Geochemical and petrological evolution of La Palma (Canary Islands) and its rift zone during the last 1.0 Ma Dissertation for the doctorate degree of the Department of Geoscience at the University of Bremen submitted by Karsten Galipp Bremen 2005 “Ach was alles Unsinn: Lava und Magma und Fumarolen und Eruptionen - im Berg sitzt ein scheußlich schwarzer Teufelskerl, der mit glühenden Steinen schmeißt.” Urmel aus dem Eis, Max Kruse Tag des Kolloquiums: 23.06.05 Gutachter: Prof. Dr. M. Olesch Dr. habil. T. H. Hansteen Prüfer: Prof. Dr. O. Brockamp Dr. A. Klügel Table of contents Abstract _________________________________________________________________ 1 Zusammenfassung ________________________________________________________ 3 1. Introduction _________________________________________________________ 6 2. Magma storage and underplating beneath Cumbre Vieja volcano, La Palma (Canary Islands) _____________________________________________________ 21 Andreas Klügel, Thor H. Hansteen, Karsten Galipp Submitted to Earth and Planetary Science Letters: Received revised manuscript, 18 February 2005 3. Thermobarometry ___________________________________________________ 46 Changing depths of magma fractionation and stagnation during the evolution of an oceanic island volcano: La Palma (Canary Islands) Karsten Galipp, Andreas Klügel, Thor H. Hansteen Submitted to Journal of Volcanology and Geothermal Research: Received 03.11.04 4. Geochemistry_______________________________________________________ 78 Petrogenesis and geochemical evolution of lavas from La Palma (Canary Islands): implications for plume-lithosphere interaction Karsten Galipp, Andreas Klügel, Tor S. Johansen, Folkmar Hauff, Kaj Hoernle To be submitted to Chemical Geology 5. Geochronology ____________________________________________________ 124 6. Conclusions _______________________________________________________ 140 Appendix ______________________________________________________________ 143 Erklärung ______________________________________________________________ 198 Danksagung____________________________________________________________ 199 Lebenslauf _____________________________________________________________ 200 Abstract Abstract Rift zones on ocean island volcanoes have a strong control over magmatic processes such as differentiation, crustal assimilation and magma transport mechanisms. Our knowledge of the magmatic evolution beneath rift zones at ocean islands is still limited. The aim of this thesis is to better understand the geochemical and petrological evolution of a rift zone in space and time focusing on La Palma (Canary Islands) as a case study. In order to model the magmatic evolution of the rift zone major, trace elements and radiogenic isotope compositions of representative rocks from the last 1.0 Ma were determined. This timespann encompasses the volcanic activity of the Taburiente shield volcano (1.2 - 0.4 Ma) and the Cumbe Nueva ridge (830 – 560 ka) as well as the short Bejenado volcanic phase (560 – 490 ka) which followed the Cumbre Nueva collapse (~560 ka), and the presently active Cumbre Vieja rift phase (125 ka to present). Clinopyroxene-melt thermobarometry and microthermometry of fluid inclusions were applied to constrain magma pathways and main levels of magma stagnation. CO2-rich fluid inclusions in Taburiente dunite xenoliths correspond to equilibrium pressures of 0.4 to 0.6 GPa (10 – 17 km). Inclusion data of Cumbre Nueva ankaramites and Bejenado cumulate xenoliths yield pressure estimates of 0.25 to 0.45 GPa (8 – 14 km). In comparison, fluid inclusions in Recent Cumbre Vieja rocks yield a pronounced pressure range of 0.20 to 0.40 GPa (5 – 14 km), which partly overlaps with the Cumbre Nueva data but is shallower than that for Taburiente. Clinopyroxene-melt barometry of phenocrysts and glassy groundmass yield equilibrium pressures between 0.60 and 1.05 GPa (19 – 34 km, Taburiente) and 0.50 to 0.85 GPa (16 – 28 km; Cumbre Nueva and Bejenado). This pressure range partly overlaps with that for Cumbre Vieja which is characterized by values of 0.45 to 0.64 GPa (14 – 25 km) but has a tendency to higher pressures. The combined data indicate that two separate magma stagnation levels existed for each rift phase: (1) a main fractionation level within the upper mantle, and (2) intermittent stagnation close to the Moho for the Taburiente volcano and above the Moho for Bejenado, Cumbre Nueva and Cumbre Vieja. A possible relationship of the magma plumbing systems of the extinct Taburiente shield volcano and the active Cumbre Vieja rift can be excluded due to the thermobarometric data obtained. Trace element and isotope ratios reveal distinct geochemical signatures for the active Cumbre Vieja and the extinct volcanoes. Sr, Nd and Pb isotopic ratios for