The Origin of Diverse Intraplate Volcanism in the Central and Equatorial Atlantic Ocean
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The origin of diverse intraplate volcanism in the Central and Equatorial Atlantic Ocean DISSERTATION zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel Vorgelegt von Xiaojun Long Kiel, 2019 Erster Gutachter: ....................................................................................... Prof. Dr. Kaj Hoernle Zweiter Gutachter: ...................................................................................PD. Dr. Thor Hansteen Tag der mündlichen Prüfung: .................................................................................... 21.11.2019 .................................... Der Dekan Hiermit erkläre ich, dass die vorliegende Doktorarbeit selbständig, abgesehen von der Beratung durch den Betreuer und ohne Zuhilfnahme unerlaubter erstellt habe. Weder diese noch eine ähnliche Arbeit wurde an einer anderen Hochschule im Rahmen eines Prüfungsverfahrens vorgrlrgt, veröffentlicht oder zur Veröffentlichung vorgelegt. Auch wurde mir kein akademischer Grad entzogen. Ferner versichere ich, dass die Arbeit unter Einhaltung der Regeln guter wissenschaftlicher Praxis der Deutschen Forschungsgemeinschaft entstanden ist. Kiel, den .......................................................................................................................... Xiaojun Long ABSTRACT Despite decades of research, the origin of oceanic intraplate volcanism and the location of its magma source(s) are still a matter of debate. Intraplate volcanism was originally tightly connected with the mantle plume concept (e.g. Wilson 1963; Morgan 1971; 1972a, b, Richards et al., 1989). Many observations, however, were difficult to explain with just a single concept or do often not agree with the classical plume/hotspot model. Therefore, a large number of alternative models were suggested (e.g. see Foulger et al. 2005 for overview). This thesis focuses on diverse forms of intraplate volcanism in the Central and Equatorial Atlantic whose origins were still unclear: 1) the Researcher Ridge on the western flank of the Mid-Atlantic Ridge (MAR) at ~15° N; 2) Isolated seamounts or seamount clusters in the Canary basin; and 3) the Bathymetrists Seamount belt in the Sierra Leone basin. This thesis aims to decipher the origin and evolution of these submarine structures. Researcher Ridge is a submarine trail of partly coalescent volcanic edifices on the western flank of the Mid-Atlantic Ridge (MAR) at ~15° N. The projection of the WNE-ESE oriented Researcher Ridge intersects the well-known 14° N MAR anomaly (where the MAR axis is both bathymetrically elevated and geochemically enriched). Only one rock sample from Researcher Ridge was previously analyzed, confirming its volcanic origin and possessing a geochemically enriched composition. Therefore, a genetic relationship of Researcher Ridge and the nearby 14°N anomaly could be assumed. The Chapter 2 of this thesis presents the detailed geochemical and geochronological investigations of lavas from the Researcher Ridge and the neighboring 14° N MAR anomaly, to gain insights into their origin and potential relationship. The data reveal that Researcher Ridge lavas have geochemically enriched ocean island basalt compositions with isotopic signatures trending towards the FOZO or I HIMU mantle end member composition. Combined with an age constrained for one of its seamounts that is distinctly younger than the underlying ocean crust, Researcher Ridge is interpreted as a classic hotspot track. The lavas from the 14° N MAR anomaly have a similar, but less enriched isotopic compositions than Researcher Ridge, supporting plume-ridge interaction. It is suggested that the 14° N MAR anomaly is caused by recent deflection of upwelling Researcher Ridge plume material towards the westward migrating MAR. Once the plume was captured by the spreading ridge, off-axis hotspot track volcanism ceased, resulting in a seamount gap between the eastern end of the Researcher Ridge and the 14° N MAR anomaly. This study was published in spring 2019 in the journal Lithos (Long et al. 2019). In contrast to well-studied large elongated seamount chains, small solitary seamounts or small isolated clusters of seamounts have generally been overlooked in geological research. However, these seamounts occur in large numbers (probably in the millions) and populate the world's ocean basins, making their investigation of great significance. Most of these seamounts do not fit into any currently accepted models (e.g. they are not associated with a linear hotspot track or cannot be related to plate boundary processes) and their formation could represent a new kind of intraplate volcanism. Therefore, the second objective of this thesis (presented in the Chapter 3) is to reveal the origin of three representative isolated seamounts/seamount clusters in the Canary basin by detailed geochemical and geochronological research. The results suggest that the three different volcanic features apparently formed in different geodynamic settings (intraplate, on a spreading ridge, and along a fracture zone) despite their relative proximity (within ~1000 km of one another). Lavas from all three seamounts/seamount clusters, however, display compositional trends between a depleted (Mesozoic MORB) end member and a common enriched (Canary or Cape Verde mantle plume-like) component in all isotope systems. Seismic tomographic models suggest that slow velocity anomalies in the regional mantle II (generally reflecting hotter temperatures) are not restricted to the main plume conduits beneath the Canary/Cape Verde archipelagos, but seem to be more widespread. It is proposed that small, isolated seamounts could have formed through melting of these small-scale isolated upwellings in the upper mantle or by secondary “plumelets” that originate from larger anomalies beneath. This process would represent a new type of intraplate volcanism that may be extremely widespread but has been poorly studied to date. This work was submitted for publication in the journal Terra Nova (Long et al., under review). The third objective of this thesis focusses on seamounts that form a broad belt instead of a narrow hotspot track. Therefore, detailed geochemical on lavas from the broad Bathymetrists Seamount belt in the eastern Equatorial Atlantic was conducted (Chapter 4). The elongated shape of several of its seamounts suggests either a formation by shallow processes or a least a structural control of the volcanism (e.g. volcanism along local fracture zones). On the other hand, the seamounts could have been formed by the postulated Sierra Leone plume (which is proposed to be currently located near the MAR at ~600 km distance from the nearest Bathymetrist Seamount). The results reveal that the Bathymetrists Seamounts have formed in an intraplate setting but exclude shallow decompression melting. The isotopic compositions of the seamount lavas show a strong HIMU signature similar, although not identical with the nearby St. Helena hotspot composition. Applied plate tectonic reconstructions do not support the hypothesis that this seamount province was created by the Sierra Leone plume, instead it is proposed that the Bathymetrists Seamounts were generated by a distinct hotspot. The Bathymetrists Seamounts provide another example of extreme HIMU end member composition besides St. Helena (in the South Atlantic) and the Cook-Austral islands (in the South Pacific) supporting the recently published view of Homrighausen et al. (2019) that this distinct end member component is more widespread in the sources of intraplate volcanism than just at the two type localities noted above. III References Foulger, G. R., 2005. Mantle plumes: Why the current skepticism. Chinese Science Bulletin 50, 1555-1560. Long, X., Geldmacher, J., Hoernle, K., Hauff, F., Wartho, J. A., Garbe-Schönberg, D. and Grevemeyer, I., 2019. Age and origin of Researcher Ridge and an explanation for the 14° N anomaly on the Mid-Atlantic Ridge by plume-ridge interaction. Lithos 326, 540-555. Morgan, W. J., 1971. Convection plumes in the lower mantle: Nature 230, 42-43. Morgan, W. J., 1972a. Deep mantle convection plumes and plate motions: AAPG bulletin 56, 203-213. Morgan, W. J., 1972b. Plate motions and deep mantle convection: Geol. Soc. Am. Man 132, 7-22. Richards, M. A., Duncan, R. A., & Courtillot, V. E., 1989. Flood basalts and hot-spot tracks: plume heads and tails. Science 246, 103-107. Wilson, J. T., 1963. A possible origin of the Hawaiian Islands: Canadian Journal of Physics 41, 863-870. IV ZUSAMMENFASSUNG Trotz jahrzehntelanger Forschung ist die Ursache von ozeanischem Intraplattenvulkanismus und dessen Magmaquelle(n) immer noch umstritten. Obwohl ursprünglich eng mit dem Mantelplume-Konzept verbunden (z. B. Wilson 1963; Morgan 1971; 1972a,b; Richards et al., 1989), lassen sich viele Beobachtungen nur schwer mit einem einzigen Mechanismus erklären oder stimmen oft gar nicht mit dem klassischen Plume-/Hotspot-Modell überein. Daher wurde eine Vielzahl alternativer Modelle vorgeschlagen (siehe Foulger et al. 2005 für eine Zusammenfassung). Diese Dissertation befasst sich mit verschiedenen Formen des Intraplatten-Vulkanismus im zentraler und äquatorialer Atlantik, deren Entstehung bisher ungeklärt war: 1) dem Researcher Ridge an der Westflanke des Mittelatlantischen Rückens bei ~ 15°N; 2) mit solitären Seamounts oder isolierten Gruppen von kleinen Seamounts im Kanarischen Becken; und 3) mit dem breiten Gürtel der Bathymetrists Seamounts im Sierra Leone-Becken. Ziel