalent to mid-ocean ridge basalts (MORB), and especially dryer as compared to South Arch volcanic field lavas, which are wetter relative to MORB. The H20/Ce ratio is uncorrelated with major element composition or extent or depth of melting, but is instead related to position relative to the Hawaiian plume and mantle source region composition, consistent with a zoned plume model in which mantle components within the plume are drier than the exterior.
In front of the plume core (i.e., South Arch, which is in the precursory alkalic eruption stage), overlying upper mantle is metasomized (metamorphosed) by hydrous regions of the Hawaiian plume that segregated and partially melted early, and becomes wetter. Downstream from the plume core (Kauai-Oahu, North Arch), some of the metasomized upper mantle is uplifted by and mixed with plume material, yielding H20/Ce similar to Pacific mid-ocean ridge basalt. Within the plume (Loihi), the lavas are relatively dry and have H20/Ce less than or equal to MORB. The deep mantle source of the plume is thought to represent subducted oceanic lithosphere and sediments, and the drier H20/Ce may reflect efficient dehydration of subducted ocean crust and sediments during recycling into the deep mantle. High 3He/4He ratios are not the result of degassing of the volatile-rich plume, but instead probably result from heated lower mantle material entrained along the outer rim of the plume.
Reference: J.E. Dixon and D.A. Clague (2001) Volatiles in basaltic glasses from Loihi Seamount, Hawaii: evidence for a relatively dry plume component, Journal of Petrology, 42(3): 627-654. [Abstract]
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