Die Ursache für Variationen der vulkanischen Aktivität entlang pazifischer Hotspot-Tracks: Wechselwirkung zwischen Plume und Lithosphäre oder pulsieren des Plumes
Physik des Erdkörpers
Zusammenfassung der Projektergebnisse
New precise radiometric ages and geochemical data for three long-lived, co-eval Pacific volcanic seamount chains allow the age-progressions, and variations in volcanic flux and geochemistry with time to be reconstructed and compared. Age progressions along parts of the Easter and Foundation seamount chains are non-linear due to the presence of microplates. As these microplates moved over the mantle hotspot, interaction with the spreading ridges at the microplate edges resulted in complex age progressions, and the subsequent volcanoes moved relative to each other after formation. For the Easter seamount chain, we find that after taking microplate formation and rotation into account, the volcanoes of the Line Islands and Tuamotu Plateau can be linked to the present location of the Easter hotspot, significantly extending the known distribution of Easter hotspot volcanism (O’Connor et al., 2022a, in prep.). Our new geochemical data show that lava composition is a function not only of distance to the hotspot at the time each volcano was built, but also the age and thickness of the underlying lithosphere, and interaction with spreading ridges at the microplate edges. Our new geochemical data for volcanoes from the Hawaiian Seamount Chain which were formed 55 to 22 Ma ago record an interesting period in the evolution of this seamount chain. At 51 Ma, an abrupt change in plate – hotspot relative motion led to the formation of the Hawaiian-Emperor ‘bend’, and immediately thereafter the volcanic flux above the Hawaiian hotspot decreased drastically, before increasing again approximately 25 Ma ago. Our new geochemical analyses (Regelous et al., 2022, in prep.) of lavas from this period show that during the low-volume period from 51 to 25 Ma, Hawaiian volcanoes were dominantly built of alkalic basalt, rather than the tholeiitic basalt that makes up about 90% by volume of most Hawaiian volcanoes. The similarity in Sr, Nd and Pb isotope compositions of the ‘low-volume’ volcanoes to those before and after suggest that the volcanic flux variations are not due to less enriched and less fusible material in the mantle source, but rather due to lower degrees of mantle melting. We propose that the change in plate – hotspot relative motion at 51 Ma disrupted the thermal structure of the upwelling mantle, leading to cooler upper mantle temperatures and lower degrees of melting. Combining our new age data for the Easter and Foundation seamount chains, with published data for these and the Hawaiian chain, allows us to compare in detail the temporal and spatial distribution of volcanism, and volcanic flux rates along three key parallel Pacific hotspot tracks. High-precision ages for these low-volume, rapidly formed volcanoes and ridges point to a relationship between changes in Pacific plate motion and mantle plume dynamics. This leads to our main finding – that synchronous increase in Pacific intraplate volcanism and faster age progression are linked to jumps in the direction of the Pacific Plate rather than simply plate speed-up or plume dynamics. The volcanic flux at the Hawaiian hotspot has varied significantly over the past 80 Ma, with variations over about 10 Myr periods. In addition to the low-flux period starting at 51 Ma, 25-30 Ma ago there was a sharp increase in the Hawaiian volcanic flux by a factor of ~4 that appears to be associated with an increase in Pacific plate motion from ~60 km/Ma to ~100 km/Ma. These variations remain unexplained by classic plume theory, but have previously been assumed to result from changes in the fertility, temperature or volume of the upwelling mantle material. Our new data for the Foundation Chain indicate that similar flux variations, at broadly the same time, also characterise this seamount chain. This indicates that flux variations at Hawaii cannot predominantly be due to changes intrinsic to the Hawaiian plume, but result from wider-scale tectonic processes. The amount of volcanism at Hawaii and other Pacific hotspots dips ~25-22 Ma and surges ~17-13 Ma and again ~6 Ma in conjunction with increases in Hawaii swell volume (O’Connor et al., 2022b, in prep.). We find that episodes of more northern (absolute) plate motion at ~25-22 Ma, ~17-13 Ma and ~6-2 Ma have an influence on age progressions, Sr-Nd-Pb isotopic composition, volcano morphology, and volcanic flux throughout the Pacific. Our results therefore support an absolute plate motion model based on twelve Pacific seamount chains (WK08- G) predicts that changes in the geometry of various hotspot chains reflect northward jumps in the direction of Pacific absolute plate motion at ~25 Ma, ~17-14 Ma and ~6 Ma. Moreover, these changes coincide with the timing of known tectonic events around the Pacific Rim (Wessel & Kroenke, 2007; 2008; Jicha et al., 2019). These synchronous changes at Hawaii and other Pacific hotspots suggest that surges in Pacific intraplate volcanism and faster age progression at Hawaii and other hotspots reflect jumps in the azimuth of the Pacific plate, which increases plate tensile stress and alters plume-asthenosphere interaction. Our ongoing study of the PP-RR and the Glimpse region will investigate further how the tectonicmagmatic development of Pacific intraplate volcanism results from shifts in plate motion rather than changes in the dynamics of Pacific plumes.
