Klügel, Andreas (1997) Ascent rates of magmas and genesis, transport and reactions of mantle and crustal xenoliths of the 1949 eruption on La Palma (Canary Islands). (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 209 pp.

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Abstract

The 1949 San Juan eruption on La Palma was a chemically and mineralogically zoned multivent eruption. Following 13 years of weak seismic precursors and 3 months of strong seismicity, the eruption began on June 24 with 2 weeks of phreatomagmatic activity of Duraznero crater (1900m asl) producing tephrite. The activity subsequently changed to Llano del Banco (1300m asl) at 3 km distance, a fissure issuing initially tephritic and later basanitic lava during 18 days, causing a voluminous lava flow that entered the sea. A third crater, Hoya Negro ( l 850m asl), simultaneously erupted basanitic, tephritic, and phonotephritic lava in violent phreatomagmatic explosions from July 12 to 26. After 3 days of quiescence at all vents, Duraznero and Hoya Negro became re-activated on July 30 during the final eruptive phase. Duraznero erupted basanitic lava in fire fountains until the eruption terminated 12 hours later, producing an 8 km long lava flow down the eastern slope that reached the sea. The final basanite contains about 1 vol.% of crustal and mantle xenoliths: (a) 41% tholeiitic MORB gabbro fragments from the Mesozoic oceanic crust; (b) 34% hauyne-bearing alkaline gabbros interpreted as fragments of crustal intrusions; (c) 21 % olivine-bearing pyroxenites and kaersutitites representing magmatic cumulates formed within the crust and also the mantle; (d) 4% basanitic nodules interpreted as dike fragments; (e) < l% of various types of fused to pumiceous crustal rock fragments; (f) < l% peridotites (spinel dunites and harzburgites) from the upper mantle. Tephrite and basanite magmas from the 1949 eruption were derived from partial melting of a HIMU-type garnet lherzolite. Both subsequently fractionated dominantly olivine and clinopyroxene at > 1.1 GPa and at 0.6-0.9 GPa as indicated by clinopyroxene-melt thermobarometry. Reversely zoned phenocrysts indicate mixing with more primitive magmas at mantle depths, in the case of basanites no longer than weeks to months prior to eruption. Tephrites and basanites represent two distinct magma batches that may have had a common parental magma. During their ascent to the surface, both magmas passed crustal reservoirs within days to weeks without significant mixing. Some of the Hoya Negro magma became contaminated at shallow levels by assimilation of crustal rock fragments and/or an evolved magma batch. Peridotitic mantle xenoliths from the 1949 and other Recent eruptions on La Palma became mineralogically and chemically modified during prolonged reaction with their host magma. They show two distinct generations of fractures having magma contact: old fractures representing most of each xenolith's surface are characterized by 0.9-2 mm wide diffusion zones in adjacent olivine, and crystalline veins or selvages with cumulus textures. Young fractures show narrow diffusion zones (<0.02 mm) and no selvages. Diffusion zones resulted from element exchange between peridotite and host magma at their contact. A model of Fe-Mg interdiffusion in olivine gives an age of 8 to 83 years and <4 days for old and young fractures, respectively. These results imply a multistage ascent: peridotite xenoliths generally had contact with their host magmas for years to decades prior to eruption, possibly indicating a period of wall-rock fracturing. Subsequent magma batches transported the xenoliths to crustal reservoirs. Here they became deposited and were surrounded by cumulates during years to decades, a time during which no new fractures were formed. Final transport to the surface lasted hours to days, as documented by young fractures caused by decompressive strain during rapid ascent. Such auto-fragmentation upon decompression can account for the angularity of many xenoliths observed. C02-dominated fluid inclusions in peridotite and pyroxenite / kaersutitite xenoliths indicate nearly exclusively crustal pressures with a maximum between 200 and 350 MPa (7-11 km depth). These pressures reflect the most prominent magma reservoirs beneath La Palma where rising batches of mafic magma were stored long before an eruption occurred. This is consistent with ultramafic cumulates and tholeiitic MORB-gabbros from the lower crust being the most abundant xenolith types on La Palma. The possibly sill-like reservoirs are partly filled with crystal-rich mush braking ascending dikes and facilitating horizontal magma transport. They may form the base of the rift zone postulated for the island. As a result, mantle xenoliths were transported to the crust by magma batches other than those carrying them to the surface. Seismic precursors suggest that rising magma batches entered the crustal reservoirs beneath the center of the island and subsequently migrated southward. The seismic precursors of the 1949 eruption also correlate with a year-long storage of the erupted mantle xenoliths in the crustal reservoirs.

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