the older Taburiente and Bejenado volcanoes show considerable variability (87Sr/86Sr = 0.703001 - 0.703110, 143Nd/144Nd = 0.512880 - 0.512971, 206Pb/204Pb = 19.142 - 19.947, 207Pb/204Pb = 15.544 - 15.639; 208Pb/204Pb = 38.829 - 39.758), whereas the range for Cumbre Vieja is rather limited (87Sr/86Sr = 0.703087 - 0.703169, 143Nd/144Nd = 0.512888 - 0.512901, 206Pb/204Pb = 19.497 - 19.715, 207Pb/204Pb = 15.609 - 15.888; 208Pb/204Pb = 39.159 - 39.488) and has a tendency to more depleted compositions than Taburiente. Much of the isotope 1 Abstract variations can be explained by binary mixing between an enriched HIMU-type plume component and a more depleted lithospheric component. Cumbre Vieja rocks are systematically more enriched in incompatible trace elements (Ba, Rb, P, Th, U, Nb, Sr, LREE) and show larger La/Yb (27.1 - 49.1) and Sm/Yb (4.6 – 7.3) ratios than the older Taburiente, Cumbre Nueva and Bejenado rocks (La/Yb = 19.1 – 37.2; Sm/Yb = 3.7 – 6.3). Remarkably, La/Yb and Sm/Yb ratios of Cumbre Vieja lavas are also higher than those of most ocean island basalts (OIB) wordwide. Slight concave-upward chondrite-normalized rare earth element (REE) patterns, negative K anomalies as well as relative depletion in Rb and Ba on mantle-normalized diagrams of fractionation-corrected La Palma basalts require the presence of residual amphibole in their melting region. It is suggested that this amphibole is of metasomatic origin as a consequence of infiltrating melts, which were derived from rising plume material penetrating the lithosphere at its base during the early submarine stage of La Palma volcanism. Model calculations for the variation of trace element ratios as a function of source composition and degree of melting illustrate that the observed rare earth element ratios and the high contents of P, Sr, Th, U and LREE of Cumbre Vieja lavas cannot be derived by a single-stage melting process from amphibole-bearing garnet lherzolite, garnet pyroxenite, or a combination of both. The compositions rather require assimilation of ~1% apatite by Cumbre Vieja melts during their ascent through the lithosphere. The geochemical data indicate that there is no progressive geochemical evolution of La Palma magmas but a compositional break between Taburiente volcanism in the north and Cumbre Vieja volcanism in the south. This conclusion is also supported by significant differences of Nb/U ratios between Cumbre Vieja lavas (33.8 – 52.3) and those of Taburiente and Bejenado (49.7 – 87.6), the latter being clearly above the global OIB range (47 ± 10) probably due to amphibole assimilation. The combined data suggest that the active Cumbre Vieja rift and the extinct volcanoes represent distinct volcanic systems, rather than a progressively evolving volcano. Incremental 39Ar/40Ar laserdating was carried out in order to extend the existing age data set of La Palma and to adress the question whether the apparent gap in volcanic activity between 410 and 125 ka could be related to sampling bias or a volcanic hiatus. However, 39Ar/40Ar age determination and step heating experiments of basaltic groundmass and glass separates yielded geologicallly unreasonable results due to strong contamination of the samples by atmospheric argon. Therefore, only one Bejenado (550 ± 20 ka) and one Cumbre Nueva (730 ± 70 ka) sample revealed reliable crystallization ages. 2 Zusammenfassung Zusammenfassung Riftzonen von Ozeaninsel-Vulkanen haben einen starken Einfluß auf magmatische Prozesse wie Differentiation, Mischung und Assimilation. Jedoch sind die Prozesse der Magmenentwicklung von Riftzonen nur wenig erforscht. Ziel dieser Arbeit ist es deshalb, ein besseres Verständnis über die zeitlich-räumliche geochemische und petrologische Entwicklung einer Riftzone zu erhalten. Als exemplarisches Fallbeispiel wurde La Palma gewählt. Um die zeitliche magmatische Entwicklung zu rekonstruieren, wurden die Zusammensetzungen der Haupt- und Spurenelemente sowie die Sr-Nd-Pb Isotopen- Verhältnisse repräsentativer Gesteine der letzten 1.0 Ma ermittelt. Diese Zeitspanne umfasst sowohl das Aktivitätsstadium des Taburiente Vulkans (1.2 – 0.4 Ma) und des Cumbre Nueva Rückens (830 – 560 ka), als auch die kurze Phase des Bejenado Vulkans (560 – 490 ka) nach dem Cumbre Nueva Kollaps (~ 560 ka), sowie das aktive Cumbre Vieja Riftstadium (von 125 ka bis rezent). Tiefen von Magmenreservoirs und temporäre Stagnationsniveaus wurden anhand eines Klinopyroxen-Schmelz-Barometers und mikrothermometrischen Untersuchungen von Fluideinschlüssen bestimmt. CO2-reiche Fluideinschlüsse in Taburiente Xenolithen weisen auf Gleichgewichtsdrücke von 0.40 bis 0.60 GPa hin, entsprechend Stagnationstiefen von 10 bis 17 km. Fluideinschlußdaten von Cumbre Nueva Ankaramiten und Bejenado Kumulaten ergaben Drücke